A new frequency plan and power deployment rules

Transcription

1 Deliverable D14 A new frequency plan and power deployment rules October 2009 ReDeSign Research for Development of Future Interactive Generations of Hybrid Fibre Coax Networks Information for Publication: Version: 1.0 Status: Public (PU)

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3 Contents Management Summary Introduction The future HFC capacity challenge Operational and business restrictions Scope of the ReDeSign studies Upstream capacity The technical issues Currently available solutions Allocation of an additional return band spectrum Summary and conclusion Downstream capacity The DVB-C2 signal level Performance simulations Network and network load scenario s Network signal quality parameters Results Summary and conclusion Summary and Conclusion New Frequency Plan

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5 Management Summary According to the early concept of the cable distribution network, the customer should be able to connect his terrestrial receivers directly with the cable network. This completely legitimate and logical requirement has shaped the HFC frequency plan. With the advent of digital transmission technologies, this linkage of the cable frequency plan to the terrestrial frequency plan has become obsolete; however, most of the transmission systems still respect the historical HFC frequency plan. A major disadvantage of the current frequency plan is the upstream band which is limited to 65 MHz. This frequency limitation is associated with the conservation with the FM radio band (87,5 108 MHz). In this study we have reconsidered the use of the HFC spectrum assuming that in the all digital era all historical restrictions can be abolished thus allowing an operator to redefine the frequency plan according to his needs, with an appropriate balance between up and down stream spectrum. Considering the outcome of the studies, we have to conclude that operators are still strongly bonded to the existing frequency design of the cable networks and to the terrestrial use of the ether. Therefore, a complete redefinition of the frequency plan appears not possible. Transmission capacity is determined not only by the available frequency spectrum, but by the applicable carrier signal and distortion signal level as well. Therefore, we have analyzed the possibilities to expand the upstream and downstream capacity from the integral viewpoint of available spectrum, the possible signal level and the distortion signal level. In principle, the frequency plan should be defined to maximize the total upstream and downstream network transmission capacity in a balanced manner, as demanded by the market. However, as elaborated in the report, a complete abandonment of the historically defined frequency plan appears impossible. The two primary reasons to conserve the existing frequency plan are: A majority of the cable operators foresees delivery of FM radio signals for at least a full decade There are many options to expand the downstream transmission capacity or to implement capacity saving solutions. Because of this an expansion of the downstream band beyond 865 MHz is rated the least. Combining both observations fixates the current frequency plan almost completely. Irrespective of this fixation of the frequency plan, we have studied the options to expand or to maximize the upstream and downstream capacity. Regarding the upstream capacity, the operator response on earlier ReDeSign network questionnaire reveals that in most cable networks the upstream band is not used efficiently. In many networks the upstream band is yet not extended up to 65 MHz whereas ingress noise levels prohibit the use of high (64 QAM) modulation schemes. In the report we provide a review of the solutions to reduce the ingress noise. Operators should first resolve this problem of ingress noise, possibly in combination with the extension of the upstream band up to 65 MHz, to maximize the upstream capacity and to warrant economical use of EuroDOCSIS equipment. Having thus upgraded the upstream channel, they can keep track with the customer capacity demand by adding more EuroDOCSIS channels and/or splitting the upstream segments. Next, once the above capacity expansion solution is exhausted, operators will face the challenge to expand the capacity beyond this level. The capacity of the MHz band is fully used, and operators are forced to find new spectrum for the upstream band, which requires a substantial network upgrade. Basically, there are two options, extension of the MHz band to higher frequencies, or the creation of a new frequency band in the UHF band, be New Frequency Plan

6 yond 865 MHz. The first option requires a solution to deliver FM radio to those customers that use this service. In addition, the EuroDOCSIS technology has to be adapted. In the DOCSIS specification, the upstream band is already extended to 85 MHz, so technically there is no serious problem; however, operators do depend on the willingness of the manufacturers. Likely, the definition of a UHF return band is more promising. By using VHF-UHF frequency converters placed in the network, investments and network adaptations can be limited. This solution is shown in the figure below. In this option, the customer equipment still transmits in the MHz band and in the lower part of the coaxial network upstream signals are conveyed at these frequencies, but higher up in the network the MHz signals are converted to a frequency beyond 950 MHz. This way, the upstream capacity can be boosted by a factor of 10 or more, whereas it requires limited network adaptations and no adaptation of the EuroDOCSIS equipment. Branching point Frequency Upconverter n m GHz 1 GHz 1 GHz 2 65 MHz n o e MHz m branches MHz As pointed out above, operators are not specifically inclined to extend the downstream band egde beyond 865 MHz because of the numerous ways to make a more efficient use of the spectrum from 85 up to 865 MHz. At the level of the network layer, the replacement of analogue signals by digital carriers and the deployment of DVB-C2 are the basic elements to implement this approach. In the ReDeSign studies we have addressed a crucial issue of this approach: the capability of the existing European HFC networks to support the DVB-C QAM modulation mode. Application of this mode requires a high DVB-C2 signal level, and the question is whether a sufficiently high signal level can be deployed without degradation of the analogue TV, DVB-C and DVB-C2 signals by the distortion products (intermodulation products) associated with the non-linear nature of the active components. To warrant a realistic result, a number of operators provided data from their networks. Four networks scenarios were studied with cascades of 2, 4, 5 and 15 amplifiers respectively. The coaxial parts and the amplifiers were completely specified, including noise and non-linear behavior of the latter. Three network loads included: a mixed analogue, DVB-C and DVB-C2 scenario (20 PAL, 30 DVB-C and 43 DVB-C2), an all digital DVB-C and DVB-C2 scenario (15 DVB-C and 78 DVB-C2). For these scenario s, we calculated the signal quality parameters like SNR for the digital carriers and CNR and CINR for the PAL signals as a function of the DVB-C2 signal level. These calculations showed that in case of the mixed load scenario DVB-C QAM modulation can de used in all four networks. In case of the all digital scenario, three out of the four networks support the use of DVB-C QAM modulation. This result suggests that DVB-C QAM modulation can be applied in many European HFC networks; however, not in all networks New Frequency Plan

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8 1 Introduction Mastering radio communications via the ether has been one of the major accomplishments of the 20 th century. Amongst these, analogue terrestrial TV appeared very successful. In response of the market demand for more TV channels, cable networks and cable technology was developed. These networks were designed for the distribution of TV services based on the terrestrial analogue TV transmission technology so that the customer could use the standard TV and FM radio sets. Therefore the design reflects the radio technologies and the spectrum allocations of the terrestrial broadcast services: Band I (TV, MHz), Band II (FM Radio, 87,5 108 MHz), Band III (TV, MHz) and Band IV/V (TV, MHz). Up to today, cable or HFC networks reflect this inherited design. Cable networks are fully owned and managed by private companies, and by their nature these networks hardly interfere with terrestrial communication services. Thus, in principle, the operator has a high level of freedom to choose the radio design of its networks. Currently, we are witnessing a period of fast change of the customer service demand. At the same time, many new technologies and solutions are developed to address the market demand. For these new services new user equipment is needed that not necessarily requires the historical cable spectrum design. Therefore, it appears logical to reconsider the spectrum design of cable networks as well. In this report, we present the results of our analysis of the cable radio design and spectrum use. The point of departure is given by the future capacity and services demand, the existing networks and the existing and known forthcoming technologies. Like in all media, the transmission capacity of an cable network is limited by i) the available frequency spectrum, ii) the signal level and iii) the distortion signal levels. Operators will face a dramatic change of the exploitation of these resources, spectrum, signal level and distortion signal levels. Here we present such an analysis. First, in the sections 1.1 and 1.2 we will consider the forthcoming network capacity demand in reference to the known network capabilities and the practical and operational limitations of changing the frequency plan respectively. This first analysis shows that operators are still strongly bonded to the existing frequency design of the cable networks and to the terrestrial use of the ether. Because of these limitations, we have adapted the scope of our studies to warrant a good alignment of the ReDeSign studies and the operator s interests. This narrowed scope is presented in section 1.3. In the next Chapters 2 and 3, we discuss the possibilities and the limitations of expanding the upstream and downstream transmission capacity of the network. 1.1 The future HFC capacity challenge In the forthcoming years, the market demand of digital broadband services and media will challenge the HFC network capacity limits of the cable operators. Although the HFC networks have a scalable architecture allowing incremental capacity expansions, every network has a maximum capacity limit. Considering the large variation between the European HFC networks and regional market conditions, some operators face this capacity limit in the more near future whereas others have sufficient capacity and may serve the markets for a decade or more. The network and business questionnaires of the ReDeSign WP2 and WP3 studies included questions regarding the network capabilities and the expected services demand. Analysis of these data provided a more quantitative picture of the network capacity challenge, as briefly New Frequency Plan

9 summarized in the following. A more extensive discussion can be found in the March 2009 issue of the Broadband Journal. 1 Capacity [Gbps] MHz 550 MHz DS Capacity (Nodes 600 HP) upgrade toward DVB-C2 Today 1-2 years 2-5 years 5-10 years Time Capacity [Gbps] MHz 550 MHz DS Capacity (Nodes 200 HP) upgrade toward DVB-C2 Today 1-2 years 2-5 years 5-10 years Time Internet VOD HD VOD SD Switched HDTV Switched SDTV HDTV SDTV PAL/SECAM Figure 1 Potential network capacity per fiber node (nodes of 600 HP and of 200 HP) for a networks with 550 MHz and 862 MHz downstream band edge versus future (average) capacity needs assuming capacity saving solutions are timely implemented. The data is based on input of European MSOs. In Figure 1 we match the expected downstream capacity demand versus the HFC capacity limit in Gbps per network node 2. In this analysis, all currently known capacity saving solutions, like the use of H.264 video compression and the use of switched TV are taken into account. In the forthcoming years, operators will gradually replace part of the analogue TV channels by DVB-C2 channels with a larger capacity per 8 MHz channel, which adds to the expansion of the maximum capacity as shown by the dashed curves in Figure 1. The figure shows the network capacity limit for networks which can be segmented in small nodes of 200 homes passed (HP) and large nodes of 600 HP, for networks with a frequency edge of 550 MHz and 862 MHz. These data show that networks with 862 MHz frequency edge that can be segmented into small nodes of 200 HP can support the market service demand for more than a decade; however, networks with larger nodes and/or a frequency edge of less than 862 MHz will face shortages within a decade. Figure 2 Future upstream capacity demand for segments of 600 HP and 200 HP, in case of an asymmetrical service demand. 1 Jan de Nijs,, Tim Gyselings and Carsten Engelke, Securing Europe s Cable Future, Broadband, Vol. 31, No 1, p. 72, March An analogue TV channel is represented by a virtual capacity of 52 Mbps New Frequency Plan

10 In Figure 2 and Figure 3 we show the capacity analysis for the upstream channel for state-ofthe-art HFC networks with a 65 MHz return channel and with a low ingress noise level, again for nodes of 600 and 200 homes passed. In such networks, operators can allocate four 6.4 MHz upstream channels with 16 QAM modulation, with a total capacity of 80 Mbps. Often, it is impossible to deploy 64 QAM modulation or a fifth 6.4 MHz channel. Therefore, 80 Mbps can be considered as the maximum upstream capacity per network segment in case of stateof-the-art networks. Figure 2 shows the data for the case that the current asymmetric traffic profile is conserved whereas in Figure 3 it is assumed that the market will require a more symmetric traffic profile because of the success of new services like videophone, camera surveillance of private homes, network back up services and P2P services. The analysis shows, that when the future market will not develop a demand for symmetric services, state-of-the-art HFC networks will be able to serve the market for the next decade; however, when the market will require symmetric services, capacity shortages will develop in time, as shown in Figure 3. The 80 Mbps for the state-of-the-art networks will not be sufficient, and operators will have to find ways to further expand the upstream capacity. In the latter figure, also the maximum upstream capacity limit of 180 Mbps is indicated. This limit is associated with the deployment of 6 upstream channels of 6.4 MHz and 64 QAM modulation. Hence, operators will be challenged to master the deployment of QAM channels. Figure 3 Future upstream capacity demand for segments of 600 HP and 200 HP, in case of an asymmetrical service demand. In this ReDeSign study, we give an in-depth analysis of the possibilities to tackle the downstream and upstream capacity challenges. Since the upstream and downstream paths have a completely different electric nature, the capacity challenges will be discussed in separate chapters. In many HFC networks the upstream capacity is rather limited because of the limited frequency spectrum (up to 65 MHz) and the high return band ingress noise levels. As such, there is a serious concern whether this band may provide the capacity demand of the future market. This issue and the conceivable remedies are treated in Chapter 2. In the downstream channel, analogue TV currently requires half of the available spectrum. Typically about 50% of the frequency spectrum is used for analogue services. Operators are considering a substantial reduction of the analogue service packages and the deployment of DVB-C2 transmission systems to create the required capacity for digital services. To accommodate this change of network load, a revision of the RF planning of the HFC networks New Frequency Plan

