Abstract. 1. Introduction. 2. OFDM Signal. where

Transcription

1 Enhancng Data Rates usng OFDM Technques or Varous Applcatons Mukul Kabra and S.L. Maskara Dhrubha Amban Insttute o Inormaton and Communcaton Technology, Gandhnagar , Gujarat (Inda) Emal: {mukul_kabra, maskara@da-ct.org} Abstract Orthogonal Frequency Dvson Multplexng (OFDM) has turned out to be a useul technque or hgh data rate transmsson,. n lmted bandwdth channel and. n adng envronment. Ths paper gves an overvew o the concept, mplementaton and derent applcatons o the OFDM technque. Some mportant ssues related to mplementaton o OFDM systems have been dscussed.. Introducton The Orthogonal Frequency Dvson Multplexng technque has the potental o enhancng data rates n a band-lmted channel n general and under adng condton n partcular. Instead o transmttng hgh data rates on a sngle carrer requrng hgh bandwdth, n OFDM, the hgh data rate sgnal s splt nto many low data rate streams, whch are then transmtted on multple closely spaced orthogonal carrers. In act the bandwdth o each subcarrer should be made small compared wth the coherence bandwdth o adng channel. In other words, the symbol perod o a sub-stream s made large compared to the delay spread o the tme dspersve rado channel. Implementaton o OFDM transmtter s consderably smpled because o use o Inverse Dscrete Fourer Transorm usng dgtal sgnal processng technques. These eatures have resulted n wde range applcatons o OFDM, both n wrelne as well as wreless communcaton systems. In ths paper a bre descrpton o the OFDM sgnal and ts transmsson and recepton schemes have been gven as a background materal. Then the perormance o an OFDM n a adng envronment has been dscussed. Some mportant ssues n the mplementaton o OFDM systems have been hghlghted. A ew promnent applcatons o OFDM have been descrbed beore concludng the paper.. OFDM Sgnal In OFDM dgtal data s transmtted usng large number o narrow bandwdth carrers whch are regularly spaced n requency, ormng a block o spectrum. Subchannel spacng ( ) s made equal to the recprocal o symbol perod (T s ), so that the modulated sgnals would be orthogonal and could readly be separated by correlaton usng a conventonal matched lter or a correlator[]. Let = / T s (where T s s the useul symbol perod over whch the recever ntegrates the demodulated sgnal). Then k th carrer (at baseband) s wrtten as: jπk t φ ( = e () k where k s the requency o the kth subcarrer. and the orthogonalty condton that carrers should satsy s: τ Ts + τ * φ ( t ) φ ( t ) = 0 ; k p k p = T s ; k = An OFDM sgnal conssts o such orthogonal subcarrers modulated by parallel data streams as shown n gure. Fgure. Frequency doman OFDM sgnal Mathematcal expresson or contnuous tme representaton o OFDM symbol x( becomes []: j k t x t = π ( ) X ( k) e ; 0 t T (3) S k = 0 where = ; basc crteron or OFDM (4) TS When we are usng dscrete tme then t wll become n t = TS ; n = 0,, Κ (5) t n and t = = (6) T S ( ) = j πkn x n X ( k ) e ; n = 0,, Κ k = IFFT p () (7)

2 Here X(k) s the kth complex data symbol (typcally taken rom a M-PSK or M-QAM symbol constellaton). Equaton () represents the common procedure o demodulatng a carrer by means o multplyng t by the same requency carrer ( beatng t down to dc ) and then ntegratng the result. Any other carrers wll gve rse to beat tones, whch are at nteger multples and have an nteger number o cycles durng the ntegraton perod, thus ntegrate to zero. I the ntegraton perod spans two symbols, not only there wll be same carrer ISI, but n addton there wll be nter-carrer ntererence (ICI) as well. Ths happens because the beat tones rom other carrers may no longer ntegrate to zero they change n phase and/or ampltude durng the perod. Ths s avoded by addng a cyclc prex (trck to avod resdual ISI), whch ensures that all the normaton ntegrated comes rom the same symbol and appears constant durng t. Instead o usng an empty guard space, t s lled wth the cyclc extenson o the OFDM symbol, gure. Introducton o Cyclc Prex adds energy loss proportonal to the