Abstract
Cooperative communication is a promising transmission paradigm which can exploit the resources of neighboring nodes in a network to provide spatial diversity in fading channels without need for physical arrays. However, the half-duplex constraint of nodes in practical systems has induced a loss of bandwidth efficiency because of extra bandwidth need from the cooperative transmission by relay nodes, which calls for new research efforts to improve the spectral efficiency. Therefore, in this thesis we have focused on exploiting novel multilink adaption strategies for half-duplex cooperative communications to further improve their performance and combat the decrease in spectral efficiency. The significance of node cooperation can be achieved by relaying protocols, such as Amplify-and-Forward (AF), Decode-and-Forward (DF). Firstly, the performance gains of cooperative communications were analyzed in terms of average symbol error probability (SEP) in fading channels, with proposed tighter bounds for AF relaying system. Next, we intensively investigated adaptive transmission to further improve the performance of cooperative communications from the following aspects. Under AF relaying, adaptive power allocation has been considered for a bidirectional relay network, with two proposed power allocation strategies. The proposed strategies require only channel mean strength information at the transmitter side, and can provide up to 15% improvement in terms of the transmission rate and 20% in terms of the outage probability. When considering DF relaying protocol, relay nodes can transmit in different modulation formats with in the source node. In this context, we have investigated the multi-link adaptive modulation under power and bit error rate constraints. Several adaptive modulation schemes were proposed to improve the link spectral efficiency up to 4.5 bits/s/Hz from 0.5 bits/s/Hz in fixed relaying, providing channel state information at the transmitters. Both single and parallel-relay networks were considered with proposed opportunistic relaying policies for parallel-relay network. Coded cooperation is another cooperative protocol developed from DF relaying in conjunction with channel coding. Two adaptive coding strategies were proposed by exploiting the properties of Rate Compatible Puncture Covolutional (RCPC) codes used in coded cooperation. It has been found that the two proposed strategies can offer up to 2 dB gain in terms of error probability and tripled code rate, respectively. Our investigation has revealed that multi-link adaption can lead to significant performance improvement with large spectral efficiency gains to half-duplex cooperative communications.