11 is needed. Operators have to reconsider the use of the available frequency spectrum and they have to define the appropriate power levels for the DVB-C2 transmission systems. In Chapter 3 we will discuss the network issues related to DVB-C2 deployment In the following section 1.2 of this introduction, we will first elaborate the network capacity issue from the operator and network management viewpoint. This analysis reveals some preferences and restriction regarding the redesign of the HFC frequency spectrum use. Based on these preferences and restrictions, we have narrowed the scope of the ReDeSign studies in section Operational and business restrictions The network transmission capacity is basically limited by the available power budget and by the available frequency spectrum. Expansion of the network capacity thus can be accomplished by expansion of the power budget and/or by expansion of the frequency spectrum, whereby often a larger frequency spectrum will require a larger power budget to provide the signal power for the additional carriers. The reduction of the analogue service packages is one option to create signal power for digital services. From the practical, operational and business perspectives, however, there are several objections against a too radical reallocation of power budget and frequency spectrum. Moreover, on the short term, today s operational and business requirements may exceed the long term importance of the sufficient up and downstream network capacity. Therefore, any long-term solution must be properly aligned with these more short-term operational and business restrictions. In the WP3 questionnaire, operators were consulted regarded some of the restrictions 3. In this subsection we present a high-level review of the operational and business aspects interfere with the expansion of the network transmission capacity Operator valuation of evolutionary upgrading technologies Currently, the market is flooded by a multitude of innovative cable technologies. Economic solutions for expanding the network capacity are developed like advanced node splitting solutions. Furthermore, technologies are developed to use the existing capacity in a more effi- Table 1 Average rating (Scale 1-10) evolutionary upgrades technologies Upgrade option Network segmentation 8,1 Statistical multiplexing 5,6 Rating cient manner, for example improved modulation codes (DVB-C2), switched digital TV and advanced video codecs like H.264. In parallel to the development and roll out of these innovative technologies, the market conditions are gradually changing. For example customer acceptance of digital TV services is increasing, thus diminishing the inconveniences of reducing the analogue packages. Analogue switch off 5,3 Better modulation codes QAM sharing Switched digital TV Extension up to 1 GHz Extension beyond 1 GHz 5,0 4,9 4,5 3,2 2,9 In the ReDeSign network and services survey, we have asked the operators which evolutionary solutions or techniques for expanding the network capacity or for using the existent capacity in a more efficient manner they appreciate most 3. The response is summarized in Table 1. The result confirms the preference for network segmentation as the most appropriate solution to expand the network capacity. Next, a number of new (digital) technologies and the switch off of analogue ser- 3 Service Requirements Report, ReDeSign D10, June New Frequency Plan

12 vices are more or less equally preferred with a rating of 4.5 up to 5.6. Finally, the extension of the frequency range beyond 862 MHz is considered as the least favourable solution. In the opinion of the operators, extension of the frequency range is a last solution. Clearly, when considering a new frequency plan, this opinion must be taken into account Analogue TV As indicated earlier, analogue TV consumes about 50% of the frequency spectrum. Reduction of the analogue TV package thus releases spectrum and signal power. The reduction of analogue TV paves the way to deploy DVB-C2 modulation which requires a sufficiently strong signal level for the 4K QAM modulation mode. Today the analogue TV offer comprises about 40 channels 3. The increasing market acceptance of digital TV makes it possible to gradually reduce the analogue packages. However, analogue TV is considered as a unique selling point since the signals are distributed throughout the house in the most convenient manner, allowing customers to view television in all rooms of the home without the need of additional equipment like STBs. Therefore, cable operators foresee an analogue package of 20 TV channels over a time period of 5 10 years from now Deployment of higher order modulation techniques Reduction of the analogue service packages results in a release of spectrum resources that can be reused for other (digital) services. The current DVB-C technology dates from more than a decade ago. It is the result of the status of the technology at that time and of the (less demanding) market conditions. Modern technology, and in particular modern error protection algorithms, permits a more efficient use of the frequency spectrum and the market demands for higher capacity. DVB-C2 has been developed to address both issues. It provides a higher throughput per 8 MHz channel in a more efficient manner than DVB-C. In particular DVB-C2 supports 4096 QAM modulation providing a bitrate of 84 Mbps per channel, albeit at an expected signal level higher than that needed for DVB-C 256 QAM modulation FM radio services FM radio is transmitted in the frequency band from 87 MHz up to MHz. In most networks, the FM band demarcates the lower edge of the downstream band. As such, the FM band is the main constraint of the upstream band. To extend the return band to frequencies above 65 MHz, operators have to sacrifice the FM radio band. In their feedback on the ReDeSign questionnaire, the cable operators expressed the business relevance of the FM radio services. Currently, all operators, apart from an incidental exception, do offer FM radio services as part of the basic analogue subscription. Over period of 10 years from now, 60% of the operators still will deliver FM radio services. This response indicates that FM radio is considered a crucial service that cannot be simply terminated without a negative business impact. Only with the availability of a convenient and economic alternative, operators could consider the clearance of the FM band Downstream band frequency extensions To expand the network capacity, the extension of the downstream frequency band toward higher frequencies may appear a most straightforward solution. However, in practice, such an extension appears rather complex. Still many European HFC networks have a down New Frequency Plan

13 stream band edge of less than 862 MHz. Therefore, below we briefly consider the extension up to 862 MHz, up to 1 GHz and beyond 1 GHz Extension up to 862 MHz Today s HFC networks often have an upper frequency edge of 862 MHz. This frequency corresponds with the frequency edge of the terrestrial analogue TV band (Band V). Consumer equipment like TV sets and many EuroDOCSIS cable modems are not designed for the reception of signals with a higher frequency. Because of the limited availability of such equipment, 862 MHz is a logical and practical upper edge for today s HFC networks. Nevertheless, many HFC networks have an upper frequency edge below 862 MHz. Some networks have a frequency edge of 550 MHz, or even below. These band edges are no hard limitations, but the frequencies above can not be used in an efficient manner. From a RF planning viewpoint, the use of the higher frequencies is less attractive because of the increasing attenuation of the coaxial cables. A channel at a higher frequency will consume a relatively larger part of the available signal power. Because of this, operators have a preference to allocate the channels at the lowest frequencies. In practice, an operator will make optimum use of the available power budget provided by the amplifiers; he will maximize its network load in terms of the number of analogue and digital channels. As such, in a properly designed and operated network, the downstream band edge and the available power budget are properly balanced and it will not be possible to add a substantial number of additional channels. To add a substantial number of additional channels, the operator will have i) to add frequency spectrum by shifting the down stream band edge to a higher frequency and ii) he will have to increase the power budget. In practise, he will have to replace the network amplifiers by ones with a higher output power. Often, the spacing between the amplifiers is rather large, and operators have not only to replace the amplifiers, but they have to add and re-space amplifiers in the cascades to shorten the distance between the consecutive amplifiers, which, of course, is most costly. In the current market there is a growing demand for digital services whereas customers are more willing to accept a reduction of the analogue service package. Therefore, an operator has to make a trade-off between i) further segmentation of the network, ii) replacement of analogue channels by digital ones, iii) the replacement and possible re-spacing of the amplifiers in order to raise the network power budget and to extend the downstream frequency band Frequency extensions up to 1 GHz For networks with a frequency edge of 862 MHz, extension beyond this frequency up to 1 GHz may appear an attractive solution, though the general problem of enhanced signal attenuation at higher frequencies will require a re-planning of the signals, possibly in combination with the replacement and/or a re-spacing of the amplifiers. Using the band from 862 MHz up to 1 GHz, however, will invoke some currently unknown problems associated with the terrestrial use of the radio spectrum by others. Most of the terrestrial spectrum between 87 MHz and 862 MHz is allocated to radio and TV broadcast services 4. Operators are hindered by terrestrial radio and TV broadcasting transmitters; however, the problems are limited to homes with inferior in-home coaxial networks in the vicinity of the transmitter. In contrast, the spectrum ranging from 862 up to 1 GHz is not allocated to ,5 MHz FM radio band; MHz (Band III) digital radio and digital TV broad casting; (Band IV/V) analogue and digital TV New Frequency Plan

14 terrestrial broadcast services but, amongst others, to mobile communication systems (GSM). GSM-900 uses MHz for uplink transmissions and MHz for the downlink. In addition, spectrum is allocated for mobile communications for railways (GSM-R) MHz (uplink) and MHz (downlink). Because of the different radio services in the 862 MHz 1 GHz band, mobile communications instead of terrestrial broadcasting, a completely different the interference scenario will occur. In case of mobile communications, customers will use their mobile terminals in home. The isolation of today s state-off-the art cable equipment like STBs and modems is insufficient to avoid interference between the cable RF signals and the transmit signals of the mobile terminals. In addition, in case of a low quality inhome network, the coaxial cables will act as an antenna that receives the mobile transmit signals, thus aggravating the interference problem. Because of these ingress problems, European operators are not eager to use the band from 862 MHz up to 1 GHz, as confirmed by the ReDeSign questionnaire Frequency extensions beyond 1 GHz Beyond 1 GHz frequencies, operators have to anticipate further operational problems. Signal attenuation will become even more pronounced than at the frequencies below 1 GHz, in particular in case of bamboo coaxial cables that have specific bands of enhanced attenuation at these frequencies. In addition, the operation of network passives will fail because these are not designed for frequencies above 1 GHz. The isolation of the passives will drop dramatically, thus contributing to further signal loss. Furthermore, at higher frequencies, the isolation of the coaxial cables will drop whereas at the output of the amplifiers a higher signal levels is needed to compensate the larger coaxial attenuation. Therefore, conceivably, excessive egress problems may arise beyond the regulatory EMC limits of cable networks Summary and conclusion In the above section we have provided a brief overview of the business perspective and technical limitations of changing the frequency plan. In summary it is argued that: There are many solutions to expand the network capacity and to make a more efficient use of the existing capacity. Taking this situation into account, operators indicate that extension of the downstream band beyond 862 MHz is the least favourite solution, The analogue packages will be reduced, thus creating room to increase the digital services. However, over a decade operators will still offer some 20 analogue channels, FM radio services are considered as a crucial part of the cable services portfolio that cannot be switched off unless a good and convenient alternative transmission solution is offered. Customers should not be disturbed with difficult solutions, To fully use the potential network capacity created upon the reduction of analogue TV services, operators must deploy 1024 QAM or higher order modulation scheme, The use of frequencies above 862 MHz is for different technical disadvantages very unattractive New Frequency Plan

15 1.3 Scope of the ReDeSign studies In this treatise, we are considering the possibilities to expand the downstream and upstream network transmission capacity. From the fundamental viewpoint there are three approaches to create more capacity: Reduction of noise and distortion signal levels in combination with the use of higher order modulation schemes, Raising the carrier signal levels in combination with the use of higher order modulation schemes, Extension of the frequency bands. In the following parts of this report, we will study the above approaches in more detail for both the downstream and the upstream frequency band. At the very beginning of this chapter we have argued that, considering the historical roots of the current radio design of cable networks and the forthcoming market demands, a complete reconsideration of the RF spectrum use was appropriate. However, taking the overall business and technical perspective into account, some a priori restrictions of the scope of the revision of the radio design appear appropriate: To expand the downstream network capacity, extension of the frequency band beyond 862 MHz appears a less attractive solution than the deployment of a transmission technology that supports higher order modulation schemes in combination with all other capacity saving solutions. Therefore, the ReDeSign studies should respect this 862 MHz as the upper band edge limit, To expand the upstream network capacity, we cannot a priori indicate a preferential solution (higher signal levels, reduction of noise level or extension of the upstream band), though extension of the frequency band beyond 65 MHz should not be considered as a first solution because it requires the termination of FM Radio services in the 87,5 107 MHZ band. Therefore, the scope of the ReDeSign studies must include all three approaches to expand the upstream capacity, taking in to account that extension beyond 65 MHz is the least favorable New Frequency Plan

