IEEE logo

OFDM Demodulation in Underwater Time Reversed Shortened Channels

J.P. Gomes
ISR - Instituto Superior Técnico, 1000 Lisboa, Portugal
A. Silva
and S.M.Jesus ,
ISR, Universidade do Algarve, 8005-139 Faro, Portugal

Comments: download pdf file .
Ref.: Proc. OCEANS'08, Quebec, Canada, 2008.

Abstract This work addresses the problem of OFDM transmission in dispersive underwater channels where impulse responses lasting tens of miliseconds cannot be reliably handled by recently proposed methods due to limitations of channel estimation algorithms. The proposed approach relies on passive time reversal for multichannel combining of observed waveforms at an array of sensors prior to OFDM processing, which produces an equivalent channel with a shorter impulse response that can be handled much more easily. A method for tracking the narrowband residual phase variations of the channel after Doppler preprocessing is proposed. This is a variation of an existing technique that can improve the spectral efficiency of OFDM by reducing the need for pilot symbols. This work also examines techniques to handle sparse impulse responses and proposes a channel estimation method where an l1 norm is added to the standard least-squares cost function to transparently induce sparseness in the vector of channel coefficients. Algorithms are assessed using data collected during the UAB’07 experiment, which was conducted in Trondheim fjord, Norway, in September 2007. Data were transmitted with bandwidths of 1.5 and 4.5 kHz, and recorded at a range of about 800 m in a 16-hydrophone array. Significant multipath was observed over a period of at least 30 ms.

“©1992-2007 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for  advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.”