Adaptive Modulation for OFDM Systems J.Faezah 1, and K.Sabira 2

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1 Iteratioal Joural of Commuicatio Networks ad Iformatio Security (IJCNIS) Vol., No., August 9 Adaptive Modulatio for OFDM Systems J.Faezah, ad K.Sabira Cetre for Foudatios Studies ad Extesio Educatio, Multimedia Uiversity, 755 Melaka, Malaysia. Departmet of Computer ad Commuicatio Systems Egieerig, Faculty of Egieerig, Uiversiti Putra Malaysia, Serdag, Selagor, Malaysia sabira@eg.upm.edu.my Abstract: The goal for the fourth geeratio (G) of mobile commuicatios system is to seamlessly itegrate a wide variety of commuicatio services such as high speed data, video ad multimedia traffic as well as voice sigals. Oe of the promisig approaches to G is adaptive OFDM (AOFDM). I AOFDM, adaptive trasmissio scheme is employed accordig to chael fadig coditio with OFDM to improve the performace. I this paper, we have cosidered oly adaptive modulatio. First we have ivestigated the OFDM system performace of ucoded adaptive modulatio usig quadrature amplitude modulatio (QAM) ad phase shift keyig (PSK). To further ehace the system, we employ covolutioal codig to OFDM system. The obtaied results show that a sigificat improvemets i terms of bit error rate () ad throughput ca be achieved demostratig the superiority of the adaptive modulatio schemes compared to fixed trasmissio schemes. Keywords: Adaptive modulatio, orthogoal frequecy divisio multiplexig (OFDM), chael codig, bit error rate(), throughput.. Itroductio Orthogoal Frequecy Divisio Multiplexig (OFDM) is a special form of multi-carrier trasmissio techique i which a sigle high rate data stream is divided ito multiple low rate data streams. These data streams are the modulated usig subcarriers which are orthogoal to each other. I this way the symbol rate o each subchael is greatly reduced, ad hece the effect of itersymbol iterferece (ISI) due to chael dispersio i time caused by multipath delay spread is reduced. Guard iterval ca also be iserted betwee OFDM symbols to reduce ISI further. The orthogoality betwee subcarriers ca be maitaied, eve though the sigal passes through a time-dispersive chael by cyclically extedig the OFDM symbols ito guard iterval. The mai advatages of OFDM are its multipath delay spread tolerace ad efficiet spectral usage by allowig overlappig i the frequecy domai. Aother sigificat advatage is that the modulatio ad demodulatio ca be doe usig iverse Fast Fourier Trasformatio (IFFT) ad Fast Fourier Trasformatio (FFT) operatios, which are computatioally efficiet. I a OFDM trasmissio system, each subcarrier is atteuated idividually uder the frequecy-selective ad fast fadig chael. The chael performace may be highly fluctuatig across the subcarriers ad varies from symbol to symbol []. If the same fixed trasmissio scheme is used for all OFDM subcarriers, the error probability is domiated by the OFDM subcarriers with highest atteuatio resultig i a poor performace. Therefore, i case of frequecyselective fadig the error probability decreases very slowly with icreasig average sigal-to-oise ratio (SNR) []. This problem ca be mitigated if differet modulatio schemes are employed for the idividual OFDM subcarriers. Ulike adaptive serial systems, which employ the same set of parameters for all data symbols i a trasmissio frame, adaptive OFDM schemes have to be adapted to the SNR of the idividual subcarriers. This will substatially improve the performace ad data throughput of a OFDM system. For example if the subcarriers that will exhibit high bit error probabilities i the OFDM symbol to be trasmitted ca be idetified ad excluded from data trasmissio, the overall ca be improved i exchage for a slight loss of system throughput. However the potetial loss of throughput due to seclusio of faded subcarriers ca be mitigated by employig higher order modulatio modes o the subcarriers exhibitig high SNR values. May adaptive trasmissio techiques have bee preseted i the literature. The combiatio of adaptive modulatio with OFDM was proposed as early as 989 by Kalet which was further developed by Chow [] ad Czylwik []. Specifically the results obtaied by Czylwik showed that the required SNR for the target - ca be reduced by 5dB to 5dB compared to fixed OFDM depedig o the sceario of radio propagatio. The performace of turbo-coded adaptive modulatio are ivestigated i []. Three differet modulatio mode allocatio algorithms were discussed ad compared. Further studies o the applicatio of turbo code i adaptive modulatio ad codig is coducted i [5]. This paper proposed a optimal approach based o predictio of the average over all subcarriers. I [6], a adaptive OFDM system with chageable pilot spacig has bee proposed. The results showed that a sigificat improvemet i the performace is achieved with sacrificig a small value of the total throughput of the system. A work is doe o several strategies o bit ad power allocatio for multi-atea assisted OFDM systems i [7]. They foud out that sometimes power ad bit adaptatio is required for efficiet exploitatio of wireless chaels i some system coditios. The performace aalysis of OFDM systems with adaptive subcarrier badwidth is ivestigated by [8]. Further ivestigatios o subcarrier adaptive modulatio scheme of