16 2 Upstream capacity During the first meeting of the ReDeSign Operator Forum, some operators have expressed their concerns concerning upstream capacity shortages. According to their viewpoint, the forthcoming demand for upstream channel capacity will create the first major bottle neck. In this chapter we study the possibilities to expand the upstream capacity in detail. The scope of this study has been limited to solutions that conserve the current frequency division duplex architecture, taking into account the business limitations of paragraph 1.2. As pointed out earlier, we will analyze the possibilities of i) expanding the frequency spectrum, ii) applying higher signal levels and iii) reducing noise levels. In the following we provide a technical overview of the issue (paragraph 2.1), a treatment of the solutions compliant to the current HFC radio design (paragraph 2.2) and options that require a reconsideration of the HFC radio design (paragraph 2.3). We will end this chapter with a brief summary and conclusion of the analysis. 2.1 The technical issues Spectrum availability and quality In the current state-off-the art HFC networks, the upstream channel is located in 5 65 MHz frequency band. However, before the advent of two-way services, Band I (47 68 MHz) was used for analogue TV broadcasting. Because of this historical use, in many cable networks Band I still is used for broadcast services, thus limiting the upstream band. In the ReDeSign network questionnaire, operators were asked to provide the upstream band edge. The response is summarized in Table 2. This results shows that although many operators have upgraded their networks up to a 65 MHz band edge, still about half of the operators have not upgraded their networks, or only partially Table 2 Upstream band edge as obtained from the ReDeSign questionnaire. The most common band edge in the range is indicated between brackets. Band Edge Occurrence MHz (65MHz) 53% MHz (52MHz) 12% < 50 MHz (42 MHz) 35% Next to this limited upstream band edge, the background noise levels are the principle cause of the limited capacity. In the mid and late nineties of the past century, the noise characteristics of the upstream channel has been extensively studied for reasons of the development of the upstream transmission technology. 5,6,7 The upstream noise enters the cable system via 5 P.J. Snijders and C-J L. Van Driel, Channel modelling of the return channel in a broadband communication CATV network, 28 th European Microwave Conference, Amsterdam New Frequency Plan

17 the customer s in-home networks. The use of inferior quality cables and splitters are a source for ingress of all types of unwanted signals. Such noise emanating from all homes connected to a cable node is aggregated at the hub or head end. From a practical viewpoint, a cable node can be considered as a vast distributed antenna with hundreds up to a thousand antennas, receiving and aggregating all electro magnetic signals from the homes. Moreover, today s homes have various coaxial branches, thus forming dipoles. Such dipoles further enhance the sensitivity of the in-home networks, thus aggravating the ingress problem. An interesting treaty can be found in reference 8. The distortion signals picked up in the home environment can be distinguished in three kinds of different nature and origin: Universal broadband noise: a roughly db increase of the noise floor below 25 MHz Narrowband interferers: short wave radio transmitters (world radio etc) Impulse and burst noise: caused by human activity in the home (switching on/off equipment etc) which creates wideband impulses and bursts of several µs with a signal level of hundreds of mv (up to volts). The activity of many of these sources shows a periodic cycle during the day. Figure 4 Continuous ingress noise taken from reference 9. Similar figures can be found in reference C. Eldering et al. CATV return path characterisation for reliable communications, IEEE Communications Magazine, 62 68, August K. Haelvoet et al. Procedure for measurement and statistical processing of upstream channel noise in HFC networks, IEEE MTT 0S Digest, K. Mothersdal, Ingress Safe, one small step for man, one giant step for your return path, Broadband, Volume 30, April 1, S.Pfletschinger, Multicarrier modulation for broadband return channels in cable TV systems, PhD Thesis University of Stuttgart, New Frequency Plan

18 In contrast to the noise signals, upstream signals are transmitted rather incidentally. Moreover, only one cable modem per RF channels is allowed to transmit at the time, thus excluding concurrent transmissions in the same upstream channel. Table 3 Optical node topology data taken from the ReDeSign reference architectures Reference Network topology Tree & Branch Hybrid Star Average homes passed per fiber node Number coaxial branches per fiber node 400/ /1000/ /2/3 1/2/3 2/ The cable networks Networks do exhibit a large variety in network parameters and noise levels. The ReDeSign questionnaire provided network figures from 21 European operators. After analyses of these data, the architecture of the European networks was captured in a limited number of reference architectures. 10 In Table 3 we have listed the relevant topology data. Because of the large variation in the European networks, unique topology figures could not be established, but instead different optional values were specified, as shown in Table 3. In this table, the average number of homes passed per fiber node is presented showing the large range of 400 up to 2000 homes passed that found. Operators have reported the maximum node size as well, yielding even twice as large nodes. Thus in reality, optical nodes of 4000 homes passed do occur regularly. Next to the node size, the number of coaxial branches emanating from the optical node is included in the reference architecture, showing that each node serves one up to four coaxial subnetworks. Table 4 Operator US channel parameters Bandwidth (MHz) 1,6 3,2 6,4 Modulation scheme QPSK 3% 17% 6% 16 QAM 6% 30% 13% 64 QAM 3% 13% 8% To assess the quality and performance of the upstream band in European cable networks, the ReDeSign network questionnaire encompassed some questions regarding the upstream 10 Reference Architectures Report, ReDeSign Deliverable 10, Octobre 31, New Frequency Plan

19 transmission profiles that are used. Operators will use the transmission profile with the largest bit rate that can be deployed in their networks. Hence, this information reflects the quality of the upstream path. This information is shown in Table 4. This table shows the occurrence of the combinations of the bandwidth and modulation mode that is used. The figures reveal that the most attractive profile (6,4 64 QAM) corresponding with a bitrate of 30 Mbps can be used in few networks only. In case of the majority of the networks, the transmission profile is limited to ones with a bitrate of 10 Mbps or less. 2.2 Currently available solutions The data of Table 2, Table 3 and Table 4 reveal that in many cable networks not all currently available technology to maximize the upstream capacity are currently deployed. In these networks, substantial capacity gains can be obtained by implementation of today s technology: Further segmentation of the network Extension of the upstream band edge up to 65 MHz Higher return band signal levels Reduction of the noise level In this section we give an overview of the conceivable options. First we will address the options to make maximum use of the current cable architecture and available transmission technology (DOCSIS). In this case, the capacity is limited by the DOCSIS technology that supports upstream channels up to 65 MHz and a spectral efficiency of about 5 bits/hz. In general, only the band from about 30 MHz up to 60 MHz can be used, thus limiting the maximum capacity to 150 Mbps per cable segment. In the further sections, we discuss new options that require new technologies, but that can surpass this 150 Mbps limitation Segmentation of the return path Today, many proven solutions for segmentation of the return path are commercially available. These solutions are based on the use of wavelength division multiplexing of the upstream frequency band, thus providing a scalable solution that allows an operator to split the nodes when needed. Splitting a node yields segments with approximately half of the number of homes. Thus, the aggregated ingress noise of the homes is halved as well, thus allowing the operator to apply a higher mode modulation scheme Extension of the US band edge up to 65 MHz In many networks, the US band is extended up to 65 MHz already. This upgrade can be considered as well-known and proven. Extending the US band edge, not only brings new spectrum, but moreover spectrum of a better quality allowing for a higher modulation scheme Higher return band signal levels As a first remedy to a low signal noise ratio, one could raise the signal level of the upstream signals. However, the upstream signal level is limited to a value of 114 dbµv to avoid harmful egress 11. The maximum transmit signal level of the EuroDOCSIS technology already is set to this maximum value and as such the signal level cannot be further raised. 11 IEC New Frequency Plan

20 2.2.4 Reduction of noise level Table 4 reveals that many operators are troubled by high noise levels in their networks. As such, reduction of the noise levels has to be considered as a serious option to expand the upstream capacity. Below we briefly discuss a number of remedies Full replacement cables and splitters in the home environment Inferior quality of in home components and a bad installation is the major cause of ingress. Conceivable, an operator could reinstall the customer in home networks, however, in practice this is a very difficult approach since each home has to be visited. Operators can encourage the use of high quality components in combination with professional installation practices, for example by creating awareness with the customers or by introducing a component quality hallmark that is supported by manufacturers, vendors and consumer stores Replacement of in home splitters The ingress signals from two in-home cable branches are strongly correlated as pointed out in reference 8. These distortion signals of different in home coaxial branches are induced by the same in-home events or received from the same external terrestrial radio sources. Because of this common source, the induced distortion signals of two branches are strongly correlated. The currently used splitters simply combine these signals. Since they are in phase, the voltages are summed. In reference 8 it is proposed to replace the current splitters by ones that create a 180 degrees phase shift in one of the branches so that correlated signals arriving from the different branches are cancelled (partially). It is shown that such splitters allow a reduction of 10 db or more of the noise level. All distortion signals, impulse and burst noise, narrow band interferers and the universal broadband noise below 25 MHz are effectively reduced. Such a solution appears attractive since splitters are relatively easy to replace as compared to a full rebuild of the customers in home network, although a visit of a technician is still required and in practice appointments with customers are difficult to make and second and even third visits are required. In networks with a high noise level, a 10 db reduction of the noise level allows a twofold capacity expansion of a DOCSIS upstream channel. In addition, it will allow extension of the usable upstream band to lower frequencies Deployment of ingress blocking systems In the late nineties of the past century, the concept of ingress blocking has been developed. Currently commercial systems like that of Proxilliant are on the market. An interesting whitepaper can be found on the John Weeks Enterprise site 12 An ingress blocking system acts as a control port that generally disconnects a cable upstream segment, but that connects the segment when an upstream DOCSIS packet is transmitted. The blocking system responds to the increased signal level of the preamble of the DOCSIS packet. The system is not channel specific: irrespective of the transmit frequency and bandwidth of the packet, the full upstream band is conveyed. As such, the noise reduction strongly depends on i) the number of DOCSIS upstream carriers deployed and ii) the number of homes connected to the network segment that is controlled by the blocking system. The noise reduction R noise (that is the ratio of the noise level at the CMTS without and with blocking system) is given by: R noise = N node / (N block segment x N carriers ) This formula provides an approximation of R noise for the case of N node /N block segment >> N carrier New Frequency Plan

21 With: N block segment = number of homes connected to the segment controlled by the ingress blocking system N node = number of homes served by the fibre node N carriers = number of DOCSIS upstream carriers When deployed sufficiently deep in the cable networks, say one blocking system serving a cable segment of 20 homes in a network with fibre nodes of a 1000 homes, the blocking system provides db noise reduction. Such a gain is substantial, and it will allow expanding the capacity of an upstream channel by a factor 2 to 4, for example an operator using QPSK modulation and 1.6 MHz upstream channel may upgrade to 16 QAM modulation and a 3.2 MHz channel. Furthermore, the reduced noise level allows the extension of the usable up stream band toward lower frequencies. As such, the solution is attractive for very noise networks. A disadvantage of the ingress blocking solution is the reduced noise suppression when deploying several upstream carriers in the same segment. Because of this, the blocking equipment should be installed near the homes thus creating small blocking segments Deployment of wall outlet with a separate FM radio, TV and EuroDOCSIS outlets Conventionally, the in-home wall outlet is equipped with separate TV and FM sockets, where the FM and TV ports respectively pass on the FM band and the bands 5 70 MHz and MHz. Without replacement of this wall outlet, a customer can connect its in-home network to the TV port and connect a cable modem everywhere in the home provided the signal strength is sufficient. In this case, all ingress distortion signals received by the in-home are injected in to the return channel. Alternatively, an operator can replace the wall outlet by one with a dedicated data port for connecting the cable modem. Only this data port allows bidirection communications. Next the cable modem is connected by a short lead to the data port whereas the in-home network is connected to the TV socket. In such a wall outlet, the upstream band of the TV socket is blocked and thus the in-home network will not contribute to the ingress noise in the cable return path. A major disadvantage of this solution is the need of a separate cable modem and of dedicated in-home data network based such as WiFi or 10/100baseT Ethernet. A major advantage is that the use of a dedicated three port wall outlet resolves the ingress problem at the root. It disconnects the in home network from the 0 65 MHz upstream channel Analysis and summary noise reduction solutions In this subparagraph we have briefly discussed a large number of currently known solutions to increase the upstream capacity. The information from the ReDeSign questionnaire shows that many networks are not completely upgraded yet, and as such major capacity gains can be realized when properly upgrading the networks. In addition, different improvements of the in-home network are possible and operators can continue with further splitting the upstream segments. The data of Table 4 show that some first operators have mastered the successful deployment of the 6,4 MHz bandwidth / 64 QAM modulation profile. As such, the current information obtained from the operators shows that the operators still are in the process of restructuring and improving their networks. Considering the large number of options to reduce the ingress noise problems, we can assume that in the end all cable networks will be able to support 6,4 MHz bandwidth / 64 QAM modulation profile. In the end, a capacity of 120 up to 150 Mbps per segment can be obtained. For completeness we have to emphasis that the implementation of the above improvements is complex and demanding; there is no cheap solution or quick gain. One thus can expect that operators will implement the solutions when the market forces them to do so, and not earlier. Ranking the solutions then installation of a wall outlet with a dedicated EuroDOCSIS New Frequency Plan

