Algorithms for the estimation of a channel whose impulse response is characterized by a large number of zero tap coefficients are developed and compared. Exploiting the sparsity of the channel, the estimation problem is transformed into an equivalent on-off keying (OOK) detection problem, whose solution gives an indication on the position of the zero taps. The proposed schemes are compared to the standard least squares estimate (LSE) via simulations in terms of mean square error (MSE) and bit error rate (BER). A scheme based on sphere decoding appears to give the best performance while maintaining moderate complexity.

Abstract-This paper is concerned with timing recovery for ultra-wideband communication systems operating in a dense multipath environment. Two timing algorithms are proposed that exploit the samples of the received signal to estimate the start of the individual frames with respect to the receiver's clock (frame timing) and the location of the first frame in each symbol (symbol timing). Channel estimation comes out as a by-product and can be used for coherent matched filter detection. The proposed algorithms require sampling rates on the order of the inverse of the pulse duration. Their performance is assessed by simulating the operation of coherent and differential detectors. Their sensitivity to the sampling rate is discussed, and the effects of the multiple access interference are evaluated.

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In this work, multi-hop cooperative schemes for underwater sensor networks are studied and shown to be highly energy efficient. Also, the signal model for a multipath channel is derived taking into account the interference from other nodes. It is shown that while the effects of uncompensated intersymbol interference (ISI) can be quite detrimental, the network exhibits enough robustness against multiuser interference.

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Synchronization and channel acquisition for ultrawideband signals are investigated. The channel impulse response is estimated via two approaches: a least-squares method which ignores channel structure, and a subspace technique which finds channel sparseness and then exploits this structure for final channel estimation. Symbol and frame synchronization is also accomplished using least squares methods. Performance is investigated through the evaluation of mean-squared channel estimation error and probability of error with a RAKE receiver employing the channel estimates for an indoor wireless channel. It is observed that the subspace method exploiting the clustered property of the channel yields the best performance at the expense of complexity. GLOBECOM

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