부산대학교 도서관

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 E b /N o , and at lower E b /N o , both the source-relay and source-base station links need to be considered. From our numerical simulation, we also noticed that the cross-over E b /N o , point (around which frequency spreading gives better performance than time spreading) moves towards the lower E b /N o , 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 λ (=source node power/total power) is defined and BER performance is evaluated for different values of λ 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-torelay, source-to-destination and relay-to-destination) have equal gains, power ratio is found to be λ ≈ 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 λ increases above the optimal value at higher E b /N o . On the other hand, the performance remains almost the same when the decrement in λ is less than the optimal value. (b) For a network with stronger source-to-relay link, the optimal λ remains almost the same as the case with equal channel gains at higher E b /N o ; however, the optimal power ratio moves toward lower value of λ of 0.65 at lower b /N o . (c) The optimal λ remains almost the same with different time-frequency spreading factors.