22 port appears the most effective and future proof solution because it disconnects the in-home network from the 0 65 MHz upstream path, thus eliminating the problem at the source. 2.3 Allocation of an additional return band spectrum In the above section we have studied the possibilities to expand the upstream capacity by expanding the upstream band up to 65 MHz and/or by reduction of the ingress noise signals. It is argued that when applying the appropriate measures, an upstream capacity of Mbps per segment would be feasible. Although some small capacity gains beyond 150 Mbps are conceivable in case of some specific networks, any substantial gains will not be possible. In other words, with an upstream capacity of 150 Mbps or perhaps a slightly larger bitrate, the limits of the current cable network design are fully reached. These solutions are fully compliant with the current HFC radio design. For a further substantial increase of the upstream capacity, drastic changes in terms of expanding the spectrum for upstream signals are inevitable. In this subsection we will discuss the conceivable options Extension of the current 5-65 MHz VHF return band A most evident and logical option to expand the upstream transmission capacity concerns the annexation of the lowest part of the downstream band. Moreover, such an annexation of neighboring spectrum would yield high-quality spectrum. This option is illustrated in Figure Spectrum issues As pointed out in the introduction, the extension of the upstream band edge toward higher frequencies requires the sacrifice of the FM radio band. The key question thus appears whether customers will accept the switch-off of FM radio or not. As a rule, customers will accept such a switch-off in case there are sufficient alternatives that are well received in the market. For FM radio, already many alternatives do exist (DAB, DVB-T, DVB-C, internet radio, ); however, on the other hand one should note that in particular FM radio is much appreciated for its high-quality audio signal, large number of channels including local channels and good reception in cars. Therefore, although the switch-off of terrestrial FM radio is under consideration, termination of this service within the next decade appears uncertain. As such, if cable operators will decide to stop broadcasting FM radio services, likely they will be the first to so, which may result in public arousal and at a time that alternatives are still insufficiently developed o m Branching point 30 MHz MHz o e MHz m branches MHz MHz Figure 5 Architecture of an E2E VHF return band solution. The 120 MHz upper edge is arbitrary chosen and intended as an illustration only New Frequency Plan

23 A possible, but likely not appealing, midway solution could be a partial annexation of the FM band, say an extension of the upstream band edge up to 80 MHz so that the 100MHz 108 MHz part of the FM band is conserved. In this case, an operator could add 2 6,4 MHz channels, or 60 Mbps per segment. An alternative solution could be the transmission of the FM signals at a higher frequency using block conversion technology. Next, those customers that appreciate reception of the FM radio signals must be provided with an appropriate frequency down converter. Such a solution can be developed by an individual manufacturer since it doesn t require cooperation between manufacturers to specify a standard. If an operator would decide to shift the upstream band up to frequencies beyond 85 MHz, he could as well extend the upstream band well beyond this frequency up to 100 or 120 MHz, which would respectively bring say 6 up to 8 extra upstream channels of 30 Mbps each. In case of networks with modular components (amplifiers, nodes etc) with replaceable diplex filter and upstream amplifiers, the costs of upgrading the network are the least, albeit certainly not negligible; however, in case of fully integrated components, an operator faces a large network investment Transmission system issues Apart from the HFC network adaptations needed to extend the upstream band, transmission technology capable of using these higher frequencies is needed. The current EuroDOCSIS technology only supports upstream frequencies up to 65 MHz. However, the newest American DOCSIS technology (CM-SP-PHYv3.0-I ) already supports the upstream frequencies up to 85 MHz. For the adaptation of the EuroDOCSIS equipment the operators are dependent on the willingness of the vendors to implement this solution in EuroDOCSIS. They will have to discuss this option with this industry. For frequencies above 85 MHz, there currently is no (Euro)DOCSIS upstream technology. Such technology needs to be developed by the industry. Clearly, for the deployment at neighboring frequencies, no new design of the upstream technology is needed. In summary we can conclude that an extension of the downstream band beyond 65 MHz will require a substantial network upgrade. Nevertheless, there appear no fundamental obstacles or great technological uncertainties, only many practical problems that certainly should not be underestimated. Therefore we may conclude that operators and manufacturers together should be able to develop the equipment and solutions when the market requires for such higher upstream capacity. Assuming an extension with for example 65 MHz, this will triple the total upstream capacity Creation of a new UHF return band > 862 MHz Apart from extending the existing upstream band, one can consider the creation of a new UHF upstream band above 862 MHz. Such a solution can be based on the use of a frequency up converter and down converter. To convey the signals, the operator will have to install UHF return band amplifiers and diplex filters in the coax network. Both components of this solution can be considered as known technologies that can be developed by a manufacturer. Furthermore, this solution is fully compliant with today s DOCSIS upstream transmission technology so that there is no need to introduce new transmission technology. To separate the new upstream band from the downstream band, a diplex filter is needed. Such a diplex filter will consume a substantial frequency band of the spectrum that cannot be used for other services. In Figure 6 an example of such a filter is shown. Simulation of this specific filter show that the -3dB stop band edges are situated at the frequencies of 883 and 965 MHz. Thus at least 80 MHz of spectrum is needed as a guard band. Taking this mini New Frequency Plan

24 mum band into account, an upstream channel could start from the midst of the 900 MHz band. Figure 6 Example of a UHF diplex filter (9 th order elliptical filter) Basic spectrum considerations In section 1.2 we have argued the inappropriateness of the use of the spectrum above 862 MHz for downstream services. The major disadvantages are i) interference from terrestrial mobile communications systems, ii) doubts regarding the electrical performance of the coaxial cables at frequencies above 1 GHz and iii) a failing performance of the passives in the network. For upstream services, in contrast, some of these disadvantages conceivably are less a handicap. Regarding the first concern, interference of a nearby mobile transmitter like a GSM cell phone, one should note that the cable downstream signal is very susceptible for such a distortion because of the low signal levels (about dbµv). In contrast, the upstream signals will have a 40 db higher signal level and, moreover, the (Euro)DOCSIS upstream transmitter has been designed for operations in a environment with impulse and burst noise events. Therefore, these frequencies appear more appropriate for upstream than for downstream use. However, interference between cable and terrestrial radio services is a mutual problem. When applying high cable signal levels, there is a risk of distorting the terrestrial mobile communication signals, such as the down stream signal from the mobile base station to the user terminal. In particular if the cable modem is connected via an (inferior) in home network that is installed and/or managed by the customer, there is a serious risk of such harmful interference. The second issue concerns the quality of the coaxial cables for carrying signals above 1 GHz. Dependent on the type and brand, the signal attenuation may increase dramatically for specific frequencies whereas screening effectiveness may reduce as well. As such, the signal balance may change completely. In the distribution part of the coaxial network, however, the output signal of the amplifier is split one or several times to feed different coaxial branches. In contrast, the upstream signals of the different coaxial branches are combined. Thus the signal loss for the upstream signals will be less than that for the downstream signals. Conceivably, this smaller signal attenuation due to combining instead of splitting the signal provides a sufficient margin to compensate for the higher attenuation of the cables and New Frequency Plan

25 for lowering the return signal level to control the egress of the cables. In case of the coaxial trunk feeding the coaxial distribution network, however, the signal loss due to splitting the signal power is rather limited, and as such there is less or no margin to absorb the higher attenuation of the cables and for lowering the signal level to reduce the egress. Next to the above, one should take into account that the coaxial part of the network is composed of different types of coaxial cables, which will complicate the identification of the most appropriate frequency band to allocated a new return path. The passives in the network have not been designed for these high frequencies, and as such one may expect a rapid performance decline for frequencies above 1 GHz. Likely, an operator will have to replace the passives. From the above, one may conclude that the creation of such a return band, assuming it s a viable option, concerns a large network upgrade, worth doing only when an operator may gain a fourfold or preferentially eightfold upstream capacity. For such a four or eightfold capacity expansion, respectively some 120 or 240 MHz of spectrum is needed. Per coaxial segment this would yield an upstream capacity of about 500 or 1000 Mbps. If the upstream band starts off from say 950 MHz, then the spectrum up to at least 1070 or 1190 MHz is needed. Table 3 lists the typical topology parameters of the reference architectures. In the worst case situation nodes of an average size of 2000 homes passed and with 1-3 coaxial branches from the node are found. For the extreme case of a single coaxial branch, 2000 homes have to be served. In case of a market development toward symmetrical services, a capacity of about 3000 Mbps is needed by the end of the next 10 year period, see reference 1 and or. Clearly, the creation of a 240 MHz return band in the UHF band is not sufficient to serve such a demand. However, in the case that the node of 2000 homes is composed of 3 separate coaxial branches, than the new return band will provide this capacity of 3000 Mbps. One should note that this analysis refers to the real worst-case networks and assuming a market development toward symmetrical services. For networks with smaller fibre nodes and larger number of coaxial branches emanating from the node, the maximum capacity per home will proportionally larger. Thus, although this solution will not be applicable for all European networks, it will nevertheless cover the needs of a vast majority of all European networks. In the above capacity analysis we have not considered the reduction of the noise level associated with the reduction of the size of the coaxial segments. In principle, a fourfold expansion of the upstream spectrum will yield segments of one-fourth of the homes passed or a 6 db noise level reduction Implementation options Conceivably, there exist two options to implement such a new return band: From the home to the head end: an E2E UHF return band The frequency up converter is placed in the customer home, conceivably integrated with the EuroDOCSIS cable modem. The down converter is located in the head end, or possibly integrated in the (EuroDOCSIS) upstream receiver. This solution is illustrated in Figure 7. From a concentration point in the network to the head end: hybrid VHF/UHF return band The frequency up converter is placed in the coaxial part of the HFC network at a network aggregation point, as shown in Figure 8. All MHz return band signals from the homes in the associated coaxial segment are aggregated and up converted to a frequency band above 862 MHz. In the following we will discuss both options separately New Frequency Plan

26 1 2 3 n Branching point 1 GHz 1 2 m o e m branches Figure 7 Architecture an E2E UHF return band solution In case of the E2E UHF return band, the frequency up converters can be directly connected to the cable modem, or even integrated in the cable modem. At higher frequencies, the inhome network will be less prone to ingress noise associated with in-home human activity. Therefore, this implementation of the up converter directly connected to or integrated with the cable modem offers the advantage of a substantial reduction of this kind ingress noise. However, as said, at these frequencies mobile communication devices may interfere with the cable upstream service and vive versa, the cable UHF upstream signals may degrade the mobile downstream signals. Thus, this solution will require dedicated frequency planning to avoid harmful interference with terrestrial services. As an alternative, the up converter could be integrated in the customer wall outlet or with the multitap serving the coaxial drop lines. In such a solution, the in-home network will carry the conventional MHz upstream signals, and not the alternative UHF return band signals. Thus, less interference with terrestrial communication systems can be expected. However, the advantage of a reduced ingress in the MHz upstream channel is sacrificed as well. The second implementation option is based on placing the return band up converter at a concentration point close to the optical node. In such an architecture, the interference with the terrestrial radio services is minimized; however, at the expense of a raised ingress of MHz distortion signals. Comparing the practical implementation of both options, than it shows that the option where the upstream band conversion is placed as close as possible to the optical node requires a substantially smaller network upgrade. Only the parts where a new UHF upstream band is needed require an upgrade in terms of placing the diplex filters, upstream amplifiers and likely replacing the passives. Moreover, only the cables of the coaxial trunk will carry the UHF return band signals, and not all the cables from the home to the node. This spatial limitation of the UHF upstream band thus enlightens the problem of finding suitable spectrum New Frequency Plan

