Doppler compensation method using carrier frequency pilot for OFDM-based underwater acoustic communication systems

In this paper, we propose a new method to compensate Doppler frequency shift for OFDM-based underwater acoustic communication systems. In order to save bandwidth, we do not use additional signal header (preamble) in each OFDM frame as proposed in many conventional approaches. Instead, the central sub-carrier is reserved for pilot transmission. This subcarrier is so-called as the carrier frequency pilot (CFP), which is used to detect the Doppler frequency. At the receiver, two synchronization steps are deployed. The first step performs coarse synchronization. In this step, the Doppler frequency is roughly estimated on the basic of the detected carrier frequency. In the second step, we use the CFP to regulate the estimated Doppler frequency. This regulation is called as fine synchronization. The advantage of the proposed method is to reduce the OFDM frame length. Therefore, the system bandwidth can be increased. The proposed method is able to track the fast time-varriation of the Doppler frequency, which is a typical characteristic of the underwater channel. To verify the perfomance of the proposed method, we have tested our approach in real underwater channel with the speed of the relative Rx movement of 3m/s. Experimental results show that the estimated Doppler frequency matches well with the theoretical calculation.