A Subcarrier Allocation Scheme for MC-DS-CDMA Systems in the Presence of Multiple Access Interference

Subcarrier allocation for multiuser MC-DS-CDMA systems is much more complicated when compared to OFDM systems due to the presence of multiple access interference (MAI) when multiple users transmit simultaneously on the same subcarrier. In this paper, we propose a novel suboptimal algorithm to adaptively allocate subcarriers for MC-DS-CDMA systems, with the effect of MAI taken into consideration. An earlier proposed algorithm minimizes the bit- error-rate (BER) of respective users rather than minimizes the overall system BER. Each user performs channel estimation, orders subcarrier channel gains and feedbacks an index vector to the base station (BS). BS uses these index vectors from all the users, together with subcarrier allocation coefficients of the previous updating stage, to adaptively generate subcarrier allocation coefficients of the current updating stage so that BER performance is optimized. However, since the assignment process depends on the earlier allocation coefficients of other users, an error can propagate and affect the decision to be made in the next stage. In the proposed approach, the problem is formulated using Quadratic Programming (QP), and a single objective function is used to optimize the overall system BER. The algorithm uses the actual values of channel gains and does not depend on any information from the previous updating stage. Thus, the discrepancy propagation is avoided. We use Kuhn-Tucker conditions to convert the formulated QP to a linear programming (LP) problem, and the solution can be obtained using standard LP solvers.