Low-Density Parity-Check Code And Its Application In The Coded Co-Operration Over Relay Channels

Multipath fading is a main obstacle for wireless communication to achieve high data rate and high reliability, an effective technique to combat multipath is diversity technique. As a typical spatial diversity technique, multiple-input multiple-output (MIMO) is able to improve reliability and effectiveness to a great extent. Although transmit diversity is clearly advantageous on a cellular base station, it may not be practical for other scenarios. Specifically, due to size, cost, or hardware limitations, a mobile unit may not be able to support multiple transmit antennas. Recently, it has been proved that, cooperative diversity by exploiting relay technique, has greatly improved the achieved system performance. By combining cooperative diversity with forward error correction coding brings further improvement. Low Density Parity Check (LDPC) codes are good error-correcting codes, which can approach Shannon's capacity limit, and the complexity of the decoding algorithm is linear with the code length. In order to take advantage of LDPC effectively, the LDPC coded cooperation scheme is the topic of the paper.In this paper, we first introduce coded cooperation strategies for relay channels based on LDPC codes. Due to the sparsity of its parity check matrix, LDPC codes can be decoded only with linear time complexity using iterative decoding algorithm. The key factor is to construct check matrix with sparsity and to keep the sparsity of its check matrix during encoding process. The emphasis has been laid on the random construction, algebraic construction and hybrid construction for binary and multicomponents, regular and irregular LDPC codes. Sum-product decoding algorithm in probability domain and the log likelihood ratio domain, together with the simplified minimum sum decoding algorithm are presented for comparsion. The LDPC code and its performance are validated via simulations.Next, the problem of cooperative communication with relays is highlighted in this thesis. Based on relay communication theory and relay communication model, this thesis presents a tutorial overview of LDPC codes under AF and DF relay strategies. The effects of some factors such as the distance between relay nodes and source nodes, the relay strategy, and the number of relay have been validated by simulation. It is unveiled through simulation results that, the channel condition or the distance between source and relay is the key factor which dominates the performance of codes cooperation based on LDPC codes. Based on the analysis of the key factors which affect the achived performance of the LDPC coded cooperation, the thesis investigates the adaptive cooperation scheme such as adaptive cooperating nodes selection and adaptive punctured LDPC coded cooperation scheme. It is shown that, the product of average channel gain of S-R link and R-D link is a promising criterion when the system needs to determine which relay should be selected to cooperate with the source node. Besides, it seems that the punctured LDPC coded cooperation is an effective way to realize the parity symbol forwarding based coded cooperation to achieve performance improvementl, especially when the distance between the source node and the relay is far enough.