Study On Channel Codes In Wireless Relay Systems

How to increase the transmission rate and improve the error-correcting performance of system have attracted more and more attentions with the rapid development of wireless communication. With the emergence of the relay technology, the coverage of communication network is extended, extra spatial diversity gain is achieved and the network capacity is also increased. Therefore, the design of error control coding scheme for wireless relay communication has been one of the focuses recently. Based on several typical relay network models, the corresponding transmission schemes are studied. The main results of this dissertation are summarized as follows:For the relay channel with two source nodes, one relay node and one destination node, this dissertation proposes a kind of puncturing algorithm for the rate-compatible network low-density parity-check (LDPC) codes, and derives the expression of decoding error probability. To minimize the decoding error probability, an algorithm of selecting variable nodes to be deleted is proposed. Moreover, an improved differential evolution algorithm is used to optimize the degree distribution of the proposed network LDPC code. Simulation results show that the proposed algorithms can provide good error-correcting performance.For the relay channel consisting of multiple source nodes, multiple relay nodes and one destination node, this dissertation studies the relay transmission scheme based on Reed-Solomon (RS) code and LDPC code. In order to combat the effects of error propagation in error-prone multiple-user multiple-relay (MUMR) wireless networks, a threshold based decode-and-forward (DF) relaying strategy is proposed. That means the relay node only forwards those bits whose log-likelihood ratios (LLRs) are larger than this threshold. Furthermore, this dissertation investigates the optimization of the threshold by minimizing the bit error rate (BER). Both separated cooperation and combined cooperation strategies are discussed. Simulation results indicate that the proposed strategy can effectively decrease the error probability of the system over both additive white Gaussian noise (AWGN) channel and fading channel.For the DF relay channel with one source node, one relay node and one destination node, this dissertation proposes a type-II hybrid automatic repeat-request (HARQ) based bilayer LDPC code by employing a half-rate invertible LDPC code, designs a joint decoding algorithm and analyzes the decoding threshold. In the proposed code, the relay node first encodes the message with a half-rate invertible LDPC code, and then encodes the coded parity check bits with a high rate LDPC code. Compared with the classic bilayer LDPC code, the designed bilayer LDPC code can significantly decrease the error probability of system.