Array processing involves manipulation of signals induced on various antenna elements. Its capabilities of steering nulls to reduce cochannel interferences and pointing independent beams toward various mobiles, as well as its ability to provide estimates of directions of radiating sources, make it attractive to a mobile communications system designer. Array processing is expected to play an important role in fulfilling the increased demands of various mobile communications services. Part I of this paper showed how an array could be utilized in different configurations to improve the performance of mobile communications systems, with references to various studies where feasibility of apt array system for mobile communications is considered. This paper provides a comprehensive and detailed treatment of different beam-forming schemes, adaptive algorithms to adjust the required weighting on antennas, direction-of-arrival estimation methods-including their performance comparison-and effects of errors on the performance of an array system, as well as schemes to alleviate them. This paper brings together almost all aspects of array signal processing.

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Application of antenna arrays to mobile communications. II. Beam-forming and direction-of-arrival considerations

Real-time ultrasonic imaging systems have been available for more than thirty years. During this time much has occurred to the basic architecture and functions of these clinical systems and their beamformers, which are, in many ways, the most important components of these systems. This talk will review some of the changes that have occurred and will discuss current trends in beamformer design. Throughout most of the 30 years of real time imaging, analog beamformers have been the mainstay of all instruments. The common methods for the implementation of analog beamformers for annular, linear/curved arrays, and phased arrays will be reviewed and the distinguishing characteristics identified. At the present time the industry is undergoing a major shift toward digital beamformation with the introduction of several commercial systems. Given that the earliest digital systems were available roughly 15 years ago, it is fair to say that the introduction of digital beamformers has been relatively slow. Today this shift seems to be gaining momentum. Some of the factors causing these shifts will be reviewed along the the common types of designs and implementations. The ongoing search for improved imaging performance will continue to introduce new challenges for beamformer designers. Among already proposed imaging methods and techniques are elevation focusing (1.5D arrays), synthetic apertures, 2D and sparse arrays, phase aberration correction, and others. The most common complication introduced by these is a significant increase in channel count. Further, there will also be beamformer design issues related to signal processing. These include not only Doppler processing but also the processing of signals due to novel contrast agents. Finally, with advances in computer and microelectronics technologies, the way we view beamformation may have to undergo sizeable changes. These topics will be reviewed not only as technical challenges but also in light of the constraints introduced by today's marketplace.

Evolution of ultrasound beamformers

There are many new challenges to be faced in implementing signal processing algorithms and designing energy-efficient DSPs for microsensor networks. We study system partitioning of computation to improve the energy efficiency of a wireless sensor networking application. We explore system partitioning between the sensor cluster and the base station, employing computation-communication tradeoffs to reduce energy dissipation. Also we show that system partitioning of computation within the cluster can also improve the energy efficiency by using dynamic voltage scaling (DVS).

Energy-efficient DSPs for wireless sensor networks

Photoacoustic signal generation can be used for a new medical tomographic technique. This makes it possible to image optically different structures, such as the (micro)vascular system in tissues, by use of a transducer array for the detection of laser-generated wide-bandwidth ultrasound. A time-domain delay-and-sum focused beam-forming technique is used to locate the photoacoustic sources in the sample. To characterize the transducer response, simulations have been performed for a wide variety of parameter values and have been verified experimentally. With these data the weight factors for the spectrally and temporally filtered sensor signals are determined in order to optimize the signal-to-noise ratio of the beam former. The imaging algorithm is investigated by simulations and experiments. With this algorithm, for what is to our knowledge the first time, the three-dimensional photoacoustic imaging of complex optically absorbing structures located in a highly diffuse medium is demonstrated. When 200-mum-diameter hydrophone elements are used, the depth resolution is better than 20 mum, and the lateral resolution is better than 200 mum, independent of the depth for our range of imaging (to ~6 mm). Reduction of the transducer diameters and adaptation of the weight factors, at the cost of some increase of the noise level, will further improve the lateral resolution. The synthetic aperture algorithm used has been shown to be suitable for the new technique of photoacoustic tissue scanning.

Image reconstruction for photoacoustic scanning of tissue structures.

A microphone array speech enhancement algorithm based on analysis/synthesis filtering that allows for variable signal distortion. The algorithm is used to suppress additive noise and interference. The processing structure consists of delaying the received signals so that the desired signal components add coherently, filtering each of the delayed signals through an analysis filter bank, summing the corresponding channel outputs from the sensors, applying a gain function to the channel outputs, and combining the weighted channel outputs using a synthesis filter. The structure uses two different gain functions, both of which are based on cross correlations of the channel signals from the two sensors. The first gain yields the GEQ-I array, which performs best for the case of a desired speech signal corrupted by uncorrelated white background noise. The second gain yields the GEQ-II array, which performs best for the case where there are more signals than microphones. The GEQ-II gain allows for a trade-off on a channel-dependent basis of additional signal degradation in exchange for additional noise and interference suppression.

Analysis/synthesis-based microphone array speech enhancer with variable signal distortion

Time domain and frequency domain concepts which aid in the design and efficient implementation of a digital beamformer have been described at various times in the literature. The numerous beam-former structures that result are discussed with an emphasis on hardware requirements and spectral areas of application. Time domain procedures which include delay-sum, partial-sum, interpolation and shifted-sideband beamforming, and frequency domain techniques which include the application of discrete Fourier transforms and phase shift beam-forming are considered. Hardware considerations are primarily in the areas of analog-to-digital conversion, data storage, and computational throughput requirements.

R. Mucci

Papers

A comparison of efficient beamforming algorithms