COMMITTEE T1 TELECOMMUNICATIONS Working Group T1E1.4 (DSL Access) CONTRIBUTION. A Comparison of Candidate Spectral Plans for Asymmetric-Optimized VDSL

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1 COMMITTEE T1 TELECOMMUNICATIONS Working Group T1E1.4 (DSL Access) T1E1.4/ Baltimore, MD, August 3 7, 1999 CONTRIBUTION TITLE: SOURCE: PROJECT: A Comparison of Candidate Spectral Plans for Asymmetric-Optimized VDSL Broadcom Corporation Next Level Communications VDSL ABSTRACT Four candidate spectral plans are studied for applicability to residential broadband service deployment based on asymmetric VDSL.. Three of the spectral plans are new, and one has been previously proposed by others. The analysis results indicate that this previously-proposed plan is not suitable for residential broadband, as it gives too much bandwidth to the upstream channel and not enough to the downstream. This fourth plan also requires an extra band transition above MHz, which is unnecessary from a differential capacity perspective and harmful in the realistic case of analog-assisted duplexing. Among the remaining plans, Plan #3 has the advantage of highest downstream capacity on the long loop, while Plan #1 has the highest downstream capacity for the short and medium loop. The lower frequency placement of the asymmetric upstream band imparts to Plan #1 a number of other practical advantages. It is proposed that Plans #1 and #3 be considered as the two candidates for an asymmetric optimized VDSL spectral plan. NOTICE This contribution has been prepared to assist Standards Committee T1-Telecommunications. This document is offered to Working Group T1E1.4 as a basis for discussion and is not a binding proposal on source companies. The proposed requirements are subject to change in form and numerical value after more study. Source companies specifically reserve the right to add to, amend, or withdraw the statements contained herein. * CONTACT: David C. Jones; dcjones@broadcom.com ; Tel: (949) ; Fax: (949) Sabit Say: ssay@nlc.com; Tel: (973) ; Fax: (973)

2 1. Introduction: A companion contribution [1] anticipates two spectral plans for VDSL, one optimized for asymmetric deployment and the other optimized for symmetric. This contribution investigates the asymmetric performance characteristics of four candidate spectral plans. Three of these plans are newly designed, and one has been previously proposed.[] Each of the four spectral plans is studied under four different assumed conditions, generated by the cross-product: (Analog-assisted duplexing vs. Pure digital duplexing) X (Amateur egress notching vs. No notching). By considering service set requirements and recognizing the unavoidable capacity loss due to each transition band split, it is concluded that one of the three new plans is optimal for an asymmetric VDSL roll-out.. Asymmetric-Optimized Spectral Plans: We analyze asymmetric-optimized spectral plans, taking into account the target service set and deployment topology most likely associated with an asymmetric VDSL roll-out..1 Implications of Service Set and Deployment Scenarios As described in [1], asymmetric VDSL will be most commonly be used as a residential broadband (i.e., SDV) access network extension cord. In this case the service set to be provided would consist of each of the following: broadcast entertainment video, video on demand, an embedded high-speed data service, and a full complement of derived narrow band telephony channels (e.g., 384 kbit/s). The high speed data service and the narrow band telephony channels would be transported across the VDSL PHY as symmetric virtual circuits. The video channels would be highly asymmetric, with large amounts of data downstream and a small upstream control channel. Lines within about 1500 ft of the VTU-O could be immediately provisioned at rates which anticipate HDTV. Those further out could support multiple HDTV channels either immediately or in the future through the addition of a second VDSL line. Such doubling of pairs would make derived narrow band capability very important, in order to avoid eventual demand for pairs outstripping supply in the distribution network. Indeed, even without VDSL deployment, many operators are currently turning to derived multi-line POTS systems as the only way to keep exploding demand for second and third lines from outpacing the availability of copper pairs. The embedded symmetric data service is also an important component of the asymmetric VDSL service set. Many if not most VDSL-based broadband early adopters will already have cable modem or DSL-based data services in their homes. Any full broadband offering on the part of a telephone company will have to feature a data service which can compete with both cable modems and DSL offerings from both incumbent and competitive local exchange carriers (ILECs and CLECs). In order to justify the construction, capital, operations, backbone, and content costs associated with the deployment of such a system, it is necessary for the network operator to achieve the widest percent coverage possible. This means that the vast majority of lines would be served through a FTTCab architecture. A small number of customers very close to the central office/exchange, and customers served in small CLEC niche applications, might indeed be served through FTTEx, but large-scale asymmetric VDSL deployment will almost certainly be FTTCab-based. From this discussion, two important facts emerge: Point 1) Discussions of asymmetric VDSL should concentrate on FTTCab deployments. FTTEx-based asymmetric lines may still exist, but will be statistically much less relevant. Point ) All asymmetric VDSL lines, regardless of length or noise conditions, will have to deliver the symmetric bandwidth necessary for the data service and the derived narrow band services, plus the asymmetric bandwidth needed for some minimal number of video channels. We estimate this minimal capacity at Mbps upstream and 16 Mbps downstream. Lines of shorter length or better noise conditions should use most or all of the corresponding increase in total capacity for the downstream channel, in order to deliver increased numbers of, or increased quality, digital video channels.. Candidate Asymmetric-Optimized VDSL Spectral Plans Page

