R&D Whie Paper WHP 120 Sepember 2005 Digial on-channel repeaer for DAB A. Wiewiorka and P.N. Moss Research & Developmen BRITISH BROADCASTING CORPORATION
BBC Research & Developmen Whie Paper WHP 120 A. Wiewiorka and P.N. Moss Digial on-channel repeaer for DAB Absrac The provision of full coverage o he public can be a difficul ask in erresrial broadcasing. Someimes, due o echnical challenges and cos consrains, i may no be possible o deploy convenional signal disribuion o ransmiing saions. In hese cases an on-channel repeaer may be he only viable opion. This paper describes a BBC digial on-channel repeaer designed o exend he coverage of DAB services, believed o be he firs device of his kind esed for broadcasing purposes in he UK. The uni receives a signal from a disan ransmier and re-broadcass an amplified and slighly delayed version of i on he same frequency. In order o avoid insabiliy caused by parasiic coupling beween he receiving and ransmiing anennas, sray ransmied signals are cancelled by an adapive filer wih a Leas-Mean-Square esimaor. The heory of operaion is explained and hardware implemenaion is discussed. Resuls of a field rial are also presened. This documen was originally published in he Proceedings of he Inernaional Broadcasing Convenion, Sepember 2005. BBC 2005. All righs reserved.
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DIGITAL ON-CHANNEL REPEATER FOR DAB A. Wiewiorka and P.N. Moss BBC Research & Developmen, UK ABSTRACT The provision of full coverage o he public can be a difficul ask in erresrial broadcasing. Someimes, due o echnical challenges and cos consrains, i may no be possible o deploy convenional signal disribuion o ransmiing saions. In hese cases an on-channel repeaer may be he only viable opion. This paper describes a BBC digial on-channel repeaer designed o exend he coverage of DAB services, believed o be he firs device of his kind esed for broadcasing purposes in he UK. The uni receives a signal from a disan ransmier and re-broadcass an amplified and slighly delayed version of i on he same frequency. In order o avoid insabiliy caused by parasiic coupling beween he receiving and ransmiing anennas, sray ransmied signals are cancelled by an adapive filer wih a Leas-Mean-Square esimaor. The heory of operaion is explained and hardware implemenaion is discussed. Resuls of a field rial are also presened. INTRODUCTION Ensuring maximum possible coverage of erresrial broadcas services can be echnically difficul and expensive. The line-of-sigh naure of modern broadcasing imposes he requiremen for a large number of ransmiers as well as a complicaed signal disribuion sysem involving UHF, microwave, saellie, or cable links. Apar from he problems of opography, here may also be issues of co-exisence wih oher operaors, who migh own or conrol suiable ransmier sies or he means of signal disribuion. In circumsances when radiional soluions for coverage exension canno be deployed, an on-channel repeaer may be he only viable alernaive. The BBC firs idenified he poenial need for on-channel repeaers in relaion o hole-filling in he coverage of is Digial Audio Broadcasing (DAB) services. An on-channel repeaer reransmis a signal on he same frequency as i receives, so ha no frequency ranslaion is required, as is he case in RF or saellie links. However, due o unwaned coupling beween he receiving and he ransmiing anennas, he device can also receive is own oupu, hus causing insabiliy and relaxaion oscillaions. For his reason analogue on-channel repeaers require large anenna isolaion and are suiable mainly for poin-o-poin links raher han broadcas purposes such as DAB. The device described in his paper employs suiably adaped echo cancellaion echniques used exensively in digial signal processing for audio applicaions. Alhough hese mehods have been known for a considerable ime, a commercially viable implemenaion using digial hardware running a he speed required for digial broadcasing only became possible around 2002. The heory of operaion is explained in he nex secion, ogeher wih he mahemaical basis of he adoped soluion. The imporan aspecs of operaional use, such as he suiabiliy of a ransmier sie are addressed. Boh analogue RF and digial design issues and consrains are presened followed by he descripion of he adoped implemenaion. Finally he resuls of a field es are shown and discussed.