2 Iteratioal Joural of Commuicatio Networks ad Iformatio Security (IJCNIS) Vol., No., August 9 precoded OFDM is preseted i [9] uder multipath chaels. Subcarriers are grouped ito subbad This paper is orgaized as follows: The system model is described i sectio. The adaptatio procedure ad switchig thresholds used i the simulatios are preseted ext. Sectio discusses the adaptive modulatio systems simulatio results. Fially coclusios are made i sectio.. System Model I this paper, subbad adaptive trasmissio schemes are employed to reduce the complexity. I subbad adaptive OFDM trasmissio, all subcarriers i a AOFDM symbol are split ito blocks of adjacet subcarriers referred to as subbads. The same mode is employed for all subcarriers of the same subbad.[]. The choice of the modes to be used by the trasmitter for its ext OFDM symbol is determied by the chael quality estimate of the receiver based o the curret OFDM symbol. Perfect chael estimatio is assumed i this paper. I this simulatio the istataeous SNR of the subcarrier is measured at the receiver. The chaels quality varies across the differet subcarriers for frequecy selective chaels. The received sigal at ay subcarrier ca be expressed as : R = H X + W () where H is the chael coefficiet at ay subcarrier, X is the trasmitted symbol ad W is the Gaussia oise sample. So the istataeous SNR ca be calculated usig where SNR H = () N N o is the oise variace [7]. The coservative approach i threshold based adaptatio is by usig the lowest quality subcarrier i each subbad for cotrollig the adaptatio algorithm []. It meas that the lowest value of SNR will be used i mode selectio. By usig this method, the overall i oe subbad is ormally lower tha the target. If the overall ca be closer to the target by choosig a more suitable modulatio mode or code rate, the throughput of the system will be higher [5]. Therefore a better adaptatio algorithm is used i this paper to provide a better trade off betwee throughput ad overall by choosig a more suitable scheme for each subbad. Istead of usig the lowest SNR i each subbad, the average value of the SNR of the subcarriers i the subbad is goig to be used. o. Block diagram The average istataeous SNR for each subbad are calculated The average istataeous SNR are compared with the thresholds value to choose the mode The iformatio about modulatio ad codig used are set to the trasmitter usig a feedback chael. Figure. Adaptatio procedure The block diagram of this system is show i Figure. The chael estimatio ad mode selectio are doe at the receiver side ad the iformatio is set to the trasmitter usig a feedback chael []. I this model the adaptatio is doe frame by frame. The chael estimator is used to estimate the istataeous SNR of the received sigal. Based o the istataeous SNR calculated, the best mode will be chose for the ext trasmissio frame. This task is doe by the mode selector block. At the trasmitter the adaptive modulator block cosists of differet modulators which are used to provide differet modulatio modes. The switchig betwee these modulators will deped o the istataeous SNR This block diagram is used to describe two types of adaptive modulatio schemes which is based o MQAM ad MPSK scheme. The goal of adaptive modulatio is to choose the appropriate modulatio mode for trasmissio i each subbad, give the local SNR, i order to achieve good trade-off betwee spectral efficiecy ad overall.. Switchig Thresholds. The switchig threshold for activatig differet modes ca be determied by extesive simulatio of the fixed mode modulatio system. The switchig algorithm used for the adaptive modulatio schemes are preseted i Table till Table. The desig parameters used i the simulatio are listed i Table 5. I this simulatio, two types of adaptatio modes will be used. The first oe is adaptive modulatio without trasmissio blockig ad the secod oe is adaptive modulatio with trasmissio blockig. I adaptive modulatio without trasmissio blockig, data will be costatly trasmitted i this scheme eve though the chael is i deep fades. If the chael quality is very bad, a robust modulatio mode will be used ad whe the chael quality is good a spectrally efficiet modulatio will be used.

