| Cooperative relaying has been identified as an effective technique to enlarge coverage and improve the performance of cell-edge users. However, relaying transmissions require extra radio resources, which lower the spectrum efficiency and become the bottleneck of improving throughput. To solve this problem, network coding and multiple-input and multiple-output (MIMO) provided two promising methods, and have drawn much attention. Due to the complementation in managing interference, the joint design of network coding and MIMO is studied in this dissertation to further explore the potential. To study the performance and the optimization of network coding in MIMO relay systems, network coding schemes based on amplify-and-forward (AF), denoise-and-forward (DNF) and decode-and-forward (DF) strategies are designed, and the main contributions can be summarized as follows:1. On the study of diversity performance for network coding in two-way AF relay MIMO systems with single-pair of usersFor two-way relay MIMO systems, all the existing approaches fail to provide a full-diversity beamforming scheme for network coding transmissions. In this dissertation, such a beamforming scheme is proposed, and the closed-form expressions for outage probability are provided with and without channel estimation error, and the diversity performance of the proposed scheme can be demonstrated by using asymptotic analysis. Our study solves a key technique problem that whether network coding can achieve the same reliability as the conventional one-way relay schemes. 2. Performance analysis of network coding transmissions in two-way AF relay MIMO systems with multi-pair of usersThe beamforming design for network coding is proposed in two-way AF relay MIMO systems. In particular, the inter-pair interference can be removed by designing the beamforming at the relay, and one degree-of-freedom can be saved due to the use of network coding, which can improve the multiplexing gains. When the perfect CSI is available, the performance of proposed scheme is studied, and the explicit expressions of ergodic capacity and outage probability are derived, respectively. Then the proposed scheme is extended to a more general scenario, where a diversity-enhanced scheme is designed based on the selection of beamforming matrices. Finally, the simulation results are given to show the performance gains, and verify that the joint design of network coding and MIMO technique can improve the performance of interference relay channels. When the number of user pairs is set as6, and the average signal-to-noise (SNR) is larger than18dB, our proposed scheme can improve the summation of user ergodic capacity by4bit/s/Hz compared with the time-sharing based network coding scheme. Compared with the conventional zero-forcing beamforming scheme, the average ergodic capacity can be improved by0.5bit/s/Hz when the number of user pairs and the average SNR are set as4and30dB, respectively.3. Performance analysis of network coding in asymmetric two-way AF relay MIMO systems with multiple usersA network coding transmission scheme is designed for the joint transmission of uplinks and downlinks in asymmetric two-way AF relay MIMO systems with multiple users, which can improve the spectrum efficiency. Then the ergodic capacity of each user is studied with both perfect CSI and imperfect CSI assumptions, and the explicit expressions are obtained. With generalized settings of relay antenna number, a relay beamforming vector selection scheme is designed, which can improve the diversity gains of proposed scheme. Finally, the simulation results are provided to show the performance of proposed scheme and the impacts of channel estimation errors. Compared with the coventional network coding scheme with zero-forcing beamforming, our proposed scheme can improve the average user ergodic capacity by0.5bit/s/Hz when the number of users is6, and the average SNR is larger than22dB.4. On the design of network coding in two-way DNF relay MIMO systemsSince both AF relay and DF relay network coding scheme have flaws, a DNF relay network coding scheme is studied in two-way MIMO relay systems. By using linear receivers and a modulo-based remapping function, the complexity of DNF network coding can be reduced. Then the beamforming design is optimized based on the mean square error (MSE) criterion, and a global optimal scheme and a sub-optimal scheme with lower computational complexity are designed, respectively. Finally, the proposed scheme is extended to a multi-pair scenario, and followed by the simulation results to show that the proposed DNF scheme can achieve a better tradeoff between complexity and performance. Compared with DF relay based network coding scheme, the bit error rate (BER) can be decreased from0.18to0.03when each node is equipped with two antennas, and the average SNR is20dB. Such a BER performance is the same as AF relay network coding scheme, but with lower computational complexity.5. On the design of network coding for asymmetric symbols in DF multi-way transmissionsThe conventional studies of physical-layer network coding is based on the assumptions that both channel statement and transmit messages are symmetric. We design a network coding scheme for asymmetric symbols, and illustrate it in an asymmetric MIMO scenario, where the multi-way transmissions and base station broadcasting can be accomplished jointly. To approach the optimal MSE performance, an iterative beamforming design is developed. Then a channel diagonalization-based beamforming design with low complexity is proposed. The simulation results show that the proposed scheme can improve the spectrum efficiency by50%, and extend the potential application scenarios of network coding. |
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