OFDM synchronization scheme for Power Line Telecommunications (PLT)

OFDM synch oniza ion scheme o Powe Line Telecommunica ions (PLT) V.Baena, J.G anado, M.A.Agui e, A.To alba, L.G.F anquelo Dp o de Ingenie ía Elec ónica. Uni e sidad de Se illa. Camino de Los Descub imien os s/n. 41092-Se illa (SPAIN) E-mail: {baena, joaquin, agui e, o alba, leopoldo}@g e.esi.us.es Abs ac This pape p esen s a new scheme o OFDM ime and equency synch oniza ion wi h applica ion in Powe Line Telecommunica ions (PLT). Simula ion esul s show an excellen beha io , e en o he low alues o SNR in he synch onize inpu inhe en o PLT. The synch onize has been p o o yped on an FPGA p io o be in eg a ed in he single-chip PLT sys em. Keywo ds: OFDM ime and equency synch oniza- ion, Powe line communica ion, PLT, FPGA. 1. In oduc ion The basic idea o mul ica ie modula ion (MCM) is o di ide he a ailable spec um in se e al sub chan- nels. In a classical FDM (F equency Di ision Mul iplexing) sys em, na ow-band signals a e gen- e a ed independen ly, assigned o a ious equency bands, pa allel ansmi ed, and sepa a ed by il e s a he ecei e [1]. In an OFDM (O hogonal F equency Di ision Mul- iplexing) sys em, he in o ma ion is pa allel mapped in o N-QAM (Quad a u e Ampli ude Modula ion) signals and mul iplexed using a FFT (Fas Fou ie T ans o m) [2]. This echnique has been conside ed o b oadband applica ions including bo h wi ed and wi eless ap- plica ions. In he case o adio ansmissions, OFDM is used in no malized Digi al Te es ial Video B oadcas ing (DTVB) and Digi al Audio B oadcas - ing (DAB), bo h s anda ds om ETSI. In addi ion, ETSI-BRAN amily o ecommenda ions (B oad- band Radio Access Ne wo ks echnical body) has selec ed his ansmission echnique o HIPERLAN/2 (High Pe o mance Local A ea Ne - wo k) [3]. Wi ed applica ions, such as ADSL (Asymme ic Digi al Subsc ibe Line) o HDSL (High-bi - a e DSL), employ OFDM echniques (also called DMT: Disc e e Modula ion Technique) o deli e high bi a es o he end use . [4]. Recen ly OFDM has been sugges ed o Powe Line Telecommunica ions (ETSI-PLT Technical Body and [5]). This echnology will be able o p o ide a new local b oadband access as well as indoo da a ne wo king using o dina y powe lines ins alled in e e y home and o ice. This pape p esen s an algo i hm o p o ide bo h, coa se and ine synch oniza ion, o an OFDM sys- em in Powe Line Telecommunica ions (PLT). The ou line o his pape is as ollows: Sec ion 2 desc ibes he p inciples o classical OFDM sys ems. I will summa y he key aspec s o he OFDM modula ion, as well as i s ad an ages and d awbacks. Sec ion 3 illus a es he e ec s o syn- ch oniza ion e o s in OFDM. Sec ion 4 p esen s a synch oniza ion scheme o p o ide ime and e- quency synch oniza ion in a PLT anscei e , and some simula ion esul s. Sec ion 5 shows a ha dwa e implemen a ion on an FPGA and, inally, in Sec ion 6 some conclusions a e d awn. 2. OFDM desc ip ion The i s OFDM sys em was p oposed in 1971 by Weins ein and Ebe [6]. Since powe ul silicon echnology was no a ailable a his ime, he de el- opmen o OFDM based sys ems was ce ainly delayed un il nowadays. In an OFDM sys em (see igu e 1), he incoming in o ma ion signal S is pa allel mapped using an o dina y cons ella ion o ob ain complex samples. N o hese complex samples Xk,p (k = 0,.., N-1) a e ans o med by an iFFT o cons i u e he p- h OFDM symbol (see equa ion 1). As a esul , xn.p is a disc e e base band sequence o N-QAM ca ie s. ∑ − = − =1 0 2 ,, N k j N kn pkpn eXx π n = 0,1,..,N-1 (1) A gua d in e al, called Cyclic P e ix (CP), wi h M samples, is added o he ou pu o he iFFT in o de o a oid