Subspace-based OFDM Carrier Frequency Offset Estimation in the Presence of DC Offset

In an OFDM receiver with direct-conversion architecture, DC offset (DCO) and carrier frequency offset (CFO) cause a severe performance degradation. Recently, by investigating the subspace of OFDM signal after removing the coarse DCO estimate obtained via a time-domain average, we have proposed a null space based estimator (NSE), for blind CFO and DCO estimation. In this paper, we analyze the cost function of the NSE, and show that the full performance NSE is complexity reducible, which can indeed lead to a common null space based estimator (CNSE). Also, we develop a maximum-likelihood estimator (MLE), and prove that the CNSE is equivalent to the MLE. Furthermore, the Cramer-Rao bound of CFO estimation in the presence of DCO is given. Finally the analysis results are verified by simulations.