length o the CP, but the zero ICI generally compensate or ths loss. OFDM was not used promnently or many years because or a large number o sub channels, the arrays o snusodal generators and coherent demodulators requred n a parallel system become unreasonably expensve and complex. The recever needs precse phasng o the demodulatng carrers and samplng tmes n order to keep crosstalk between sub channels acceptable. In practce, complete dgtal mplementaton could be bult around usng specal purpose hardware perormng FFT and IFFT [], whch s mathematcally equvalent o dscrete DFT and IDFT respectvely, but more ecent or mplementaton. DFT transorm correlates the nput sgnal wth each o ts snusodal uncorrelated bass unctons. The correlaton or a gven subcarrer only sees energy or that correspondng subcarrer. Ths separaton o sgnal energy s the reason that the OFDM subcarrers overlap wthout causng ntererence. Fgure. Use o Cyclc Prex to combat multpath Theoretcally, M-ary dgtal modulaton scheme usng OFDM can acheve bandwdth ecency β (dened as bt rate per unt bandwdth) o log M bts/s/hz (or orthogonal requency spacng and strctly band-lmted spectra δ= /). The bt rate or a correspondng M-ary system s log M/(T S /). Thereore the [] β becomes log M = (8) β + δ In realty, however the spectra overlow ths mnmum bandwdth by some actor α such that δ=(+α) and the ecency becomes log M (9) β = log M α + To obtan the hghest β n an OFDM system, must be large and α must be small. Theoretcally, gven subcarrers, the total bandwdth requred s (see Fg. ): BW total = (+)/T s = (+) = (-) +δ 3. OFDM Transmt-Receve Schemes Fgure 3. OFDM Transmt-Receve scheme The requency-selectve rado channel may severely attenuate the data symbols transmtted on one or several sub-carrers, leadng to bt-errors. Channel codng and nterleavng (requency doman) [3] s used to correct or the erroneous bts. Channel encoder can be a convolutonal encoder wth or wthout puncturng, turbo coder or Reed Solomon Coder. Hperlan/ standard uses one o the /, 3/4 or 9/6 rate convolutonal coder dependng on channel condton. OFDM systems utlzng error correcton codng are oten reerred as coded OFDM (COFDM) systems. Seral to parallel converted low data rate FEC coded and nterleaved bt stream rst converted to some n-phase (X I (k)) and quadrature-phase X Q (k) component usng M-QAM or M- PSK modulaton. Ths process wll map the gven M bts to a symbol X(k) = X I (k)+jx Q (k), whch represents phase and ampltude o a partcular k th subcarrer. At the transmtter, OFDM system treats these symbols as though they are n the requency doman. The IFFT takes n symbols at a tme where s the number o subcarrers n the system, whch depends on the applcaton. ot all the carrers are used to transmt data. As n IEEE 80.a standard, 4 subcarrers are used or plot sgnal and subcarrers are let blank (ncludng dc) out o 64 avalable subcarrers. Each nput symbol acts lke a complex weght or the correspondng orthogonal snusodal bass uncton. The IFFT output s the summaton o all snusods. The block o output samples rom the IFFT makes up a sngle OFDM symbol. Cyclc Prex (whch s the copy o ew last samples o the OFDM tme symbol) s added to the output o IFFT and then parallel to seral converson takes place as shown n gure 4. CP s

3 usually 0 to 5% o OFDM symbol length, as longer CP adds to energy loss. Fgure 4. Baseband mplementaton o OFDM scheme For most wreless applcatons the OFDM sgnal s generated at base-band usng complex samples (WLA), then modulated up to the requred RF requency usng an IQ modulator (analog technques or a Dgtal Up Converter). A transmtted RF sgnal s always a real sgnal as t s just a varaton n eld ntensty. It s however possble to drectly generate a real OFDM sgnal usng IFFT but n that case or pont IFFT wll be requred or data carrers. Input gven to the IFFT wll be conjugate symmetrc so that the output o IFFT s real. 4. Perormance o OFDM n Fadng Channels The multpath adng channels manests tsel n two eects. Frst, tme spreadng (n τ) o the symbol duraton wthn the sgnal, and/or, a