Adaptive postbeamformer interference canceler with improved performance in the presence of broadband directional sources

Abstract

A postbeamformer interference canceler (PIC) is a beam space processor that processes the signals derived from an antenna array by forming two beams using fixed beamforming weights. One beam, referred to as the signal beam, is formed to have a fixed response in the known signal direction. The processed output of the second beam, referred to as the interference beam, is subtracted from the output of the signal beam to form the output of the PIC. In the presence of broadband directional sources, the formation of a beam is usually carried out by a set of steering delays followed by a weight and sum network. For this case, the processing of the interference beam output is carried out by a tapped delay line filter. When the two beams of the PIC are formed using conventional weights and the tapped delay line filter weights are adjusted to minimize the mean output power of the PIC, a substantial amount of the signal gets suppressed. It is due to the presence of signal component in the interference channel. This paper considers a frequency domain structure of the PIC (FDPIC) in the presence of broadband directional sources; derives conditions required for the performance of the FDPIC to be equal to the performance of the time-domain structure of the PIC (TDPIC); presents an algorithm to calculate the fixed weights of the two beams at each frequency bin such that the FDPIC maintains a specified frequency response in the signal direction and its optimized performance is better than that of the TDPIC using conventional beam weights; and suggests a scheme to select a gradient step size, which allows the misadjustment of the convergence time constant of the iterative gradient algorithm to be specified.