Design, Evaluation and Comparison of Four Large Block FEC Codecs, LDPC, LDGM, LDGM Staircase and LDGM Triangle, plus a Reed-Solomon Small Block FEC Codec
Abstract
This paper introduces with a great level of details the design and the performance evaluation of four large block FEC codes: regular LDPC (Low Density Parity Check), LDGM (Low Density Generator Matrix), LDGM Staircase and LDGM Triangle, that are all capable of operating on source blocks that are composed of several tens of thousand of packets. If all four belong to the category of codes on graphs, they differ by their encoding algorithm. We evaluate and compare the performances of all four codes, as well as a Reed-Solomon codec which serves as a reference and gives an idea of the level of performance achievable with small block codes. The comparison focuses on three major aspects: the global decoding inefficiency (caused either by the FEC code or by the need to split the object into several blocks), the encoding and decoding speeds, and the memory requirements at the encoder and decoder. From these performance results, we identify the situations where they best operate. Unlike many other works in the coding theory area, this paper deliberately skips theoretical aspects to focus on practical results. Besides this work does not consider techniques that are known to be patented (e.g. the use of irregular graphs), no matter how efficient they may be. This deliberate choice stems from our desire to design patent-free, open source FEC codecs. Our conclusion is that the LDGM Staircase/Triangle codes have excellent performances and open new opportunities in the area of bulk data distribution or storage.