Effect of Spatial Decorrelation on Nulling Performance of Linear Adaptive Array

Abstract

Abstract : The presence of a disturbed ionosphere can cause signal decorrelations in time, frequency, and space. This report is concerned with the impact of spatial decorrelations on the performance of an adaptive array used to null interfering sources and jammers. Attention is concentrated on the effect of spatial decorrelation between spaced broadbeam antenna elements rather than decorrelation across individual antenna apertures. While fully adaptive arrays are considered, more analysis and evaluation is carried out for the sidelobe canceller (SLC) configuration. Complex Gaussian channel fluctuations are assumed for the disturbed ionosphere. Three types of output SNR analyses are conducted: non-fading, fading with array time constants short enough to adapt to fading, and fading with array time constants too slow to adapt to fading. Most detailed analytical results are carried out for one or two antenna elements and slow fading time constants. However, some more general results on processing gain are obtained for a linear array of M SLC antennas and slow fading time constants where processing gain is defined here as the ratio of output SNR with the array functioning to achieve optimum performance to output SNR with no adaptive array. Curves and simple analytic expressions are given relating processing gain to a suitable decorrelation parameter. The latter, in turn, is related to the rms angle of arrival fluctuatiions. Error rate expressions are developed for coherent and incoherent FSK, PS K, and DPSK modems when used in conjunction with an SLC adaptive array.