Maximum Likelihood Receiver for MMSE Relaying

One major goal for future wireless communication systems is a uniform user experience. In order to solve this problem, additional relay nodes can be used to support the communication. Up to now mainly two relaying functions are considered, namely Amplify-Forward (AF) and Decode-Forward (DF). DF can either mean a channel decoding at the relay or a hard decision directly on the received signal which will be called DetF here to avoid confusion. But both have certain drawbacks: AF suffers from noise amplification whereas with DetF useful information is lost due to the hard decision. For this purpose a relay function minimizing the mean squared error called Estimate-Forward (EF) has been proposed in the literature combining the benefits of AF and DetF. If more than one relay is considered or if the signal received directly from the source contains useful information, these signals should be combined properly. In the case of nonlinear functions at the relay like for DetF or EF, the disturbance of the signal received from the relay is not Gaussian any more and simple weighted linear combining like Maximum Ratio Combining for Gaussian channels is suboptimal. In this paper we will derive a Maximum Likelihood detector for EF with higher order modulation schemes. This receiver results in significant gains in terms of bit error rate and mutual information and therefore leads to a higher user throughput compared to state-of-the-art techniques. Moreover, these gains are obtained without increasing the computational complexity of the terminals significantly.