Multiuser Detection for Decode-and-Forward Cooperative Relaying in DS-CDMA Systems

In this paper, we consider the uplink of a Direct Sequence Code Division Multiple Access (DS-CDMA) system, where the source users cooperate in relaying each other´s message to the destination node based on the decode-and-forward relaying protocol. We study the following partner selection strategies: 1. each user helps all other users (All-Cooperate); 2. each user helps all successfully decoded users (Sue-Cooperate); 3. each user helps a user with the maximum observed signal-to-(interference plus noise) ratio (Max-Cooperate). To mitigate the multiple access interference (MAI) due to the usage of non-orthogonal spreading codes, we develop MMSE multiuser detectors at both the cooperative users and the destination node and evaluate the bit-error-rate (BER) performance. Simulation results show that the cooperative relaying significantly outperforms the direct transmission. Among the three partner selection strategies, Max- Cooperate achieves the lowest average BER. It is found that when the number of users increases, the BER performance of direct transmission gets worse, but the BER performance of Max- Cooperate improves significantly. For All-Cooperate and Sue- Cooperate selection strategies, the number of users does not have much impact on the BER performance.