Adaptive OFDM system for multi-rate multi-user services in wireless communications

The paper investigates the performance and throughput of an adaptive bit allocation and variable rate algorithm depending on channel state information (CSI) and transmit power in a multi-user OFDM transmission. The purpose of the multi-rate OFDM system using adaptive modulation is to increase bandwidth efficiency by controlling constellation size and to adapt the proper service for each user according to channel conditions and the transmit power while maintaining the transmission´s quality of service (QoS) at the receiver. A fixed-rate adaptive OFDM system only exploits instantaneous channel state information by using several modulation levels, but it is inappropriate to multi-rate multi-user services. On the contrary, the multi-rate adaptive OFDM system is very efficient for a multi-user system requiring various services, because this system exploits not only instantaneous channel state information but also the transmit power. Computer simulation results show that the capacity of the adaptive multi-rate OFDM system can be adaptively changed for various services. A performance and throughput analysis of this system in multipath fading channels with different RMS delay spread is also presented.