3 Points 1 and in Section.1 can be used as the starting point for the design of asymmetric-optimized spectral plans. The implication of Point is that the upstream band must be placed low enough in frequency so that sufficient upstream bandwidth for the data and narrow band services is guaranteed to be available, even on the worst channels. This suggests the three new candidate asymmetric-optimized spectral plans shown in Figure 1 through Figure 3. In this contribution these plans are referred to as Spectral Plan #1, Spectral Plan #, and Spectral Plan #3, respectively. All three of these plans feature two downstream and one upstream band for asymmetric service, plus a second highfrequency upstream band for spectrally compatible symmetric service. An optional low-frequency upstream band below 138 khz could also be considered, but the results presented in this contribution assume that an upstream band below 138 khz is not used. This assumption is made because use of VDSL frequencies below 138 khz will have an adverse impact on the required physical size of transceiver and POTS splitter magnetics. In addition, applications requiring VDSL transport over ISDN Basic Rate on the same line will require that this band not be used for VDSL. Spectral Plan #4, shown in Figure 4, was previously proposed in []. Not counting an upstream band below 138 khz, this spectral plan requires 3 downstream and upstream bands for asymmetric service, plus a third high-frequency upstream band for symmetric service. As with Spectral Plans 1 3, the upstream band below 138 khz is disabled in this contribution in order to allow for circuit magnetics with a smaller footprint. All four candidate spectral plans could have their upstream capacities boosted by an equal amount through the re-insertion of this low frequency upstream band, so for comparison purposes the status of this band is moot. All four plans use the ADSL downstream band as the starting point for the first VDSL downstream band. With reference to Point #1 above, this actually violates both the ANSI and ETSI FTTCab masks. As with the deleted low frequency upstream band, use or disuse of this first downstream band will effect the performance of all of the spectral plans equally. However, to avoid further modifications to Spectral Plan #4 as proposed in [], this contribution assumes the use of this first downstream band, at a transmit PSD level of 60 dbm/hz. In fact, the spectral plans are compared with all bands using transmit PSDs of 60 dbm/hz. The only exception to this are the amateur bands which, for the cases which assume amateur notching, carry no significant transmit energy. Page 3

4 TX PSD (dbm/hz) DS1 US1 Downstream US Figure 1: Spectral Plan #1 TX PSD (dbm/hz) Downstream 1 US1 Downstream US Figure : Spectral Plan # TX PSD (dbm/hz) Downstream 1 US1 Downstream US Figure 3: Spectral Plan #3 TX PSD (dbm/hz) DS1 US1 DS US Downstream 3 US Figure 4: Spectral Plan #4 Page 4

