Channel Estimation And Synchronization For OFDM Underwater Communication

Underwater acoustic channel is the most challenging channel in the world due to its time varying and frequency-selective characters. These facts obstruct the realization of most of high bit rate underwater communication systems.The growing demand for high speed underwater telemetry systems inspires researchers and institutions to study new methods and develop new systems. Some coherent systems exhibit good performance by exploiting sophisticated adaptive equalizers to combat multipath fading. Multicarrier modulation in the form of orthogonal frequency division multiplexing (OFDM), on the other hand, converts a frequency selective channel into a set of parallel frequency flat sub-channels, thus greatly simplifying receiver equalization to only one tap. Moreover, OFDM is the most bandwidth efficiency way so far as we know which could work in underwater acoustic channel.Lately, a lot of literatures have discussed underwater communication in multicarrier modulations but few of them study channel estimation and synchronization systematically.In this dissertation, a channel estimation scheme is proposed with no channel priori statistics required at all. Three kinds of pilot pattern are examined and each pattern is suitable for a particular environment. Channel estimation based on LS algorithm is vulnerable to noise and Inter Channel interference (ICI), so noise reduction is a compulsory task. Pilot noise reduction is adopted in transform domain, because the large value of the transform domain of channel frequency response will only appear in the "low frequency", if synchronization is perfect. Channel interpolation is performed by an ideal low pass filter based on multirate signal processing theory. This filter is realized by an FFT/IFFT based transform domain filter, so the computation burden is significantly reduced.A new 350ms length synchronization frame is proposed and timing, carrier frequency offset correction, sampling clock correction and channel estimation could all be done in this frame. A new fine timing synchronization algorithm is introduced which is called transform domain synchronization. This algorithm utilizes the cyclic property of the transform domain of channel frequency response estimation.Numerical simulation and experiments demonstrate that OFDM technique is robust with respect to frequency selectivity and time selectivity.