Research On Interference Alignment For Mimo Relay Systems

Relay and Multiple-Input-Multiple-Output (MIMO) techniques have been studied as the candidates for the Long Term Evolution Advanced (LTE-A) s-tandard proposed by the3rd Generation Partnership Project (3GPP). Thus, the MIMO relaying technique, which combines both of the aforementioned tech-niques, may be the most competitive technique in future mobile communication system. However, the performance of the MIMO relaying system, especially the multi-user two-way/multi-way relaying system, is severely restricted by the multi-user interference. Developing efficient method to manage the interfer-ence is the key to explore the potentials of MIMO relaying technique. Recently, interference alignment (IA), as a novel interference management method, has been attracting growing interest for its outstanding effect in scenarios like in-terference channel. Meanwhile, IA also provides new ideas for the interference management in MIMO relaying system. The dissertation mainly focuses on the MIMO relaying system and studies the transmission scheme based on IA in three typical scenarios, namely the multi-way relaying X channel, Y channel, and the multi-pair two-way relaying. Additionally, the beamforming design and user scheduling problems about the transmission scheme have also been studied. The major work and contributions are summarized as follows.(1) A transmission scheme based on IA is proposed for the multi-way relaying X channel. With the help of the transmission scheme, the informa-tion exchange is completed in two time slots. Compared with the traditional schemes, the spectrum efficiency is significantly improved. The feasibility con-dition of the transmission scheme is also studied, which determines the range of application. Additionally, the degrees of freedom (DOF) achieved by the proposed transmission scheme is analyzed, and the result is confirmed by the numerical simulations. It is shown that the proposed scheme achieves higher DOF than the conventional scheme based on zero-forcing (ZF) transceiver.(2) The exact DOF of a particular Y channel is studied. To begin with, we assume full cooperation between users and obtain a DOF upper bound accord-ing to the cut-set theorem. Then a transmission scheme based on asymmetric interference alignment is proposed. And according to the theoretical analy-sis and numerical results, the DOF upper bound is achieved by the proposed scheme. Combining the upper bound and the achievability, we conclude that the exact DOF is obtained. Moreover, an iterative beamforming optimization algorithm is proposed to further improve the sum rate performance, which op-timizes the beams of the users and the relay jointly. At last, the performance advantage and the convergence speed are demonstrated by the simulation re-sults.(3) A transmission scheme based on asymmetric interference alignment is proposed for the multi-pair two-way relaying channel. And the DOF achieved by the scheme is verified by theoretical analysis and simulation, which shows that the proposed scheme achieves much high DOF than traditional ones. Then, a joint beamforming optimization algorithm is proposed for the proposed trans-mission scheme. And the simulation results demonstrate the performance ad-vantage and the convergence speed of the beam designs. Moreover, a semi-orthogonal user scheduling method is proposed, which effectively harvests the multi-user diversity gain and further improves the sum rate.