Cooperative-diversity networks technology is a promising solution to the high data-rate coverage required in future wireless communications systems. There are two main advantages of this technology:​ the low transmit RF power requirements and the spatial diversity gain. Different cooperation diversity protocols have been proposed for wireless networks. The basic idea is that in addition to the direct transmission from the transmitter to the receiver, there are other nodes, which can be used to enhance the diversity by relaying the source signal to the destination. This thesis investigates the performance of cooperative diversity networks using various relaying and cooperative techniques.

This thesis addresses five cooperative diversity cases. In the first case, we analyze the error and outage performance of cooperative diversity networks using amplify-and-forward and adaptive decode-and-forward relaying over independent non-identical flat Nakagami-m fading channels. We derive closed-form expressions for the error and outage probabilities and analyze their dependence on the channel parameters. In adaptive decode-and-forward relaying, among M relays that can participate, only C relays (C ≤ M), with good channels to the source, decode and forward (retransmit) the source information to the destination. In amplify-and-forward all the M relays can participate in resending the source signal to the destinations. In both techniques, the destination combines the direct and the indirect signals using the maximum ratio combining (MRC) technique. Results reveal that fixed protocols for amplify and forward increase the performance unlike decode-and-forward which need an adaptive protocol to improve the performance.

In the first case we assume that the destination has perfect knowledge of the channel state information (CSI) of all links. While in some scenarios the CSI can be acquired using the pilot symbols or training sequences, it may not be possible in some systems, particularly with fast fading channels. In the second case we relax this condition by assuming that no CSI is needed at the relays and the destination using differential equal gain combining (EGC) in cooperative diversity networks for both techniques amplify-and-forward and decode-and-forward. In this case, we derive exact closed-form expressions for the average bit error rate and outage probability. Furthermore, we find the SNR moments, the average signal-to-noise ratio (SNR) and the amount of fading. Numerical results show that the differential EGC can benefit from the path-loss reduction and outperform the traditional multiple-input single output (MISO). Also, numerical results show that the performance of the differential EGC is comparable to the maximum ratio combining (MRC) performance.

In the previous two cases, all the relay nodes relay the source signal using orthogonal channels (time slots, carriers or codes) to avoid cochannel interference. Hence, for a regular cooperative diversity network with M relays, we need M + 1 channels (one for the direct link and M for the M indirect links). This means that the number of required channels increases linearly with the number of relays. In the third case, we investigate the performance of the best-relay selection scheme where the “best” relay only participates in the relaying. Therefore, only two channels are needed in this case (one for the direct link and the other one for the best indirect link) regardless of the number of relays (M). We show that the best-relay selection not only reduces the amount of required resources but also can maintain a full diversity order (which is achievable by the regular multiple-relay cooperative diversity system but with much more amount of resources).

Another method to save the channel resources, incremental relaying technique, is examined in the fourth case. In incremental relaying technique we restrict the relaying process to the bad channel conditions only. Incremental relaying cooperative relaying networks exploit limited feedback from the destination terminal, e.g., a single bit indicating the success or failure of the direct transmission. If the destination provides a negative acknowledgment via feedback, the relay retransmits in an attempt to exploit spatial diversity by combining the signals that the destination receives from the source and the relay. Closed-form expressions for the bit error rate and the outage probability are determined. Results show that the incremental relaying cooperative diversity can achieve the maximum possible diversity, compared to the regular cooperative diversity networks, with higher channel utilization (channel capacity).

In the last case, we propose and analyze a novel relaying technique that combines incremental relaying with the best relay selection. We will derive exact closed-form expressions for the error probability and outage probability for this algorithm. The main advantage of this algorithm is that we use the channel resources in a controlled manner to always enhance the spectral efficiency of the cooperative networks.