Bit Error Probability of Cooperative Diversity for M-ary QAM OFDM-based system with Best Relay Selection

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1 011 Internatonal Conerence on Inormaton and Electroncs Engneerng IPCSIT vol.6 (011) (011) IACSIT Press, Sngapore Bt Error Proalty o Cooperatve Dversty or M-ary QAM OFDM-ased system wth Best Relay Selecton Pussadee Kratpongvooth and Asst. Pro Dr Suvepon Sttchvapak Telecommuncaton Engneerng, Engneerng, Kng Mongkut s Insttute o Technology Ladkraang 3 Moo Chalongkrung Road, Ladkraang, Bangkok 1050 Thaland Astract. Cooperatve relay networks n OFDM-ased systems have een shown to mprove perormance n wreless communcaton systems as a orm o spatal dversty, ut are not readly avalale cooperatve dversty or M-ary QAM OFDM-ased system where the est relay only partcpates n the relayng. In ths paper, we nvestgate the error rate perormance o cooperatve Dversty or M-ary QAM OFDM-ased system wth est-relay selecton. The results present t error proalty o M-ary QAM OFDM-ased system wth est-relay selecton or derent values o the numers o relays (M), the numers o suchannels and modulaton scheme. M-ary QAM OFDM-ased system wth est-relay selecton provdes hgher perormance. Keywords: OFDM, M-ary QAM, cooperatve dversty, est relay selecton 1. Introducton Relays are exploted to mprove perormance n wreless communcaton systems. The advantages o the cooperatve dversty protocols come at the expense o a reducton n the spectral ecency snce the relays must transmt on orthogonal channels n order to avod ntererng etween the source node and each other as well. Hence or a regular cooperatve dversty network wth M relayng nodes, M+1 channels are employed, whch ncurs a andwdth penalty. Ths prolem o the necent use o the channel resources can e elmnated wth the use o the est-relay selecton scheme. In such a scheme, the est relay node only s selected to retransmt to the destnaton. Hence, two channels only are requred n ths case (regardless o the numer o relays). In ths paper, we nvestgate the error rate perormance n a multple path amply-andorward relay wth est-relay selecton network usng orthogonal requency dvson multplexng (OFDM) sgnals. Multple-relay cooperatve dversty n M-ary QAM OFDM-ased systems wth Best Relay Selecton has etter perormance than the regular cooperatve system.. System Model.1. M-ary QAM OFDM-ased system Fg. 1. shows the system conssts o an OFDM-ased mplementaton, wth M-ary QAM on each suchannel. Ths s smlar to the conguraton used or DMT-ased Asymmetrc Dgtal Suscrer Lne (ADSL), ut wth xed modulaton on each suchannel. [1] The sngle-carrer system was extended to a multcarrer (multple suchannels) system y dvdng the avalale andwdth nto suchannels. The M-ary QAM OFDM system was smulated or M=4, 16, 64 and 56, over =4, 16, 64 and 56 suchannels respectvely. We reer to these comnatons o M-ary modulaton scheme (used per suchannel) and suchannels as MQAM- (e.g. 64QAM-16 s 64-QAM modulaton over 16 suchannels). + Correspondng author. Tel.: ; ax: -. E-mal address: pggy_a@hotmal.com 95

2 Fg. 1: Illustraton o M-ary QAM on OFDM Relay etwork System. Accordng to [1], the BER expresson or M-ary QAM OFDM s: For M=4 (QPSK): For M=16, 64, 56, : 1 erc ( e = P 1) E 1 0 (1) M 1 ( 1) 3.log M. E erc 1 M log M ( M 1). e = P.. Multple-Relay Cooperatve Dversty Systems wth Best Relay Selecton System Fg. shows a source node (S) communcates wth the destnaton (D) through the drect lnk and the ndrect lnk (through the est relay). Ths est-relay selecton scheme allows the destnaton to get two copes o the source sgnal. The rst one s rom the source (drect lnk). The channel coecents etween the source S and the th relay R (h ), etween R and D(g ) and etween S and D( ) are lat Raylegh adng coecents. In addton, h, g j, and are mutually-ndependent and nondentcal or all and j. We also assume here, wthout any loss o generalty that addtve whte Gaussan nose (AWG) terms o all lnks have zero mean and equal varance 0. 0 () Fg.. Illustraton o the cooperatve dversty network wth the est-relay selecton scheme 96

3 ( ) Assumng that the relayng gan equals 1/ E h + [6] (to keep the relay power wthn ts constrants, s o especally when the adng coecent (h) s low), where Es s the transmtted sgnal energy o the source, t s straghtorward to show that the end-to-end SR o the ndrect lnk S R D can e wrtten as S R D h g = + +1 h g (3) Where = h E / s the nstantaneous SR o the source sgnal at R and = g E / g s the h s o s o nstantaneous SR o the relayed sgnal (y R ) at D, where E s the sgnal transmtted energy o the relay. The est relay wll e selected as the one that acheves the hghest end-to-end SR o the ndrect lnk. Then assumng that maxmum rato comnng (MRC) technque s employed at the destnaton node, the total SR at the destnaton node can e wrtten as tot = h + max g h + g + 1 (4) where = E / s o s the nstantaneous SR etween S and D. In order to use the total SR n the outage and error perormance calculatons, (3) should e expressed n a more mathematcally tractale orm. To acheve t, we proposed n [7] a tght upper ound or S R D, gven y = mn (5) The PDF o can e expressed n terms o the average SR h = E E / h s and = E( g ) E / (where g s o / E( ) s the statstcal average operator) as ( ) ( ) where. Usng the value o, we = 1/ e = h g /( h + g ) can rewrte the total SR n (4) as + (6) where mn = max mn h g more tractale than the exact value n (4) 3. Smulaton Result ( ) ( ( ) S R D tot ( ). Ths approxmaton o the end-to-end SR n (6) s analytcally We used an amply and orward cooperatve dversty system where a source node communcates wth a destnaton node drectly and ndrectly. h g ( ) o Bt Error Rate (BER) Fg. 3: Comparson o the est-relay selecton schema and the regular cooperatve dversty Fg. 3., We smulated BER versus SR or QPSK OFDM nto 16 suchannels wth the est-relay selecton. Fg. 3. compares BER perormance o the est-relay selecton schema and the regular cooperatve dversty or derent values o the numers o relays(m). At BER=10-5, QPSK-16 wll requre a E / o = E / o