possible ISI (In e Symbol In e e ence). FFT Y N-2, p Y N-1, p Y 0, p Y 1, pSignap demappe . . . R 0, p R 2, p R N-2, p R N-1, p S/P y N-2, p y N-1, p y 0, p y 1, p S/P CP S/P iFFT X 0, p X 2, p X N-2, p X N-1, p Sp x 0, p x 2, p x N-2, p x N-1, p P/S CP Signap mappe . . . S 0, p S 2, p S N-2, p S N-1, p u n h c ( ) y n ADC Fs Fs DAC I-Q mod Fc I-Q demod Fc Fig. 1. OFDM sys em o e iew This cyclic p e ix is a copy o he las M samples o he OFDM symbol ha is p e ended o be ansmi - ed and i is a pu e sys em o e head ha educe he base band bandwid h equi ed by a ac o =N/(M+N). η The disc e e base band signal is analogue con e ed (DAC-Fs) and up mixed o he channel equency Fc. The esul ing signal is exp essed in equa ion 2, whe e is he absolu e ime, T is he OFDM symbol du a ion, N is he numbe o sub ca ie s and Xk,p is he cons ella ion poin ca ied by he k- h sub ca ie o he p- h OFDM symbol. The indi idual spec a a e now o hogonal sinc unc ions and he o al bandwid h B is di ided in o N equidis an na ow band sub channels. ∑∑ ∞+ −∞= − −=      +−         = p N Nk F T pT j pk c eX z 12/ 2/ 2)(2 , )( π η π (2) The mos impo an ad an age o OFDM sys ems when compa ed o single ca ie sys ems is ob ained in b oadband applica ions o e equency selec i e channels ( adio channels, powe line channels, e c). Equaliza ion in OFDM is educed o a simple mul i- plica ion o each sub ca ie by a complex ac o , whe eas equaliza ion in single ca ie ansmission may no be easible o in oduces la ge delays. OFDM p oduces much g a e immuni y o impulse noise and as ades due o i s long symbol ime. In addi ion, he cyclic p e ix inclusion educes ISI, e en when using an Analogue F on End (AFE) wi h la ge o de FIR il e s. Di icul ies ega ding OFDM a e: peak- o-mean- powe a io ha equi es an ex emely high linea ampli ie o educe OOB (Ou -O -Band) In e e - ence, and he equi emen o accu a e ime and equency synch oniza ion. 3. Synch oniza ion e o s in OFDM Time and equency synch oniza ion be ween ans- mi e and ecei e a e o c ucial impo ance in e ms o sys em pe o mance [3]. A equency misma ch be ween ansmi e and ecei e causes a los o o hogonali y ha will e- duce he use ul signal ampli ude and will lead o In e Ca ie In e e ence (ICI). Bo h impai men s cause an impo an BER deg ada ion. OFDM sys- ems a e o de s o magni ude mo e sensi i e o equency o se and phase noise han single ca ie sys ems [7]. A ime o se in he FFT ime window es ima ion causes phase o a ion in equency domain. The ou pu symbol wi hin he OFDM symbol is o a ed by a di e en angle. F om subca ie o subca ie , he angle inc eases p opo ionally o he equency o se . In OFDM sys ems wi h cohe en de ec ion his o a ion has o be p ope ly co ec ed. Howe e , unde non-cohe en de ec ion, his inc emen al o se does no dec ease sys em pe o mances since he in o ma ion is ca ied in phase o se s be ween con- secu i e symbols. I he es ima ed s a posi ion o he FFT window loca es wi hin he da a in e al, he sampled OFDM symbol will con ain some samples ha belong o o he OFDM symbol. The phase o a ion imposed by OFDM symbol synch oniza ion e o can hus be co ec ed by app op ia ely o a ing he ecei ed signal, bu he dispe sion o signal cons ella ion caused by ISI o ms a bi e o BER loo due o he p esence o un eco e able samples. In conclusion, he use o a synch oniza ion scheme which a oids OFDM symbol es ima ion e o , will lead o an e ec i e dec ease in he leng h o he cyclic p e ix, educing i s o e head. In his case, he cyclic p e ix leng h can be educed un il he loo le el imposed by he mul i-pa h and ading channel ea u e. The mos impo an synch oniza ion me hods in OFDM a e