A Low-Complexity Residual CFO Estimation Scheme for OFDM Systems

A low-complexity residual carrier frequency offsets (CFOs) estimation scheme of a multiuser uplink orthogonal frequency division multiplexing (OFDM) system is presented as an improvement to the traditional time-multiplexed method. This method employs an orthogonal code, such as the Frank-Zadoff code, in the training sequence. Each antenna transmits this code sequence according to a prearranged schedule. Hence, the base station estimates the uplink CFO of each path in an efficient time and spectral manner. Compared to the time-multiplexed method, the total required training time is reduced approximately 50% and the computation requirement is also reduced approximately 50% as the number of users grows large. Moreover, the overhead for the total uplink frame at the BS is independent to the number of users. Simulations of a 2xl systems show that this new method has a smaller variance of the estimation error than the time-multiplexed method when the signal-to-noise ratio (SNR) is low, and the same variance of the estimation error as the time-multiplexed method when the SNR is high in the COST207 typical urban.