Investigations on cooperative transmission for the multiple access relay channel

In the past decade network coding has become a broad research area. Since the benefit of relays is well-known they have been considered within countless studies. Furthermore, it seems to be widely accepted that user cooperation promises a huge benefit. This paper considers a two-user relay network with a TDMA structure. Several cooperative and non-cooperative transmission schemes are analyzed. The cooperative ones will process the incoming information from several users at the relay jointly before forwarding it to the destination. The relay is restricted to forward information in just one time slot. Thus, it can either forward a subset of the received information or it has to change the coded modulation scheme by increasing the code rate or choosing a larger modulation alphabet. Using numerical simulations, five different strategies are analyzed. It will be illustrated that the performance of cooperative schemes deteriorates significantly when the relay is not able to decode successfully. Especially in asymmetric scenarios, the gains obtained by cooperation for the weaker user are much less than the corresponding losses of the stronger user so that the overall system performance degrades.