Training Design for Repetitive-Slot-Based CFO Estimation in OFDM

Carrier frequency offset (CFO) estimation is a key challenge in wireless systems employing orthogonal frequency-division multiplexing (OFDM) modulation. Often, CFO estimation is carried out using a preamble made of a number, say J , of repetitive slots (RS). We here focus on the issue of optimal RS preamble design using the Cramer-Rao bound (CRB) averaged over the channel, which is assumed to be Rayleigh. We show that the optimal value of J is a tradeoff between the multipath diversity gain and the number of unknowns to be estimated. In the case of correlated channel taps, we also show that uniform power loading of the active subcarriers is not optimal (in contrast with the uncorrelated case) and better power loading schemes are proposed. The proposed power loading schemes consist of allocating more power to activated carriers with higher signal-to-noise ratios. Simulation-based performance results of the maximum likelihood estimator support the CRB-based theoretical results.