27 Branching point Frequency Upconverter n m GHz 1 GHz 1 GHz 2 65 MHz n o e MHz m branches MHz Figure 8 Architecture of a hybrid VHF / UHF return band solution This solution of a UHF return band has the feature that it pairs frequency stacking of 4 or 8 upstream channels with a deeper segmentation of the HFC network without the need to deploy fiber beyond the optical node. This is shown in Figure 8. Assuming that this architecture allows a further segmentation by another factor of 3, it can boost the upstream capacity by a factor of Summary and conclusion Summarizing, the creation of upstream channels above 1 GHz will be challenging, however, this solution allows a vast and scalable expansion of the upstream capacity. Analysis of the spectrum indicates a preference to use the spectrum above 862 MHz for upstream signals, and not for downstream signals; this spectrum is used for terrestrial mobile services and the cable downstream signals will be more prone to interference than cable upstream services. An attractive solution appears the creation of an UHF upstream band from a concentration point A near the optical node to the head end, with the frequency conversion point located at this point A. From the home up to concentration point A the conventional MHz band is conserved. This solution minimizes the network upgrade while minimizing interference with terrestrial services. Nevertheless, one should be aware that here we have presented a conceptual analysis, and no proof or results from trials. Therefore, this architecture must be considered as a technical proposal as input for the development of a solution to expand the upstream capacity. 2.4 Summary and conclusion In this chapter we have studied the options to expand the upstream capacity of cable networks. First we have presented an overview of the current status of the networks. This overview shows that in many networks the upstream path can be classified as far from state of the art; not the full frequency range up to 65 MHz is used, and ingress levels are rather high, inhibiting the use a 6.4 MHz bandwidth and/or 64 QAM modulation. The operators will have to extend the upstream band up to 65 MHz in combination with a reduction of the ingress noise level. Assuming that all the appropriate measures are implemented, a maximum capacity of 120 up to 150 Mbps per segment can be obtained, and not more New Frequency Plan

28 Table 5 Summary and comparison return path capacity expansion options Current state-of-the art Extended VHF return Band E2E UHF Band Hybrid VHF/UHF Band Bandwidth (indicative) 35 MHz Control ingress noise Main issues 90 MHz Control ingress noise Replacement diplex filters in amplifiers and taps Sacrifice FM radio / or alternative radio service Adaptation EuroDOCSIS 120/240 MHz 120/240 MHz Control ingress noise Managing of UHF harmful interference to other (terrestrial) services Installation frequency VHF/UHF converters, UHF diplex filters and UHF return band amplifiers and replacement passives Adaptation of the optical node Control ingress noise Limited installation VHF/UHF converters, UHF diplex filters and UHF return band amplifiers and replacement passives; part of the coaxial networks toward the homes does not need adaptation Adaptation of the optical node To create a capacity above this Mbps limit, there are two options: i) extension of the existing return band toward frequencies above 65 MHz, an extended VHF return band or ii) the creation of an new UHF upstream band above 950 MHz. The first option, extension of the 65 MHz band will only yield a double or triple upstream capacity at most. Moreover, it requires the sacrifice of the FM radio band. The second option, a new UHF return band, is more challenging, but will provide much more capacity. This solution offers a combination of stacking 4 up to 8 upstream bands in say 120 or 240 MHz and of a further splitting of the upstream segments at a network level below the optical node. This new UHF extension band can be implemented either as an E2E technology or as a hybrid VHF/UHF solution. In this hybrid solution, the network adaptations are limited to the coaxial network parts near the optical node and the optical node itself. Table 5 provides a concise summary of the options. To conclude this chapter we have to note that this treatise is far from conclusive. It should be considered as a desk study of the options to expand the upstream capacity. The analyses shows that operators that have not upgraded the return band to 65 MHz should first implement such an upgrade. In parallel they should resolve the ingress noise issue. If a further capacity expansion is needed, than possibly the hybrid VHF/UHF return band solution appears most attractive. It combines the largest capacity expansion with a limited network upgrade New Frequency Plan

29 3 Downstream capacity In chapter 1 and in particular in paragraph 1.3 we have argued that an expansion of the frequency band beyond 862 MHz appears not attractive from the technical as well as the operational viewpoint. Instead, a better use of the available power budget or an expansion of the power budget appears the preferred solution. In this chapter we will assess the possibilities to expand the downstream capacity without expansion of the frequency spectrum. Instead, we will analyze the benefits of the use of DVB-C2 in a quantitative manner. This analysis boils down to three central questions: 1. what is the minimum DVB-C2 signal level needed for proper reception, 2. what is the maximum tolerable DVB-C2 signal level? 3. what is the maximum DVB-C2 modulation scheme? In this chapter we will address these question in integral manner and in relation to the other HFC network signals like PAL and DVB-C carriers. To warrant a result relevant for the European situation, we have closely worked together with some European cable operators. They provided the technical specification of their networks as input for the ReDeSign studies. In this chapter, first we will evaluate the DVB-C2 signal level requirements in section 3.1. Next in 3.2 (Simualtions) 3.3 (scenarios) (Results) we describe the technical basis and approach of our studies. 3.1 The DVB-C2 signal level The minimum signal level In contrast to the HFC network, the cable operator can not plan or manage the customer inhome network. Because of this, he will offer a minimum signal of high quality that allows satisfactory reception in case of an elementary in-home network, for example a network with two coaxial leads feeding a TV set/stb and a cable modem respectively. A customer, of course, is free to install a more extended in-home network with coaxial branches to different rooms; however, in this case it s the customer s own responsibility to deliver a sufficient signal level in the rooms. The above user case provides a more or less deterministic case to specify the nominal signal levels of the services as delivered by the operator. Below we follow a reverse analysis from the tuner of the customers TV set, STB and cable modem up to the system outlet of the HFC network of the operator Signal Requirements The international standards like the IEC provides signal requirements obtained from studies, measurements and practical experiences from operational networks. In Table 6 we list some of the signal requirements that are relevant for the current analysis. Since DVB-C2 is not included in the IEC standard, the currently known requirement for the signal to noise ratio are included. These values are obtained from the DVB TM-C New Frequency Plan

30 Table 6 Signal requirements for analogue and digital services Analogue (IEC ) DVB-C (IEC-60728) DVB-C2 Signal customer wall outlet (dbµv) Carrier-to Random Noise Ratio (db) Carrier to Composite Beats Ratio (db) FM PAL SECAM 64 QAM (38 Mbps) 256 QAM (52 Mbps) 1k QAM (70 Mbps) 4k QAM (84 Mbps) Sensitivity tuners The sensitivity of a PAL receiver is defined as the minimum signal level at the tuner input socket needed for synchronization and to recover the video and audio signals with a sufficient quality. Similarly, the sensitivity of a QAM-receiver is defined as the minimum input level at the input of the receiver for which Quasi Error Free (QEF) reception is possible when only the wanted signal is present. In this context, QEF is defined in EN , where QEF means less than one uncorrected error event per hour. This requirement corresponds to BER = 1e-11 at the input of the MPEG-2 multiplexer. The required receiver sensitivity (for PAL and digital services) is the result of a straightforward power budget calculation as illustrated in Figure 9. receiver sensitivity receiver Implementation Loss signal level theoretical required SNR for QEF 0 thermal noise floor Figure 9: budget calculation for receiver sensitivity In this calculation, the thermal noise floor for the broadband cable technologies like PAL and DVB-C and DVB-C2 is about 4 dbµv. 14 This value refers to the weighted carrier-to-composite ratio for summed clusters in negative modulation, see IEC , paragraphs and New Frequency Plan

31 PAL tuners in modern TV sets and state-of-the-art digital receivers have an implementation loss associated with the analogue RF processing and, in case of a digital tuner, the digital processing. Although state-of-the-art receivers require an implementation loss of 8 db, we assume a loss of 11 db because such an implementation loss matches with the loss in case of the existing EuroDOCSIS cable modems. In addition we assume a 1 db higher implementation loss for DVB-C QAM modulation because of the more advanced digital processing. In the customer home, the services are conveyed to the different receivers like TV set, cable modem or STB. The operator delivers the signals to the wall outlet which as a rule is located in the meter cupboard in the home. Therefore, an operator should ad a signal margin for the signal transport from the wall outlet to the respective receivers. This margin concerns a business choice. Table 7 Receiver sensitivity and minimum signal level at the cable system outlet. PAL DVB-C DVB-C2 64 QAM 256 QAM 1k QAM 4k QAM Implementation loss (db) Minimum SNR taken from Table 6 (db) Noise floor (dbµv) Sensitivity (dbµv) (43) (47) Nominal signal level at the input port of the wall outlet (dbµv) Minimum IEC requirement taken from Table 6 (dbµv) As a rule, the HFC networks are designed following a set of design rules, or a reference design, that specifies the worst case home connection in terms of i) a maximum number of cascaded amplifiers between the optical node and the homes, ii) a maximum length of the coaxial cable between the node and first amplifier, two subsequent amplifiers and the end amplifier and the home and iii) the number of coaxial branches or homes fed by an amplifier. This set of design rules thus specifies the worst-case connection between the node and a home. When constructing a real network, the neighborhood with all its geographical features is mapped on the reference architecture. As a result of this blueprinting, most of the home connection will have better signal levels and signal-to-noise ratio than the nominal signal levels and nominal signal-to-noise ratio of the reference design. Because of this, an operator may choose to warrant a good reception of all cable signals in case of an elementary inhome network fed by a worst-case coaxial cascade from the node to the customer wall outlet, and no more. Since most homes will have a better signal quality than the nominal signal 15 The Recommendation of the ITU BT.804 provides a sensitivity figure of 50.8 dbµv for a UHF PAL receiver. This figure is some 3 db less than our estimate. 16 The figure between brackets represent the sensitivity typically for consumer modems (cf. Arris, Motorola and Thomson) New Frequency Plan

32 quality associated with the worst-case cascade, a more extended in-home network will work properly in most homes. If not, the customer should install a consumer electronics VHF/UHF amplifier. The above elementary in-home network would allow connection of a single TV set and a cable modem. In case of analogue reception, the TV set is directly connected to the wall outlet. In contrast, in case of digital reception, the TV set is connected via a STB. In this elementary in-home scenario the nominal signal from the cable plant has to be split once, either by the wall outlet (separate FM, TV and data sockets) or with an external splitter (wall outlet with FM and TV socket only). Next a coaxial cable of say 10 m is needed to connect the TV set, STb or cable modem. Typically such an in-home coaxial cable will have an attenuation of 20 up to 30 db per 100 m at a frequency of 865 MHz. As such, the in-home signal attenuation would add up to about 7 db. Taking this 7dB margin in to account, we can calculate the nominal signal level at the input port of the wall outlet. In Table 7 we have included the nominal signal levels. In addition the tables lists the minimum signal levels for PAL and DVB-C taken from the IEC standard. Comparison of this minimum signal level and the nominal signal levels shows the values for the analogue TV services and for DVB-C do match properly The maximum signal level Apart from a minimum signal level needed for a good reception, one has to consider the maximum signal level that the network can deliver as well. Dependent on this maximum signal level, an operator can apply the 4096 QAM modulation mode or not. SNR CINR DVB-C2 [db] Impulse noise AWGN + AWGN -4-5 BER Measured Carrier signal level [db µv ] LOG (BER) Figure 9 Schematic diagram of the Signal to Noise Ratio (SNRDVB- C2) versus the carrier signal level for a single amplifier loaded with 96 digital carriers. The figure illustrates the nature of the distortion signals. The bit-error-rate (BER) refers to the BER before error correction. By gradually increasing the DVB-C2 signal level, the composite signal power of the cascade will gradually increase. At a specific DVB-C2 level, the composite signal power will reach the maximum level that the cascade can tolerate. Beyond this level, the generation of intermodulation products will become noticeable, gradually deteriorating the quality of the signals. For a single amplifier this behavior is schematically shown in. This figure is an abstraction of different measurements. This figure shows the ratio of the carrier signal level and the sum of the thermal noise and intermodulation products (SNR DVB-C2 ). For a low carrier level, say below 110 dbµv, the generation of intermodulation products is negligible and the SNR DVB-C2 increases linearly with the carrier level. Beyond 110 dbµv, the generation of intermodulation products becomes noticeable, as shown by the deviation from the line with slope equal to 1. Beyond 112 dbµv, the intermodulation products dominate the distortion signal, resulting in a New Frequency Plan