THEORY OF OPERATION An on-channel repeaer receives a weak signal hrough a receiving anenna and reradiaes an amplified version of his signal, on he same frequency, using anoher anenna. Due o he physical proximiy beween he anennas, as well as reflecions off he ground and nearby objecs, here may be a significan amoun of unwaned feedback from he oupu of he repeaer ino is inpu. If no removed, such feedback causes insabiliy and relaxaion oscillaions ha render he signal unusable. s() Weak Received Signal x() ε() Unwaned Feedback Adapive Filer h Filer Esimaor Decorrelaing Delay Reference Signal y() Srong Transmied Signal In-band inerference from adjacen channel Figure 1 Simplified block diagram of an on-channel repeaer. The cancellaion of parasiic feedback is performed by an adapive finie impulse response filer which models he pah beween he anennas. The filer impulse response h can be esimaed using correlaion mehods, provided ha he delay hrough he sysem is sufficienly long o avoid he effecs of auocorrelaion. In oher words, he reransmied signal, and hence he feedback, mus be uncorrelaed wih he received signal, so ha i can be unambiguously idenified. In fac, apar from delayed ε(), y() may conain a number of inerfering signals such as inermodulaion producs from he power amplifier or even inermodulaion producs from adjacen channels, if presen. The simples, and in mos cases enirely sufficien, mehod of esimaing filer aps is he Leas Mean Square (LMS) algorihm. Given he inpu x() and oupu y(), he objecive is o minimize he error ε(), leaving only he received signal s(). This signal is recovered by subracing from x() he oupu y() filered by h, as shown below: ε ( ) = x( ) h T y...(1) Vecor y conains y(), y(-1) o y(-k-1) where K is he lengh of h. Ideally all remnans of y() are removed from x(), and he energy in ε() is minimized. In order o achieve his goal he filer aps h mus be esimaed. Firs he expecaion of squared magniude of ε() is defined and differeniaed wih respec o h: E 2 T 2 ε ( ) = E x( ) h y...(2) d E ε ( ) dh 2 T * * { y [ x( ) h y ] } = E[ y ε ( ) ] = E...(3) The aps are hen updaed wih an appropriaely scaled conjugae of he insananeous value of he derivaive: h = h λε...(4) * 1 + ( ) y The value of λ is chosen as a compromise beween he speed of convergence and signal-o-
noise raio. In pracice, faser convergence allows he algorihm o follow rapid changes in he feedback pah caused by Doppler pahs and muliple reflecions, a he expense of increased noise in he filer ap esimaes h, which affecs feedback cancellaion. Similarly, in he case of slow convergence and in he presence of Doppler, ε() may conain a significan remnan of y(), also causing insabiliy. Because he level of his residual feedback is proporional o he loop gain of ε, he amoun of Doppler variabiliy in x() imposes an upper limi on he gain of he repeaer. As discussed in Marple (1), convergence also depends on he eigenvalues of he covariance marix R yy =E(yy*). Opimal behaviour occurs when all eigenvalues are equal, bu if R yy is no full rank, or some eigenvalues are close o zero, convergence is significanly impaired. One of he consequences of poor convergence migh be large amouns of noise generaed in unused areas near he edges of he Nyquis band. For his reason, he choice of sampling rae has o be carefully addressed and bes resuls are achieved if he inpu signal is noiselike in naure. In conras, oher algorihms such as Fas Recursive Leas Squares esimae he auocorrelaion of he recovered signal and have assured convergence properies (1). Implemenaion of Fas RLS is however considerably more demanding of hardware resources. THE ON-CHANNEL REPEATER AND THE DAB COVERAGE PROBLEM The reasons for filling gaps in coverage of a broadcas service can be divided ino wo broad caegories: Coverage exension: If signal srengh is insufficien in a paricular area, he repeaer can be used as an addiional ransmier in he Single Frequency Nework (SFN). In his case he repeaer mus have good inpu sensiiviy. If he area o be covered is large, he required gain could be as high as 100dB. Hole-filling: Signal srengh is sufficien bu signals on adjacen channels are so srong ha domesic receivers may no have sufficien dynamic range o recover he waned channel successfully. In exreme cases, in areas very close o adjacen channel ransmiers, he inermodulaion producs from he adjacen channels inerfere direcly wih he waned signal even hough specral mask requiremens have been fulfilled. When used in such an environmen, he repeaer mus exhibi very large dynamic range and very good seleciviy. Indeed, he device mus remove he inermodulaion producs of cosied adjacen channels ha arrive a is inpu ogeher wih he unwaned feedback. To do his he use of a common anenna feed is necessary. Figure 2 shows ypical signal levels observed in an area of Noingham, UK, where he waned signal on channel 12B is 43dB below he adjacen Level (db) 0 Minimum received signal ha mees EN50248:2001-30 12B -43-45 -80 176kHz 12C 225.648 227.360 Criical mask for 12C Frequency (MHz) Figure 2 Recepion condiions in he presence of a srong adjacen channel.