3 Iteratioal Joural of Commuicatio Networks ad Iformatio Security (IJCNIS) Vol., No., August 9 OFDM Trasmitter Data Geerator Ecoder Serial to Parallel Adaptive Modulator IFFT Add Guard Iterval Parallel To Serial Mode Selector Chael Estimator Rayleigh Fadig Gaussia Noise Serial to Parallel Remove Guard Iterval Data Out Decoder Parallel To Serial Adaptive Demodulator FFT OFDM Receiver Figure. Block diagram I adaptive modulatio with trasmissio blockig, trasmissio will be disabled whe the chael is i deep fade. This mode is itroduced because the sigal quality is too bad to guaratee a required trasmissio []. Data will be trasmitted if the chael quality improved. Mode Modulatio Thresholds QAM SNR 5.6 db 8QAM 5.6 db<snr 8.6 db 6QAM 8.6 db<snr.5 db QAM.5 db<snr.6 db 5 6QAM SNR>.6 db Table. Switchig threshold for MQAM without trasmissio blockig Mode Modulatio Thresholds No Tx SNR< db QAM db<snr 5.6 db 8QAM 5.6 db<snr 8.6 db 6QAM 8.6 db<snr.5 db 5 QAM.5 db<snr.6 db 6 6QAM SNR>.6 db Table. Switchig threshold for MQAM with trasmissio blockig Mode Modulatio Thresholds QPSK SNR 6.6 db 8PSK 6.6 db<snr db 6PSK db<snr 8. db PSK SNR>8. db Table. Switchig threshold for MPSK without trasmissio blockig Mode Modulatio Thresholds No Tx SNR< db QPSK db<snr 6.6 db 8PSK 6.6 db<snr db 6PSK db<snr 8. db 5 PSK SNR>8. db Table.Switchig threshold for MPSK with trasmissio blockig Parameter Value IFFT Size 5 Number of subcarriers 5 Number of subbad Number of subcarriers 6 per subbad Guard Time Duratio 8 Frame size 6 SNR - db Modulatio scheme MPSK, MQAM Codig rate /, /,/ ad /. Badwidth 5MHz Carrier Frequecy GHz Samplig Frequecy 5.MHz Table 5. System Parameters. Simulatio Results ad Discussio I this simulatio, the performace of adaptive modulatio is ivestigated i terms of throughput ad performace. To highlight the advatages of adaptive modulatio compariso is made to fixed modulatio system. Iitially the ucoded performace of adaptive modulatio based o MQAM ad MPSK scheme are ivestigated. Next covolutio codig is employed to the adaptive modulatio system. Fially compariso is made betwee the proposed system ad the coservative adaptatio system.. Adaptive Modulatio for MQAM systems... Adaptive Modulatio without Trasmissio Blockig (AMQAM-No blockig) I the adaptive modulatio system which did ot icorporate trasmissio blockig, the performace was better compared to the performace of fixed modulatio system as show i Figure.