tme varant behavor (n. In such case receved sgnal y( can be expressed as a convoluton o the transmtted sgnal x( wth the channel mpulse response h(t,τ) plus nherent AGW n(. y( = x( h( t, τ ) + n( = x( t τ ) h( t, τ ) dτ + n( ( 0) and the baseband mpulse response h b (t,τ) o a multpath channel can be expressed as h ( t, τ ) = b L =0 a ( t, τ ) exp [ j( π τ ( + φ ( t, τ ))] δ ( τ τ ( ) () c where a (t,τ), φ (t,τ) and τ represents attenuaton, phase and delay o the th multpath component, respectvely. For AWG channel and lat adng channel OFDM behaves same as a sngle carrer system. One tap equalzer s used to correct the ampltude and phase change because o the lat adng channel. For AWG channel, BER or BPSK and QPSK s gven by: E b d Pe = 0.5erc F. where F = () o + CP P where CP s length o cyclc prex, d s no o data carrer and p s no o plot carrer used n OFDM system. The expresson o BER or QPSK n Raylegh adng channel wth perect estmaton s: µ γ P e = 0.5 where µ = (3) µ + γ 3 Eb γ =. F. the average receved SR per symbol (4) o Advantage o OFDM comes when the sgnal bandwdth s more than the channel coherence bandwdth (requency selectve channel). In such case sgnal degradaton occurs (because o ISI) n sngle carrer system. Let us consder a specc case o multpath adng n WLA, n whch requred data rate s hghest possble 54 Mbps. For ths we use 64 QAM modulaton, then symbol rate wll be approxmately 0 Msps (or smplcty). Let us assume RMS delay spread to be σ =600ns = 0.6µs (worst case). Channel coherence bandwdth wll be /(5σ)=333 KHz whch s greater than the subchannel bandwdth 3.5 khz. Table. Comparson o sngle carrer and OFDM sgnal n multpath adng channel Parameters Sngle Carrer OFDM Total # o subcarrers 64 Data Rate per subcarrer R b 0 Msps 56 Ksps Symbol Tme (T S = / R b ) 0. µs 6.4 µs s σ > T S / 0 YES O Fadng Channel Type Frequency Selectve Flat ISI wll be reduced t s lat adng. Further use o cyclc prex (ncreases the tme duraton o OFDM symbol) completely removes the ISI. 5. Some Implementaton Issues 5. The Peak Power Problem n OFDM The OFDM sgnal s bascally a sum o complex random varables. All the sgnal components may add up n phase and produce a large output or they may cancel each other producng zero output at derent tmes. Thus the peak to average rato (PAR) o the OFDM system s very large. Power ampler at the transmtter ront-end must have a wde lnear range to nclude the peaks n the transmtted waveorm, whch s a costly aar. Further, ths also results n hgh power consumpton. The DAC s and the ADC s must also have a wde range to avod clppng. PAPR s reduced by: Sgnal Dstorton Technque (clppng, wndowng and lnear peak cancellaton technques) Codng Technques [5] Scramblng Technques [4] 5. Synchronzaton n OFDM Systems In OFDM systems orthogonalty o the subcarrers s crucal. Symbol tme and carrer requency osets [6] may cause the loss o subcarrer orthogonalty. I not compensated, they may lmt the perormance o an OFDM system because they cause ISI and ICI. Phase nose s also present n all practcal oscllators and t manests tsel n the orm o random phase modulaton o the carrer. The synchronzaton process s normally splt nto an acquston phase and a trackng phase, the characterstcs o the random requency and tmng errors are known. In acquston phase, an ntal estmate o

4 the errors s acqured, usng more complex algorthms and possbly a hgher amount o synchronzaton normaton n the data sgnal, whereas later the trackng algorthms only have to correct or small short-term devatons. Synchronzaton n OFDM systems s done usng: Cyclc Extenson Tranng Sequences The maxmum requency error that can be estmated usng tranng sequence s gven by: = (5) max DT s where D s the delay between the dentcal samples o two repeated symbols. 