On The Performance Limit Of Multi-user MIMO Two-way Relay Channels

Wireless relay has been an important ingredient in both ad hoc and infrastructure-based wireless networks.It shows great promises in power reduction,coverage extension and throughput enhancement.Compared with traditional one-way relaying,two-way relaying can almost double the spectral efficiency with physical-layer network coding.The multi-user MIMO twoway relay channel is a natural generalization of the two-way relay channel,which is a fundamental channel model in the wireless communications.In this thesis,we focus on the degrees of freedom(DoF)analysis of the multi-user MIMO two-way relay channels.DoF characterizes the independent number of data streams the system can transmit and is thus a measure of asymptotic channel capacity at high signal-to-noise ratio(SNR).Due to the diversification of two-way communication and the arbitrary number of antennas at each node,it is extremely challenging to analyze the DoF of multi-user MIMO two-way relay channels.The main contributions of this thesis are as follows:1.We propose a new transmission scheme,named as generalized signal alignment(GSA),and present its sufficient and necessary conditions.It overcomes the antenna configuration limitations of the conventional signal alignment schemes and is applicable to any antenna configurations.By using GSA,we analyze the maximum achievable degree of freedom of the general multi-user MIMO two-way relay channels.Furthermore,we present the asymtotic sum DoF when the users in the system tend to infinity.2.We characterize the optimal DoF region for the asymmetric MIMO Y channel.Given the outer bound of the DoF region,we divide the DoF tuples in the outer bound into two cases.Then,for each case,we prove that the DoF tuples are achievable by using techniques including antenna deactivation,pairwise signal alignment and cyclic signal alignment.The main novelty of our proof lies in the appropriate combination of antenna deactivation,pairwise signal alignment and cyclic signal alignment to achieve the DoF outer bound without specifying the antenna configuration.3.We characterize the optimal DoF region for asymmetric two-pair MIMO two-way relay channel.For the asymmetric data exchange,the relay not only needs to form the network-coded symbols but also need to process the additional asymmetric data streams.Given the DoF outer bound,we only need to divide all the possible antenna configurations into three cases.We then prove that the DoF tuples in the outer bound in each case are achievable.After obtaining the optimal DoF region,determining the optimal sum DoF becomes a linear programming problem.By analyzing this problem,we find that enforcing symmetric data exchange within each user pair does not lose any optimality in terms of the sum DoF.Based on this finding,the linear programming problem is greatly simplified and we are able to obtain the optimal sum DoF explicitly at all antenna configurations.4.We characterize the DoF region of the asymmetric MIMO two-way X relay channel by using outer bound and inner bound.By analyzing the properties of the DoF-region outer and inner bounds,we find that,in the case of symmetric data exchange,our inner bound coincides with the outer bound and thus our proposed transmission strategy is optimal.Furthermore,we derive the optimal sum DoF in the special case when the users within a same group are equipped with the same number of antennas,which provides a new insight to analyze the optimal sum DoF.Finally,it is shown through numerical results that the proposed DoF-region outer and inner bound coincide with most cases of antenna configurations.