Modified OFDM Model Based on Sub-Sliding Window Technique

Transcription

1 wwwccseetorgcis Computer ad Iformatio Sciece Vol, o ; Jauary Modified OFDM Model Based o Sub-Slidig Widow Techique Salih Mohammed Salih Electrical Egieerig Departmet, College of Egieerig, Uiversity of Abar Abar, Iraq Tel: dr_salih_moh@yahoocom Waleed Amee Mahmoud Electrical Egieerig Departmet, College of Egieerig, Uiversity of Baghdad Baghdad, Iraq Tel: profwaleed5@yahoocom Abstract I moder wireless commuicatio systems, Doppler frequecy reductio is employed for efficiet trasmissio of data i oisy eviromets Achievig a very less bit error rate BER) has bee the major task i the orthogoal frequecy divisio multipleig OFDM), which is a popular multicarrier modulatio techique i IEEE 8a wireless systems ad is curretly beig used i the IEEE 86e WiMa) stadard This paper presets a modified sub-slidig widow SSW) techique that is used to improve the performace of OFDM modem for the physical layer of IEEE 8a stadard To carry out the simulatios, the bit error rate performace of the modified ad stadard models is compared uder the additive white Gaussia oise AWG) ad flat fadig chael The simulatio results show that the combiatio of SSW ad OFDM techology is superior to the OFDM stadard techology Keywords: SSW, OFDM, Doppler Frequecy, AWG, IEEE 8a Itroductio I multicarrier approach the origial data stream is divided ito umber of parallel streams The each stream is modulated with differet carrier frequecy ad set across the chael This actually reduces iter-symbol iterferece as each stream travels at a differet frequecy So cosiderig the umber of users, multicarrier approach will be the best choice for G wireless systems Richard, 6 ad Va, ) Amog the multi-carrier modulatio schemes CDMA ad OFDM are the two techologies the researchers cocetrate o CDMA is used i G wireless systems OFDM has gaied more iterest i recet times ad is used by may broadbad trasmissios like IEEE 8a ad 8g Richard, 6) OFDM could be used both as the multiple access techology ad modulatio scheme i G But OFDM i itself faces a lot of challeges Richard, 6) As specified by IEEE 8a, the physical layer is based o a 5-carrier OFDM modulatio scheme icludig 8 subcarrier for data symbols ad pilot subcarriers for chael estimatio process Yu, 9) I the stadard model, the trasmitter icludes a symbol mapper, iverse fast Fourier trasform IFFT) ad cyclic etesio blocks IFFT block geerates subcarriers ad the cyclic etesio block adds the guard time iterval ito the OFDM symbol The receiver cosists of the guard time removal, fast Fourier trasforms FFT), symbol demapper ad sychroizer blocks Further studies carried out i the recet years They aimed at reducig the BER by usig the chael estimatio techiques Due to the o-orthogoality betwee the real ad imagiary parts i OFDM sigals at the receiver due to the chael effect, the demodulated real valued OFDMQAM symbol always has imagiary valued itrisic iterferece from eighborig symbols Jifeg, 9 ad Floch, 995) This hiders covetioal chael estimatio methods for OFDM from beig directly applied to OFDMQAM Therefore, chael estimatio has bee cosidered a big problem for OFDMQAM i dispersive chaels ad attracted umerous research efforts I order to reduce the itrisic iterferece to a miimum, a pilot-based chael estimatio scheme has bee proposed i Javaud ) ad a preamble-based chael estimatio method i Kag, 7), where i both cases a group of adjacet symbols are carefully selected so that the itrisic iterferece at the cetral symbol positio ca be greatly reduced I Holtzma, 995), a autocorrelatio based scheme for maimum Doppler-frequecy estimatio was proposed for sigle-carrier systems, where the estimate is obtaied usig the evelope of the received sigal I Ca ), the correlatio betwee the tail of the OFDM symbol ad the guard iterval was eploited to ISS E-ISS

