Fast-varying Doppler compensation for underwater acoustic OFDM systems

This paper presents a practical low-density parity-check (LDPC) coded OFDM system designed for the underwater acoustic channel with its attendant multipath channel and large Doppler shift. The Doppler shift and channel state information (CSI) are assumed unavailable to both to the transmitter and the receiver. Due to the slow speed of sound in water, even small platform motions can affect the performance of the wideband system significantly, thus tracking the time-varying Doppler is essential for mobile underwater communication. An iterative receiver is proposed, which performs a Doppler estimation, channel estimation, and decoding in an iterative manner. The Doppler and channel tracking is done based on symbol by symbol, thus desirable for high speed mobile platform.