Study On Precoding Design And Performance Optimizing Based On MIMO Two-way Relay Systems

Since MIMO technology can improve the utilization of the spatial resource and enhance the reliability, it has been widely used in wireless communication networks.Physical layer network coding(PNC) has the ability to expand the coverage area of cells, strengthen the quality of service on the edge of cells and resist to the interference among users. Combing PNC and MIMO, it can further broaden the ubiquitous service and improve the system performance. As a key point of the next generational wireless communication systems, two-way relay receives considerable attention for the higher spectrum efficiency. For the reason, this paper mainly discuss the precoding algorithms and the optimal resource problems of two-way relay systems around three core points of multi-relay, multi-user and high dimension of codewords. Three aspects as follows constitute our main content:(1) Base on the recent study of power optimization mainly focus on the level of subcarriers for Multiple-Input and Multiple-Output-Orthogonal Frequency Division Multiplexing(MIMO-OFDM), the precoding scheme of two-way relay channels is proposed to divide complicate MIMO-OFDM two-way channels into multiple independent physical network coding(PNC) sub-channels. On that basis, the system is extended to the multi-relay model and the appropriate relay is chosen to the scheme that subcarriers in frequency and sub-channels in space domain pair by the criterion of maximizing the SNR of the receiving signals at users and which can improve the performance of the relay. Finally, Joint the optimal relaying selecting, giving each node a constrained total power and Lagrange dual method, we obtain the optimal system resource allocation. Simulation results show that there is 3.7bits/s/Hz spectral efficiency gain when the proposed scheme of joint optimization compares to the scheme of considering subcarriers pairing only, on the case of SNR 30 dB and 4 relays.When 16 relay, the proposed scheme has 1.1bits/s/Hz spectral efficiency gain compared to 4 relays and close to the upper bound of system performance.(2) For Multi-user MIMO(MU-MIMO) and considering the interference among multiple users become the important factor that affects decoding correctly, a scheme of precoding is designed for two-way relay systems, based on the receiving criterion of Minimum Mean Square Error(MMSE). Taking the convenient in mathematical analysis into account, users are distributed in two symmetrical groups in the way of exchanging information. To reduce the complexity of proposed precoding scheme asmuch as possible, we employ the QR decomposition and complex lattice reduction(CLR) transform to replace the two times singular value decomposition(SVD) of conventional BD precoding algorithm by introducing a combined channel inversion to eliminate the multiple-user interference(MUI). Performance analysis confirmed that the proposed the CLR- MMSE algorithm not only has a better BER performance,but also can simplify users’ receiver and greatly reduce the complexity of the precoding compared to the MMSE and BD algorithms. Finally, for 6 users, numerical simulations demonstrate that the proposed algorithm also has the advantage of the achievable rate compared to MMSE.(3) Considering the transmission of high dimension codewords and susceptible to overlap on the constellation diagram of relay when joint Quadrature Amplitude Modulation(QAM) and PNC and lead to increase the error probability of decoding, a precoding algorithm is proposed and based on the lattice code(LC) and simultaneous wireless information and power transfer(SWIPT), which can quantize source information adding dither and form the lattice modular arithmetic at the side of launch, and the modulus at the relay is still a one-to-one correspondence to this, the constructed codeword to avoid overlapping. According to the critical rate of error bit rate with a regional expression, the constraints of convex optimization are obtained.Then, through alternating optimization and the tool box CVX, we get the optimal power split factor and the receiving beamforming matrix of the relay node. Finally,the simulation results demonstrate that the proposed scheme not only has a better performance of the achievable system rate, and a better BER performance compared against by that the equal power allocation scheme.