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Abstract

The paper proposes an iterative Hidden Markov Model (HMM) for decoding a Low Density Parity Check (LDPC) code. It is demonstrated that a first-order HMM provides a natural framework for the decoder. The HMM is time-homogeneous with a fixed transition matrix and is based on a random walk through the encoded frame bits. Each hidden state contains a pair of two encoded bits, and parity checks are naturally incorporated into the observation model. The paper shows that by implementing a forward-backward smoothing estimator for the hidden states, decoding is efficient and requires only a small number of iterations in most cases. The results show that the LDPC decoding threshold is significantly improved compared to belief propagation (BP) on a Tanner graph. Numerical results are presented showing that LDPC codes under the proposed decoder yield a frame error rate (FER) and decoding threshold comparable to that of a Polar code where Successive Cancellation List (SCL) - Cyclic Redundancy Check (CRC) decoding is deployed. This is shown to be achieved even if the frame length is short (on the order of $512$ bits or less) and a regular LDPC code is used. 1