4 Iteratioal Joural of Commuicatio Networks ad Iformatio Security (IJCNIS) Vol., No., August QAM 8QAM 6QAM QAM 6QAM Adaptive Modulatio - QAM 8QAM 6QAM QAM 6QAM Adaptive Modulatio Figure. Performace of adaptive modulatio without trasmissio blockig (AMQAM-No blockig) The performace of adaptive modulatio system was better tha the most robust modulatio mode QAM systems i terms of performace. This pheomeo was also observed i []. As ca be see, the remais costat at - after 5 db sice the scheme is desiged to meet the target of -. Aother aspect of performace improvemet is i terms of the decrease of SNR required to meet the target. Adaptive modulatio scheme is able to meet the target at 5 db compared to more tha 8 db for QAM fixed modulatio scheme. Figure shows the throughput performace of adaptive modulatio scheme without trasmissio blockig. I terms of throughput performace, it is foud that o fixed OFDM system provides better performace while simultaeously providig better throughput. The fixed OFDM system ca achieves a higher throughput by usig higher level modulatio such as 6QAM but poor performace. This shows that fixed systems either achieve good throughput performace or good performace but ot both Figure. Throughput performace for adaptive modulatio without trasmissio blockig (AMQAM-No blockig).. Adaptive Modulatio with Trasmissio Blockig (AMQAM-Blockig) Figure 5 presets the performace AM scheme utilizig trasmissio blockig. As see clearly a sigificat improvemet i performace for AM with trasmissio blockig is achieved. The performace for all SNR values is successfully decreased to meet the target -. At low SNR, the system will eter trasmissio blockig mode ad this will results i o error. Therefore the overall ca be reduced to meet the target. The performace is ot recorded for the SNR rage of ( - db) because o bits are trasmitted at all. - - QAM 8QAM 6QAM QAM 6QAM Adaptive Modulatio Specifically, for low SNR values from (- db), the throughput performace is similar to QAM. This ca be explaied by the fact that at low SNR values the domiat modes used is QAM. As SNR icreases more that db, the throughput performace of AMQAM No blockig is better tha the fixed system with the icrease i throughput aroud Bps/Hz. Adaptive modulatio system provides the best throughput without sacrificig performace. It is also observed that the throughput achieved by adaptive modulatio system is the highest compared to fixed modulatio system. Therefore it is proved that adaptive modulatio techiques ca be used i order to achieve a give target level of by choosig the appropriate modulatio modes for the istataeous chael coditios Figure 5. performace for adaptive modulatio with trasmissio blockig (AMQAM-Blockig) I terms of throughput performace, AMQAM-Blockig scheme outperform the fixed modulatio scheme as show i Figure 6. At low SNR (- db) a slight degradatio of throughput performace aroud. Bps/Hz ca be observed. The decreased i the throughput at low SNR is because most subcarriers will eter o trasmissio mode thus o bits are trasmitted. As the SNR icreases more tha db, a icreased i the throughput aroud Bps/Hz ca be observed.