p esen ed in [8]-[15]. In o C P eamble BA Fig. 2. Bu s s uc u e 4. Synch oniza ion algo i hm A ime equency synch oniza ion scheme o bu s based ansmissions is p oposed he e. I will be in eg a ed in o a PLT sys em. As o he bu s based ansmission sys ems, he OFDM ame is s uc u ed as shown in igu e 2. The p eamble is depic ed in igu e 3 and consis s o h ee di e en sec ions (A, B and C). Sec ion A is conside ed o p eamble de ec ion, gain adjus men (no mally pe o med by an ex e nal P og ammable Gain Con olle ), and coa se iming es ima ion. Fine equency and ime uning a e done in he B sec ion. The C-Field is ese ed o channel es ima ion. Sec ion A B 32 B 32 B 32 Sec ion B B 32 C 32 C 64 C 64 Sec ion C A 16 A 16 A 16 A 16 A 16 -A 16 Fig. 3. Heade de ail desc ip ion An au o-co ela ion scheme has been selec ed o p eamble de ec ion and coa se iming synch oniza- ion ( igu e 4). The ecei ed signal is delayed by he co ela ion delay D (16 samples). Conjuga e com- plex samples o he delayed e sion a e mul iplied by he ecei ed samples. Resul ing p oduc s a e eed in o he mo ing a e age block, whose window size is W=64 samples, and hen hey a e pos -p ocessed o h eshold de ec ion and maximum sea ch in o de o ind he co ec iming. D (.)* Mo ing A e age |(...)| 2 Mo ing A e age |(...)| Recei ed da a X(i) Y(i) X(i)/ Y(i) R ( Fig. 4. Au oco ela ion scheme Figu e 5 shows he au o-co ela o ou pu o sec ion A in p esence o AWGN inpu noise. No e ha a h eshold alue is equi ed a) o minimize he p ob- abili y o alse p eamble de ec ion wi hin he da a ield o a MAC (Medium Access Laye ) ame, and b) o il e ou small peaks in he au o-co ela o ou pu due o inpu noise. Howe e , a la ge h esh- old alue dec eases he p obabili y o co ec p eamble de ec ion wi hin he p eamble ield o a MAC ame. A e exhaus i e simula ion, i has been ound ha a h eshold alue o 0.55 ep esen s a good comp omise. Noise and mul i-pa h signal p opaga ion o e powe - line channels p oduces b oade peaks a he au o- co ela o ou pu , educing he accu acy o he im- ing eco e y p ocess. This coa se synch oniza ion can educes he bu den o he ine iming p ocess. Simula ions esul s show ha he maximum e o in oduced by he coa se iming p oces, when he peak o he R(i) signal is abo e he h eshold alue, is only +/-4 samples. Fine iming is achie ed by using ma ched il e s. Since he maximum e o is +/-4 samples, a bank o nine ma ched il e s is equi ed, one o each possi- ble sample delay. I has o be no ed ha wi h he ou B- ields o he p eamble only h ee complex mul iplica ion and h ee addi ions pe sample a e needed, a much lowe load han he 32 complex mul iplica ions and he 32 addi ions needed i a c oss co ela ion is done o e all he incoming sam- ples. Figu e 6 shows he il e s ou pu s in he case o AWGN inpu noise. Due o equency de ia ion be ween ansmi e and ecei e oscilla o s, he ecei ed base band signal has a ime a ying phase componen which de e io- a es he OFDM demodula ion. Fig. 5. Au o-co ela o ou pu . A sec ion 100 200 300 400 500 600 700 80 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1R(i), AWGN, SNR=15dB Sample index R(i) −5 −4 −3 −2 −1 0 1 2 3 4 5 0 0.5 1 1.5 Fine iming, AWGN, SNR=15dB Co ela o index Co ela o s ou pu s The p ocess o he es ima ion o equency de ia- ion is depic ed in igu e 7 and i is done in pa allel wi h he ine iming p ocess. S a ing wi h he i s sample o he i s B- ield, he incoming signal is delayed by he delay D (32 samples). The conjuga e complex samples o he delayed signal a e mul i- plied by he ecei ed samples. The ou pu is hen a e aged o e 96 p oduc s (un il he end o he B- ield). S aigh o wa d analysis shows ha he angle o he esul Y is p opo ional o equency de ia ion. Fig. 6. Ma ched il e s ou pu . B sec ion. F equency co ec ion can be easily pe o med by complex mul iplica ion o he in-phase and quad a- u e n- h inpu sample wi h , whe e ∆ω is he es ima ed equency de ia ion. nj eω∆ D(.)