Experimental Results on the Coexistence of TV Broadcasting Service with LTE Mobile Systems in the 800 MHz Band

Transcription

1 Experimental Results on the Coexistence of TV Broadcasting Service with LTE Mobile Systems in the 800 MHz Band M. Ferrante, G. Fusco, E. Restuccia Istituto Superiore delle Comunicazioni e delle Tecnologie dell Informazione of MiSE Rome, ITALY M. Celidonio, P.G. Masullo, L. Pulcini Fondazione Ugo Bordoni Rome, ITALY Abstract Latest technological developments have allowed a more efficient use of the frequency spectrum, providing resources for the deployment of fourth generation broadband mobile networks, named LTE, in a portion of UHF band previously allocated to TV broadcasting. However, to cope with this new scenario, the designing criteria of the TV receiving installations need to be reviewed as well as, for existing installations, the problems of coexistence must to be addressed carefully. Accordingly, this study focuses on laboratory results of experimental tests carried out to evaluate interference effects occurring on DVB-T channels and in presence of different MATV configurations, due to radio emissions generated by LTE Base Stations in the 800 MHz band. Keywords LTE, downlink, DVB-T, Measurements, Interference I. INTRODUCTION The current generation of mobile networks continues to transform the way people communicate and access information. Further development and implementation of technologies, enabling network access for person-to-person and person-to-machine connectivity, are redefining the end user mobility concept: over time, any mobile app and any mobile service will be given the potential to connect to anything at anytime, from people to physical things. This is the promise of 4G LTE (Long Term Evolution) and, more concretely, of future 5G systems to expand the possibilities of what mobile networks can do, and to extend the range of services they can deliver. Accordingly, a flexible and efficient use of all available non-contiguous spectrum portions in different network deployment scenarios, in addition to the freeing up of additional spectrum, will be required to support thousand-fold capacity increases by In particular, the new spectrum allocation at 800 MHz for mobile use, resulting from the digital switchover (DSO) process from analog to digital television (DTV) system, that was concluded at the end of the year 2013 in almost all European countries, is providing many new challenges for the wireless industry. But, in this reference framework, some coexistence issues are possible. More in detail, the deployment of mobile services in the MHz band can inject interfering signals into the inhome TV receiver or set top box in different ways. This can degrade the perceived quality of the broadcast TV service (QoS). Traditionally, radio interferences between different services have been usually mitigated by using guard bands or by ensuring sufficient geographical separation. For LTE mobile network deployments planned in the UHF released band ( MHz), the guard band has been reduced to just 1 MHz from broadcasting DTT service (see Fig. 1). The close proximity of cellular and digital TV assignments might consequently result in interference to digital TV service in some areas, particularly near the edge of coverage. The transmitter powers, sites, network delivery infrastructure, antennas and network topologies for DTT broadcast networks are quite different from mobile cellular ones. As a result of these differences and the very small guard band adopted, a number of interference scenarios are going to emerge. Effects on existing TV receiver installations are of particular importance due to their design and realisation criteria based on the availability of the entire TV UHF band up to 862 MHz and consequently without selective protection against unforeseen other services inside this band. This paper examines the potential interference caused by the downlink emissions, from the LTE radio base station to the TV antenna, by means of laboratory simulation experimental results obtained in different setup scenarios. To this aim, after an introductive overview of regulatory aspects of the 800 MHz band in section II and a brief description of the considered experimental setups in section III, main resulting measurements obtained in the ISCTI (Istituto Superiore delle Comunicazioni e delle Tecnologie dell Informazione) laboratory about the evaluation of the effects produced by an interferential LTE signal on a DVB-T signal will be presented in section IV. Finally, some conclusive remarks will be reported in section V. II. 800 MHZ BAND REGULATORY ASPECTS Starting from September 2008, in consequence of the switchover process from analogue to digital broadcasting TV system, the European Parliament [1] has encouraged Member States to make available the digital dividend portions of UHF

