Study Of OFDMA For BWS

Motivated by the increasing demand of the modern society for high-speed reliable information exchanges whoever, whenever, wherever, whatever, broadband wireless systems (BWS) are envisioned for future mobile communication networks. It has been widely acknowledged that the terrestrial BWS faces a formidable challenge due to the frequency-selective multipath fading propagation environment. Multi-carrier modulation (MCM) is a preferable physical layer technology for BWS, since it can efficiently mitigate the frequency-dependent distortion across a wide frequency band and simplify the equalization in a multipath fading environment. BWS also relies on the multiple access technique to divide the common transmission medium among multiple users. As a unique multiple access system based on the MCM technology, the orthogonal frequency division multiple access (OFDMA) is a promising technology for BWS.This dissertation first introduces OFDMA systematically, The multiple access technique in OFDMA is discussed along with the comparison of different subcarrier assignment schemes. Second we deal with the effects of residual timing and frequency offsets on the symbol error rate (SER) performance of an orthogonal frequency-division multiple access (OFDMA) system. Proper frequency and timing synchronization is necessary to maintain orthogonality among the active users. So how to overcome these difficulties and improve the system performance has became an important research topic. After Theoretical analysis, we simulations and analyses by computer show that the frequency and timing offsets affect the OFDMA sub-carriers differently.