Hybrid DF and SIR Forwarding Strategy in Conventional and Distributed Alamouti Space-Time Coded Cooperative Networks

In this paper, we propose a hybrid decode-and-forward and soft information relaying (HDFSIR) strategy to mitigate error propagation in coded cooperative communications. In the HDFSIR approach, the relay operates in decode-and-forward (DF) mode when it successfully decodes the received message; otherwise, it switches to soft information relaying (SIR) mode. The benefits of the DF and SIR forwarding strategies are combined to achieve better performance than deploying the DF or SIR strategy alone. Closed-form expressions for the outage probability and symbol error rate (SER) are derived for coded cooperative communication with HDFSIR and energy-harvesting relays. Additionally, we introduce a novel normalized log-likelihood-ratio based soft estimation symbol (NL-SES) mapping technique, which enhances soft symbol accuracy for higher-order modulation, and propose a model characterizing the relationship between the estimated complex soft symbol and the actual high-order modulated symbol. Furthermore, the hybrid DF-SIR strategy is extended to a distributed Alamouti space-time-coded cooperative network. To evaluate the~performance of the proposed HDFSIR strategy, we implement extensive Monte Carlo simulations under varying channel conditions. Results demonstrate significant improvements with the hybrid technique outperforming individual DF and SIR strategies in both conventional and distributed Alamouti space-time coded cooperative networks. Moreover, at a SER of , the proposed NL-SES mapping demonstrated a 3.5 dB performance gain over the conventional averaging one, highlighting its superior accuracy in estimating soft symbols for quadrature phase-shift keying modulation.