Timed Arrays

8ADAPTIVE ARRAYS

An adaptive array improves signal detection by modifying its antenna pattern characteristics in response to environmental or operational changes [1]. Adaptive arrays are timed arrays, since the array performance changes with time. This chapter begins with the development of the signal correlation matrix which is at the heart of direction finding and adaptive nulling algorithms. Since these algorithms primarily rely upon complex and expensive digital beamforming (DBF), some other cheaper alternative adaptive nulling approaches are presented. Next, reconfigurable arrays are introduced. They use mechanical and electrical switching mechanisms to remake the array and alter its performance in a desirable way. Finally, the chapter ends with an overview of several other adaptive arrays that prove useful in various systems.

Adaptive arrays originated in the 1950s with the invention of the Van Atta retrodirective array [2]. Shortly afterward, Howells proposed a method of steering a null in the direction of a jammer [3] in order to improve signal reception. Howells and Applebaum successfully tested a five-loop sidelobe cancelling array, but the information was not reported for many years [4]. The next major contribution was the least mean square (LMS) algorithm, and it caught on very quickly [5]. This approach became the canonical adaptive algorithm from which many variations have sprouted.

Adaptive arrays sense the presence of interference then place nulls in the directions ...