5 .3 Analog Band Splitting Filter Requirements It now appears widely accepted [3] [4] [5] [6] [7] that some form of analog filtering is required as part of an FDD implementation strategy, even with 1 bits of effective ADC/AFE precision. Apparently the only remaining question is the amount of analog filter stopband attenuation that is required. The simple equations of [4] can be used to provide the answer. For 1 effective bits of ADC/AFE precision (which typically would require a 1 bit hardware ADC plus first or second order over-sampling techniques), a 36 MHz sampling rate, 1 db trans-hybrid loss, ADC clipping probability of 1E-07, and an effective ADC quantization noise floor of dbm/hz, then 1 db of band split filter stop band attenuation is required. This stop band attenuation figure can be reduced by increasing either the ADC clipping probability or the effective quantization noise floor, but neither of these steps is advisable for a field-deployed system. Indeed, normal sample-to-sample variability in the performance of manufactured ADCs would argue for specifying a higher stop band attenuation figure, not lower. Assuming 1 db of target attenuation for now, we next consider the corresponding transition bandwidth requirement. (Analog filter transition bandwidth is expressed as the ratio f stopband / f passband for a LPF, and the reciprocal of this for a HPF.) Of course the transition bandwidth depends on the filter type, order, and (if applicable) passband ripple amount. To minimize the transition bandwidth we consider only elliptical filters, but require small (< 0.18 db) passband ripple as doing otherwise results in poor return loss.[8] Too keep the filters simple and the number of analog components small, we would like to consider only 3 rd order filters. However, analysis indicates [8] [9] that a 3 rd order 1 db diplexing filter pair with 14 db return loss and inductor unloaded Q of 100 requires a transition band ratio of 47%. This means that if a low frequency band stops at f 0 Hz, then the next higher band cannot start until 1.47 f 0 Hz. This is a very large loss, and for this reason the order of the diplexing filter pairs considered in this contribution is being increased to 4 th order. A 4th order 1 db diplexing filter pair with 14 db return loss and inductor unloaded Q of 100 requires a transition band ratio of 18%. This means that if a low frequency band stops at f 0 Hz, then the next higher band starts at 1.18 f 0 Hz. For the analog-assisted duplexing cases this 18% roll-off is imposed onto each spectral plan at each boundary between upstream and downstream bands. This results in the modified analog-assisted spectral plans of Figure 5 - Figure 8. These modified spectral plans each contain the 18% roll-off factor between each band boundary described above. In each case all of the bandwidth loss due to the necessary filtering is taken from the downstream channel. This was done solely for ease of comparison with the digital duplex case. Capacity loss due to the necessary analog filters could just as easily be taken from the upstream channel, or some combination of upstream and downstream. Page 5

6 TX PSD (dbm/hz) DS1 US1 Downstream US Figure 5: Spectral Plan #1 with 1 db Analog Band Split TX PSD (dbm/hz) Downstream 1 US1 Downstream US Figure 6: Spectral Plan # with 1 db Analog Band Split TX PSD (dbm/hz) Downstream 1 US1 Downstream US Figure 7: Spectral Plan #3 with 1 db Analog Band Split TX PSD (dbm/hz) DS1 US1 DS US Downstream 3 US Figure 8: Spectral Plan #4 with 1 db Analog Band Split Page 6

