Research Of Relay Technology In 4G Mobile Communication Systems With Limited Feedback

Recently, there is a growing interest in various forms of cooperative communication protocols that provide diversity and multiplexing gain for communication via wireless relays. Also, relay is considered for Long Term Evolution-Advanced (LTE-A) as a tool to improve e.g. the coverage of high data rates, group mobility, temporary network deployment, the cell-edge throughput. In practical implementation, relay selection is a useful cooperative technique since it only activates the best relay to forward source information to the destination. Apart from simplicity of signaling, relay selection avoids complex synchronization schemes (needed by most distributed space-time coding schemes) and reduces the power consumption of the terminals.Firstly, in chapter 3, in a multi-relay scenario, when a direct link is absent between the source and destination, two relay selection protocols with limited feedback are proposed:Best Relay Selection without direct link protocol and Cycle Relay Selection without direct link protocol. Traditional Distributed Space-Time-Coded (DSTC) protocol requires all relay nodes to participate in forwarding. While Best Relay Selection protocol only chooses one relay with the highest instantaneous channel gain to the destination for forwarding, which could achieves the same Diversity-Multiplexing Tradeoff (DMT) performance of DSTC. But it has been well-known that repeating the source's transmission limits the spectral efficiency in relay network. To recover a good part of the rate loss, a Cycle Relay Selection with limited feedback protocol is proposed. The basic operation of the protocol is as follows:in each time slot, the source transmits a new packet for the benefit of the relays. Simultaneously, the best relay forwards a packet for the benefit of the destination, interfering with the reception of other relays. All relays attempt to decode in the presence of interference, to be able to participate in the next round of transmission. It is shown that Cycle Relay Selection protocol has superior DMT performance at high multiplexing gains over existing protocols.Secondly, in chapter 4, the case is considered where a direct link is available between the source and destination, in addition to the relayed links. For this configuration, two relay selection protocols with limited feedback are proposed:Best Relay Selection with direct link protocol and Adaptive Cycle Relay Selection (ACRS) with direct link protocol. Best relay selection with direct link protocol lets the source transmit most of the time and always allows source transmission to be forwarded only by the best transmitter available (maybe source itself) if needed, leading better DMT performance over the Best Relay Selection without direct link protocol. But when the direct link is unavailable for a long time, the relays must repeat the signal in a two-hop fashion, making a great loss in spectral efficiency. Accordingly, Adaptive Cycle Relay Selection with direct link protocol is proposed. If the direct link is in outage for many time slots, ACRS will try to make each packet transmission finished in less than two time slots, for the purpose of compensating the loss in spectral efficiency. An adaptive method is introduced to make sure that the system could perfectly respond to any kind of channel situation. As shown in the dissertation, ACRS can achieve better DMT performance compared with previous multiple relay DF scheme across a large range of spectral efficiencies. Simulation results also show that it outperforms a variety of existing schemes in terms of outage performance.Finally, the work of this dissertation is summarized and the future research points are suggested.