Performance Analysis of Subcarrier Allocation in Two Dimensionally Spread OFCDM Systems

Orthogonal frequency code division multiplexing (OFCDM) has recently been introduced as a candidate for 4th generation wireless systems. This signalling scheme utilizes code spreading in the time and frequency domains simultaneously to improve frequency diversity and minimize MAI simultaneously. As a result, it is capable of outperforming multicarrier CDMA systems that employ one-dimensional spreading. In this paper, a novel adaptive subcarrier allocation algorithm is developed for OFCDM to improve the overall BER performance for all spreading configurations. This algorithm assigns users to subcarrier groups that provide favorable fading characteristics, while simultaneously reducing the amount of interference caused to other users. The proposed algorithm is shown to provide a performance improvement ranging from 1.5 dB with 2times16 spreading, to 7 dB with 16times2 spreading. The algorithm is also shown to maintain or improve the BER floor for each OFCDM spreading configuration.