We will review prior work in the area of channel estimation for wide band channels such as ultrawideband radio and underwater acoustic systems. In such systems, individual multipath can often be resolved resulting in channels with some structure. Sparse and clustered channel estimation strategies will be considered. In some cases, however, experimental propagation data suggests that such wideband channels are more complex. Thus, herein we also propose a novel channel model based on both diffuse and sparse components. Tailored to this hybrid model, channel estimators are designed for different scenarios which differ in the amount of side information available at the receiver. Additionally, an Expectation-Maximization based algorithm to estimate the power delay profile of the diffuse component is proposed. The proposed channel estimation methods are compared to unstructured and purely sparse estimators. The numerical results show that the new channel estimation schemes considerably improve the estimation accuracy and the bit error rate performance over conventional channel estimators. Further, a mean-squared error analysis of the proposed estimators is conducted in two asymptotic regimes: high SNR and low SNR enabling a simple characterization and thus comparison of the proposed estimators.
