Joint timing and frequency synchronization for OFDM underwater acoustic communications

Timing and frequency synchronization is critical to the performance of orthogonal frequency-division multiplexing (OFDM) underwater acoustic (UWA) communications. In this paper, we propose a joint estimation scheme for fine timing, Doppler scaling factor, and residual doppler or carrier frequency offset (CFO), based on pilot symbols. The estimation problem boils down to an optimization problem over a three-dimension space. As the cost function has no closed form and a brutal search method demands prohibitively-high complexity, an iterative procedure is proposed to find the solution at a reasonable complexity. The joint estimation is performed in a block-by-block fashion and to further save computation, estimation results from a previous block can be used as initializations of the current block. At-sea experimental data collected in the SPACE08 UWA communication experiment was used to verify the performance of the proposed method. Attributed to the joint synchronization, decent detection performance was achieved for data with relative transceiver motion.