2 frequency bands to other services, in order to obtain a more efficient use of the spectrum. This position has been successively strengthened by the Communication [2] sent in 2009 to the European Parliament by the European Commission, focused on a study witnessing the added value in terms of cost/benefit ratio which would be possible to obtain in case the digital dividend band MHz was reserved to electronic communications services (mobile wireless broadband) in the whole European Union. Accordingly, in occasion of the ITU World Radiocommunication Conference (WRC) 2007, this band was allocated for mobile services on a co-primary basis with broadcasting. In particular, the harmonised plan for the 800 MHz band in ITU Region 1 (consisting of Europe, Africa and parts of the Middle East), divides the band into two 30 MHz blocks for FDD downlink and uplink transmissions, as described in the Decision 2010/267/UE [3], that in Europe have been reserved to the deployment of the new fourthgeneration broadband mobile systems, commonly known as LTE. In the lower part of the band, from 791 MHz to 821 MHz, 6 blocks 5 MHz bandwidth have been selected for the downlink communications and the same spectrum range has been devoted to uplink communications in the upper part of the band, from 832 MHz to 862 MHz. The adoption of the above mentioned frequency allocation determines, between uplink mobile service and broadcasting DTT service, a 42 MHz frequency separation, considering also the halfway 11 MHz duplex gap which could be exploited by other unspecified services. Fig. 1: Bandwidth allocation in ITU-Region 1 for UL and DL LTE transmissions at 800 MHz Additionally, in occasion of the last WRC-12, held in Geneva, further UHF spectrum to mobile services has been allocated [4], even if on a co-primary basis with other services, in order to meet the demands of future additional bandwidth. In ITU Region 1 the new mobile spectrum allocation involves the frequency band from 694 to 790 MHz, and is proposed to come into force in 2015, in order to enable the conclusion of the necessary technical studies regarding the availability and assignment of the new band. III. EXPERIMENTAL SCENARIO The receiving antennas used in radio and television plants installed before 2013 were designed to receive the third VHF band and the fourth and fifth UHF band up to 862 MHz, corresponding to the edge of the channel 69. The LTE mobile radio service, currently deployed in the frequency band ranging from 791 MHz to 862 MHz, makes television receiving systems subject to interference effects that can be summarized into two major categories: the ones occurring on the antenna TV, with or without the presence of a masthead amplifier, and the ones occurring in the in-home TV set. These interferences can be produced both by the downlink connection (from the base station to user terminal) and by the uplink connection (from user terminal to base station). Measurements and analysis reported deal with potential interference caused by the downlink emissions by means of laboratory simulations carried out using the experimental setup shown in Fig. 2. The DVB-T signal is generated by the Rhode-Schwarz SFU broadcast test system, which is modulated by the output MPEG-2 transport stream provided by the R&S DVG generator. On the other side, the LTE downlink signal is generated by the R&S SMBV100A vector signal generator. The two signals, DVB-T and LTE, are combined using a Directional Coupler operating in the frequency band MHz, which provides an effective decoupling of about 50 db between the two signal generators. Furthermore, impedance adapters (50Ω, 75Ω) have been used to connect professional instruments and commercial TV equipment. Concerning the device under test (DUT) two different options have been considered: a DVB-T TV set selected from those normally available on the market, directly connected to the receiving antenna; a complete chain of a typical TV receiving plant, including a masthead (broadband) amplifier and a variable attenuator that simulates the cable and the distribution network losses resulting in the individual apartments, in order to have a -50 dbm DVB-T signal power level at the input of the TV receiver. During the experimental study, three DVB-T receivers (indicated in the following as TV1, TV2 and TV3), implementing two different tuning techniques (CAN and Silicon tuner), have been tested. In particular: - CAN tuner (also known as super-heterodyne) is placed in a metal enclosure to prevent interferences and the RF input signal is mixed with a Local Oscillator to obtain an Intermediate Frequency (IF) signal. The super-heterodyne architecture used in this kind of tuners leads to an image frequency 72 MHz above the tuned signal. - Silicon tuner uses Large Scale of Integration (LSI) chips. The receivers using this kind of tuner are not affected by the problem of image frequency, because the input signal is directly converted in base band or in a very low IF.