33 steep decline of the SNR DVB-C2. Measurement of the bit error rate (before error correction) shows the onset of bit errors at an carrier level associated with the maximum of the SNR DVB- C2 curve. In the ReDeSign project, we have studied single amplifiers and cascades of amplifiers, with an all digital and a mixed analogue digital load. In all cases comparable SNR curves were obtained, see for example Deliverable 8 and Deliverable 10. As such, this curve reflects the universal behavior of single components and cascades. 3.2 Performance simulations In this subsection we provide the technical outline of the network simulations that we have used to assess the DVB-C2 signal level that a network may tolerate. The approach of the analysis which is based on a limited number of representative network scenarios. For these scenario s we study the feasibility to deploy DVB-C2, and in particular the maximum tolerable DVB-C2 signal level. The approach is described in subsection Next, in the following subsection we give an overview of the scenarios Technical approach of the estimations In subsection 3.1 we have estimated the signal level requirement at the HFC network system outlet. The next question thus is, whether the HFC network can deliver such a level, and in particular for how many channels while conserving a sufficient signal quality for all services, FM radio, analogue TV, DVB-C and the new DVB-C2. The relevant subset of signal quality requirements is listed in Table 6. Stated different, we could formulate the question as follows: What is the maximum tolerable signal level for DVB-C2 for a representative HFC network with a represented network load which includes a number of DVB-C2 carriers? In our studies we have addressed this question with the aid of the UTOPIC cable RF planning tool. With this tool, the levels and the quality of the signal at the outlet of a coaxial cascade of a cable network can be calculated. Figure 10 shows the concept the tool. For input, the tool requires: full specification of the coaxial topology including the length and frequencydependent attenuation of the coaxial cables and all losses related to the coaxial branches the specification of the amplifiers in terms of gain, slope, noise figure and CENELEC CSO and CTB figures specification of the composite input signal. For each signal, the kind (FM radio, PAL, DVB-C, unmodulated carrier), the carrier frequency and the signal level have to be specified. With this information, the tool calculates: all output levels, the random noise distortion signal being the sum the cumulative thermal noise generated by the amplifiers and the 2 nd and 3 rd order broadband intermodulation products, and the 2 nd and 3 rd order narrow band intermodulation products (Composite Cluster Beats) New Frequency Plan

34 Specification Network: Topology Coaxial cables Branching/splitting. Specification Components Lasers Optical nodes Amplifiers System Performance Calculation System Performance signal output level SNR CINR (BER) Specification Load PAL (f, signal level) DVB-C (f, signal level) DVB-C2 (f, signal level) FM (f, signal level) Figure 10 Schematic diagram of system performance calculations For the narrowband interference of analogue services associated with the 2 nd and 3 rd order composite beats, the appropriate definitions as specified in the IEC (paragraphs 4.3 and 5.9.3) are used, including the video weighting functions. A description of the tool can be found in the December 2008 issue of the Broadband magazine 17. As an illustration of the tool, we show a part of the spectrum ( MHz) of the signal delivered at the system outlet in Figure 11. This illustration is obtained for a simulation of a cascade with a load of FM radio, PAL TV, DVB-C and DVB-C2 signals. PAL Carrier + Video PAL Audio (FM) Random Noise Composite Cluster Beats Figure 11 Sample of the signal at the system outlet of a cascade as obtained from UTOPIC. The figure shows a number of PAL channels, showing the carrier, the (modeled) video signal and FM audio signal of each channel separately. In addition the random noise (thermal noise generated by the amplifiers + the broadband intermodulation products associated with the digital carriers) and the composite cluster beats generated by the intermodulation of PAL signals are indicated. Figure 12 and Figure 13 provides some further illustration of the UTOPIC tool. It shows the signal levels of the FM, PAL, DVB-C and DVB-C2 carriers, of the random noise distortion signals and of the narrow band composite cluster beats. 17 Jeroen Boschma, UTOPIC, a new RF planning tool for cable networks, Broadband, Vol. 30, Issue 3, December New Frequency Plan

35 100 Signal Level dbµv FM PAL DVB-C DVB-C MHz 400 MHz 600 MHz 800 MHz Figure 12 Full spectrum at the output port of a trunk amplifier of a cascade with a load of 25 FM radio channels, 20 PAL, 30 DVB-C and 43 DVB-C2. In black and red, the random noise (thermal noise and broadband intermodulation products) and the narrowband composite cluster beats are shown. The spectrum shows the slope needed to compensate the higher attenuation for higher frequencies. The signal levels in dbµv refer to the level as measured with a spectrum analyzer with 300 khz band width resolution. Thus the real DVB-C and DVB-C2 levels are about 14 db higher than shown in the figure. dbµv FM PAL DVB-C DVB-C2 AT V SAT V QAM FM Carrier IM2+IM3 Noise Random Noise Composite Cluster Beats dbµv FM PAL DVB-C2 DVB-C AT V SAT V QAM FM Carrier IM2+IM3 Noise Random Noise Composite Cluster Beats MHz MHz Figure 13 Full spectra at the system outlet of a cascade with a load of 25 FM radio channels, 20 PAL, 30 DVB-C and 43 DVB-C2. In black and red, the random noise (thermal noise and broadband intermodulation products) and the narrowband composite cluster beats are shown. The left figure shows a simulation in case of a low DVB-C2 signal level, the right figure for a high DVB-C2 level. For the high DVB-C2 signal level, a 20 db increase of the random noise level is observed. This increase reflects the enhanced intermodulation of the DVB-C2 carriers. In contrast, the composite cluster beats remain the same because the PAL and FM signal levels are not changed Simulation of the impact of the DVB-C2 signal level For FM radio, analogue TV and DVB-C signals, the required signal level is well established. Although conceivable technological tuner improvements could allow some reduction of these New Frequency Plan

36 signal levels, we should not anticipate for such a reduction in thus study. Instead we will assume that an operator will offer a larger signal level. This approach provides a build-in robustness of the simulations. Operators are all anticipating a gradual reduction of the analogue service offer to provide room for more digital services, and as such the studies should cover this development. Considering the known signal levels for FM, analogue TV and DVB-C services and the forthcoming reduction of the analogue TV offering, we have arranged the studies as follows: four representative networks were specified three different network loads were specified: o o o mixed analogue and DVB-C (reference load) mixed analogue, DVB-C and DVB-C2 all digital, DVB-C and DVB-C2 the reference scenario provides a programming check and reference values for the signal quality for the scenarios with a DVB-C2 load, the DVB-C2 signal level was stepwise increased and the CINR (carrier-to intermodulation and noise ratio) for the analogue, DVB-C and Signal to Noise Ratio of the DVB-C and DVB-C2 signals were calculated. The FM, analogue TV and DVB-C2 signal levels were fixed. Thus, this calculation shows the impact of the DVB-C2 carriers on the services. As said, in the approach that we have followed, we have calculated the SNR DVB-C2 curve assuming a channel load with constant signal levels for the FM, PAL and DVB-C carriers, but a gradually increasing DVB-C2 signal level. These simulations yield the SNR DVB-C2 curves as shown in. From these curves, the maximum signal level can be established that can be used for DVB-C2. An operator should apply a signal level below the level associated with the maximum of the SNR DVB-C2 curve Match between simulation and reality In a preceding study, we have studied the quantitative nature of the intermodulation phenomenon. Amongst others, we have compared the measured SNR of a digital carrier and the simulated SNR. For the simulations we used a component model comprising the 2nd and 3rd order non-linear behavior. The weighting functions to describe this 2nd and 3rd order non linear behavior were obtained from a standard CENELEC CSO/CTB measurement with a load of 42 unmodulated carriers. After specification, the same amplifier was exposed to a composite load of 96 DVB-C carriers. The carrier signal level was gradually increased and for three frequencies (120 MHz (f1), 416 MHz (f2) and 854,5 (f3) MHz, we measured the SNR curves. With the use of the specification of the 2nd and 3rd order non-linear behavior, we calculated the SNR curves for a digital load of 96 carriers and for the frequencies f1, f2 and f3. The results of the measurement and of the simulation are plotted together in Figure 14. For different components, we performed the same measurement and simulation, with a similar result. As discussed in Deliverable D10, the simulation using a model limited to 2nd and 3rd order intermodulation, systematically yields a too low estimation of the distortion signal level. Further analysis revealed that for digital systems, the 5th order non-linear products dominate the degradation of the SNR of the component, and not the 2nd or 3rd order products New Frequency Plan

37 Hybrid 1 SNR (db) Figure 14 Comparison of measured and predicted SNR for three frequencies. This figure is taken from the ReDeSign Deliverable D10, Me- measurement thodology for Specifying HFC Networks and simulation Components Output level (db) Apart from an explanation we have so far not developed an appropriate solution. Therefore, we will follow the most practical approach. From Figure 14 we can read that the simulation provides an under estimation of the maximum of the SNR curves. It shows that the measured and simulated SNR curves start deviating at a DVB-C2 carrier level of 4 db beneath the carrier level that corresponds to the maximum SNR of the simulated curve. In all cases, this 4 db back-off with reference to the carrier level related to the maximum SNR of the simulated curve provides a safe margin for the performance degradation associated with the 5th order non-linear behavior. Therefore, we should apply this 4 db margin to any simulated SNR curve based on a component model limited to 2nd and 3rd order non-linear behavior. In summary, we propose a technical approach as follows. Four representative networks and three representative network loads are defined. For the combination of the networks and loads, the SNR DVB-C2 curves are calculated. From these curves, we have extracted the tolerable signal level for DVB-C2, taking a 4 db margin into account. In the following subsection we will discuss the networks and network loads in more detail. 3.3 Network and network load scenario s Network scenario s Since the results and conclusions of this ReDeSign study should reflect the European cable networks, it is most crucial to use representative networks that cover somehow the range of European networks. In the ReDeSign Deliverable 6, the Reference Architectures Report, we have specified a number of coaxial architectures. However, because of the large variation in coaxial architectures found in Europe, these reference architectures specify ranges for network parameters. The reference architectures lacked sufficiently specified network figures that could be used in the calculation. Therefore, we have sought a close cooperation with some cable operators that were willing to share their network information with the ReDeSign project. Since the information is confidential, we only provide an anonymous summary of the cascades. The reference architecture or reference cascade specifies the worst-case cascades that can be found in the real network. It specifies the node and all the amplifiers and the maximum signal attenuation between two subsequent amplifiers or between the end amplifier and the customer wall outlet. When building a network, the reference architecture is projected or mapped on the geographical topology in terms of streets, homes, railways, rivers etc of a neighborhood. This projecting or mapping yields the detail topological network lay out in terms of the sites of the optical node and the amplifiers, the number of sub-branches fed by a New Frequency Plan

38 node or amplifier and the length of the coaxial cable connecting the node with the first amplifiers, the following consecutive amplifiers and the end amplifier with the customer outlet. Because of the above process of mapping the reference cascade, each home connection will be unique. As a rule, the signal loss between two consecutive amplifiers, or between the end amplifier and the customer wall outlet will be less than specified by the reference architecture. As a result, the quality of the signal delivered at the customer wall outlet will be better than the signal quality of the reference architecture, or at least equal. The reference architecture thus specifies the worst-case home connection. All home connections will have a signal with a quality equal or better than that of the reference cascade. Section 30 db -30 db 30 db -30 db 30 db -30 db 30 db system inlet + slope + slope + slope 30 db -30 db 30 db 40 db - 40 db + slope + slope system outlet Figure 15 Specification of coaxial cascade. The gain and loss values indicated in the figure are chosen arbitrary. For the studies we have used four reference cascades with a different number of amplifiers: an optical node with respectively 2, 4, 5 and 15 amplifiers. Each section encompasses an amplifier (or the node) and the coaxial network connecting to the subsequent amplifier or to the customer wall outlet. Table 8 Summary of the ReDeSign Reference Cascades used in this study. Node + 2 Amplifiers Section 1 Section 2 Section 3 U CS0=-60dBc / U CTB=-60dBc MHz Node + 4 Amplifiers Section 1 Section 2-4 Section 5 U CS0=-60dBc / U CTB=-60dBc MHz Node + 5 Amplifiers Section 1 Section 2-5 Section 6 U CS0=-60dBc / U CTB=-60dBc 110? MHz Node + 15 Amplifiers Section 1 Section 2-11 Section U CS0=-60dBc / U CTB=-60dBc MHz New Frequency Plan