2 wwwccseetorgcis Computer ad Iformatio Sciece Vol, o ; Jauary estimate the Doppler frequecy f d ), where the effects of itersymbol iterferece ISI) was ot cosidered I Krasy, ), the estimatio of f d is obtaied via a maimum-likelihood ML) based time-domai method for TDMA ad CDMA systems The applicatio of this algorithm to OFDM systems was preseted i Yucek, 5), where time domai chael estimatios were used to obtai the maimum Doppler frequecy estimatio I this model, the chael is ot estimated based o pilot subcarriers, but by usig preambles ad isertig frequet mid-ambles I its frequecy domai approach, the ICI is igored Thus, a error floor is observed I Gao, 9) a method for estimatig the Doppler spread i mobile OFDM systems was described The estimatio was based o phase message of time domai chael estimatio over several OFDM symbols I Salih, ), a slidig widow SW) techique was proposed to ehace the system performace of the OFDM modem for the physical layer of IEEE 8a stadard The performace of such system was improved uder the flat ad the AWG chael The author i Salih, ) used additioal two blocks FFT ad SW blocks) at the trasmitter side ad the iverse of these blocks at the receiver side i additio to the other mai blocks ie IFFT at the trasmitter side ad FFT at the receiver side) The SW block of legth 6-bits was used i Salih, ) which is the same size of FFT block I this paper, we split the block of SW ito sub-slidig widow SSW) based o shiftig the sigal vector bit-by-bit to the right i the trasmitter side ad addig them vertically as show i sectio The trasmitted vector of legth will be divided ito -subsets ad we apply the SSW ad the IFFT o each subset The reverse steps will be applied at the receiver to recover the trasmitted sigal The modified method ca improve the system performace without usig the blocks of FFT at the trasmitter ad its iverse at the receiver sides respectively Effect of Doppler Frequecy Motio of a atea produces Doppler shifts of icomig received waves OFDM has bee adopted by may wireless stadards such as IEEE 8 ad 86 ad has bee implemeted i may practical systems Kowledge of mobile speeds is critical i improvig the performace of multi-cell wireless commuicatio systems The mobile speed iformatio is also very critical for implemetig a umber of physical ad etwork layer fuctios such as adaptive ad fast lik adaptatio, ad accurate chael predictio Thus, the scheduler gai due to multiuser diversity ad spectral efficiecy of the system ca be icreased Yag, 6) The comple basebad represetatio Steele, 99) of a wireless chael impulse respose ca be described by: h t, ) t) ) ) where is the delay of the l-th path ad t) is the correspodig comple amplitude ad δ is the delta fuctio Due to the motio of the vehicle, t) s are wide-sese statioary WSS) arrowbad comple Gaussia processes, which are idepedet for differet paths The Doppler frequecy fd v fc c ), is related to the vehicle speed ad the carrier frequecy f c, where c is the speed of light OFDM System The time domai samples of a OFDM symbol ca be obtaied from data symbols as: j k e,,, X IFFT k k ) k where i, k is data symbol of the k-th subcarrier of the i-th OFDM symbol ad is the umber of subcarriers After the guard iterval additio, the samples X i, are trasmitted over the liear time depedet chael described i ), with additive white Gaussia oise AWG) Z i,, with zero mea ad variace of I this z paper, we assume the chael to be costat over a OFDM symbol, but time-varyig across OFDM symbols is a reasoable assumptio for low mobility At the receiver, assumig perfect sychroizatio, the received samples ca be epressed as: y X * h t, ) Z ) where * stads for covolutio After removig the guard iterval, the receiver demultiplees the received samples by usig the FFT as: j k y y e, k,, Yi, k FFT ad the demodulated sigal Yk ca be epressed as: Y H t f Z k i, k, ) k 5) ) Published by Caadia Ceter of Sciece ad Educatio