4 db., QPSK-16 or numers o relays (M) equal 1 wll requre a E / o = 6.9 db., QPSK-16 or numers o relays (M) equal wll requre a E / o = 3.47 db., QPSK-16 or numers o relays (M) equal wth estrelay selecton wll requre a E / o = 3.18 db., QPSK-16 or numers o relays (M) equal 3 wll requre a E / o =.5 db., QPSK-16 or numers o relays (M) equal 3 wth est-relay selecton wll requre a E / o =.05 db., QPSK-16 or numers o relays (M) equal 4 wll requre a E / o = 1.77 db., QPSK-16 or numers o relays (M) equal 4 wth est-relay selecton wll requre a E / o = 1.9 db. The est-relay selecton whch numers o relays (M) equal provdes a perormance mprovement o nearly 0.9 db. The est-relay selecton whch numers o relays (M) equal 3 provdes a perormance mprovement o nearly 0.45 db. The est-relay selecton whch numers o relays (M) equal 4 provdes a perormance mprovement o nearly 0.48 db. E / o Fg. 4: Comparson o the est-relay selecton schema or derent values o the numers o suchannels Fg. 4., We smulated BER versus SR or QPSK OFDM wth the est-relay selecton or numers o relays (M) equal 3. Fg. 4. compares BER perormance o the est-relay selecton schema or derent values o the numers o suchannels. At BER=10-5, A sngle carrer wll requre a E / o = 5 db. By sudvdng the sngle carrer nto 4 suchannels wll requre a E / o = 3.7 db. By sudvdng the sngle carrer nto 16 suchannels wll requre a E / o =.09 db. By sudvdng the sngle carrer nto 64 suchannels wll requre a E / o = 1.93 db. Sudvdng the avalale andwdth thus provdes a perormance mprovement o nearly db. Bt Error Rate (BER) Bt Error Rate (BER) Fg. 5: Comparson o the est-relay selecton schema or M-ary QAM OFDM-ased system Fg. 5., We smulated BER versus SR or M-ary OFDM nto 16 suchannels wth the est-relay selecton or numers o relays (M) equal 3. Fg. 5. compares BER perormance o M-ary QAM OFDM- 98 E / o

5 ased and wth the est-relay selecton schema. At BER=10-5, 4QAM-16 wll requre a E / o = 6.9 db. 4QAM-16 wth the est-relay selecton wll requre a E / o =.05 db. 16QAM-16 wll requre a E / o = 10.7 db. 16QAM-16 wth the est-relay selecton wll requre a E / o = 6 db. 64QAM-16 wll requre a E / o = 15 db. 64QAM-16 wth the est-relay selecton wll requre a E / o = 10. db. The cooperatve dversty wth est-relay selecton provdes a perormance mprovement o nearly 4.7 db. 4. Concluson The results show M-ary QAM OFDM-ased system wth Best Relay Selecton the est-relays selecton cooperatve has BER etter than the regular cooperatve dversty. It was ound that the perormance gan etween M-ary QAM OFDM-ased system wth Best Relay Selecton the est-relays selecton cooperatve and the regular cooperatve dversty (rrespectve o modulaton scheme used) was aout db. When we change modulaton scheme or hgher capacty, we can ncrease numer o suchannel and use cooperatve dversty wth est-relay selecton or hgher perormance. In ths paper, we nvestgate only amply and orward cooperatve dversty system n the multple path. For example, decode and orward, the sngle path can e consdered. 5. Reerences [1] van Wyk, J. and Lnde, L., Bt error proalty or a M-ary QAM OFDM-ased system, n IEEE Trans. on Wreless Comm.,, pp. 1-5, 007 [] Salama S. Ikk and Mohamed H. Ahmed, Perormance o Multple-Relay Cooperatve Dversty Systems wth Best Relay Selecton over Raylegh Fadng Channels, n Hndaw Pulshng Corporaton EURASIP Journal on Advances n Sgnal Processng, vol. 008, Artcle ID , pp. 7, March 008 [3] J. G. Proaks, Dgtal Communcatons, ch. 5--, pp. 57 8,ch , pp McGraw-Hll, 4th ed., 001. [4] D. Yoon, K. Cho, and J. Lee, Bt error proalty o M-ary quadrature ampltude modulaton, n Proc. IEEE VTS-Fall VTC 00, (Boston, MA,USA), 4-8 Septemer 000. [5] Y Zhao, Ravraj Adve and Teng Joon Lm, Symol Error Rate o Selecton Amply-and-Forward Relay Systems, IEEE Trans. on Wreless Comm., vol. 10, no.11, pp , ovemer 006. [6] M. O.Hasna andm.-s. Aloun, Harmonc mean and end-toend perormance o transmsson systems wth relays, IEEE Transactons on Communcatons, vol. 5, no. 1, pp , 004. [7] S. S. Ikk and M. H. Ahmed, Perormance analyss o cooperatve dversty wreless networks over akagam-m adng channel, IEEE Communcatons Letters, vol. 11, no. 4, pp ,