Multiuser Detection of M-QAM Symbols via Bit-Level Equalization and Soft Detection

Building upon a unified framework for CDMA multiuser detection proposed in our prior work, we investigate the detection of M-QAM symbols in a multiuser CDMA channel. The solution proposed may be summarized as a generic iterative detection scheme for coded interference channels called bit-level equalization and soft detection (BLESD). It is shown that this novel approach avoids the exponential complexity of a posteriori probability (APP) detection by optimizing a closely-related, but much more manageable, objective function called variational free energy. It also fundamentally differs from the conventional symbol detector, in that data symbols are transparent to the new detector. Instead, soft estimates of the bits that make up the symbols are directly and naturally obtained at the detector output, in terms of posterior probabilities given the channel observation, facilitating efficient message-passing in joint detection and decoding