A multiple access/multiple user system called collision frequency shift-keying (CFSK) is investigated. Maximum-likelihood sequence detection (MLSD) provides optimal multiuser detection for the CFSK system but is too complex for practical applications so iterative detection is investigated as a suboptimal approach. The MAP-consensus decoder is given as an example of iterative detection and studied for both the synchronous and asynchronous system. A channel interleaver is introduced so that the performance of the MAP-consensus decoder is asymptotically optimal. The metric of the CFSK system has nonpolynomial complexity so suboptimal metrics with linear complexity are considered. Simulation results show that iterative detection with reduced complexity metrics achieve good performance for moderate spreading factors.

The MAP consensus decoder using iterative detection for a collision frequency-shift-keyed (CFSK) system is investigated in this paper. This consensus decoder decodes each user's message in a joint and iterative manner. For a specific user, interference from other users is canceled by the iteration process. The metric of the CFSK channel will introduce high complexity to the MAP algorithm when the system is highly loaded. The K -most probable combinations (KMPC) approach and two-most probable combinations (TMPC) approach are applied to simplify the metric. Analysis shows that the TMPC method keeps fairness for all the users, even though some users put high weights on the probabilities of part of the frequencies.