Showing papers by "Milica Stojanovic published in 2000"

Underwater acoustic networks

Abstract: With the advances in acoustic modem technology that enabled high-rate reliable communications, current research focuses on communication between various remote instruments within a network environment. Underwater acoustic (UWA) networks are generally formed by acoustically connected ocean-bottom sensors, autonomous underwater vehicles, and a surface station, which provides a link to an on-shore control center. While many applications require long-term monitoring of the deployment area, the battery-powered network nodes limit the lifetime of UWA networks. In addition, shallow-water acoustic channel characteristics, such as low available bandwidth, highly varying multipath, and large propagation delays, restrict the efficiency of UWA networks. Within such an environment, designing an UWA network that maximizes throughput and reliability while minimizing the power consumption becomes a very difficult task. The goal of this paper is to survey the existing network technology and its applicability to underwater acoustic channels. In addition, we present a shallow-water acoustic network example and outline some future research directions.

Hypothesis-feedback equalization for direct-sequence spread-spectrum underwater communications

Abstract: Direct-sequence code-division multiple-access is considered for underwater acoustic communication networks. Unlike in the majority of spread-spectrum radio systems, intersymbol interference cannot be neglected, and time variability of the channel requires that receiver adaptation be performed at the chip, rather than the bit rate. Adaptive decision-feedback equalization, which has successfully been used for single-user underwater communications, is not directly applicable to spread-spectrum signals because of the delay in the despreading process and the lack of reliable chip decisions. To overcome this problem, a receiver is proposed which feeds back hypothesized, rather than the actual decisions. Numerical examples demonstrate the receiver's ability to cope with time varying channel distortions and preserve the processing gain when conventional, symbol-rate adaptive methods fail.

Towards robust adaptive acoustic communications

Abstract: Modulation and detection techniques used for underwater acoustic communications include phase coherent (PSK and QAM) and noncoherent (FSK) techniques. The objective of this work is the development of algorithms that can select the best technique for a given channel. The system of interest operates in shallow water over several kilometers in the 10-15 kHz band. A wideband probe is used at the beginning of each data packet to measure the multipath and the Doppler spread, as well as the SNR. The choice of modulation is based on these measurements. The spread factor of the channel determines whether phase coherent communications are possible. If so, an equalizer is employed to combat any intersymbol interference. If the channel varies too rapidly, noncoherent signaling is chosen. Guard intervals are used to eliminate the effect of multipath. Computer simulations and theoretical analysis are presented to quantify the performance of candidate modulation methods for varying channel parameters.

Differentially coherent diversity combining techniques for DPSK over fast Rayleigh fading channels

Abstract: The performance of differential phase-shift keying (DPSK) is considered for fading channels with nonnegligible time-variation, as well as signal or noise correlation between diversity branches. A differentially coherent maximum-ratio combiner is derived, showing optimal weight dependence on the fading statistics. The bit error rate (BER) is evaluated analytically for Rayleigh fading, and comparison is made between the optimal and the equal-gain differentially coherent combiner. The optimal combiner provides improvement in all situations, and reduces the error probability floor. Performance improvement is higher for higher Doppler spreads, with the exact amount depending on diversity correlation. For low diversity correlation, equal-gain combining performance stays close to optimal.

Survey and analysis of underwater acoustic channels for coherent communication in the medium-frequency band

Abstract: A comprehensive set of acoustic communication data is analyzed using a complete receiver algorithm which includes a Doppler pre-processor, equalizer configuration algorithm and the decision feedback equalizer (DFE). The signal used for the tests is 1250 symbol per second QPSK (2500 bps) transmitted at a carrier of 2.25 kHz. Source-receiver ranges of up to 44 km were achieved at this data rate. The test environments include the New England Continental Shelf (less than 200 m), the shelf break (200-1200 m) and deep water (2500 m). In each environment the impulse response, the ambiguity function and the performance level of the receiver are presented. The link was very reliable to the maximum test range of 44 km on the shelf and to 20 km at the shelf break. In deep water the performance was good at less than 10 km and also at 35-44 km.