Inspired by the idea of multiple-input and multiple-output (MIMO) systems,diversity can be achieved when multiple copies of the transmitted signal are received at the destination through cooperative relaying, resulting in improved reliability using single-antenna terminals. In cooperative communications the relaying scheme plays a fundamental role in exploiting the achievable diversity gain. Two techniques have been largely studied, namely, Amplify-and-Forward(AF) and Decode-and-Forward (DF). To avoid unnecessary consumptions of resources, relays should be used only when the direct transmission is failed, which is known as incremental relaying. The drawback of incremental relaying is the degraded bandwidth efficiency, due to information repetition. Particularly, incremental relaying based on AF suffers noise amplification, while it incurs decoding delay using DF. How to balance the use of relays in order to achieve a good trade-off between reliability and bandwidth efficiency needs a special treatment.
This work proposes a hybrid relaying scheme that exploits the principal benefits of AF and DF in the context of incremental relaying. AF is performed in the first phase using a small number of relays, while the second phase is launched using DF only if the first one fails. To facilitate performance evaluation, important performance metrics are obtained in closed-form expressions. Both numerical and simulation results show that the proposed scheme achieves higher reliability and makes better use of the radio resources.

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