5.3 Channel Estmaton Data detecton n coherent OFDM recevers requre an accurate (or near accurate) estmate o Channel State Inormaton (CSI). In non-coherent methods, the detecton s perormed based on the derental normaton avalable between successve symbols, so there s no need or channel estmaton. Channel estmators are plot symbol asssted (PSAM), tranng symbol asssted and blnd estmaton. A mxture o blnd method wth lmted tranng symbols s called sem-blnd technque. For OFDM systems PSAM on lat adng channels nvolves the sparse nserton o known plot symbols n a stream o data symbols. Most documented channel estmaton concepts, namely LS, MMSE and LMMSE, consst o two steps, one or both o whch use the correlaton o the channel. Frst, the attenuatons at the plot postons are measured and possbly smoothed usng the channel correlaton. These measurements then serve to estmate (nterpolate) the complex-valued attenuatons o the data symbols n the second step. Ths second step uses the channel correlaton propertes ether wth nterpolaton lters or wth a decson-drected scheme. Dependng on the plot pattern, the estmaton strateges dverge. 6. Applcatons o OFDM Advanced and ast dgtal sgnal processors have made the mplementaton o FFT, IFFT and other sgnal processng very easy and cost eectve. Thereore, OFDM applcatons are ncreasng day by day. The prmary applcatons are n Wreless LA, Dgtal Subscrber Lnes. DAB, DVB-T systems. It has also been suggested or Power-lne communcaton systems, Orthogonal Frequency Dvson Multple Access systems and other applcaton based on Mult Carrer CDMA. Some o these applcatons are descrbed here. 6. Wreless LA Applcatons Date rates n wreless applcatons are manly lmted because o multpath adng channel. HperLA 4 (European standard) and IEEE 80.a pushes the perormance o WLA systems, allowng a data rate o 6 Mbps to 54 Mbps. User allocaton s acheved usng TDM, and subcarrers are allocated usng a range o modulaton schemes, rom BPSK up to 64QAM, dependng on the lnk qualty. Convolutonal coder (33, 7 octal and K=7) and nterleavng s used. IEEE80.a [7] has the same physcal layer as HperLA (table ) wth the man derence between the standard correspondng to the hgher-level network protocols used. Table. Physcal layer parameters or the IEEE 80.a standard Parameter Value Data Subcarrers 48 Plot Subcarrers 4 Channel Spacng 0 MHz Carrer Spacng ( ) 3.5 khz omnal Bandwdth 6.5 MHz =(3.5 khz x 5) Useul Symbol Perod 3. µsec ( = / ) Guard Perod 0.8 µsec Modulaton Schemes BPSK, QPSK, 6 QAM, 64 QAM IFFT 64 Codng Rate /, /3, 3/4 Relatvely small amount o delay spread (00 ns) s encountered n ndoor envronment applcatons or whch ecency o OFDM s very hgh. In outdoor envronments however, drectonal antennas need to be employed (to reduce the eect o delay spread more than 800ns). 6. Dgtal Subscrber Loop (xdsl): Subscrber lnes are tme-nvarant, extremely non-lat channel wth hgh mpulsve nose. Sngle tone system wll begn to make errors at requency nulls and mpulse ampltude, whch wll not aect multtone because o narrowband channels (4 khz) and adaptve bt loadng. In DSL we can transmt data up to 5 Mbps usng Dscreet Mult Tone (OFDM) on the same copper wre par, whch s used to transmt no more than 64 kbps usng conventonal PCM. In ADSL, DMT uses 49 channels n the requency range o 6 khz to. MHz n downstream and 5 channels between 6 khz to 33.8 khz n upstream. The carrers are spaced at 4.35 khz. Table 3. Comparson o varous DSL standards Modem Data Rate Modulaton Bandwdth Ecency HDSL 048 kbps BQ bts/s/hz ADSL Downstream Mbps Upstream kbps CAP / DMT 8 bts/s/hz Downstream VDSL 3-5 Mbps < 4 CAP / DMT Upstream bts/s/hz.5-.3 Mbps -pont complex to real IFFT s used nstead o -pont IFFT to transmt the sgnal over a baseband or onedmensonal channel. TCM s used n DMT or channel codng. In DMT, throughput s maxmzed or a gven target o overall error probablty by varyng the constellaton sze

