Showing papers by "Milica Stojanovic published in 1995"

Sparse equalization for real-time digital underwater acoustic communications

Abstract: Due to the very long reverberation time of many ocean channels, the size of the adaptive filters required for conventional equalization becomes large, rendering the computational complexity of the adaptive receiver unacceptable for many cases of practical interest. To overcome this problem we exploit the natural sparseness of the reverberation pattern. By focusing only on those intervals which contain a significant portion of the signal energy, the sparse equalization method provides data detection using a minimum complexity adaptive receiver subject to an upper bound on the signal estimation error. Experimental results demonstrate an order of magnitude reduction in computational complexity with a negligible loss in performance.

Analysis of the impact of channel estimation errors on the performance of a decision-feedback equalizer in fading multipath channels

Abstract: A coherent receiver with a decision-feedback equalizer (DFE) operating on a Rayleigh fading channel under a suitable adaptive algorithm is considered. In the analysis of a DFE, a common assumption is that the receiver has perfect knowledge of the channel impulse response. However, this is not the case in practice, and for a rapidly fading channel, errors in channel tracking can become significant. We analyze theoretically the impact of these errors on the performance of a multichannel DFE. The expressions obtained for the achievable average MPSK bit error probabilities depend on the estimation error covariance. In order to specify this matrix, we focus on a special case when a Kalman filter is used as an optimal channel estimator. In this case, the probability of bit error can be assessed directly in terms of channel fading model parameters, the most interesting of which is the fading rate. Our results show the penalty imposed by imperfect channel estimation, as well as the fading-induced irreducible error rates. >

Reduced‐complexity spatial and temporal processing of underwater acoustic communication signals

Abstract: Multichannel processing of high‐speed underwater acoustic communication signals requires computationally intensive receiver algorithms. The size of adaptive filters, determined by the extent of ocean multipath, increases with signaling rate and limits system performance through large noise enhancement and increased sensitivity of computationally efficient algorithms to numerical errors. To overcome these limitations, reduction in receiver complexity is achieved by exploiting the relationship between optimal diversity combining and beamforming. Under relatively simple conditions, two adaptive receivers, one based on diversity combining which does not rely on any spatial signal distribution, and the other based on optimal beamforming, are shown to achieve the same performance. The beamforming approach, however, leads to a receiver of lower complexity. Carrying these observations over to a general case of broadband transmission through an unknown channel, a fully adaptive receiver is developed which incorporates a multi‐input multi‐output, many‐to‐few combiner, and a reduced‐complexity multichannel equalizer. Receiver operations are optimized jointly to ensure minimum mean‐squared error performance of data detection. Results of processing experimental shallow water data demonstrate the capability to fully exploit spatial diversity of underwater multipath while keeping the complexity at an acceptable level.

Underwater acoustic communications

Abstract: In recent years, underwater acoustic (UWA) communications have received much attention as their applications are beginning to shift from military towards commercial. UWA communications are made difficult by the combined effect of multipath propagation and high temporal and spatial variability of the channel conditions. Until recently, the design of communication systems has mostly relied on the use of noncoherent modulation techniques. However, to achieve high data rates on the severely bandlimited UWA channels, bandwidth-efficient modulation techniques must be considered, together with array processing for exploitation of spatial multipath diversity. The new generation of underwater communication systems employing phase-coherent modulation techniques will achieve at least a ten fold increase in data throughput. The communication scenario in which the modern UWA systems will operate is that of an underwater network consisting of stationary and mobile nodes. Current research focuses on the development of efficient signal processing algorithms, multiuser communications in the presence of interference, and design of efficient modulation and coding schemes. >

Analysis of the Impact of Channel Estimation Errors on the Performance of a Decision-Feedback Equalizer in Fading

Abstract: A coherent receiver with a decision-feedback equalizer (DFE) operating on a Rayleigh fading channel under a suitable adaptive algorithm is considered. In the analysis of a DFE, a common assumption is that the receiver has perfect knowledge of the channel impulse response. However, this is not the case in practice, and for a rapidly fading channel, errors in channel tracking can become significant. We analyze theoretically the impact of these errors on the performance of a multichannel DFE. The expressions obtained for the achievable average MPSK bit error probabilities depend on the estimation error covariance. In order to specify this matrix, we focus on a special case when a Kalman filter is used as an optimal channel estimator. In this case, the probability of bit error can be assessed directly in terms of channel fading model parameters, the most interesting of which is the fading rate. Our results show the penalty imposed by imperfect channel estimation, as well as the fading-induced irreducible error rates. >

Multisensor multiuser receivers for time-dispersive multipath fading channels

Abstract: Receiver structures based on joint MMSE diversity combining, equalization and multiple access interference suppression are discussed. It is shown that receiver complexity can substantially be reduced by exploiting the structure of multipath. Experimental results, obtained in an underwater acoustic channel, demonstrate superior capabilities of the receivers proposed.