Soft iterative decoding for overloaded CDMA

The probabilistic association algorithm (PDA) is proposed as a quasi-optimal solution for synchronous overloaded CDMA detection. Overloaded CDMA is useful in applications in which the band is a limited resource and more users are to be conveyed on the same channel. The PDA algorithm, already successfully applied to underloaded CDMA, is extended to the more difficult overloaded CDMA problem. PDA uses dynamic soft updates for the a posteriori probabilities and it is derived under the Gaussian assumption for the superposition of multiuser interference and Gaussian noise. A discussion on the separability of the two-class problem suggests how a dynamic PDA (D-PDA) may solve the problem more effectively in comparison to a fixed-order PDA (F-PDA). Performance results are reported in this paper and show that D-PDA can provide good decoding with limited computational complexity with the bit error rate going to zero as the signal-to-noise ratio increases.