| Spatial diversity is one of the well known diversity methods used in combating fading channels. Recently, cooperative diversity has been widely studied in literature as a spatial diversity technique. Different from multiple-input multiple-output (MIMO) systems, each user in the cooperative network is employed with a single transmit/receive antenna. In this thesis, we propose a cooperative diversity technique for asynchronous direct sequence code division multiple access (D8-CDMA) over frequency selective slow fading environment. First we assume the single cooperation relay case, where the bit-error-rate performance of the system is studied for both cases of perfect and imperfect inter-user channel (user-relay link). In order to mitigate the multi-access interference (MAI), decorrelator multiuser detectors are introduced at both relay and base station sides. Its effect on performance is studied and compared to the performance of the conventional matched filter receiver. Additionally, the performance of the system is studied and compared for different multi-path diversity scenarios in the inter-user and uplink channel. Furthermore, a coded multi-relay cooperation technique is proposed, where channel coding is introduced to minimize errors over the inter-user channel. All users are embedded with convolutional encoder and a Viterbi decoder. We study the performance of the coded system for different number of cooperating relays and over different multi-path diversity scenarios. Both simulation and analytical results are compared. Finally, we conclude that for a communication network to benefit from the cooperation diversity technique, a reliable communication link between active users and the cooperating relays should be secured (inter-user channel). We show that for an active user cooperating with V relays over a P-path frequency-selective fading channel, the expected diversity degree is P(V + 1). |
