Exploiting Irregular Variable Node Degree in a MIMO System

Low density parity check (LDPC) codes have shown excellent performance in error correction capability rivaling and even surpassing turbo codes. Multiple-input-multiple-output (MIMO) antenna technology has demonstrated the means of delivering increased diversity gain and increased throughput. This paper investigates a sub-class of LDPC codes, irregular LDPC codes, having the property whereby the number of edges incident on a node of a particular set is not uniform in the factor graph representation of the code. This non-uniform property confers differing levels of error protection to the coded bits. A singular value decomposition (SVD) of the channel matrix decouples the MIMO channel into a number of parallel sub-streams ranked by relative channel gain. By analysing the irregular LDPC code structure and coupling the coded bits with the SVD ranked sub-streams a technique is proposed for improving performance when using irregular LDPC codes in an SVD MIMO system. Simulation of the proposed scheme for BPSK and QPSK modulation schemes in a number of different indoor environments has shown a significant reduction in the observed error floor and up to a 2.2 dB gain at a bit-error rate of 1times10-4