39 Figure 15 shows a cascade of a number of sections. For each section the following figures were specified: Node/amplifier: o Noise figure o Worst case CSO and CTB values as specified in the IEC Coaxial part of the segment: o Maximum branching loss associated with splitters or multitaps as specified by the reference cascade o Maximum coaxial loss at the frequencies of 200 MHz and 862 MHz as specified by the reference cascade. Table 8 gives a summary of the 4 reference cascades used in this study. This specification is not complete, but it gives an indication of the cascades Network load scenario s Next, in Table 9 we have specified the network loads that we have used for the simulations. In all cases we used a load of 93 equidistant channels of 8 MHz for PAL, DVB-C or DVB-C2 channels. As a rule, the PAL channels were placed at the low frequency side, the DVB C2 channels at the high frequency and the DVB-C channels in the middle. In addition, the load included 25 FM radio channels located in the ,5 MHz FM band. Throughout the simulations, the same signal levels delivered to the customer wall outlet for FM radio, PAL TV and DVB-C were used. In contrast, the signal level of the DVB-C2 carriers was varied over a large range. As shown in the table, a DVB-C signal level with a 4 db backoff with reference to the PAL carrier level was used. Similarly, a 10 db back off was applied for the FM signal level. Table 9 ReDeSign network load, signal levels and cumulative digital capacity Network Load Digital Capacity Scenario FM PAL DVB-C Frequency edge DVB-C2 DVB-C2 DVB- 256 C QAM 4096 QAM QAM Reference MHz 2,7 2,7 Scenario A MHz 4,5 5,1 Scenario B MHz 6,2 7,3 Signal outlet dbµv The simulations were configured such that a flat spectrum was delivered at the customer wall outlet. Similarly, a flat spectrum was delivered to the input port of each amplifier of the cascade. To this end, a sloped amplifier gain was applied which results in a sloped output spectrum with carrier levels that gradually increase for higher frequencies. An absolute PAL signal level of 69 dbµv was chosen, which is somewhat higher than usually delivered by the operators. However, one should note that this value refers to the level at the input port of the wall outlet, so, at the TV output socket a 3 db weaker signal will be delivered New Frequency Plan

40 3.4 Network signal quality parameters With the aid of the UTOPIC tool, we calculated the levels and quality of all the signals as delivered at the system outlet, for the cascades specified in paragraph in combination with the loads given in paragraph The tool calculates the cumulative thermal noise generated by all amplifiers and all 2 nd and 3 rd order intermodulation products. The non-linear distortion products generated by intermodulation of analogue signals (composite beats) on the one hand, and those generated by intermodulation of digital carriers or of a digital carrier with an analogue carrier on the other hand, are accounted separately because of their different nature. The first, intermodulation between two or more analogue carriers produces the well known narrowband composite cluster beats. Such narrowband distortions interfere with the PAL video signal, thus generating an unwanted hatching of the video picture. In contrast, distortion products generated by intermodulation of digital carriers or of a digital carrier with an analogue signal resemble broadband random noise. Because of this, the signal power of the thermal noise of the amplifiers and of these broadband intermodulation products are summed, and presented as a single random noise distortion level. Thus the UTOPIC tool calculates the composite cluster beat level and the random noise signal level. Figure 12 and Figure 13 show the spectrum of such a simulation of a cascade with a load of 25 FM channels, 40 analogue TV channels and 53 DVB-C carriers, applying a 300 khz simulation resolution. Inspection of Figure 12 and Figure 13 teaches that the signal quality of the different PAL and digital carriers varies from carrier to carrier. For practical reasons related to handling this frequency dependency, we will define a single-valued worst-case signal quality figures for each of the signals. In Table 10 we have listed the terms and definitions of the signal quality parameters calculated by UTOPIC. Table 10 Terms and Definitions signal quality parameters PAL signals Carrier-to-Noise Ration CNR PAL Worst-case ratio of the carrier level and random noise level all PAL signals as measured following IEC p 4.6 Composite Intermodulation Noise Ratio Digital signals Signal-to-Noise Ratio CINR PAL SNR DVB-C SNR DVB-C2 Worst-case ratio of the carrier level and the composite beats level of all PAL signals as measured following IEC p Worst-case ratio of the signal level and the random noise level of all digital signals as measured following IEC p In the simulations, the DVB-C2 signal level is varied. For each DVB-C2 signal level, the output spectra of the signals and noise and intermodulation products are calculated, and the worst-case values of the CNR PAL, CINR PAL and SNR DVB-C and SNR DVB-C2 are calculated. Next the impact of the DVB-C2 level is visualized by plotting these quality parameters New Frequency Plan

41 3.5 Results Scenario A (mixed load) To warrant a proper delivery of all cable services, all following requirements of Table 6 should be met. In the following we will present and discuss the signal quality for the different cascades with a mixed load (Scenario A). We will successively consider the DVB-C2 signal level, the SNR DVB-C2, the CNR PAL and the CINR PAL. For network load scenario A with 43 DVB- C2 channels (see Table 9), we calculated the SNR DVB-C2 in case of the four specified reference cascades (see Table 8). DVB-C2 signal level In Figure 16 we show the SNR DVB-C2 curves. All curves reveal the onset of the generation of 2 nd and 3 rd order intermodulation products. The DVB-C2 signal level of the cascades show a clear quantitative differences with a somewhat lower signal level for the cascade with 5 amplifiers (red curve) and the highest signal level for the cascade with 4 amplifiers (bleu curve). For the cascade with 5 amplifiers, the maximum SNR DVB-C2 is found for a DVB-C2 signal level of 67 dbµv. As discussed in paragraph 3.2.3, we have to account for the fact that the degradation is caused by the 5 th order non-linear behavior whereas the curves only include the generation of 2 nd and 3 rd order intermodulation products. As discussed in paragraph 3.2.3, a 4 db correction should be applied as an additional margin. For the cascade with 5 amplifiers (red curve), the maximum DVB-C2 signal level thus becomes 63 dbµv. Since this value is substantially more than the minimum signal level for 4096 QAM (58 dbµv) we can conclude that DVB-C2 with 4096 QAM modulation can be deployed in this cascade. SNRDVB-C2 (db) Node + 2 Amps Node + 4 Amps Node + 5 Amps Node + 15 Amps 4096 QAM 1024 QAM 4 db DVB-C2 Carrier Level (dbµv) Figure 16 SNR DVB-C2 vs. DVB-C2 carrier level at the input port of the wall outlet for all reference cascades of Table 8 and for a load of 25 FM channels, 20 PAL channels, 30 DVB-C channels and New Frequency Plan

42 DVB-C2 channels as specified in Table 9. In the figure we have indicated the DVB-C2 sensitivity limits for 1024 QAM and 4096 QAM modulation from Table 7 and the 4dB margin as discussed in paragraph The three remaining cascades all support higher DVB-C2 signal levels than the cascade with 5 amplifiers. Therefore, we can conclude that all four reference cascades can provide a sufficient DVB-C2 signal level for the 4096 QAM modulation mode. DVB-C2 Signal-to-noise ratio For DVB-C QAM modulation, a minimum SNR of 35 db is needed. Figure 16 shows that this SNR DVBC-2 is met for all signal levels. PAL Carrier-to-noise ratio The CNR PAL will be affected by the broadband intermodulation products generated at a high DVB-C2 signal level. For higher DVB-C2 signal levels, the broadband intermodulation products of the digital carriers will gradually exceed the thermal noise. This effect is also shown in Figure 13. In Figure 17 we show the effect for the different cascades. Because of the large number of amplifiers, the worst CNR PAL is found for the cascade of the node with 15 amplifiers. In the figure, both the minimum DVB-C2 signal level and the IEC CNR PAL requirement are shown. The result shows that in the operational DVB-C2 signal level range, the CNR PAL requirement is not violated. 65 CNR PAL (db) CINRPAL (db) QAM Node + 2 Amps Node + 4 Amps Node + 5 Amps Node + 15 Amps IEC DVB-C2 Carrier Level (dbµv) Figure 17 Degradation of the PAL Carrier-to-Noise Ratio (CNR PAL ). Since intermodulation of digital carriers generates broadband random noise, the CNR PAL will degrade with increasing DVB-C2 signal level. The IEC standard specifies a 44 db CNRPAL requirement, as indicated in the figure. The result shows that for these cascades the CNR PAL requirement is not violated for DVB-C2 levels up to say 6 db above the 4096 QAM minimum level. PAL Composite Intermodulation Noise Ratio To conclude, we have to verify the composite cluster beat levels. Figure 13 demonstrates the effect of raising the DVB-C2 signal level on the composite cluster beats. Comparison of the spectra for a low DVB-C2 signal level (left window) and a high DVB-C2 level (right window) teaches that the composite cluster beats are not affected at all. Although this may appear awkward and against once expectations, it is correct. The explanation is rather simple. Only New Frequency Plan

43 the intermodulation of the analogue signals, and in particular of the PAL carrier, contributes to the composite cluster beats. Raising the DVB-C2 level raises the overall level of the intermodulation products; however, only the level of broadband intermodulation products increases, and not the level of the narrow band cluster beats. Therefore, we can conclude that the phenomenon of composite cluster beat generation is isolated from the digital carriers. Only raising the signal level of the analogue signals or changing the number of analogue signals will affect the composite cluster beats level. From the above, we can conclude that the CINR PAL should be reduced upon the reduction of the analogue package. Figure 18 shows the CINR PAL levels for the different cascades for the reference load with 40 PAL channels and for the mixed load of scenario A with 20 PAL channels. In all cases an improved CINR is found upon the reduction of the analogue package. CINR PAL (dbµv) Node 20 PAL (Scenario A; mixed load) 40 PAL (Reference scenario) IEC Ampl + 4 Ampl + 5 Ampl + 15 Ampl Figure 18 Impact of the network load on the composite intermodulation noise ratio of the PAL signal (CINR PAL ). The red bars indicate the CINR PAL for the reference load including the 40 PAL channels. In blue we show the CINRPAL for the mixed load of scenario A. In addition, the IEC requirement is indicated. The figure shows an overall improvement of the CINRPAL due to the reduction of the analogue package from 40 to 20 channels. In summary, the simulations show that in the four cascades with the load of scenario A, the cascade can provide a DVB-C2 signal level needed for 4096 QAM modulation without any harmful interference to the PAL, DVB-C and DVB-C2 services. Apparently, 4096 QAM can be deployed in all these networks. In the above study, we considered the replacement of a substantial number of PAL and DVB- C channels by DVB-C2 channels. An operator will consider this a huge change. Likely, he will prefer a more gradual change, replacing a single carrier at a time, or a few at most. A straightforward way to look at this issue is, whether or not he can replace an analogue channel or a DVB-C channel by a DVB-C2 channel. Our analysis shows that in case of the four cascades, DVB-C2 can be deployed at a signal level at least up to 63 dbµv. In three of the four cascades an even a higher DVB-C2 level can be delivered; however, already 63 dbµv is 5 db above the minimum level as specified in 3.1. As such, 63 dbµv can be considered as a robust signal level. In our scenario, we have assumed a 65 dbµv level for the DVB-C carriers, see Table 9. Therefore one may replace a DVB-C carrier by DVB-C2 without negatively affecting the distortion signal levels. Next, we shall briefly consider the replacement of a PAL channel. In the scenario s we used a PAL carrier level of 69 dbµv at the network side port of the customer wall outlet. This 69 dbµv carrier level corresponds with a signal level of 63 dbµv of the time-averaged PAL signal. Replacement of a PAL channel by a DVB-C2 carrier with a signal level of 63 dbµv thus appears neutral from the viewpoint of the power budget. However, the reduction of the analogue package will yield somewhat lower composite cluster beat levels whereas an increase of the number of digital carriers will result in a slightly higher level of the broadband intermodulation products. Stated differently, some intermodulation signal power is moved from the cluster beats to the broadband random noise. For the cascade composed of the node and 5 amplifiers, such an increased level of the broadband intermodulation products could be unac New Frequency Plan

44 ceptable, because the system already operates at the maximum tolerable signal level for DVB-C Scenario B (All digital) Like for the mixed analogue digital scenario A, we have calculated the SNR DVB-C2 curves for the all-digital scenario B. In scenario B, 15 DVB-C channels and 20 PAL channels are replaced by an additional 35 DVB-C2 carriers. The time averaged signal level of a PAL carrier is about 6 db less than the PAL carrier level. As such, at a DVB-C2 signal level of 63 dbµv, the composite signal level of the network loads A and B are approximately equal. For a higher DVB-C2 signal level, the network will carry a larger composite signal power. Therefore, for a high DVB-C2 signal level, the cumulative signal load of the network will increase. Associated with this increased network load, an enhanced intermodulation can be expected. In Figure 19 we show the SNR DVB-C2 curves for all four cascades. In grey, we have indicated the curves for the network load of scenario A. As argued, an enhanced level of intermodulation is seen for the higher DVB-C2 signal levels. SNRDVB-C2 (db) Node + 2 Amps Node + 4 Amps Node + 5 Amps Node + 15 Amps 1024 QAM 4 db 4096 QAM DVB-C2 Carrier Level (dbµv) Figure 19 SNR DVB-C2 curves in case of the all-digital scenario (B) at the input port of the wall outlet for all reference cascades of Table 8 and for a load as specified in Table 9. In the figure we have indicated the DVB-C2 sensitivity limits for 1024 QAM and 4096 QAM modulation from Table 7 and the 4 db margin as discussed in paragraph The grey lines show the SNR DVB-C2 curves for the load of scenario A with 20 analogue TV channels with a 69 dbµv carrier level. For all cascades, the maximum DVB-C2 signal level that the network can support is reduced. When applying the 4 db margin, it shows that in case of the cascade with 5 amplifiers the minimum and maximum DVB-C2 signal levels are coincide: 4096 QAM modulation appears possible, but the network is loaded up to the maximum. The network load is critical, and there is a serious doubt whether the 4096 modulation mode will perform satisfactory in this cascade. The 3 other cascades can support a substantially larger DVB-C2 signal level; there still is a sufficient margin to warrant a proper performance of the DVB-C QAM mode New Frequency Plan

