Censoring and combining in cooperative communications assisted by a relay

A cooperative communication system in which a censorial relay is adopted to assist binary data transmission from a source to a destination is addressed in this paper. A censorial relay is also called a selective DF (decode-and-forward) relay. It censors (i.e. does not pass forward) decisions that are deemed as not reliable enough. The reliability of decisions is measured in terms of the magnitude of the log likelihood ratio (LLR) of a received data bit (from the source). If this magnitude is below some preset threshold, then the data bit is censored. At the destination, the received signals from the source and the relay are lineally combined. The ratio between the coefficients for combining must be properly chosen in order to minimize the BER. In many works found in the literature, those coefficients for combining are chosen according to the maximum ratio combining (MRC) rule. However, it will be shown that MRC is not the optimal choice. We will show what the optimal choice is. Moreover, we will also show what the optimal LLR threshold for censoring is. The key reason for the feasibility of finding the optimal combining coefficients and the optimal censoring threshold is that a closed-form BER formula is derived (and then, of course, is utilized). To further take advantage of this BER formula, it is also used to perform power allocation (PA) between the source and the relay. Results from theoretical analysis and simulation are generated to evaluate the effectiveness of the proposed system. Those results are also compared to some existing relay-assisted cooperative communication systems found in the literature.