channel 12C. According o EN50248:2001, Secion 7.3.3, a consumer DAB receiver mus be able o decode a signal surrounded by adjacen channels a +30dB and cope wih any signals 5MHz away from he cenre frequency a +40dB. Even before allowing an addiional margin of 10dB for fading, i is clear ha he signal on 12B is unlikely o be decoded by a ypical commercial receiver. A decision on wheher o deploy an on-channel repeaer should be based on he measuremen of anenna isolaion A A and he minimum received signal srengh P RX. Given he arge power P TX, he power of inermodulaion producs P IP from adjacen channels (if any), and he cancellaion capabiliy of he repeaer A R, he esimae of signal o inerference raio S is: P TX P IP = + + 10 S AA AR PRX 10log 10 10 + 10 (db)...(5) To ensure sabiliy S should no be smaller han 15dB. In he absence of inermodulaion his amoun of feedback corresponds o approximaely 3dB peak-o-peak ripple in frequency response of he sysem and is seen by a DAB receiver as addiional delayed componens of he channel response. Large amouns of uncorrelaed inerference and noise can also be oleraed because DAB uses differenial QPSK modulaion which produces 10-3 bi error rae a 11dB carrier-o-noise raio. The saisics of OFDM signals are noise-like and hus ideal for processing wih an LMS algorihm. Anoher advanage is he exisence of he guard inerval, 246µs in he case of DAB mode I, approximaely one fifh of symbol duraion. This allows he repeaer o inroduce a processing delay sufficien boh o cancel disan reflecions and o decorrelae he feedback signal from he inpu signal. On he oher hand, he delay mus be shor enough no o disurb he operaion of he DAB single frequency nework. I is expeced ha some receivers ha do no posiion heir FFT window o include all significan componens of he channel response may exhibi problems wih ime synchronisaion. IMPLEMENTATION Performance requiremens The repeaer was designed for a ypical siuaion shown in Figure 3. A is main inpu, fed from he receiving anenna, he device should cope wih a parasiic feedback o received raio of 30dB, while he cancellaion of his feedback should be a 45 o 50dB in order no o degrade he signal o noise raio of he recovered signal. If he repeaer is co-sied wih an adjacen channel ransmier, he preferred soluion is o ransmi from he same anenna as he adjacen channel. The reference inpu of he repeaer can hen be fed wih a combined Adjacen channel Feedback o received raio Received Reransmied Residual +23 0-30 Level (db) Adjacen channel Cancellaion of parasiic feedback Frequency Figure 3 Design crieria for he on-channel repeaer.
signal ransmied hrough he anenna ha conains no only he waned bu also he adjacen channel, so ha is inermodulaion producs can be cancelled as well, avoiding undesirable re-radiaion. This is a consequence of he fac ha signal y() in Figure 1 need no be relaed o he recovered signal ε(). Indeed hose wo signals mus no be correlaed wihin he ime period equal o he lengh of he adapive filer. The adjacen channel signals can be up o 53dB larger han he received waned signal, imposing sringen requiremens on seleciviy of filers, lineariy of mixers and dynamic range of digial-o-analogue converers. A furher consrain affecing he design of he digial filers is he requiremen o keep he processing delay as shor as possible, preferably below 20µs, i.e. less han one enh of he guard inerval. Circui descripion The prooype on-channel repeaer is implemened as a single hardware uni wih a 0dBm oupu driver. Reference Inpu RF Filer Main Inpu RF Filer Band III Local Oscillaor IF Filer LO IF Filer A D C A D C Subsampling and filering 20MHz Subsampling and filering LMS Esimaor Adapive Filer Embedded Processor AGC Digial Processing D Upsampling A C 20MHz Figure 4 Block diagram of he BBC digial on-channel repeaer. USB Inerface Signal Monioring Oupu 0dBm RF Filer LO The waned DAB signal on Band III, received using a direcional anenna, is fed ino he main inpu of he repeaer uni. The firs module is an RF filer designed primarily o rejec an image frequency before downconversion. A high level mixer is hen used o conver o a 20MHz IF where a high order LC IF filer is applied o remove mos of he energy of he adjacen channels. The received signal may of course be buried under a feedback signal some 30dB sronger. In addiion, he unfilered remnans of he adjacen channels mus