Research On Diversity Performance Of Opportunistic Cooperation In Wireless Relay Systems

As an effective method to combat wireless fading, cooperative diversity technique has been researched extensively and intensively recently. Through sharing the resources such as antennas, geographically isolated network nodes can exploit spatial diversity gain to improve system performance by forming a virtual multiple input multiple output (MIMO) system. As an important aspect of cooperative diversity techniques, opportunistic cooperation has attracted more and more attention due to its features including excellent performance, simple implementation, good scalability et al., and has gradually become a hot issue in academic circles. In consideration of the development tendency of future communication systems, we theoretically study the diversity performance of decode-and-forward based opportunistic cooperation technique in this dissertation, deeply analyze the key factors that impact the performance of the protocol and their relations, provide necessary theoretical support to apply this technique in next generation mobile communication systems. In this dissertation, we firstly study the diversity performance of decode-and-forward opportunistic cooperative protocol with extended relay selection schemes, then evaluate the impact of different universal fading channel models on the protocol, and furthermore take the specific performance of this protocol with imperfect channel state information (CSI) into consideration, and finally we research on the achievable theoretical diversity performance of decode-and-forward opportunistic cooperative protocol enhanced by MIMO techniques. The major work and contributions of this dissertation consist in:1. Research on diversity performance of opportunistic cooperation with extended relay selection schemesFirstly the performance of decode-and-forward opportunistic cooperation with a general relay selection scheme, i.e., the Nth best relay selection, is studied under Rayleigh fading channels, where the impact of the direct link is taken into consideration. Then considering the unfair problem that might happen when traditional decode-and-forward opportunistic cooperation is implemented, we propose a relay selection scheme based on the normalized channel state and analyze its performance. Based on the Nth best relay selection, we analyze and obtain the expressions of system performance when decode-and-forward opportunistic cooperation is utilized under independent but not necessary identical Rayleigh fading channels, and take the impact of the direct link on system performance into consideration, and prove in theory that the system diversity order varies linearly with the relay selection criterion N. We also analyze the performance of decode-and-forward opportunistic cooperative protocol with normalized channel state, and prove its advantages in terms of both diversity order and fairness, which provides theoretical support when fairness of relay selection is needed in practical systems.2. Research on diversity performance of opportunistic cooperation under universal fading channel modelsThe achievable diversity performance of decode-and-forward opportunistic cooperation under universal applicable Nakagami-m fading channel model is first analyzed, where both symmetric and asymmetric network topologies are taken into consideration. Then combined with the feature of wireless propagation environment in real systems, the performance of decode-and-forward opportunistic cooperation under mixed fading channel models,?.e., mixed Rayleigh and Rician fading channel models, and more general mixed?-?and?-?fading channel models, are studied, and the impact of fading channel parameters on performance is analyzed. Theoretically we derive and obtain the expressions of end-to-end outage probability and diversity order of decode-and-forward opportunistic cooperation under Nakagami-m fading channels, where the availability of the direct link is considered and the Nth best relay selection scheme is implemented. The obtained expressions are simpler than existing ones, and they can be applied in more scenarios. Under mixed fading channel models, the expressions of end-to-end outage probability and diversity order of decode-and-forward opportunistic cooperation are derived for the first time, and the results can be utilized directly in network planning and optimization. Furthermore, the used universal fading models can describe the features of practical systems more accurately, which makes the results more appropriate be used in real networks.3. Research on diversity performance of opportunistic cooperation with imperfect CSIWe first study the diversity performance of decode-and-forward opportunistic cooperative protocol when channel estimation errors exist at receivers, and evaluate the impact of channel estimation errors on diversity order. Then we extend the imperfect CSI to both transmitters and receivers,?.e., jointly considering the impact of channel estimation errors and feedback delay, and obtain the achievable diversity performance of the system under such conditions. Theoretically, we prove the impact of channel estimation errors and channel feedback delay on performance of decode-and-forward opportunistic cooperative protocol, and derive and obtain the expressions of end-to-end outage probability and the achievable diversity order. The related results can be utilized to calibrate the performance of decode-and-forward opportunistic cooperative protocol in practical systems when the capability of channel estimation and control channels should be taken into consideration.4. Research on diversity performance of opportunistic cooperation enhanced by MIMO techniquesFirstly we study the performance improvement of decode-and-forward opportunistic cooperation when multiple antennas are implemented at source node, relay nodes, and destination node and enhanced with orthogonal space-time block code (OSTBC) transmission. Then based on the results we design a new incremental relaying transmission scheme, which can effectively improve the system performance by harvesting the diversity gain of OSTBC and opportunistic cooperation together with the spectral efficiency gain of direct transmission, and also we obtain the expressions of diversity performance in theory. Theoretically, we derive and obtain the relation among system outage probability, antenna configuration, relay deployment and network topology, and also obtain the closed-form expressions of system diversity order, which facilitates the implementation and application of decode-and-forward opportunistic cooperative protocol under multiple antenna scenarios. We propose a novel decode-and-forward opportunistic cooperative protocol by combining OSTBC and incremental relaying, and prove that this protocol not only obtains the full diversity order gain, but also improves the system transmission efficiency through making full use of resource dimension including multiple antennas and multiple relays, et al.