Frequency Synchronization for OFDM/OFDMA Systems

Orthogonal frequency division multiplexing (OFDM) has been the focus of many studies in wireless communications because of its high transmission capability and its robustness to the effects of frequency-selective multipath channels. However, it is well known that OFDM systems are much more sensitive to a carrier frequency offset (CFO) than single carrier schemes with the same bit rate. Therefore, a frequency synchronization process is necessary to overcome this sensitivity to frequency offset. Synchronization is performed in two stages: acquisition and tracking. After a first estimation and correction of the CFO performed in the acquisition stage, there still remains a residual frequency offset (RFO) due to real system conditions. Therefore, the RFO tracking has to be performed for all the receiving data. Frequency synchronization is even more complicated for uplink communications in OFDMA (orthogonal frequency division multiple access) systems because the base station (BS) has to deal with signals from different users in the same bandwidth. Each user's data is affected by a different CFO. Because of this, estimation and correction of the CFOs cannot be accomplished by the same methods as in OFDM systems.