also be accommodaed in order o avoid clipping. Consequenly, he design of he mixer and IF filer mus ensure he lowes disorion and he highes dynamic range wihin he waned bandwidh before he signal is digiised. The signal is sampled by a 14-bi analogue-o-digial converer running a 80MHz so ha he quanisaion noise is spread over he enire Nyquis bandwidh of 40MHz resuling in he effecive signal o noise raio of 93dB wih respec o full scale wihin he useful 1536kHz bandwidh. Once in he digial domain, he signal is convered o complex baseband, filered and subsampled o 1.66MHz. I should be noed ha adjacen ensembles are separaed by as lile as 176kHz, which imposes a sharp filer roll-off, resuling in non-linear group delay performance of he repeaer a he edge of he ensemble. There follows a feedback cancellaion block, which renders he signal suiable for re-broadcasing. The oupu level is
sabilised by he AGC module, which is appropriaely gaed o avoid gain changes during he DAB null symbol. The signal is hen upsampled, mixed up o an IF of 20MHz and convered back o analogue in a 16-bi converer. All digial processing is performed in a single Field Programmable Gae Array (FPGA) device, which execues approximaely 3 billion fixed-poin muliplicaions per second. A furher up-conversion o Band III is followed by an image rejecion band-pass filer and a 0dBm driver sage. This signal can hen be roued o an exernal RF power amplifier and RF channel filer, and combined wih he main ransmier oupu carrying an adjacen channel or direcly applied o he ransmiing anenna. The reference signal obained from a coupler in he anenna feed is processed in an idenical manner o he main signal, as described above, up o and including he digial downconversion and filering. The nex sage is he adapive filer which produces he cancellaion signal subraced from he main inpu. Filer aps are calculaed in an LMS esimaor module. The signal levels are moniored by an embedded microprocessor in he FPGA ha conrols he behaviour of he AGC circuiry. If an abnormal condiion is deeced, such as an overload or signal disconnecion, he oupu of he repeaer is reduced or mued. Imporan parameers, such as he lengh of he adapive filer and he oupu signal level, can be adjused by means of a USB inerface, which is also used during field ess for real-ime monioring of filer aps and oher inernal signals on a lapop compuer. A DAB demodulaor was implemened in sofware and used o assess he qualiy of he specrum and consellaion of he reransmied signal. LABORATORY TESTS In order o evaluae he repeaer performance, a number of laboraory measuremens and experimens were carried ou a he home of BBC R&D in Kingswood Warren, Surrey, UK. Firs, an HP11759D channel simulaor was used o measure he abiliy of he adapive filer o rack dynamically varying feedback beween anennas. Differen saic and Doppler channels wih muliple reflecions were successfully cancelled by he repeaer, depending on he speed of convergence and gain parameers. For example, feedback cancellaion of 34dB could be achieved for channels wih Doppler pahs varying a 10Hz, equivalen o 13km/h a 225MHz. During anoher es, involving a reransmission of 1W of DAB hrough a roofop anenna, cancellaion levels of 50dB were reached wih he ransmied signal 30dB larger han he original received signal a he main inpu. Since no significan variabiliy of he parasiic feedback was observed, he Doppler handling capabiliy could o be reduced o 0.8Hz, sufficien o follow vibraions of he anennas caused by wind as well as movemen of disan objecs. FIELD TRIAL The firs field es of he BBC on-channel repeaer was conduced a he Mapperley Ridge ransmier sie in Noingham, UK. The sie, conaining a 1kW ERP ransmier for channel 12C (227.36MHz), was chosen due o problems in recepion of channel 12B (225.648MHz) from Walham 30km away. Referring back o Figure 2, he raio of powers of signals 12B and 12C was oo high o allow reliable recepion wihin a 3.5km radius of he Mapperley sie. Thus he objecive was o ransmi a 5W ERP signal on channel 12B, having received i from Walham using a 3-elemen Yagi mouned on he ransmier mas. As shown in Figure 5 he oupu of he repeaer a 0dBm was fed ino a 40W DAB amplifier. The signal hen passed hrough a 6-caviy Kahrein channel filer o a 10dB coupler o be combined wih he signal on 12C. The reference signal was obained from a 47dB coupler.