7 .4 Differential Capacity and the Candidate Spectral Plans The four spectral plans of Figure 1 through Figure 4 (and repeated with modifications in Figure 5 - Figure 8) are of interest because they each place the first upstream band(s) relatively low in frequency, thereby meeting the goal of Point in Section.1. Relative to each other, Spectral Plans # and #3 will benefit from the lower low-frequency crosstalk noise floor at the VTU-R, compared to Spectral Plans #1 and #4. This is made clear through reference to the differential capacity plots of Figure 9 - Figure 11. These figures plot the differential capacities for short (1500 ft), medium (3000 ft) and long (4500 ft) #4 AWG loops. For all three loops the noise model used is ETSI Set A, with Mix A modified to also include 0 VDSL self-fext. 1 The curves assume a noise margin of 6 db and a coding gain of 3.5 db, for an SNR gap of 1.5 db (see [1]). From Figure 9 - Figure 11 it can be seen that the downstream channel has more capacity at low frequencies (0.5.0 MHz) than does the upstream channel. This leads immediately to the suspicion that Spectral Plans # and #3 will outperform Plans #1 and #4 in all cases. On the other hand, Spectral Plan #1 will benefit in the analog band split analysis from its lower band split frequencies, compared to the other plans. This is clear from Figure 5, which shows less white space above MHz than with any of the other plans. Plan # will experience a similar advantage compared to Plans #3 and #4. The outcome of this inherent trade-off will become clear in the simulation results of Section 3. In the analog-assisted duplexing analyses, Plan #4 will suffer worst from transition band spectrum loss due to its two additional bands. Indeed, one of the primary facts pointed out by Figure 9 - Figure 11 is that the upstream and downstream differential capacities are nearly equal above MHz. This occurs because the noise at these frequencies is dominated by self-fext, which is of course the same in both upstream and downstream directions. The implication of this is that there can be no benefit, especially in the asymmetric case, to splitting the spectrum above MHz into a large number of bands. Instead, the asymmetric service spectrum above MHz for this case only needs to be divided into two or three bands, as shown in Plans #1, #, and #3. Adding more asymmetric bands, as in Plan #4, cannot increase the total upstream + downstream capacity, and in fact will reduce it due to the additional transition band loss. All of these observations are borne out in the simulation results of Section 3. 1 ETSI Set A does not indicate the inclusion of VDSL self-fext. Whether this is an oversight or purposeful was unknown at the time of this writing. In any case, VDSL self-fext should certainly be included in this analysis, and should be added in using the ETSI crosstalk combination method, not as an entirely separate 99% worst-case additive model. This is the approach taken by this contribution. Page 7

8 10 9 Downstream Upstream Frequency (MHz) Figure 9: Differential Capacities for 1500 ft #4 AWG 9 8 Downstream Upstream Upstream 1 0 Downstream Frequency (MHz) Figure 10: Differential Capacities for 3000 ft #4 AWG Page 8

9 8 7 Downstream Upstream Upstream 1 0 Downstream Frequency (MHz) Figure 11: Differential Capacities for 4500 ft #4 AWG 3. Simulations: 3.1 Results This section presents the results of capacity simulations for the four spectral plans described in Section. All simulations assume a 1.5 db SNR gap and include noise of type ETSI Set A, with Mix A modified to also include 0 VDSL self-fext. Each spectral plan is studied for the short (1.5 kft), medium (3 kft) and long (4.5 kft) #4 AWG ANSI loops, and each of these for the following four cases: 1) Digital duplexing, no amateur band notches ) Digital duplexing, with amateur band notching 3) 1 db analog-assisted duplexing, no amateur band notching 4) 1 db analog-assisted, with amateur band notching The results are presented in Table 1 and Figure 1 - Figure 13. Page 9

