A novel adaptive modulation scheme for WIMAX OFDM system to improve QoS

WiMAX at 4.9GHz has been identified by the ITU as important in achieving harmonised spectrum for advanced public protection and disaster relief. The band is currently used for this purpose in the US for disaster relief. The most recent WiMAX standard (802.16e) supports broadband applications to mobile terminals and laptops. In particular, WiMAX technology is one of the most energy efficient systems among pre-4G systems and it is capable to provide a Broadband Wireless Access in metropolitan areas. In this paper, adaptive modulation technique is proposed with the aim of improving performances in Non Line-of-Sight communications, typical of urban environments. Through these techniques it is possible to switch the order of the modulation to better match the channel conditions even in the disaster situation, obtaining comforting results in terms of probability of error and throughput. The system has been modeled with a finite state structure in which every state consists in a possible scheme of transmission (i.e. a specific modulation and the switch among different states happens when multiple thresholds on channel attenuation are reached. The adaptation is realized at the physical level of the transmission for a WiMAX OFDMA structure. Additive White Gaussian Noise (AWGN) model is used with Rayleigh fading to resemble the real world scenario. The proposed technique is suitable for different kind of traffic to meet different QoS requirements. The Intent of this paper is to provide better performance by maintaining the block error rate under a limit and it achieves improved throughput even with less average bits per symbol in WiMAX system.