3 Fig. 2 Functional schematic of the test bed A. System parameters adopted Interfering signal In this experimental context the LTE downlink signal acts as the interferer one and has been configured on the basis of the following settings: bandwidth: 10 MHz and 30 MHz; background noise added: none; traffic load: 70% and 0% (IDLE). Victim signal The victim signal considered in this study is a reference DTT signal compliant with the ETSI EN [5], whose main features are summarized in Table I. TABLE I. MAIN FEATURES OF THE DVB-T SIGNAL (VICTIM SIGNAL) Modulation OFDM Subcarriers Code Rate Guard Interval Bandwidth (MHz) Data Rate (Mbps) 64 QAM 8K 3/4 1/ Measurements were carried out for different signal levels at DTT receiver antenna input or, in presence of a masthead amplifier, at the input of it. Nine DVB-T multiplex channels were tested, spanning from channel 52 up to channel 60 (carrier spacing 8 MHz). The calculated parameter was the protection ratio, defined as the minimum value of the ratio between the wanted and the unwanted signal powers, measured at the antenna input of the device under test [6] and evaluated in correspondence of a specific degradation threshold. This threshold was determined following the guidelines provided by the Subjective Failure Point (SFP) [6] method that is used to identify the minimum level of the unwanted signal that causes the first event of deterioration (the so-called pixeling, visible on the screen) in a predetermined observation period (about 20 s), with a fixed level of the victim signal. IV. RESULTS In this section are summarized some results obtained during the laboratory tests. A more detailed description of the overall results are reported in [7]. The I/C ratio (LTE signal power on DVB-T signal power), as a function of the TV channel frequencies, has been considered as a reference parameter. As above mentioned, data have been collected for channels ranging from 52 to 60, that are the ones adjacents to the LTE downlink band. The I/C ratio, expressed in decibels, is the opposite of the protection ratio defined above. The choice of using this ratio is due exclusively to have a more comfortable visual analysis of the obtained results. It is also important to note that the results depend on the spectral characteristics of the used LTE and DVB-T signals. Any variant of them such as, for example, DVB-T signal degradations due to propagation with multiple paths or noise platform in the LTE signal, could lead to conclusions different from those obtained in this paper. As mentioned in the previous sections, measurements have been carried out both in presence and absence of masthead amplifier and/or a low-pass filter addressed to mitigate LTE signal influences. Detailed results for each of these configurations are reported in the next subsections. A. Measurements in absence of masthead amplifier From the results obtained in absence of the masthead amplifier, it is evident an interference effect of the LTE signal that focuses mainly on channel 60. This impairment is much more evident when the level of the DVB-T signal is low (see an example of experimental results in Fig. 3). As expected, the effect is more pronounced when the signal is emitted in LTE band immediately adjacent to the DVB-T ones rather than in the case of LTE signals transmitted in the upper frequency bands. Obviously this interference is also noticeable in the case of simultaneous transmission of all LTE signals (30 MHz bandwidth signal, from 791 MHz to 821 MHz). In this case, the I/C ratio is slightly lower. Fig. 3 - I/C for a LTE signal (traffic load: 70% and bandwidth 10 MHz) centred at 796 MHz and DVB-T Signal Power C= -50dBm Furthermore, when operating with TV1 and TV2, equipped with CAN tuner, is highlighted the presence of an image frequency signal effect 72 MHz below the LTE signal

4 frequency band. This impairment may significantly affect the QoS of the tuned DVB-T signal, causing the above mentioned picture failures on the video image even for LTE interfering signal power level not so high. In particular, according to the experimental results, in presence of image frequency impairment, the LTE signal power level able to produce a DVB-T signal degradation is reduced up to around 20 db. Depending on the carrier frequency of LTE downlink interfering signal, the effect of the image channel moves from DVB-T channel 52 (central frequency: 722 MHz) with the 796 MHz LTE carrier frequency, up to channel 55 (central frequency: 746 MHz) with 816 MHz LTE carrier frequency. These effects are even more evident for lower power levels of the DVB-T signal at the input of the receiver.tests have been also performed in presence of LTE signals in IDLE condition. On the basis of the obtained results (see Fig.4), the interference effects start to appear for lower power levels (around 5-6 db) with respect to the 70% traffic load. To analyse the effects caused by the presence of this device in the MATV chain, tests were carried out in laboratory using a typical masthead amplifier in two different realistic configurations: amplifier gain: 43 db with a DVB-T input power signal level equal to -26 dbm or -31 dbm; amplifier gain: 53 db with a DVB-T input power signal level equal to -36 dbm or dbm. Fig. 5 - Performance of a typical high gain broadband amplifier in UHF band Fig. 4 - I/C comparison between a LTE signal with traffic load 70% and