Time-varying carrier frequency offset estimation in OFDM underwater acoustic communication

We consider the problem of carrier frequency offset estimation in OFDM underwater acoustic communication. In our previous work, we suggested transmitting equi-power and equi-spaced pilot tones which led to a simple carrier frequency offset estimator. Here, we extend this work in two directions: First, equi-power pilots may result in a large peak to average power at the transmitter. Thus, we propose a method to design the phases of the pilot tones so as to keep the peak to average power low, while still obtaining large enough pilot to data ratio needed to decode the pilot symbols. The design method is based on ideas adopted from phase retrieval techniques. Second, we modify the previous carrier frequency offset estimators to the case of a time-varying underwater acoustic channel. These modifications are based on modeling the carrier offset by polynomial and piece-wise models. The estimators use the correlations and intervals between the pulses, and approximate the frequency offsets by small order polynomials. Simulations results show that for constant offsets, the proposed estimators achieve similar performances as state-of-the-art techniques, while for time-varying offsets the proposed estimators are superior.