Robust initialization with reduced pilot overhead for progressive underwater acoustic OFDM receivers

Recently, a progressive receiver has been proposed to mitigate the intercarrier interference (ICI) in orthogonal-frequency-division-multiplexing (OFDM) transmissions over underwater acoustic (UWA) channels, where the ICI span gradually increases during the receiver iterations. Operating on a block-by-block basis, the progressive receiver is initialized by measurements on pilot subcarriers inserted in each OFDM block. In this paper, we propose an initialization method that exploits channel correlation across blocks to reduce the number of pilots needed. Specifically, channel estimation in the initialization phase is carried out by combining two sets of measurements: a set of artificial measurements predicted from the estimated channel of the previous block, and a set of measurements on the pilot subcarriers of the current block, where these measurements have different reliability levels. Performance results based on data recorded in SPACE08 and MACE10 experiments demonstrate the robust system performance with reduced number of pilots, where the transmitter in SPACE08 was stationary and that in MACE10 was slowly moving. Extension to the progressive receiver for multi-input multi-output (MIMO) OFDM is also pursued, where it is shown that the proposed hybrid initialization enables drastically improved receiver performance with a small number of pilots per transmitter.