45 The cascade with 5 amplifiers misses a sufficient power budget to warrant the 4096 QAM modulation in case of the all-digital load. Inspection of Table 8 shows that the 6 th section of this cascade combines a large attenuation (30 db) with a low amplifier output power (104 dbµv). Likely, the limited DVB-C2 signal level is associated with this 6 th section. Assuming that the four cascades are representative for the European networks, the results are rather encouraging for DVB-C2. The results entail a wide applicability of the 4096 QAM mode in European cable networks Impact analysis As a rule, the network(s) of any European operator will not exactly match with the reference cascades studied here, and summarized in Table 8. Clearly, simulation of all reference cascades is an impossible task, beyond the time and budget limitation of the ReDeSign project. Nevertheless, to assists operators with the question whether or not their network may, or may not, support 4096 QAM modulation, we have made a limited impact analysis of the most relevant network parameters. In brief, we have studied the effect of: A lower PAL carrier level of 63.8 dbµv and 66.8 dbµv instead of 68.8 dbµv, A 6 db higher output power of all amplifiers in the reference cascade with 2 amplifiers, 113 dbµv instead of 107 dbµv, And a 3 db reduced attenuation (@ 862 MHz) of the coaxial part of each segment of the reference cascade with 4 amplifiers. In Figure 20 and Figure 21 we summarize the result of the above studies. SNRDVB-C2 (db) PAL 69.8 dbµv PAL 66.8 dbµv PAL 63.8 dbµv DVB-C2 Carrier Level (dbµv) Figure 20 Impact of a lower PAL carrier level for a cascade of 15 amplifiers and a network load of scenario A. The effect of a lower PAL carrier level is shown In Figure 20. When reducing the PAL carrier level, one would expect that the signal power thus relieved from the analogue TV signals would contribute to a higher DVB-C2 maximum signal level. However, the figure shows that the maximum of the SNR DVB-C2 curves does not shift to a higher DVB-C2 signal level. One only observes a modest increase of the SNR value. Although the result doesn t match with ones first expectation, it appears completely logic. The time-averaged power level of a PAL signal is about 6 db less than the PAL carrier level. Therefore, a 69 dbµv PAL carrier level represents only 63 dbµv signal level. Moreover, the network load comprises 20 PAL carriers next to 43 DVB-C2 carriers. Given these figures, the signal power of the PAL signals is negligible for a DVB-C2 signal level of 65 dbµv and larger. Therefore, when reducing the PAL New Frequency Plan

46 carrier level, the SNR DVB-C2 curve will not shift to higher DVB-C2 levels. However, reduction the PAL carrier levels yields a reduced intermodulation of analogue and digital carriers, and a reduction of the broadband intermodulation noise. This reduction of the broadband intermodulation products explains the improved SNR DVB-C2 level. SNRDVB-C2 (db) 60 Low High power power (107 dbµv) High Low power (113 dbµv) DVB-C2 Carrier Level (dbµv) SNRDVB-C2 (db) Reference Architecture - 3dB per section Reference Reference Architecture Architecture 3dB per section DVB-C2 Carrier Level (dbµv) Figure 21 Impact of the amplifiers maximum output power level as specified by a CENELEC measurement with 42 unmodulated carriers and of the coaxial attenuation per segment respectively for the reference cascade with 2 amplifiers and the reference cascade with 4 amplifiers. To conclude we have assessed the effect of a higher maximum amplifier output power level and of a reduced attenuation per section of the cascade. In agreement with ones expectations, both an increased amplifier maximum output power and a reduced attenuation of the coaxial networks interconnecting the consecutive amplifiers provides a higher DVB-C2 signal level without increasing the power associated with the intermodulation products. This results in a shift of the point of maximum SNR DVB-C2 to a higher DVB-C2 signal level and a higher SNR DVB-C2 value, along the line with slope 1. Such a shift agrees with the simulations of Figure 21. These results show that replacement of the amplifiers is the most straightforward solution to raise the maximum DVB-C2 signal level. Therefore, operators with a network which does not support a sufficiently high DVB-C2 carrier level should consider the replacement of the amplifiers. Moreover, operators that currently or in the near-future face replacement of their amplifiers should take into account the issue of a sufficient maximum output power as well. 3.6 Summary and conclusion From the viewpoint of future network capacity, it is most crucial to know the maximum DVB- C2 signal level that can be applied without degrading the quality of the analogue TV, the DVB-C and DVB-C2 signals. Dependent on this maximum DVB-C2 level, the 4096 QAM modulation can be used, or not. In this chapter we have studied the issues of signal level, signal quality and maximum modulation scheme. To obtain realistic data, the studies were performed in close collaboration with a number of cable operators, using network data from their networks. The studies comprised four networks with cascades of a node and 2, 4, 5 and 15 amplifiers respectively. The coaxial parts New Frequency Plan

47 and the amplifiers were completely specified, including noise and CSO/CTB figures of the latter. Three network loads were studies: a mixed analogue and DVB-C scenario (40 PAL and 53 DVB-C 256 QAM), a mixed analogue, DVB-C and DVB-C2 scenario (20 PAL, 30 DVB-C and 43 DVB-C2), an all digital DVB-C and DVB-C2 scenario (15 DVB-C and 78 DVB-C2). For the second and third scenario, we calculated the signal quality parameters like SNR for the digital carriers and CNR and CINR for the PAL signals as a function of the DVB-C2 signal level. These calculations showed that in case of the load scenario of 20 PAL, 30 DVB-C and 43 DVB-C2 signals, DVB-C QAM modulation can de used in all four networks. In case of the all digital scenario with 25 DVB-C and 78 DVB-C2 carriers, three out of the four networks could allow the use of DVB-C QAM modulation. This result suggests that DVB-C QAM modulation can be applied in many European HFC networks; however, not in all networks. For operators whose networks cannot support DVB-C modulation, it is of interest to know which parameters affect the maximum DVB-C2 signal that can be used. Therefore, we have studied the impact of i) the PAL carrier level, ii) the maximum amplifier output power and iii) the signal attenuation associated by the coaxial segments interconnecting the amplifiers. These results show that a lower PAL carrier level has only a limited positive effect on the maximum DVB-C2 signal level. Replacement of the amplifiers by ones with a 3 db or 6 db higher maximum output power has a substantial positive effect, as surmised. Similarly, DVB-C2 signal level can be increased when reducing the attenuation of the coaxial parts. When necessary, replacement of the amplifiers has to be considered as the best solution to prepare HFC networks for DVB-C QAM deployment New Frequency Plan

48 4 Summary and Conclusion According to the early concept of the cable distribution network, the customer should be able to connect his terrestrial receivers directly with the cable network. This completely legitimate and logical requirement has shaped the HFC frequency plan. With the advent of digital transmission technologies, this linkage of the cable frequency plan to the terrestrial frequency plan has become obsolete; however, most of the transmission systems still respect the historical HFC frequency plan. A major disadvantage of the current frequency plan is the upstream band which is limited to 65 MHz. This frequency limitation is associated with the conservation with the FM radio band (87,5 108 MHz). In this study we have reconsidered the use of the HFC spectrum assuming that in the all digital era all historical restrictions can be abolished thus allowing an operator to redefine the frequency plan according to his needs, with an appropriate balance between up and down stream spectrum. Considering the outcome of the studies, we have to conclude that operators are still strongly bonded to the existing frequency design of the cable networks and to the terrestrial use of the ether. Therefore, a complete redefinition of the frequency plan appears not possible. Transmission capacity is determined not only by the available frequency spectrum, but by the applicable carrier signal and distortion signal level as well. Therefore, we have analyzed the possibilities to expand the upstream and downstream capacity from the integral viewpoint of available spectrum, the possible signal level and the distortion signal level. In principle, the frequency plan should be defined to maximize the total upstream and downstream network transmission capacity in a balanced manner, as demanded by the market. However, as elaborated in the report, a complete abandonment of the historically defined frequency plan appears impossible. The two primary reasons to conserve the existing frequency plan are: A majority of the cable operators foresees delivery of FM radio signals for at least a full decade There are many options to expand the downstream transmission capacity or to implement capacity saving solutions. Because of this an expansion of the downstream band beyond 865 MHz is rated the least. Combining both observations fixates the current frequency plan almost completely. Irrespective of this fixation of the frequency plan, we have studied the options to expand or to maximize the upstream and downstream capacity. Regarding the upstream capacity, the operator response on earlier ReDeSign network questionnaire reveals that in most cable networks the upstream band is not used efficiently. In many networks the upstream band is yet not extended up to 65 MHz whereas ingress noise levels prohibit the use of high (64 QAM) modulation schemes. In the report we provide a review of the solutions to reduce the ingress noise. Operators should first resolve this problem of ingress noise, possibly in combination with the extension of the upstream band up to 65 MHz, to maximize the upstream capacity and to warrant economical use of EuroDOCSIS equipment. Having thus upgraded the upstream channel, they can keep track with the customer capacity demand by adding more EuroDOCSIS channels and/or splitting the upstream segments. Next, once the above capacity expansion solution is exhausted, operators will face the challenge to expand the capacity beyond this level. The capacity of the MHz band is fully used, and operators are forced to find new spectrum for the upstream band, which requires a substantial network upgrade. Basically, there are two options, extension of the MHz band to higher frequencies, or the creation of a new frequency band in the UHF band, be New Frequency Plan

49 yond 865 MHz. The first option requires a solution to deliver FM radio to those customers that use this service. In addition, the EuroDOCSIS technology has to be adapted. In the DOCSIS specification, the upstream band is already extended to 85 MHz, so technically there is no serious problem; however, operators do depend on the willingness of the manufacturers. Likely, the definition of a UHF return band is more promising. By using VHF-UHF frequency converters placed in the network, investments and network adaptations can be limited. This solution is shown in the figure below. In this option, the customer equipment still transmits in the MHz band and in the lower part of the coaxial network upstream signals are conveyed at these frequencies, but higher up in the network the MHz signals are converted to a frequency beyond 950 MHz. This way, the upstream capacity can be boosted by a factor of 10 or more, whereas it requires limited network adaptations and no adaptation of the EuroDOCSIS equipment. Branching point Frequency Upconverter n m GHz 1 GHz 1 GHz 2 65 MHz n o e MHz m branches MHz Figure: Architecture of a hybrid VHF / UHF return band solution As pointed out above, operators are not specifically inclined to extend the downstream band egde beyond 865 MHz because of the numerous ways to make a more efficient use of the spectrum from 85 up to 865 MHz. At the level of the network layer, the replacement of analogue signals by digital carriers and the deployment of DVB-C2 are the basic elements to implement this approach. In the ReDeSign studies we have addressed a crucial issue of this approach: the capability of the existing European HFC networks to support the DVB-C QAM modulation mode. Application of this mode requires a high DVB-C2 signal level, and the question is whether a sufficiently high signal level can be deployed without degradation of the analogue TV, DVB-C and DVB-C2 signals by the distortion products (intermodulation products) associated with the non-linear nature of the active components. To warrant a realistic result, a number of operators provided data from their networks. Four networks scenarios were studied with cascades of 2, 4, 5 and 15 amplifiers respectively. The coaxial parts and the amplifiers were completely specified, including noise and non-linear behavior of the latter. Three network loads included: a mixed analogue, DVB-C and DVB-C2 scenario (20 PAL, 30 DVB-C and 43 DVB-C2), an all digital DVB-C and DVB-C2 scenario (15 DVB-C and 78 DVB-C2). For these scenario s, we calculated the signal quality parameters like SNR for the digital carriers and CNR and CINR for the PAL signals as a function of the DVB-C2 signal level. These calculations showed that in case of the mixed load scenario DVB-C QAM modulation can de used in all four networks. In case of the all digital scenario, three out of the four networks support the use of DVB-C QAM modulation. This result suggests that DVB-C QAM modulation can be applied in many European HFC networks; however, not in all networks New Frequency Plan

50 New Frequency Plan