Information Combining for Relay Networks

Analyzing the capacity of relay networks is often based on the max-flow min-cut theorem providing an upper bound on the true capacity which is generally still unknown. This paper analyzes the achievable rates of a simple relay network by a semi-analytical approach considering ideal capacity achieving codes of appropriate rates and optimum symbol-by-symbol soft-output decoding at the relay. Applying the information combining technique, regions are illustrated where the upper bound provided by the max-flow min-cut theorem can be reached. Moreover, the results show that the lower bound obtained by the constraint of error-free decoding at the relay can be exceeded in many scenarios. Simulations with a nearly capacity achieving half-rate repeat accumulate code confirm the semi-analytic results.