On multi-user detection in CDMA based cooperative networks

In this paper, we investigate cooperative diversity in the uplink of a code division multiple access (CDMA) system, in which users cooperate by relaying each other´s messages to the base station (BS). We assume that the spreading waveforms are not orthogonal and hence multiple access interference (MAI) might exist at both the relay nodes as well as the BS. This MAI causes cooperative diversity gain to decrease. To alleviate this problem, the maximum likelihood multiuser detector (MUD) is introduced. We assume that direct and inter-user channel state information (CSI) is present at the BS, and it can use this information to optimally combine received signals from both direct and relay paths. Simulation results show that the proposed maximum likelihood (ML) detector can exploit maximum diversity and also cancel MAI at the BS. However, the complexity of this detector increases exponentially with the number of users. To address this issue, a genetic algorithm (GA) is employed and it will be shown that the proposed detector can achieve a near-optimum bit-error-rate performance with a reasonable complexity.