5 among the subcarrers so that each acheves approxmately the same error probablty. Another advantage o DMT s that data rates can be ncremented or decremented n steps o 3 kbps rom 64 kbps, whereas CAP can only be coarsely adjusted rom 640 kbps. 6.3 Dgtal Audo Broadcastng (DAB) DAB s an European standard or dgtal broadcastng that s ntended to replace the current analog technologes such as AM and FM wth a good sound qualty and better spectrum ecency even n multpath adng channel. DAB uses DQPSK modulaton or subcarrers and has got our transmsson modes as shown n table 4. Table 4. Parameters or some DAB transmsson modes Parameters Mode- Mode- Mode- Mode- I II III IV # o subcarrers Subcarrer spacng khz 4 kkz 8 khz khz Symbol Tme ms µs µs 63 µs Guard Tme 46 µs 6.5µs 30.8µs 3 µs Carrer Frequency < 375 <.5 < 3 <.5 MHz GHz GHz GHz DAB system samples audo at a sample rate o 48 khz wth bt resoluton. Then the data s compressed n the range o 3 to 384 kbps. A rate ¼ convoluton code [3] s used or FEC. The total data rate s about.mbps. Audo rame duraton s 4ms. A null-symbol (or a slence perod that s slghtly greater than the OFDM symbol length) s used to ndcate the start o the rame. A reerence OFDM symbol s then sent to serve as a startng pont or the derental decodng o the QPSK subcarrers. 6.4 Dgtal Vdeo Broadcastng (DVB-T) DVB s also an ETSI standard or broadcastng dgtal televson over satelltes, cables and thorough terrestral transmsson. DVB-T recever nstalled n a movng vehcle provdes clear pctures and good musc qualty (as compared to analogue TV technology) and snce the technology s dgtal, multplex transmsson o maps and other navgaton normaton s possble as a supplementary data servce. Table 5. Parameters or DVB-T transmsson modes Parameters k mode 8k mode # o subcarrers QPSK, QPSK, Modulaton type 6QAM, 6QAM, 64QAM 64QAM Subcarrer spacng 4464 Hz 6 Hz Symbol Tme (T S ) 4 µs 896 µs Allowed guard nterval ( /T S ) /4, /8, /4, /8, /6, /3 /6, /3 DVB-T has a bandwdth o 8MHz. Each DVB rame conssts o 68 OFDM symbols. Each symbol s consttuted by a set o 687 carrers n the 8K mode and carrers n the K mode as shown n table 5. It uses a Reed-Solomon outer code (04,88, T=8) and nner convolutonal code (77, 33 octal) combned wth two layers o nterleavng or error-control [3]. Data rate s aected by the choce o modulaton and amount o FEC (one o /, /3, 3/4, 5/6, 7/8 rate). It uses scattered and contnual plot subcarrers or recoverng ampltude and phase or coherent demodulaton. Two-dmensonal channel estmaton s perormed usng the plot subcarrers, whch ads n the recepton o the OFDM sgnal. 6.5 Further Drectons Perhaps o even greater mportance s the emergence o ths technology as a compettor or uture 4th Generatons (4G) wreless systems, whch s expected to emerge by the year 00. More recently, OFDM applcatons were studed wthn the European 4 th Framework ACTS program. The MEDIA project nvestgated a 55 Mbps WATM network, whle Magc WAD group developed a WLA. 7. Concluson OFDM enables transmsson o hgh data rates n lmted bandwdth. For example n VDSL, t s possble to transmt data at a rate o as hgh as 5 Mbps. OFDM brngs sgncant mprovement n hgh data rate transmsson through adng channels. For example OFDM s used n WLA applcatons gvng data rates upto 54 Mbps even n presence o dspersve channel havng maxmum delay spread o 700ns. Implementaton o OFDM s now qute smple usng the FFT technque, whch could be easly realzed usng modern DSPs. Uses o transmt dversty and adaptve modulaton technques along wth OFDM can enhance the data rates urther. In OFDM PAPR, tme and requency synchronzaton and channel estmaton are mportant desgn parameters, whch should be consdered approprately. Reerences [] Chang. R.W., Synthess o band-lmted orthogonal sgnals or mult-channel data transmsson, bell system tech., 45:775-96, December 966. [] Cmn L.C. Jr., Analyss and Smulaton o a Dgtal Moble Channel Usng Orthogonal Frequency Dvson Multplexng, IEEE Trans. on Communcaton, July 985. [3] R. Wesel and J. Co, "Fundamentals o Codng or Broadcast OFDM", Proceedngs o the 9th ACSSC. [4] R.V. ee & R. Prasad, OFDM or Wreless Multmeda Communcatons, Artech House Publshers, 00. [5] Larsen Y, Leus G, Gannaks,G.B, Constant modulus and reduced PAPR block derental encodng or requencyselectve channels, Communcatons, IEEE Transactons, pp 6 63, Aprl 004. [6] Zhang Z, Long K, Lu Y, Complex Ecent Carrer Frequency Oset Estmaton Algorthm n OFDM Systems, Broadcastng, IEEE Transactons, pp 59 64, June 004. [7] IEEE P80.a/D7.0, 999.