Book: Smart Antennas (Electrical Engineering & Applied Signal Processing Series)
Publisher: CRC Press
Smart antennas involve processing of signals induced on an array of sensors such asantennas, microphones, and hydrophones. They have applications in the areas of radar,sonar, medical imaging, and communications.Smart antennas have the property of spatial filtering, which makes it possible to receiveenergy from a particular direction while simultaneously blocking it from another direction.This property makes smart antennas a very effective tool in detecting and locating anunderwater source of sound such as a submarine without using active sonar. The capacityof smart antennas to direct transmitting energy toward a desired direction makes themuseful for medical diagnostic purposes. This characteristic also makes them very usefulin canceling an unwanted jamming signal. In a communications system, an unwantedjamming signal is produced by a transmitter in a direction other than the direction of thedesired signal. For a medical doctor trying to listen to the sound of a pregnant mother’sheart, the jamming signal is the sound of the baby’s heart.Processing signals from different sensors involves amplifying each signal before combiningthem. The amount of gain of each amplifier dictates the properties of the antennaarray. To obtain the best possible cancellation of unwanted interferences, the gains of theseamplifiers must be adjusted. How to go about doing this depends on many conditionsincluding signal type and overall objectives. For optimal processing, the typical objectiveis maximizing the output signal-to-noise ratio (SNR). For an array with a specifiedresponse in the direction of the desired signal, this is achieved by minimizing the meanoutput power of the processor subject to specified constraints. In the absence of errors,the beam pattern of the optimized array has the desired response in the signal directionand reduced response in the directions of unwanted interference.