5 5 Iteratioal Joural of Commuicatio Networks ad Iformatio Security (IJCNIS) Vol., No., August QAM 8QAM 6QAM QAM 6QAM Adaptive Modulatio.5 PSK 8PSK 6PSK PSK 6PSK Adaptive Modulatio Throughput ( Bps/Hz) Figure 6. Throughput performace for adaptive modulatio with trasmissio blockig. (AMQAM-Blockig). Adaptive Modulatio for MPSK systems.. Adaptive Modulatio without Trasmissio Blockig (AMPSK-No blockig) Figure 7 shows the performace of the adaptive modulatio scheme without trasmissio blockig. It ca be see that the performace of the adaptive scheme is equal to QPSK for SNR rage db till 5 db. For SNR more tha 5 db the performace is better tha all the fixed MPSK system. However the SNR required to meet the target is quite high which is more tha 5 db Figure 8. Throughput performace for adaptive modulatio without trasmissio blockig (AMPSK No blockig).. Adaptive Modulatio with Trasmissio Blockig (AMPSK-Blockig) Figure 9 shows the performace of adaptive modulatio with trasmissio blockig based o MPSK system (AMPSKblockig). It is foud that the performace of adaptive MPSK scheme early meet the target -. It ca be see that the required SNR to achieve the target is quite high which is more tha 5 db PSK 8PSK 6PSK PSK 6PSK Adaptive Modulatio Figure 7. performace for adaptive modulatio without trasmissio blockig (AMPSK-No blockig) Figure 8 shows the throughput performace of the adaptive modulatio without trasmissio blockig (AMPSK- Noblockig). It ca be observed that the throughput performace for the adaptive scheme is better compared to the fixed MPSK scheme. For low SNR values (- db) the throughput performace is the same with QPSK. For SNR more tha db a gai i throughput aroud.- Bps/Hz ca be achieved. - PSK 8PSK 6PSK PSK 6PSK Adaptive Modulatio Figure 9. performace for adaptive modulatio with trasmissio blockig (AMPSK-Blockig) Figure depicts the throughput performace of adaptive modulatio with trasmissio blockig (AMPSK-blockig) compared to the fixed MPSK system. It ca be see clearly that the throughput performace for the adaptive systems are better compared to all the fixed MPSK systems. Based o the results obtaied i this sectio, it ca be said that the performace of adaptive modulatio based o MPSK system is ot as good as the performace of the adaptive modulatio based o MQAM systems sice the SNR required to meet the target is quite high. Therefore the ext sectio oly MQAM modulatio is goig to be cosidered i the simulatio.

6 6 Iteratioal Joural of Commuicatio Networks ad Iformatio Security (IJCNIS) Vol., No., August PSK 8PSK 6PSK PSK 6PSK Adaptive Modulatio.5 ucoded rate / rate / rate / Figure. Throughput performace for adaptive modulatio with trasmissio blockig (AMPSK-Blockig). Coded Adaptive Modulatio Schemes I this sectio the employmet of covolutioal code i adaptive modulatio system will be ivestigated. It is kow theoretically that the usage of codig ca improve the performace. Figure shows the performace of ucoded ad coded adaptive modulatio system. It ca be see that the covolutioal code provide a cosiderable codig gai.the achieved codig gai at a - is db, 7 db ad 9 db for rate /, / ad / Figure. Throughput performace for coded adaptive modulatio scheme without blockig (Coded AMQAM- No-blockig) For adaptive modulatio with trasmissio blockig, the similar performace is observed. From Figure, it ca be see that the performace improve tremedously. Specifically whe rate / code is used, the performace decreases below - for SNR more tha db. These results suggest that more reductio i the ca be obtaied by usig chael codig. - ucoded rate / rate / rate / - ucoded rate / rate / rate / Figure. performace of coded adaptive modulatio without trasmissio blockig (Coded AMQAM-No Blockig) Figure shows the throughput performace of ucoded ad coded adaptive modulatio systems. It is observed that the throughput for the coded adaptive modulatio is slightly higher aroud. Bps/Hz tha ucoded adaptive modulatio for low SNR values (- 8 db). However the throughput performace of coded adaptive