Abstract
In wireless networks, relaying techniques have been proposed to expand the coverage of communication systems and increase the spectral and energy efficiency. In particular a typical relaying scenario consists of a source, a relay and a destination. The most applicable cooperative relaying techniques are decode and forward (DF), and amplify and forward (AF). DF is a regenerative approach and performs satisfactorily when the source-relay link is strong. Thanks to its regenerative approach, relay can change its transmission format to suit the characteristic of the channel ahead. This will allow the time durations of its receive and transmit phases to be independently adjusted. In contrast AF has less complexity as well as less cost. It's a non- regenerative scheme that performs a very simple linear processing technique at the relay. Moreover AF can provide better performance especially when the source to relay link is not reliable enough. However in the traditional AF the time durations for the relay-receive and relay- transmit phases are the same. This thesis investigates a flexible cooperative AF scheme that allows separately tuneable phase durations. The achievable rates and the outage probabilities were derived for the proposed AF cooperative protocol. The presented analysis can be used to predict the AF scheme performance and to optimise the duplexing ratio for different network topologies and SNR regions. In deep static fading channel conditions, it is likely that the transmitted message is not successfully decoded at the destination in the first transmission. In these events the performance of the system can be improved by retransmitting the message with a suitable combination of channel coding and Automatic Repeat re-Quest scheme, known as Hybrid ARQ (HARQ). In this thesis, two different HARQ strategies combined with cooperative flexible AF are analysed in order to evaluate the performance of the proposed strategies without any need for time consuming Monte Carlo based evaluation methods. Furthermore, the flexible AF scheme is extended to multiple antenna scenario. Performance of the flexible AF with multiple antennas is investigated for the case that relay has both receive and transmit channel knowledge. Since the transmit channel knowledge may not always be available at the relay and also in order to reduce the complexity a novel power allocation method based on the maximization of the mutual information of the relay link is proposed when the relay has only the receive channel knowledge.