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In this article, subcarrier and power allocation schemes are proposed and analyzed for different scenarios for a two-hop decode-and-forward OFCDM based relay network. In subcarrier allocation, the effect of considering the channel state information (CSI) of source-base station and source-relay link are evaluated in a cooperative diversity system. Results show that allocation of subcarriers based on source-relay node CSI provides better BER performance at higher Eb/No, and at lower Eb/No, both the source-relay and source-base station links need to be considered. From our numerical simulation, we also noticed that the cross-over Eb/No, point (around which frequency spreading gives better performance than time spreading) moves towards the lower Eb/No, when the subcarrier allocation is done giving more weight to source-base station link rather than the source-relay link which provides additional flexibility in operating environment for OFCDM systems. In power allocation, a cooperative power allocation ratio lambda (=source node power/total power) is defined and BER performance is evaluated for different values of lambda in the relay network. It is found that there exists an optimal power allocation ratio for different operating environment such as source-to-relay channel gains and time-frequency spreading factors. It is reported that: (a) when all three channels (source-to-relay, source-to-destination and relay-to-destination) have equal gains, power ratio is found to be lambda ap 0.8 (i.e., 80% and 20% of the total power is distributed among source and relay node respectively). The performance degrades at much faster rate when lambda increases above the optimal value at higher Eb/No. On the other hand, the performance remains almost the same when the decrement in lambda is less than the optimal value. (b) For a network with stronger source-to-relay link, the op- timal lambda remains almost the same as the case with equal channel gains at higher Eb/No; however, the optimal power ratio moves toward lower value of lambda of 0.65 at lower b/No. (c) The optimal lambda remains almost the same with different time-frequency spreading factors.
Date of Publication: September 2009