modulatio deteriorated at high SNR values. This is due to the redudat iformatio overhead of the chael codig that limits the system throughput. As expected, the / codig rate results i / throughput compared to ucoded case. However rate / code offers a low trasmissio over the chael. From the results obtaied, it is cocluded that chael codig reduces the SNR required for achievig a give target at the expese of a reduced throughput performace Figure. performace of coded adaptive modulatio with trasmissio blockig. (Coded AMQAM- Blockig) Figure shows the throughput performace for Coded AMQAM-Blockig scheme. A decrease i the maximum throughput is observed due to the covolutioal codig employed. Eve though Coded AMQAM-Blockig scheme achieves a good performace, there is a trade off where the maximum throughput beig limited. Throughput(Bps/Hz) Figure. Throughput performace for coded adaptive modulatio with trasmissio blockig (Coded AMQAM- Blockig)

7 7 Iteratioal Joural of Commuicatio Networks ad Iformatio Security (IJCNIS) Vol., No., August 9. Compariso of Adaptive Modulatio Schemes. algorithm are icreased by approximately. Bps/Hz for the etire rage of SNR values. From the aalysis doe, it ca be see that the ucoded fixed schemes require the highest chael SNR of all examied trasmissio methods i order to achieve target -. By itroducig ucoded adaptive modulatio without trasmissio blockig, the performace ca be improved to meet the target. At the same time the throughput is higher compared to the fixed scheme. To improve the performace further, adaptive modulatio with trasmissio blockig is employed. It is observed that the target was achieved for all SNR whe we utilized trasmissio blockig. The results preseted here is comparable to the results preseted i []. Thus the advatage of usig a adaptive scheme with trasmissio blockig is that the performace ca be desiged to meet a certai required. However the disadvatage is that the utilizatio of the trasmissio blockig results i trasmissio latecy. The performace of adaptive modulatio ca be further improved by usig chael codig. However both coded adaptive modulatio without trasmissio blockig ad with trasmissio blockig schemes suffer from limited throughput as the chael codig limits the data throughput by a factor of its codig rate. However for low SNR values the performace of both coded AM scheme is better tha ucoded AM scheme. I summary, the AM scheme has its advatages whe compared to idividual fixed modulatio modes i terms of its performace ad throughput performace. Note however for the adaptive scheme without trasmissio blockig that the target ca oly be achieved if the chael SNR is higher tha db respectively. Table 6 shows the compariso of the adaptive modulatio schemes i terms of the SNR required to meet the target ad the maximum throughput achieved at SNR db. I this sectio, we also foud that the adaptive modulatio based o MQAM scheme is better tha adaptive modulatio based o MPSK system. Scheme SNR required to meet target(db) Maximum Throughput (Bps/Hz) Ucoded AM-No Blockig.5 Ucoded AM-Blockig -.5 Ucoded AM-No Blockig.5 Ucoded AM-Blockig -.5 Coded AM-No Blockig.5 (rate/) Coded AM-No Blockig 9.7 (rate /) Coded AM-No Blockig 6.5 (rate /) Coded AM-Blockig -.5 (rate /) Coded AM-Blockig -.75 (rate /) Coded AM-Blockig (rate /) -.5 Table 6. Performace compariso of adaptive modulatio schemes. However the performace usig the coservative adaptatio algorithm is much lower tha the target. This ca be explaied by the adaptatio regime which was based o the priciple of usig the lowest SNR of the subcarrier i each subbad for mode selectio leadig to pessimistic estimate for the etire group [5] Adaptive modulatio (proposed) Adaptive modulatio (others).5 Performace Compariso. Lastly, a compariso is made betwee the adaptive trasmissio scheme usig the proposed adaptatio algorithm ad the adaptive trasmissio scheme usig the coservative adaptatio algorithm. The coservative approach is based o the poorest subcarrier i a subbad. The compariso is