Research On The Technology Of Compress-and-Forward In Cooperative Communication And Relay System

Cooperative communications refer to systems or techniques that allow users to help transmit each other’s messages to the destination. The basic model for cooperative communications is the three-terminal relay channel, including the source, the relay, and the destination. The source broadcasts its message to both the relay and the destination. The relay processes the message it receives from the source and forwards the processed signal to the destination, which reconstructs the original message by decoding the signals received from both the source and the relay. The task of the relay is thus to facilitate joint decoding at the destination by means of spatial/temporal diversity. The cooperative relaying technique, or relaying protocol, is divided into four kinds: amplify-and-forward (AF), decode-and-forward (DF), coded cooperation (CC), and compress-and-forward (CF). CF model has many advantages. CF is desirable over DF when the signals received at the relay and the destination are highly correlated, e.g., when the relay is close to the destination. In addition, as opposed to DF, CF always outperforms direct transmission. Thus, in CF, the relay always helps the source, even if the link between the source and the relay is poor.First of all, this paper carries out the implementation and comparison of the basic CF and DF models. We use LDPC codes as the channel encoding and decoding in both CF and DF models. In the basic CF model, scalar quantization is used at the relay. In the basic DF model, the relay decodes the message and uses the same channel code as the source. Simulation results show that CF outperforms DF when the relay is nearer to the destination than the source, which is in accord with the theoretical results.In addition, we propose a method for the power allocation in CF when the achievable rate is maximized, in time-division half-duplex model. Given the total power and the distance between the source and the relay, this method bases on the formula of the maximized achievable rate, combines with the restriction of the power and uses numerical approach to confirm the time-division factor and the power allocation between the source and the relay.Finally, with the two periodical results before, based on the theory about the application of superposition coding for Wyner-Ziv problem, this paper extends the theoretical analysis, proposes a practical CF scheme in half-duplex mode by using the technique of superposition coding and provides specific coding strategy for it. Simulations in AWGN channel and block fading channel show that our proposed scheme outperforms the conventional CF scheme with scalar quantization at the relay. This coding strategy can be easily extended to the scenarios of fading channels, which are more practical in wireless communication.