On The Cooperative Diversity Scheme In Wireless Communication System

With the increase in the wireless communication demand, people are seeking better wireless transmission services. Therefore, it is important to find out a useful technique to combat effectively the detrimental multipath fading and the the imposed problems introduced by the time-varying transmission conditions. Among all candidate techniques, diversity technique is one of the most effective technologies under the constraint of the limited transmission power and the system bandwidth. Nonetheless MIMO (Multiple Input Multiple Output) technique offers an effective way to make full use of multipath transmission to increase the system capacity, it is generally not feasible to deploy multiple antenna at the mobile unit due to its size limit. Recently, the concept of cooperative diversity in the relay transmission scenarios has attracted widespread attention in the literature, which has been extended to cellular mobile communication systems, Ad hoc networks, wireless LAN and wireless sensor networks.In this thesis, two specific cooperation scenarios, i.e., the two users cooperating transmission in cellular networks and the cooperation transmission over multiple relays are concentrated. The first is the two users’ cooperative diversity scheme based on orthogonal signaling. The primary advantages of this scheme are the simple structure to be implemented, the reduction in the processing delay, both of which make the cooperative scheme attractive in terms of practical application. Unlike the ideal assumptions in the previous investigation, the following four impairments are investigated in great detail. Firstly, instead of the ideal channel estimate assumptions and the MRC receiver utilized, only co-phasing and the more realistic EGC receiver with comparable performace are considered. Secondly, the very special similar channel conditions are assumed for both cooperating users in the previous research, however, in practical applications, different users may undergo quite different signal variations due to their different surroundings. It is shown that, the user with inferior channel will definitely benefit from the cooperation in terms of reliability improvement, while it is highlighted that there will be threshold phenomena for the user with superior channel conditions. Thirdly, the influence of the phase estimation error on the achieved reliability of the cooperative diversity is investigated as well. Finally, a link-adaptive strategy is presented to enhance the cooperation performance. It is validated that, the threshold phenomena for the user with superior channel will be mitigated to a great extend by employing the proposed link adaptive cooperation.The second topic of the thesis is the multiple relay cooperation scheme for the multi-hop communication scenarios in wireless networks. And the purpose of this relative analysis is to show the contribution of different multiple relay cooperation to the resultant achieved reliability. Firstly, the approximate SER of the multiple relay cooperation scenario wherein each relay get the signal exclusively from the source (scheme 2 for short) are derived. It is shown that, such a simplified multiple relay cooperation scheme can achieve nearly the same reliability as the previsouly published multiple relay cooperation scheme in which each relay sucessively combines the signals from both the source and the previous relay (scheme 1 for short). It is shown that, scheme 2 instead of scheme 1 is more preferable for its simplicity if the channel between the relay and the destimation is good enough. Secondly, two power allocation strategies are addressed. It is shown through theorecial analysis that, the application of either FSP-ER or FSP-Near Optimal is independent on the channel condition on the direct link between the source and the destination. More specifically, it is the involved link quality between the source and the relay, and that between the relay and the destination should be utilized to determine which power allocation strategy is more preferable. And it is highlighted that, FSP-Near Optimal scheme works in the situation where the link quality between the relay and the destination is better that that between the source and the relay, otherwise FSP-ER should be used. Thirdly, a new multiple realy cooperation scheme wherein all relays direct decode and forward their received signal to the detimation (scheme 3 for short) is proposed. Obviously, scheme 3 is the simplest one among three multiple relay cooperation schemes at the cost of some degradation in the reliability. Fortuneatly, it is unveiled that, the performance can be improved significantly if opportunitic relay and the optimal power allocation techniques are employed. Moreover, scheme 3 works well if the link quality between the source and the destimation is not too bad to be utilized.The analysis and proposed schemes in this thesis could be used as a reference for the booming cooperative diversity research and applications, especially when two-user cooperative diversity scenarios in the cellular system and the multiple relay cooperation in multi-hop wireless communication network are concerned.