made oly for the adaptive scheme utilizig trasmissio blockig. This is because the performace of adaptive scheme utilizig trasmissio blockig is more superior compared to adaptive scheme without trasmissio blockig. Figure 5 shows the performace of the adaptive modulatio scheme usig the proposed adaptatio algorithm ad the coservative adaptatio algorithm. It ca be observed that both performaces maage to meet the target. I terms of throughput performace, there is a slight improvemet gaied by adaptive modulatio with trasmissio blockig usig the proposed adaptatio algorithm as show i Figure 6. It ca be see that the throughput of the AM-Blockig scheme usig the proposed Figure 5. performace of adaptive modulatio with trasmissio blockig (AM Blockig) Adaptive modulatio (proposed) Adaptive modulatio (others) Figure 6. Throughput performace of adaptive modulatio with trasmissio blockig. (AM-Blockig)

8 8 Iteratioal Joural of Commuicatio Networks ad Iformatio Security (IJCNIS) Vol., No., August 9. Coclusios I this paper, the performaces of adaptive trasmissio scheme for OFDM have bee ivestigated. The advatage of employig adaptive trasmissio scheme is described by comparig their performace with fixed trasmissio system. A better adaptatio algorithm is used to improve the throughput performace. This algorithm utilizes the average value of the istataeous SNR of the subcarriers i the subbad as the switchig parameter. The results show a improved throughput performace with cosiderable performace. Refereces [] A.Sohail ad M.N.Jafri, Adaptive OFDM over Frequecy Selective ad Fast Fadig Chael Usig Blockwise Bit Loadig Algorithm, IEEE Iteratioal Cofereece o Wireless ad Optical Commuicatio Networks,pp. -, July 7. [] A.Cyzlwik, Adaptive OFDM for widebad radio chaels, Global Telecommuicatios Coferece, vol, pp7-78, Nov 996. [] P.S.Chow, J.M.Cioffi ad J.A.C Bigham, A practical discrete multitoe trasceiver loadig algoritm for data trasmissio over spectrally shaped chaels, IEEE Trasactios Commuicatios, vol 8, pp , 995. [] T.Keller, ad L.Hazo, "Adaptive modulatio techiques for duplex OFDM trasmissio," IEEE Trasactios o Vehicular Techology, vol.9, o.5, pp.89-96, Sep. [5] Y.Lei ad A.Burr "Adaptive Modulatio ad Code Rate for Turbo Coded OFDM Trasmissios," Vehicular Techology Coferece VTC7, pp.7-76, -5 April 7 [6] A.Omar. ad A.R.Ali, "Adaptive chael characterizatio for wireless commuicatio," IEEE Radio ad Wireless Symposium, pp. 5-56, - Ja. 8 [7] M.I.Rahma, S.S.Das, Y.Wag, F.B.Frederikse ad R.Prasad, "Bit ad Power Loadig Approach for Broadbad Multi-Atea OFDM System", IEEE Trasactios Commuicatios, pp , 7. [8] S.S.Das, E.D.Carvalho ad R.Prasad, Performace Aalysis of OFDM Systems with. Adaptive Sub Carrier Badwidth, IEEE Trasactios o Wireless Commuicatios vol. 7, o., pp. 7-, April 8. [9] T.Tsugi, M. Itami, "A study o adaptive modulatio of OFDM uder impulsive power lie chael," IEEE Iteratioal Symposium o Power Lie Commuicatios ad Its Applicatios, ISPLC, pp. - 9, - April 8 [] T.H.Liew ad L.Hazo, Space-Time Trellis ad Space-Time Block Codig Versus Adaptive modulatio ad Codig Aided OFDM for widebad chaels, IEEE Trasactios o Vehicular Techology, vol 55, pp. 7-87, 6 [] L.Hazo, S.X.Ng, T.Keller ad W.Webb. Quadrature Amplitude Modulatio: From Basics to Adaptive Trellis Coded, Turbo Equalised ad Space-Time Coded OFDM, CDMA ad MC-CDMA Systems, d Editio, Joh Wiley&Sos Ltd,. [] K.M.Hadi, R.Tripathi ad K.Kat, Performace of Adaptive Modulatio i Multipath Fadig Chael, The 8 th Iteratioal Coferece o Advaced Commuicatio Techology, ICACT, vol., pp. 77-8, - February 6. [] S.Sampei ad H.Harada, "System Desig Issues ad Performace Evaluatios for Adaptive Modulatio i New Wireless Access Systems", Proceedigs of the IEEE, vol.95, o., pp. 56-7,Dec 7, [] L.Hazo, C.H.Wog ad M.S.Yee, Adaptive Wireless Traceivers, Joh Wiley &Sos,. [5] L. Khalid ad A. Apalaga, "Threshold-Based Adaptive Modulatio with Adaptive Subcarrier Allocatio i OFCDM-Based G Wireless Systems," IEEE Vehicular Techology Coferece, pp.-6, Sept. 6