The Study Of Coding Techniques In Relay-Assisted Communication

This thesis mainly studies coding in relay-assisted communication. Precoding and network coding schemes are proposed for applications in one-way relay, two-way relay and multi-way relay scenarios, respectively. The specific work is listed as follows:1) The problem of designing the precoders in a MIMO relay system is studied. Many early works are based on minimum-mean-square-error criteria, and the direct link between the source node and the destination node is neglected. In this thesis, the coexistence of relay link and direct link is considered and based on maximizing the mutual information between the source and the destination, a new joint source and relay precoder design is proposed. The derived problem is a non-convex optimization problem about the source and the relay precoding matrices. Firstly, a possible precoding structure is proposed, and then Hadamard inequality is used to turn the original problem into two convex subproblems. The closed-form solutions of the subproblems are given, and then the original problem is solved by iteration. Simulation results show that the proposed scheme converges rapidly, and provides more mutual information than existing schemes.2) A joint power allocation and relay position optimization problem of a relay-assisted multicast system in a sector of a cell is studied. Two transmission schemes are proposed:1) combining the use of space-time coding and distributed space-time code; 2) using distributed space-time coding and maximum ratio combining. The outage probabilities of these two schemes are analyzed in this thesis. Under the outage probability constraint, in order to maximize the spectral efficiency of the system, the optimization of power allocation and relay placement is studied. The adopted method is to reduce the dimension of the original problem by using Lagrange expansion to approximate part of the outage probability expression. The genetic algorithm is used to search the optimal value of the variables, and the result matches the result that is achieved by the former approximate solution. In addition, simulation results show that the spectral efficiency of the two proposed schemes is similar to each other, and both schemes’spectral efficiency is better than that of the existing scheme.3) The problem of precoder design in a relay-assisted D2D system is studied. The optimization objective is to minimize the total power consumed by the cooperating relays. The constraint is to ensure that all the users’ receiving signal-to-interference-plus-noise ratio (SINR) is greater than a given threshold. After some transformation, the original problem which uses a block diagonal matrix as its variable is turned into an optimization problem which uses a vector as its variable. Then the method of semi-definite relaxation is used, and the relaxed problem is solved. The method to find the solution to the original problem is also provided. Simulation results show that compared to existing schemes, to achieve the same performance, the proposed scheme can reduce the total power used by the relays significantly. By simulation, the effect of some parameters on the total power, such as the relay number and the antenna number, is also studied.4) A two-way communication precoder design for a multi-antenna relay system is studied. In this system, the relay helps the downlink transmission from the base station to each user and the uplink transmission from each user to the base station simultaneously. The design objective includes the transmission matrices of the base station and each user, the precoding matrix of the relay and the receiving matrices of the base station and each user. The design adopts both the idea of signal space alignment and network coding. Under the constraint of the maximum transmit power of each node, when the SINR ratio of the uplink is satisfied, how to maximize the minimum SINR of the downlink independent data stream is studied. Firstly, the original problem is decomposed into subproblems, which is then solved by using the second-order cone programming. By using the solutions of the subproblems, the matrix variables are optimized by iteration. Simulation results show that under different scenarios, the proposed scheme always converges quickly, and performs better than the comparing scheme.5) The problem about how to use a relay to assist the data exchange among the D2D users in a group is studied. When there are relatively more users, if the traditional amplify-and-forward scheme is used to assist each user sequentially, the system spectral efficiency will decline. Therefore, using analog network coding at the relay is considered, in order to let the relay assist each user simultaneously and thus the relaying time slots will be reduced. Since there are multiple users, in order to help the users fairly, it is necessary to optimize the coefficients of the analog network coding to maximize the minimum user multicast rate. After analyzing, the problem is quasi-convex and can be solved by combining linear feasibility programming and the bisection method. Simulation results show that when there are relatively more users and the signal-to-noise ratio (SNR) is relatively high, the proposed scheme is optimal. When there are relatively more users and the SNR is relatively low, amplify -and-forward scheme is optimal. When there are only a few users, direct D2D without using the helping relay performs the best.