  We consider joint detection of co-channel signals---specifically, signals
which do not possess a natural separability due to, for example, the multiple
access technique or the use of multiple antennas. Iterative joint detection and
decoding is a well known approach for utilizing the error correction code to
improve detection performance. However, the joint maximum a posteriori
probability (MAP) detector may be prohibitively complex, especially in a
multipath channel. In this paper, we present an approximation to the joint MAP
detector motivated by a factor graph model of the received signal. The proposed
algorithm is designed to approximate the joint MAP detector as closely as
possible within the computational capability of the receiver.