10 Table 1: Performance Results Loop Length Digital Duplex? Ham Notch? Spectral Plan Upstream Rate Downstream Rate 1.5 kft Yes No # Mbps 47.0 Mbps 1.5 kft Yes No # 5.64 Mbps 47.3 Mbps 1.5 kft Yes No # Mbps 46.8 Mbps 1.5 kft Yes No # Mbps 38.9 Mbps 1.5 kft Yes Yes # Mbps 41.4 Mbps 1.5 kft Yes Yes # 5.64 Mbps 41.6 Mbps 1.5 kft Yes Yes # Mbps 41. Mbps 1.5 kft Yes Yes #4 1. Mbps 34.8 Mbps 1.5 kft No No # Mbps 38.1 Mbps 1.5 kft No No # 5.64 Mbps 36.0 Mbps 1.5 kft No No # Mbps 34.1 Mbps 1.5 kft No No # Mbps.1 Mbps 1.5 kft No Yes # Mbps 34.1 Mbps 1.5 kft No Yes # 5.64 Mbps 33.4 Mbps 1.5 kft No Yes # Mbps 31.5 Mbps 1.5 kft No Yes #4 1. Mbps 0.9 Mbps 3 kft Yes No # Mbps 35.3 Mbps 3 kft Yes No # 4.74 Mbps 35.8 Mbps 3 kft Yes No # Mbps 35.6 Mbps 3 kft Yes No # Mbps 8.7 Mbps 3 kft Yes Yes # Mbps 30.9 Mbps 3 kft Yes Yes # 4.74 Mbps 31.3 Mbps 3 kft Yes Yes # Mbps 31. Mbps 3 kft Yes Yes #4 9.8 Mbps 5.5 Mbps 3 kft No No # Mbps 9.6 Mbps 3 kft No No # 4.74 Mbps 8. Mbps 3 kft No No # Mbps 7.3 Mbps 3 kft No No # Mbps 16.7 Mbps 3 kft No Yes # Mbps 6.5 Mbps 3 kft No Yes # 4.74 Mbps 6. Mbps 3 kft No Yes # Mbps 5. Mbps 3 kft No Yes #4 9.8 Mbps 15.9 Mbps 4.5 kft Yes No #1.8 Mbps 17.6 Mbps 4.5 kft Yes No # 3.1 Mbps 18.4 Mbps 4.5 kft Yes No #3.6 Mbps 0.1 Mbps 4.5 kft Yes No # Mbps 13.6 Mbps 4.5 kft Yes Yes #1.8 Mbps 15.0 Mbps 4.5 kft Yes Yes # 3.1 Mbps 15.8 Mbps 4.5 kft Yes Yes #3.6 Mbps 17.5 Mbps 4.5 kft Yes Yes # Mbps 1.1 Mbps 4.5 kft No No #1.8 Mbps 14.5 Mbps 4.5 kft No No # 3.1 Mbps 14.6 Mbps 4.5 kft No No #3.6 Mbps 17.7 Mbps 4.5 kft No No # Mbps 7.9 Mbps 4.5 kft No Yes #1.8 Mbps 13.0 Mbps 4.5 kft No Yes # 3.1 Mbps 13.4 Mbps 4.5 kft No Yes #3.6 Mbps 16.6 Mbps 4.5 kft No Yes # Mbps 7.9 Mbps Page 10

11 Plan #1 Plan # Plan #3 Plan # Case # (See Legend in Table ) Figure 1:Upstream Simulation Results Plan #1 Plan # Plan #3 Plan #4 Case # (See Legend in Table ) Figure 13: Downstream Simulation Results Page 11

