Average coset weight distributions of Gallager's LDPC code ensemble

In this correspondence, the average coset weight distributions of Gallager's low-density parity-check (LDPC) code ensemble are investigated. Gallager's LDPC code ensemble consists of regular mtimesn-LDPC matrices with column weight j and row weight k. The average coset weight distribution can be derived by enumerating the number of parity-check matrices in the ensemble satisfying certain conditions. Based on combinatorial arguments, a formula for the average coset weight distribution will be proved. From the formula, we can show some properties of the average coset weight distributions such as equivalence classes of syndromes, symmetry of the distributions, and a lower bound on coset weight