Recently, the demand for using multicarrier communication systems is growing fast and is predicted to continue in the future. OFDM systems are the most useful type of multicarrier systems due to their ability to combat frequency selective fading channels. This system is used in IEEE802.11a/g, IEEE802.16a, DVB and ADSL. OFDMA system as a combination of OFDM system with FDMA technique is considered in multiuser communication. In additional to attractive characteristics of OFDMA systems, they have some challenges such as frequency and time synchronization. Carrier frequency offset is caused by Doppler effect and/or oscillator frequency mismatch between transmitter and receiver. Same as OFDM, OFDMA systems have high sensitivity to carrier frequency offset. Incorrect carrier frequency offset compensation will destroy orthogonality between subcarriers and result in inter-carrier interference and multi-access interference. This study provides carrier frequency offset investigation and compensation in OFDMA systems. Each user will receive a signal with single carrier frequency offset in downlink mode, where the received waveform is a combination of transmitted signals from different users with individual carrier frequency offsets in uplink. Then the study and compensation of carrier frequency offset in downlink mode is similar to single user scenario. However, the carrier frequency offset compensation in uplink is a challenging task. Carrier frequency offset compensation can be performed before or after DFT block. To improve the performance of carrier frequency offset compensation in OFDMA system, some interference cancellation techniques have been proposed to reduce multiple access interference. These result in some performance gain, however, at the cost of significant increase in complexity. Reducing the level of sidelobes for each subcarrier decreases multiple access interference with less complexity than interference cancellation techniques. Pulse shaping is a common method to reduce the side-lobe level in OFDM systems. In this research we used raised cosine window in transmitter and receiver. However this method improves the performance of direct compensation technique with block allocation scheme at the cost of less bandwidth efficiency. KeyWords: OFDMA systems, Windowing, Carrier frequency offset, Carrier frequency offset compensation.