12 Table : Legend for X-Axis in Figure 1 and Figure 13 Case Number Loop Length Digital Duplex? Ham Notch? kft #4 AWG Yes No 1.5 kft #4 AWG Yes Yes kft #4 AWG No No kft #4 AWG No Yes 5 3 kft #4 AWG Yes No 6 3 kft #4 AWG Yes Yes 7 3 kft #4 AWG No No 8 3 kft #4 AWG No Yes kft #4 AWG Yes No kft #4 AWG Yes Yes kft #4 AWG No No kft #4 AWG No Yes 3. Analysis In studying the results presented in Section 3.1, the following conclusions can be made: 1) Spectral Plan #4 is inappropriate for asymmetric VDSL services. Too much bandwidth is dedicated to the upstream channel, and not enough to downstream. This is understandable given the intention that Plan #4 be used for both symmetric and asymmetric applications. However, the corresponding loss in downstream bandwidth will be unacceptable to operators contemplating a residential broadband service deployment based on VDSL. This is particularly true in the United States, where ILECs face impending competition in their core business from alternative broadband-capable carriers. ) Spectral Plan #4 suffers the most in a move from a digitally-duplexed FDD implementation to the more realistic analog-assisted duplexing case. This occurs because of the two additional bands contained in Plan #4, compared to Plans #1 - #3. Overall, Plans#1 - #3 have comparable performance to one another, making the choice between them not an obvious one. The following additional observations should be considered in making the choice. 3) Plan #3 has the highest downstream and lowest upstream capacities on the long loop. In that this upstream capacity is still greater than.5 Mbps, Plan #3 can be said to have the best performance for asymmetric services over the longest loops. 4) Upstream performance of Plans #1 - #3 is more than adequate on the short and medium loops. Hence plan comparison for the short and medium lops can be based on downstream capacity alone. 5) Plan #1 has the best performance and Plan #3 the worst on the short and medium loops. 6) Plan #1 experiences the least transition band spectral loss in the case of analog-assisted duplexing. 7) The low frequency placement of the Plan #1 asymmetric upstream band will simplify upstream power back-off for it (less dynamic range and less slope equalization), relative to the other plans. 8) Loops with inadequate splicing for broadband services exhibit loss curves with higher-than-expected slope. Plan #1 will be best for such loops, from a plant maintenance and trouble-shooting perspective, since a full-duplex link will still usually be available to aid in system diagnostics. This is less likely with Plans # - #3. 4. Summary In designing an asymmetric-optimized spectral plan, it is necessary to consider both technical issues and the service requirements and the deployment environment for an asymmetric roll-out. Based on these factors, three promising candidate spectral plans are proposed and their performance studied. Comparison is also made to the performance of a fourth spectral plan that has been previously proposed by others. Page 1

13 The analysis results indicate that this fourth plan is not suitable for the asymmetric bandwidth needs of a residential broadband deployment, as it gives too much bandwidth to the upstream channel and not enough to the downstream. This fourth plan also requires an extra band transition above MHz, which is unnecessary from a differential capacity perspective and harmful in the realistic case of analog-assisted duplexing. Among the remaining plans, Plan #3 has the advantage of highest downstream capacity on the long loop, while Plan #1 has the highest downstream capacity for the short and medium loop. The lower frequency placement of the asymmetric upstream band imparts to Plan #1 a number of practical advantages in the areas of duplexing loss, upstream power back-off, and trouble-shooting diagnostics. Based on the results of this analysis, this contribution makes the following proposal: It is proposed that Spectral Plans #1 and #3 from this contribution be considered as the two candidates for an asymmetric optimized VDSL spectral plan. 5. References [1] Broadcom and General Instrument/NLC, Separate asymmetric- and symmetric-optimized spectral plans, T1E1.4/99-397, Baltimore, MD, August 3 7, [] Alcatel, ST Microelectronics, Texas Instruments, Mitel, G.vdsl: Universal Fixed Frequency Band Proposal, ITU Q4/15 Temporary Document NG-11R, Nuremberg, Germany, August [3] Texas Instruments, G.vdsl: Feasible Implementation of Digitally Duplexed DMT VDSL with at most a 1-bit ADC, ITU Q4/15 temporary Document NG-055, Nuremberg, Germany, August [4] Broadcom Corporation, Analog Front End Dynamic Range Requirements for Programmable Spectrum VDSL Systems, T1E1.4/99-315, Ottawa, Canada, June 7 11, [5] Broadcom Corporation, The Need for Analog Suppression of Transmit Echoes in VDSL Transceivers: Measured Results, T1E1.4/99-398, Baltimore, MD, August 3 7, [6] Lucent Technologies, G.vdsl: Reducing ADC Resolution by Using Analog Band-pass Filters in FDD based VDSL, Contribution ITU-T SG15/Q4 D..., Geneva, Switzerland, June [7] Lucent Technologies, G.vdsl: On ADC and DAC Resolution Requirements for DMT/FDD based VDSL, Contribution ITU-T SG15/Q4 D..., Geneva, Switzerland, June [8] R. W. Rhea, HF Filter Design and Computer Simulation, Noble Publishing, Atlanta, GA, [9] A. I. Zverev, Handbook of Filter Synthesis, John Wiley & Sons, New York, NY, Page 13