3 wwwccseetorgcis Computer ad Iformatio Sciece Vol, o ; Jauary Modified Model Aalysis The block diagram of the modified OFDM model is show i figure ote that the stadard widow size of 6-bits for IEEE 8a is divided ito equally parts of legth 6-bits after the QAM modulatio A slidig widow techique will be applied o each part Salih, ) A slidig widow i Salih, ) is used for reducig the effect of Doppler frequecy; the author applied the SW o the frame legth of 6-bits i additio to the other FFT ad IFFT blocks added at the trasmitter ad the receiver sides respectively I this work, we wo't use the FFT at the trasmitter ad its iverse at the receiver side, i couterwise we just use the mai blocks of IFFT at the trasmitter ad FFT at the receiver side The additioal blocks i Salih, ) used to icrease the orthogoality of subcarriers ad reduce the roud off error come from the block of SW To reduce the roud of error, the slidig widow is divided ito sub-slidig widow of legth 6-bits each The slidig widow techique is used for ehacig the system performace uder the Doppler frequecy effect ote that o-coheret combiig of four short correlatio periods istead of a sigle log block correlatio is due to the potetial high roud off error which may occur at ay bit that spreads to the other adjacet bits due to the miig property of the suggested algorithm Also the frequecy offset results i phase rotatio or shifts to the sampled sigals causig reduced correlatio performace betwee the received sigal ad the trasmitted waveform The loger correlatio period leads to larger phase shift ad poorer correlatio performace A shorter correlatio period thus helps mitigate the frequecy offset effect ad the roud off error The output data vector after the QAM modulator is: k ) 8 6) The slidig widow techique ca be applied o this vector by shiftig the vector bit-by-bit from the left to the right which forms a matr ad the we add the data vertically, while eglectig the last value i each shift without circulatig The other traiig bits -bits) ad the bits from 5 to 6 which are used for sychroizatio or zero paddig I this paper, we set them to zero) are added to eq 6) such that the total vector will be of legth 6 bit The shiftig of bits will be divided ito four parts accordig to the followig step: Shift : from bit to 6 ) k ),, 7) k Shift : From bit 7 to ) k ),, 8) k Shift : from bit to 8 ) k ),, 9) k Shift : from bit 9 to 6, the details of these equatios ie eqs 7) to )) are give i appedi-a ) k ),, ) k ow the output vector from equatios 7) to ) ca be coverted from frequecy to time domai by usig the IFFT block as i the covetioal model eq )) At the receiver side, the iverse steps will be applied i order to recover the trasmitted sigal, where the last step i the trasmitter side is the first step at the receiver side ad so o The reversig of sub-slidig widow ca be eecuted as i the followig steps after the FFT block eq )): Cosider the output data from the FFT block are: FFTo p yk, k ) The the reversig of slidig widow ca be applied o this data vector as give i the et equatio y y, y y y, y y y,, y6 y6 y6 or i geeral form: y y y ) ote that the last equatio ca be applied from bit to ; because dividig of FFFs ito four sub-blocks will ot cause ay effect o the iverse of sub-slidig widow ISSW) The traiig sequece will be used to estimate the chael frequecy respose at the receiver side The chael frequecy respose ca be applied as show i figure Salih, ad Salih, 7) Re ceived Traiig i) ) Chael Estimatio i), i Trasmitted Traiig i) ISS E-ISS

4 wwwccseetorgcis Computer ad Iformatio Sciece Vol, o ; Jauary The legth of -bits of traiig vector ca be icreased by repeatig them where the total legth will be the same as the total umber of active subcarriers which are 8-bits The chael frequecy respose which is foud i the last step will be used to compesate the chael effect o the data, ad the estimated data ca be foud usig the followig equatio: Estimated Data j) Chael Estimatio j)) Re ceived Data j), where j to 8 ) The fial step is to apply the QAM demodulatio at the recovered data after chael estimatio ad compesatio Effectiveess of Slidig Widow Techique The effect of Doppler spread i wireless chaels i OFDM systems is importat, sice chael variatios durig oe OFDM symbol cause Iter Sub-Carrier Iterferece ICI), which degrades its performace As the fluctuatio of trasmitted sigal shape reduces, the Doppler frequecy has less effect o the system performace Salih, ) To show the effect of our suggested SW techique o the BER performace of OFDM model, let us cosider that a sie wave sigal is discretized ito bit ad trasmitted over wireless chael This sigal is subjected to a Doppler frequecy of of the carrier frequecy with respect to the mai trasmitted sigal frequecy as show i figure The trasmitted sigal shape has blue solid lie curve TX without SW), while the received sigal has black lie curve RX without SW) From this curve it is clear that there is a phase shift betwee them due to Doppler frequecy effect If we set the threshold at '' level, the the received sigal is cosidered as logic '' if it is more tha this level, else it will be of logic '' value From this figure, there is a error of 7-bits due to Doppler Effect o the received sigal If the SW techique is applied o the trasmitted sigal the its shape will chage black lie with a circle) ote ), the other lie black with a square); represets the received sigal after addig the Doppler frequecy The last dotted lie represets the received sigal shape after applyig the iverse of slidig widow ISW) From this curve dotted Lie), it ca be see that the sigal shape, after applyig the SW techique, is earer to the shape of the mai trasmitted sigal TX without SW), ad the error is about of the total error that appears i RX without SW oly -bits of error relative to the 7-bits without SW), so the proposed algorithm reduces the effect of Doppler frequecy Simulatio Results The Performace of the proposed ad stadard models ca be tested uder the flat fadig ad AWG chael The stadard parameters are give i table ) Richard, 6, Salih,, ad Gav ) The Performace at the AWG Chael The performace of modified ad stadard models at the AWG chael ad at two values of costellatio mappig poits 6QAM, ad 6QAM poits ie m=6, ad 6 poits) is show i figure From this figure, it ca be see that the modified model based o SSW techique has a BER= - at SR=dB, while the covetioal model has the same BER at 7dB, which meas that we obtaied a gai of db from the modified model As the QAM poits icreases from 6 to 6 dotted curves), the BER rate for both models will also icrease Wide spa gais betwee the performaces of these models are obtaied, so that the modified model is better ad more sigificat tha the covetioal system at the AWG chael The BER icreases i the same level for both models, ad this esures that the modified model has a stable performace related to the covetioal system The Performace of Models i Flat Fadig Chael I this sectio, the performace of models i flat fadig chael ca be simulated at Doppler frequecies 5 Hz, ad 5 Hz Figure shows the performace of both models at Doppler frequecy 5 Hz A gai of 5 db at BER= - was obtaied from the modified model relative to the stadard oe at m=6 As m icreases from 6 to 6 poits, the BER for both models icreases I all rage of SR, the suggested model outperforms the other oe The BER was icreased whe the chael model varied from AWG to flat fadig The performace of both models at Doppler frequecy 5Hz is show i figure 5 The icreasig i the Doppler frequecy leads to icrease the BER for both models, but the suggested model still outperforms the other stadard model i all rage of SR The obtaied gai from the modified model is about 5 db As the umber of costellatio mappig icreases from 6 to 6, the BER of models icreases i the same amout 5 Coclusio By carryig out the aalysis, we ivestigated ew modified model of OFDM for the physical layer of IEEE 8a stadard by usig a sub-slidig widow techique The simulatio results have show that the ew Published by Caadia Ceter of Sciece ad Educatio

