On LDGM Code And Its Application In The Coded Cooperation Scheme

With the development of the mobile communication technology, the next generation of mobile communication system needs to support the high-speed of data transmission up to 1Gb/s. It is important to exploit the limited frequency resources sufficiently and efficiently in the high-speed wireless communications, which is the main problem and chanllenge to resolve for the future mobile communicaiton network. In recent years, some investigations have indicated that distributed wireless communication technology will be hopeful to turn into the key technology for resolving the high-speed data transmition and large scale network covering. Coded cooperation is a channel coded scheme suitable for the requirement of distributed wireless communication technology. The distributed disposing and cooperative communication are the essential characteristics to realize the coded cooperation.By all appearance, the coded cooperation technology with low complexity is highly desirable for the wireless cooperative communication deployment. As a special form of the LDPC codes, LDGM codes have the advantage of simplified encoding. So the LDGM-based high performence coded cooperation and adaptive coded cooperation scheme are of significant theoretic and practical value.In this parper, after a brief introduction of the LDPC coding principle, we particularly address the similarities and differences bettween LDGM code and the LDPC code. It is shown that, like the LDPC code, the performence of LDGM codes is completely dependent on its encoding construction. For a given codeword length, different encoding parameters will give rise to different performence. Next, with similar matrix parameter but different codeword length, a longer codeword will have a better performance. Furthermore, similar with LDPC, with the SNR increasing, error floor performance will present for LDGM codes as well. Because of the sparse matrix including identity matrix for LDGM codes, some bit information will not be updated during in the BP decoding, which makes LDGM have worse error floor than the LDPC codes. According to the error rate analysis over the binary symmertric channel, it is shown that degree-1 or degree-2 bit nodes are the reason for the higher error floor for LDGM codes, which could be intuitively explicated since the related messages will not be updated during the BP iterative decoding for these bit nodes. Concatenated scheme could be utilized to suppress the error floor significantly. After the explanation on the reason of the improvement on the error floor performance by using serially concatenated LDGM codes, the serially concatenated sheme based on dual diagonal identity matrix is utilized and its improvement on the error floor performance is validated through simulations. By setting the appropriate coding parameters, the serially concatenated LDGM codes could keep the low complexity as well as greatly improve the LDGM performance.Past investigation results have shown that, both Turbo codes and LDPC codes are excellent channel coding sheme, both of which could be utilized in the relay communication model to achieve the capacity-approaching coded cooperation scheme. Aiming at the half-duplex relay model, serially concatenated LDGM coded cooperation is highlighted in this thesis. It is unveiled that, concatenated LDGM could achieve the capacity-approaching performance in the source-relay-destination relay model as well. Besides the performance, the comparative simplicity for implementation makes the serially concatenated LDGM coded cooperation scheme an attractive and promising coded scheme for the practical implementation in the relay communication model. Recent investigation has suggested that, the network connection topology could be interpreted as coded constraint, with the well-known class of code-on-graph. And the recently proposed adaptive network coded cooperation(ANCC) was validated in this paper to transform the network connection topology into a class of adaptive LDGM network codes. The relation of mapping network-on-graph into LDGM code-on-graph is addressed and validated through simulations. It is disclosed that adaptive LDGM coded cooperation scheme could be realized without additional coding redundance at every individual node in network communication scenarios, thus offering a promising network-on-code-graph scheme for a distributed communication with many sources and one destination.The work in this thesis validated that the LDGM coded cooperation and adaptive network coded cooperation are effective in distributed communication environment. It is of valuable reference for the resemble investigation.