An opportunistic subcarrier allocation algorithm based on cooperative coefficient for OFDM relaying systems

The downlink subcarrier allocation in a cooperative multiuser system with an amplify-and-forward relaying is studied for OFDM systems. We use the cooperation coefficient (Γ) from the literature as a basis for subcarrier allocation. Mean of the coefficient is first derived for Rayleigh faded channel. Then, a well known subcarrier allocation algorithm namely Max-Min is modified to make use of Γ. In this algorithm, the base station (BS) allocates the subcarriers to the users based on the combination of the direct and weighted indirect subcarrier gains. How to weigh the indirect path depends on the amount of information about Γ available at BS. Three different scenarios, each with varying level of implementation complexity, are considered: (a) knowledge of the instantaneous Γ at BS, (b) knowledge of the mean of Γ at BS, and (c) an estimate of the mean of Γ at BS. Finally, the performance of the Γ-modified Max-Min algorithm is evaluated for the three scenarios using Monte-Carlo simulation. It is shown that (i) the proposed algorithm outperforms the non-cooperative counterpart in terms of total throughput, (ii) the throughput gain moderates with larger group size showing the group diversity behavior in opportunistic communication and (iii) using the mean of the coefficient, instead of instantaneous coefficient, in cooperative subcarrier allocation provides comparable performance in Rayleigh faded channel environment, giving implementation advantage to the mean-based implementation of the Γ-modified Max-Min algorithm.