5 wwwccseetorgcis Computer ad Iformatio Sciece Vol, o ; Jauary method ca improve the system performace at the AWG ad flat fadig chael We observed that the performace of sub-slidig widow model is superior to that of the covetioal model, where differet gais obtaied from the proposed model relative to the stadard model We proposed a sub-slidig widow trasceiver for OFDM over AWG ad flat fadig chael, ad aalyzed its performace We also compared the performace of models by usig two values of the costellatio of the modulatio to be optimized for SR; we foud that the modified OFDM have a advatage over stadard OFDM i terms of the simulated BER performace Refereces Richard Vu ee 6) A ew OFDM stadard for high rate wireless LA i the 5 GHZ bad, IWCMC 6, July 6, Vacouver, British Columbia, Caada Richard va ee, Prasad R ) OFDM Wireless Multimedia Commuicatios Artech House, Bosto Yu LI, Jie Zhou, Yoshikui Oozato 9) Performace Study for STC-OFDM Systems Based o IEEE8a Stadard, 5 th Iteratioal Coferece o Wireless commuicatios, etworkig ad Mobile Computig, WiCom 9, pp - Jifeg Dut, Svate Sigell 9) ovel Preamble-Based Chael Estimatio for OFDMOQAM Systems, proceedig at IEEE ICC 9 Floch B Le, Alard M ad Berrou C 995) Coded Orthogoal Frequecy Divisio Multiple, Proceedigs of the IEEE, pp Javaudi J P, Lacroi D ad Rouel A ) Pilot-aided chael estimatio for OFDMOQAM, VTC Sprig, pp Kag SW, Chag K H 7) A ovel chael estimatio scheme for OFDMOQAM-IOTA system, ETRI Joural, vol 9, pp 6 Holtzma J M, Sampath A 995) Adaptive averagig methodology for hadoffs i cellular systems, IEEE Tras o Vehicular Techology995, vol, o, pp Cai J, Sog W, ad Li Z ) Doppler spread estimatio for mobile OFDM systems i Rayleigh fadig chaels, IEEE Tras o Cosumer Electroics, vol 9, pp Krasy L, Arsla H, Koilpillai D, ad Cheakeshu S ) Doppler spread estimatio i mobile radio systems, IEEE Commuicatio Letters, vol 5, o 5, pp Yucek T, Taious R M A, ad Arsla H 5) Doppler spread estimatio for wireless OFDM systems, IEEESaroff Symposium o Advaces i Wired ad Wireless Commuicatio, pp 6 Gao Yua, Che We Xie Tao, Log Biqi 9) Doppler Spread Estimatio for o-rayleigh Fadig Chael, IEEE coferece, proceedig at ICIEA 9, pp 6-9 Salih Mohammed S ) ovel Slidig Widow Techique of OFDM Modem for the Physical Layer of IEEE 8a Stadard, Joural of Commuicatios, Vol, Issue, pp 67-7 Yag-Seok Cho Ca Ozdural O, Huapig Liu, Siavash Alamouti 6) A Maimum Likelihood Doppler Frequecy Estimator for OFDM Systems, Proceedigs at IEEE ICC 6, pp Steele R 99) Mobile Radio Commuicatios ew York: IEEE Press Gavi Yeug, Mieo Taka Rajive Bagrodia, Alireza Mehria, Babak Daeshrad ) Detailed OFDM Modelig i etwork Simulatio of Mobile Ad Hoc etworks, Proc of the 8 th Workshop o Parallel ad Distributed Simulatio PADS ), pp 6- Salih Mohammed, Ahmed A, Ada S, Ahmed 7) Improvemet model of MC-CDMA i frequecy selective fadig chael, the secod iteratioal coferece o cogitive radio orieted wireless etwork ad commuicatio, CROWCOM 7), Orlado-Florida, USA ote : The real trasmitted sigal shape has radom amplitude ad phase depedig o the iput discrete data, but we assumed it has siewave shape such that the reader ca simulate fig easily for uderstadig the idea If we take the output from the IFFT for a radom iput sigal, the all curves i fig will chage ISS E-ISS

