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 subcarriersand result in significant performance degradation, is critical to be estimatedand compensated for. In this paper, a suboptimal scheme is proposed to estimatethe CFO using null subcarriers. In the proposed scheme, the CFO is dividedinto the fractional part and the integer part, with the fractional CFO estimatedfollowed by the integer CFO estimation. Compared with previous work usingnull subcarriers and exhaustive search, the implementation complexity of theproposed scheme is significantly lower due to the fact that the main componentrequired is a simple correlator. Furthermore, with only one trainingOFDM symbol and proper null subcarrier allocation, the estimation range ofthe 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 uponsome specific binary sequences such as the proposed extended m-sequences. It willalso be shown by simulations that the same null subcarrier allocation criterionshould be employed for optimal maximum-likelihood CFO estimation.

I. Introduction

Orthogonal frequency-division multiplexing (OFDM) is a promising technology in broadband wireless communications due to its ability in mitigating multipath effects. Hence, it has been adopted as the key technology for standards such as DVB-T, MMAC, 802.11g, 802.11a, and HiperLAN [1]. However, OFDM is extremely sensitive to synchronization errors, especially the carrier frequency offset (CFO), which is induced by oscillator discrepancies between the transmitter and receiver and/or Doppler shifts. As a result, CFO estimation for OFDM is an active area of research [2].