Low-Complexity CFO Compensation for Uplink OFDMA Systems

Orthogonal frequency-division multiplexing multiple access (OFDMA) has been considered as a promising multiple access technique in OFDM-based communication systems. In the system, all users share subcarriers in a frequency-division multiplexing manner. Similar to OFDM systems, the carrier frequency offset (CFO) induces inter-carrier interference (ICI) degrading the system performance seriously. However, since the CFO for each user may be different, the compensation turns out much more complicated than that in OFDM systems. A simple method to this problem is the use of a direct zero-forcing (ZF) method which involves the inverse of a matrix. The required computational complexity becomes prohibitively high when the number of subcarriers is large. In this paper, we propose a low-complexity method to overcome the problem in an interleaved uplink OFDMA system. We explore the special structure of the CFO-induced interference matrix and develop a method to solve the matrix inversion problem with fast Fourier transform (FFT). The proposed method can reduce the required computational complexity from O(N_c ³) to O(N_clog₂N_c). From the simulations, we can see that the proposed method can perform as well as that of the direct ZF method while the required complexity is much lower