Low-complexity CFO Compensation for OFDM-based Massive MIMO Systems

Massive multiple input multiple output (MIMO) plays a pivotal role in the fifth generation (5G) wireless networks. However, its performance heavily relies on accurate synchronization. Although timing offset (TO) can be avoided by applying orthogonal frequency division multiplexing (OFDM) with an adequate length of cyclic prefix (CP), carrier frequency offset (CFO) is still a challenging issue. Especially, in the uplink of multi-user massive MIMO systems, CFO compensation can impose a substantial amount of computational complexity to the base station (BS) due to the large number of BS antennas. To resolve this problem, in this paper, we propose a low-complexity CFO compensation technique. Our solution is performed after combining the received signals at the BS antennas. We calculate the interference matrix and discuss that asymptotically, this matrix can be obtained in terms of the power delay profile (PDP) of the users' channels. Consequently, the computational complexity of the CFO compensation process is independent of the number of BS antennas and thus remains constant as this number increases. Moreover, we show that the calculated interference matrix can be diagonalized due to its circulant property, and as a result, its inverse can be obtained straightforwardly. This leads to a considerable saving in the computational cost of the receiver. Numerical results are presented to verify the performance of our proposed CFO compensation technique and to investigate its computational complexity.