Fractional Cooperation using Coded Demodulate-and-Forward

Since the introduction of cooperative diversity, many different implementations have been proposed to increase the reliability and/or power efficiency of distributed networks via relaying. One simple and flexible scheme introduced is coded demodulate-and-forward, where the relay only demodulates, instead of decodes, the received data, to create and forward a new codeword to the destination. This reduces the complexity of hardware as well as the energy consumption by the relay. In this paper, we consider another flexible feature of the coded demodulate-and-forward scheme, where the relay uses only a fraction of its codeword to assist the source, and saves the rest of the codeword for transmitting its own information. Previous schemes have generally focused on all-or-nothing cooperation where a relay either contributes all its resources or none at all to the source. Depending on the channel conditions, improved diversity order of the source codeword can be achieved with some small loss in the relay's own transmission performance. Here we identify the necessary criterion for the source to achieve a diversity order of 2.