Power and rate control effects in OFDM systems under fading environments

Abstract

The growing of recent demand for wireless digital multimedia services requires reliable and high-rate data communications over a wireless channel. Due to its robustness against intersymbol interference (ISI) and effectively mitigating the effects of multi-path delay spread of wireless radio channel, multi-carrier modulation technique such as orthogonal frequency division multiplexing (OFDM) is one of promising answers for this recent uprising demand. In this thesis, we consider a downlink in OFDM systems with multiple user access under given total transmit power and different channel conditions per subchannel. An overview of two promising next generation mobile communication standards such as IEEE802.16e and IEEE802.20 is provided in this study. Adaptive modulation method is utilized in rate control of each subchannel according to its channel condition, and the power control concept from conventional code division multiple access (CDMA) system is incorporated in our proposed scheme. We investigate the effects of power and rate control and compare them with conventional OFDM system in terms of BER and throughput performances under both slow fading and fast fading environments. Additionally, we propose an efficient multilevel outer-loop power control scheme to distribute transmit power as well as data rate in the subchannels to maximize the total output performance even further. Moreover, we examine our system with the IEEE802.16e and IEEE802.20 system parameters to observe practicality of our proposed schemes.