On Estimating the Symbol APP in MIMO Systems

The use of iterative receivers based on a-posteriori probability component decoders has proven to be an exceptionally powerful strategy for approaching the capacity of various communications channels. In systems with large dimensionality however, exact calculation of a-posteriori probabilites becomes computationally intractable, and we must resort to approximation techniques. In this work we describe a strategy for closely approximating the a-posteriori probabilities for these systems by marginalising over a small, high probability subset of possible sequences. We investigate how the required size of this subset varies with system parameters