Tree Search Detection Based on LLR Using M Algorithm in MC-CDMA Systems

摘要

In this paper, we propose a new multiuser detection scheme using Maximum Likelihood (ML) criterion and the M algorithm for Multi Carrier (MC)-Code Division Multiple Access (CDMA) systems in the down-link channel. We first describe an implementation of ML detection separating In- and Quadrature-phase components and using well-known linear filters. In the proposed algorithm, we produce hypothesis symbol vectors in a tree structure by partly changing the sub-optimum hard decisions based on the linear filter output. At each stage, we adopt the best M likely paths with respect to the true log likelihood or distance function as survivors. We determine the symbol vector which minimizes the distance function at the final stage. Although the complexity of ML detector is exponentially increasing as a function of the number of users, the proposed scheme requires by far less complexity. We demonstrate that the proposed scheme achieves equivalent Bit Error Rate (BER) performance with lower complexity in comparison with ML detector by computer simulations. Moreover we compare the proposed detection scheme with QRD-M algorithm which is based on QR decomposition combined with M algorithm.