Carrier frequency offset estimation for OFDM systems using null subcarriers

Carrier frequency offset (CFO) in orthogonal frequency-division multiplexing (OFDM) systems, which can induce the loss of orthogonality among subcarriers and result in significant performance degradation, is critical to be estimated and compensated for. In this paper, a suboptimal scheme is proposed to estimate the CFO using null subcarriers. In the proposed scheme, the CFO is divided into the fractional part and the integer part, with the fractional CFO estimated followed by the integer CFO estimation. Compared with previous work using null subcarriers And exhaustive search, the implementation complexity of the proposed scheme is significantly lower due to the fact that the main component required is a simple correlator. Furthermore, with only one training OFDM symbol and proper null subcarrier allocation, the estimation range of the proposed scheme can be the inverse of the sampling duration. In contrast, conventional schemes with the same estimation range and complexity require two OFDM symbols. To achieve good performance, it will be shown that the null subcarrier allocation should be based upon some specific binary sequences such as the proposed extended m-sequences. It will also be shown by simulations that the same null subcarrier allocation criterion should be employed for optimal maximum-likelihood CFO estimation.