A low-complexity zero-forcing CFO compensation scheme for OFDMA uplink systems

Similar to the conventional orthogonal frequencydivision multiplexing (OFDM) system, an OFDM multiple access (OFDMA) system will have a carrier frequency offset (CFO) problem. Since CFOs of all users are different, CFO compensation in the OFDMA uplink system is much more involved. A simple, yet efficient, method is the zero-forcing (ZF) compensation method. However, it involves an inverse of an NtimesN CFO-induced ICI matrix, where N is the number of subcarriers. Thus, the complexity can become very high when N is large, a case commonly seen in OFDMA systems. In this work, we propose a low-complexity ZF method to overcome the problem. The main idea is to use Newton´s method to solve matrix inversion iteratively. We explore the structure of the CFOinduced ICI matrix and develop a method that can implement Newton´s method with fast Fourier transforms (FFTs). As a result, the required computational complexity is significantly reduced from O(N ³) to O(2N log₂ N). Simulations show that, with only three iterations, the proposed method can have similar performance to the direct ZF method.