6 wwwccseetorgcis Computer ad Iformatio Sciece Vol, o ; Jauary Table Simulatio Parameters Modulatio Type=QAM Guard Period Doppler spread Pilot subcarriers umber of effective sub-carriers umber of trasmitted symbols Chael frequecy Data rate Chael model 6QAM, 6QAM 8-bits 5, 5 Hz 8 5 GHz 5 Mbps AWG Flat fadig+awg Figure Block diagram of the proposed model Published by Caadia Ceter of Sciece ad Educatio 5

7 wwwccseetorgcis Computer ad Iformatio Sciece Vol, o ; Jauary 5 TX w ithout SW RX w ithout SW TX w ith SW RX w ith SW RX after ISW Figure Doppler Effect o the system performace Modified, m=6 Modified, m=6 Stadard, m=6 Proposed, m=6 - BER SR db) Figure Performace of stadard ad modified model at the AWG chael 6 ISS E-ISS

8 wwwccseetorgcis Computer ad Iformatio Sciece Vol, o ; Jauary Modified m=6 Modified m=6 Stadard, m=6 Stadard, m=6 - BER SR db) Figure Performace of stadard ad modified models i flat fadig chael Doppler frequecy=5 Hz) Modified, m=6 Modified, m=6 Stadard, m=6 Stadard, m=6 BER SR db) Figure 5 Performace of stadard ad modified models i flat fadig chael Doppler frequecy=5 Hz) Published by Caadia Ceter of Sciece ad Educatio 7

9 wwwccseetorgcis Computer ad Iformatio Sciece Vol, o ; Jauary ISS E-ISS Appedi A The slidig widow techique from equatios 7) to ) ca be applied o 6) by shiftig the vector bit-by-bit from the left to the right as i the et equatio, while eglectig the last value i each shift without circulatig Also the shiftig of bits will be divided ito four parts accordig to the followig step: Shift : from bit to 6 5) Shift : From bit 7 to 6) Shift : from bit to 8 7) Shift : from bit 9 to 6, where the bits from 5 to 6 used for sychroizatio or zero paddig I this paper, we set them to zero 8) After applyig these formulas of SSW by shiftig the data vectors horizotally, we add the values vertically as give by the et four equatios The output from these equatios will be of the same legth of the iput data vector ie =6 bits) ) ) ) SSW ) ) ) ) 5 ) ) ) 5 ) ) ) SSW ) ) ) ) 5 ) ) ) 6 ) 5 ) ) ) SSW 5) ) ) 5) ) ) 5) ) ) 5) SSW

10 wwwccseetorgcis Computer ad Iformatio Sciece Vol, o ; Jauary Published by Caadia Ceter of Sciece ad Educatio 9 The output data from bit to 6 are: 9) From bit 7 to are: ) from bit to 8 are: ) ad from bit 9 to 6 are: ) ote that equatios 5) ad 9) represet the details of 7), equatios 6) ad ) represet the details of 8), equatios 7) ad ) represet the details of 9), ad equatios 8) ad ) represet the details of ) ) ) ) ) 5) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 5) 6) 6) ) ) ) ) ) ) ) ) ) 5) ) ) ) ) 5) ) ) ) 5) ) ) 5) 5)