In spite of the fact that exploitation of the idea of cooperative systems has surged lately, the idea of cooperative communication can be traced back more than 40 years. It has drawn significant attention in both academia and industry due to its deployment flexibility and hardware feasibility. In cooperative communication, different users, each fitted out with a single antenna, can work as collaborative partners or relays to share their physical resources to assist the source transmit signals to the destination.
Many wireless relay protocols have been proposed in the literature to reap the benefits promised by cooperative wireless communication. The two most frequently used relaying protocols are decode-and-forward (DF) and amplify and forward (AF). The main performance degradation in a DF protocol arises from error propagation during retransmission when decoding errors occur at the relay in the low SNR regime of the source-relay channel. In the AF relay protocol, the signal received by the relay is amplified and re-transmitted to the destination. Different amplifying factors may be used. In contrast to DF, the AF protocol suffers from noise amplification, but AF precludes premature decisions and thus preserves the soft information content of the received signal. DF lacks the main advantages of AF and vice versa.
One way of avoiding the error propagation and noise amplification is to calculate and forward the corresponding soft information instead of making a premature decision based on the transmitted information symbols at the relay. Soft information indicates the reliabilities or probabilities of the underlying symbols. We refer to such a protocol as soft information relaying (SIR). This work investigates soft information relaying (SIR) for low-density parity-check (LDPC) coded transmission in wireless networks. We introduce a new scheme for soft parity symbol generation at the relay, which features two key strategies: a two-step soft parity generation process, and a pre-scaling technique. The two-step soft parity generation procedure is designed to allow efficient relay processing, while yielding an overall (i.e., destination) parity-check matrix structure with desirable properties. The pre-scaling method prevents the amplitudes of generated soft symbols successively converging to zero, as happens with some existing soft forwarding methods. Finally, we propose an appropriate LLR former at the destination which is tailored to the proposed soft parity generation technique. Simulation results demonstrate that the proposed relay protocol yields an improved BER performance compared to competitive schemes proposed in the literature.