* A e age Recei ed da a Fig. 7 5. Ha dwa e implemen a ion P e ious sec ions desc ibed a new scheme o ime and equency synch oniza ion wi h applica ion o PLT. A ha dwa e implemen a ion o his scheme, called Hype synch Module (HSM), has been done using VHDL. HSM has been implemen ed ying o sa e as much silicon a ea as possible while i ing i in o he sys em clock equi emen s. FIFO 16 MULT CONJ DinRe{13}+DinIm{13} MOD 2 FIFO n n{32:128} SUM NEG ModDin 2 {27} FIFO n n{32:128} SUM NEG Value{27} IncValue{27} SUM+ ACUM Acum MOD 2 ENERGÍA POT2 ENEGÍA 2 DIV ModAcum n FsmSinc oA NewDa LoadFIFO16 PhaseB PhaseA LoadFIFON1 LoadFIFON2 Cl Sinc oA LoadFi o16 LoadFi oN2 LoadFi oN1 Fig. 8 Sec ion A a chi ec u e Fi s o all, ega ding possible ime-sha ing o ha d- wa e esou ces, coa se and ine synch oniza ion wi h sec ions A and B o he p eamble a e made in di e - en ime in e als, sha ing ha dwa e esou ces such as memo ies, adde s and mul iplie s. Second, hose a i hme ic ope a ions which a e oo slow o which consume la ge silicon a ea, such as squa e oo and di isions, a e a oided, by using powe -o - wo cod- ing o he in e nal signals. Finally, pipeline echniques has been in oduced in o de o educe delay chain and op imize c i ical pa hs. Figu es 8 and 9 shows a b ie desc ip ion o he solu ion adop ed o bo h blocks and ha dwa e esou ces consumed. They show hei own memo y blocks, bu in he inal sys em he module is sha ed. Fo a oid- ing squa e oo s all he quan i ies a e squa ed. Be o e being in eg a ed on he inal chip, HMS has been p o o yped on an Xilinx Vi ex-XV300 FPGA ( o m HADES-1 Sys em [16]) using Founda ion 3.3i The comple e sys em equi ed no mo e han 50.000 sys em ga es excluding memo ies, and wo king a 33 MHz. The whole sys em (Base Band P ocesso ) has been also in oduced using he same en i onmen and has been a ed o 80.000 sys em ga es a 33MHz. The p o o ype has been un using a XSV-800 boa d and he on-chip memo y o s imuli injec ion. The esul s has been compa ed wi h Ma lab high le el simula- ions, and all he quan iza ion e o s and o he e ec s has been alida ed he FPGA emula ion ap- p oach. Sinc oB Algo i hm Da aIn Re{13},Im{13} X COEFICIENT MEMORY(32) Dec0 Dec1 Dec2 FIFO32 + Acumula o Bank RegAng COMPARATOR F O Rom and Sma Acumula o COSINE(N*F O ) SINE(N*F O ) SEQUENCE COMPENSATION Fig. 9 Sec ion B a chi ec u e 6. Conclusions Powe line communica ion is now possible wi h signi ican ad an ages o e con en ional cable in- dus y because i uses he exis ing elec ic powe in as uc u e. Howe e , he physical medium is ha d, equi ing inno a i e solu ions. In his pape , a new scheme o ime and equency synch oniza ion o OFDM bu s ansmission is p esen ed as well as i s ha dwa e implemen a ion on an FPGA. Simula- ion esul s a e p o ided and show he e ec i eness o he p oposed solu ion, and an implemen a ion has been made using mode a e ha dwa e esou ces. Acknowledgemen s This wo k has been inanced by he Eu opean Commission unde he IST V p ojec INSONET (IN home and Soho NETwo king h ough he mains ne wo k). Re e ences [1] H. Rohling, T. May, K. B üninghaus and R. G ünheid. “ B oad band OFDM adio ansmission o mul ime- dia applica ions”. P oc. O he IEEE ol. 87, no. 10, oc obe 1999. [2] John .A.C. Bingham: "Mull ica ie modula ion o da a ansmission: an idea whose ime has come". 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