On the growth rate of irregular GLDPC codes weight distribution

In this paper the exponential growth rate of irregular generalized low-density parity-check (GLDPC) codes weight distribution is considered. Specifically, the Taylor series of the growth rate is expanded to the first order with the purpose of studying its behavior in correspondence with the small weight codewords. It is proved that the linear term of the Taylor series, and then the expected number of small linear-sized weight codewords of a randomly chosen GLDPC code in the irregular ensemble, is dominated by the degree-2 variable nodes and by the check nodes with minimum distance 2. A parameter is introduced, only depending on such variable and check nodes, discriminating between an exponentially small and exponentially large expected number of small weight codewords.