IDLE mode, centred at 796MHz, DVB-T signal power C=-50dBm, receiver type: SILICON Attenuators of appropriate value was used to keep constant at -50 dbm the power level of DVB-T signal at the input of the TV set. In Fig. 6 are summarized the I/C ratio results when a LTE signal with 70% traffic load, 10 MHz bandwidth and 796 MHz centre frequency is interfering a DVB-T signal with the above mentioned power levels and amplifier settings. In this case a TV receiver with CAN tuner is considered. However, taking into account that, when the radio base station is operating in IDLE mode, the LTE signal is at a power level approximately 10 db below the one in the case of operating mode at full load, the interference condition, with the same radio base station, occurs within a smaller interference area. B. Measurements in presence of masthead amplifier Antenna systems including an amplifier in the headend, typical in Master Antenna Television (MATV) systems, show a behaviour deserving particular attention. The amplifiers commercially available before the migration of mobile services in the UHF television band show amplification almost constant up to 862 MHz, as shown in Fig. 5, today unnecessary and even harmful. In fact, a LTE signal irradiated close to the TV antenna, if not properly limited inside the MATV system, could lead the amplifier to a saturation condition which generates high level of intermodulation products which degrades signals transmitted in part or in the whole remaining UHF television band, with the possible consequence to blind the corresponding DVB-T channels. Fig. 6 - I/C for a LTE signal (traffic load: 70% and bandwidth 10 MHz) centred at 796 MHz, receiver type: CAN Analysing the test results it is possible to conclude that: 1. for low levels of DVB-T signal the interfering signal acts mainly on the internal TV set circuits. In fact the I/C ratio follows the same behaviour previously observed during tests without amplifier; 2. for high levels of DVB-T signal the interfering signal acts mainly on the amplifier, which tends towards saturation with consequent increase of intermodulation products, as shown by the relevant very flat performance of the I/C ratio. In this case, the harmful effects are evenly distributed on all the considered channels; 3. a LTE signal with 30 MHz bandwidth in comparison to a 10 MHz one, having the same total power level, centred on

5 the same frequency, give rise to a worsening of the I/C ratio, only for low DVB-T power levels. C. Measurements in presence of masthead amplifier and low-pass filter The drawbacks presented in the previous paragraph demonstrate the need to protect MATV installations. In Italy, for example, this issue was on charge of Subcommittee 100D of the CEI (Italian Electrotechnical Committee) who, after a thorough study, has produced suggestions, included in the guide CEI [8]. The mitigation of interference effects, might be achieved with the introduction of a special filter, whose characteristics are given in the above mentioned guide, with the purpose of limiting the reception band to the one currently attributed to the broadcasting service ( MHz). An example of frequency response of LTE filter is shown in Fig. 7. Fig. 7 LTE signal filter characteristic function in the MHz frequency band The improving effect caused by the insertion of a LTE filter is evident in Fig. 8 where is shown a comparison between I/C ratio results obtained with the presence of the amplifier versus the configuration with amplifier and filter, evaluated in comparable operative conditions (LTE signal carrier: 796 MHz, DVB-T signal level around -60 dbm, CAN receiver). Note that a broadband noise platform, introduced by the LTE signal generator, represents a co-channel interference for the DVB-T signal and it could lead to different findings. As a first result it is evident an almost constant behaviour of I/C ratio in presence of the filter, even using CAN receivers, where the image frequency effect is significantly mitigated. Fig. 8 - I/C ratio performance comparison with and without LTE filter using an amplifier and a CAN receiver Some problems are still present on channel 60, mainly caused by non-ideal characteristics of the specific filter used in the experimental tests in correspondence of the cutoff frequency, considering that the commercially available filters are a trade off between technical features and cheapness. D. Practical considerations concerning the protection distance On the basis of the obtained results, illustrated in terms of I/C ratio, it is possible to calculate, at least as a first approximation, the critical interfering radiated power and the corresponding so called Protection Distance (PD) as a function of receiver system parameters. From a technical point of view, the PD is defined as the minimum spatial distance, expressed in meters, between an interfering antenna system (the LTE system) and the victim receiving antenna of the system to be protected (DVB-T receiver) in order to ensure that the interference effects at the front-end antenna of the receiver are still acceptable in terms of quality of service. Recommendation ITU-R SM [9] provides a general method to evaluate this parameter, as well as the frequency separation between the interfering and the victim antennas in order to bind the interfering signal under an acceptable power level. According to this method, the PD can be evaluated assuming that the LTE base station is in line-of-sight with the DTT receiving antenna (see Fig. 9) and that the path loss of the radio signals can be calculated using the free space propagation model, as suggested in Rec. ITU-R P [10]. Fig. 9 Interfering scenario In particular, adopting the above mentioned model in case of MATV systems, it is possible to relate the EIRP of LTE transmitter and the distance between TX and RX antennas using the expression: I [ ] 20logd m dbm G dbi 20log f MHz A db A db A db EIRP dbm r where: EIRP: equivalent isotropically radiated power by the LTE Base Station antenna in the direction of TV antenna (dbm); d: distance between LTE and TV antennas (m); f: LTE carrier frequency (MHz); p c f