47dB coupler +30dBm Reference Inpu Main Inpu On-Channel Repeaer 12B DAB Power Amplifier Channel Filer 10dB coupler Lapop Compuer Adjacen Channel Transmier 12C Figure 5 On-channel repeaer a Mapperley Ridge. A Mapperley Ridge he isolaion beween he receiving and ransmiing anenna was measured a 77dB, and he 12B signal from Walham was a 40dBm. The oal gain required for 12B, including he 10dB coupler and feeder loss, was 70dB o produce +30dBm oupu. The isolaion was observed o change wih ime, possibly due o weaher condiions, such as rain, fog and moisure on he ground. Figure 6 shows he specrum obained from he receiving anenna. The signal on 12B is a sum of he feedback from he ransmiing anenna and he original signal received from Walham. I can be seen from Figure 7, which shows he signal a he ransmiing anenna, ha he specrum of he 12B ensemble is fla, which indicaes ha he parasiic feedback has been removed. No noiceable degradaion of signal-o-noise raio was observed. A recepion survey confirmed ha he repeaer enabled successful demodulaion of 12B in he viciniy of he Mapperley Ridge sie. Due o he modes gain requiremen and higher han expeced anenna isolaion, he full capabiliy of he repeaer was no exercised. The measured level of cancelled inerfering signal was, depending on weaher condiions, 1dB o 7dB lower han he level received from Walham. This should be compared o he resul quoed in he previous secion, where he feedback was 30dB higher han he waned signal. However, an aemp o re-broadcas wihou cancellaion would have resuled in insabiliy, or a bes in very non-linear frequency response. I was also observed ha, alhough he repeaer inroduced a delay of only 20µs, some DAB receivers used for he recepion survey had occasional problems wih ime synchronisaion in areas where he signals from Walham and Mapperley Ridge were of comparable srengh. Furher work is needed o quanify how pervasive his problem migh be. CONCLUSIONS AND FUTURE WORK The BBC prooype of a digial on-channel repeaer demonsraed he suiabiliy of digial echo cancellaion echniques for exending coverage of OFDM broadcas services. Effecive anenna isolaion was increased by up o 50dB, sufficien o achieve significan sysem gain values while mainaining good signal-o-noise raio. No problems wih sabiliy were observed during field ess, as parasiic anenna coupling was changing very slowly, largely due o weaher condiions. A similar, bu compuaionally more powerful, repeaer can also be designed for DVB-T, bu he mulilevel QAM modulaion imposes sricer limis on signal-o-noise raio. Hence, he sysem gain canno be as high as in he case of DAB, especially if 64-QAM is used. To
achieve faser and more precise racking of feedback, oher algorihms, such as Fas RLS could be implemened, a he expense of much greaer compuaional complexiy. REFERENCES 1. Marple, S. L. Jr., Digial Specral Analysis wih Applicaions, Prenice Hall,1987, ISBN 0-13-214149-3. 2. ETS 300 401, Radio Broadcasing sysems: Digial Audio Broadcasing (DAB) o mobile, porable and fixed receivers. 3. EN50248:2001, Characerisics of DAB Receivers. dbm -50-60 -70-80 -90-100 -110-120 -130-140 Rx anenna "Tx ON" 3 1 12B 1 225.680-80.4000 2 227.355-73.4900 3 222.055-85.4100-150 Sar: 220.680 Sop: 230.680 Res BW: 100 Vid BW: 30 Sweep: 10 9h Feb 2005 Figure 6 Signal a he receiving anenna (afer 30dB coupler). 2 12C dbm 0-10 -20-30 -40-50 -60-70 -80-90 -100 Tx anenna "47dB" 1 12B 1 225.680-33.2700 2 227.355-9.8800 3-1 1.675 23.3900 Sar: 220.680 MHz Sop: 230.680 MHz Res BW: 100 Vid BW: 30 Sweep: 10 9h Feb 2005 2 3-1 12C Figure 7 Signal a he ransmiing anenna (afer 47dB coupler).