Soft decode-and-forward using LDPC coding in half-duplex relay channels

This paper proposes a new soft decode-and-forward (SDF) protocol using LDPC codes in the half-duplex relay channels. In order for the encoding of the additional parity-check symbols at the relay to be linear and systematic, we introduce a structured rate-compatible (RC) LDPC code. We then develop the soft-decoding and soft-re-encoding algorithms for the proposed RC-LDPC code, which allows the relay to forward soft messages to the destination when the relay fails to decode the source´s message. Furthermore, we propose a new method, which we refer to as soft fading, to compute the log-likelihood ratio (LLR) of the received signal at the destination for the SDF protocol. We show that our proposed method performs better when compared to a previous reported method in literature.