6 G r : gain of the receiving antenna with reference to an isotropic antenna at frequency f and in direction of the LTE antenna (dbi); A p : polarization mismatch loss (db); A c : TV antenna cable loss (db); A f : LTE filter loss (if present) (db); I: LTE signal level at the antenna input of the DUT (television set or masthead amplifier) in correspondence of the I/C ratio critical value (threshold level of video signal degradation) (db). A further term could be inserted in the second member of the expression to take account of a margin before the impairment. For example, considering a scenario where the interfering signal is the LTE one set with a carrier frequency f=806 MHz and a bandwidth of 10 MHz, and the victim signal is the TV channel 53 (730 MHz), the I/C ratio threshold value resulting from experimental tests was 35 db (C=-50 dbm, I=-15 dbm). In this condition, taking into account the typical values of the additional parameters reported in Table II, the resulting protection distance for some power levels of the LTE EIRP signal, are reported in Table III. TABLE II. MATV SYSTEM PARAMETERS G r 9 dbi A p 3 db (45 ) A c 3 db 0 db (without LTE filter) A f TABLE III. PROTECTION DISTANCE FOR SOME EIRP LEVELS IN THE SCENARIO CONSIDERED IN THE EXAMPLE LTE EIRP [dbm] PD [m] V. CONCLUSIONS This paper aims at presenting the test results performed in the laboratory, using a specific experimental setup, in order to simulate the interference effects that could be produced on the DVB-T service as a consequence of the introduction of the LTE mobile radio service in the digital dividend part of the UHF band. The study, which refers to typical configurations of domestic television reception plants, takes into consideration the downlink signal transmitted by a LTE base station in the MHz frequency band. The obtained results highlighted that the TV receiver features, as well as the presence or absence in the receiving chain of a masthead amplifier, also in conjunction with a commercial filter, may heavily influence, in specific circumstances, the DVB-T signal QoS perceived by the user. More in detail: - in absence of the masthead amplifier, the CAN TV receiver determines the presence of image effects on DVB-T channels 52-55, depending on the LTE signal carrier ranging from 796 to 816 MHz, with a consequential increase of the PR up to about 20 db. On the contrary, this result does not occur for the Silicon TV receiver where, only in presence of DVB-T signal power levels close to the minimum allowed or in presence of high LTE signal power levels, the reduction of DVB-T signal QoS is significant, with an almost flat behaviour in the UHF TV band; - in presence of the masthead amplifier, it has been observed a behaviour similar to the one described in the previous case for quite low power levels of the victim signal. For higher values the nonlinearity characteristic effects of the amplifier appear predominant and the receiving system becomes more affected by the interfering LTE signal gathered by TV antenna; - in presence of masthead amplifier and low-pass filter, the interference effects described in the previous cases appear mitigated, also with reference of frequency image effects introduced by CAN TV receivers. Note that the QoS of the TV channel 60, which is the closest to the LTE frequency band, results adversely affected by the commercial LTE filter adopted during the experimental tests. From these results it is clear that some interference problems exist. For this reason, if appropriate countermeasures are adopted both in the MATV plant (as the ones suggested in [8]) and in the 800 MHz LTE base stations, harmful impairments for a large portion of the DVB-T service area might be avoided. REFERENCES [1] EPR 2008/2099 (INI), Reaping the full benefits of the digital dividend in Europe: A common approach to the use of the spectrum released by the digital switchover, European Parliament Resolution,, September 2008; [2] EU COM(2009) 586, Transforming the digital dividend into social benefits and economic growth, October 2009; [3] Dec. 2010/267/EU On harmonised technical conditions of use in the MHz frequency band for terrestrial systems capable of providing electronic communications services in the European Union, May 2010; [4] WRC Resolution 232 [COM5/10], Use of the frequency band MHz by the mobile, except aeronautical mobile, service in Region 1 and related studies, Geneva 2012 [5] ETSI EN , Digital Video Broadcasting (DVB), Framing structure, channel coding and modulation for digital terrestrial television (DVB-T), April 2009 [6] Rec. 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