Orthogonal frequency-division multiplexing (OFDM) has been considered as a promising candidate technology for high speed wireless communication systems. However, the orthogonality between subcarriers is destroyed in the presence of carrier frequency offset (CFO), which results in system performance degradation. This problem is further complicated by the presence of direct current offset (DCO) in direct conversion receivers (DCRs). In this paper, we propose a novel method to efficiently estimate both the CFO and the DCO for the OFDM-based wireless local area networks (WLANs). In this method, we utilize the periodicity and correlation of the training symbols in the preamble and design a null space separation method to decouple the CFO and the DCO variables. After the CFO estimation is obtained, the DCO can be easily estimated. Simulation results show that compared to previously published methods, the proposed method achieves near Cramer-Rao lower bound (CRB) with high efficiency.
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