Joint bit loading and power allocation for downlink minimum mean square error combining based multi-carrier code division multiple access systems

The performance of a downlink multi-carrier code-division multiple-access (MC-CDMA) system heavily depends on the receiver type at the user side. The minimum mean square error-combining scheme possesses the finest performance. However, the resource allocation problem becomes a highly complex task because of multiple access interference. This study investigates a realistic scenario, where the base station determines the bit loading and power for all downlink users with both the power and bit error rate (BER) constraints to maximise the overall system throughput. The authors first derive the instantaneous signal to interference and noise ratio (SINR) and then approximate data transmission as an equivalent M-QAM system with a signal-to-noise ratio (SNR) equal to the instantaneous SINR of that user. Combined with accurate BER formulas of M-QAM system, the authors estimate the BER for each user given a set of power and bit loading configuration. Finally, the authors develop an iterative joint bit loading and power allocation algorithm to maximise the system throughput. The simulation results show that the proposed solution achieves performance close to the optimal one and maintains the BER close to the desired target.