Improved broadband adaptive beamformer performance

Abstract

The protection of wireless communications links against interference is a key concern in mission critical systems. In particular low probability of interception (LPI) systems which operate below the thermal noise floor of the receiver are particularly vulnerable. To protect against interference it is often necessary to include some form of active interference suppression. Broadband adaptive beamforming is one such technique which may be used to suppress interference by spatial and spectral filtering of the signals received by an array antenna. The hardware requirements of adaptive Beamforming systems are high in comparison to other approaches. As a result, the total number of adaptable weights realisable in the beamformer may be limited by the size, weight and power constraints of the system. This will limit the degrees of freedom in the beamformer and hence, the interference cancellation capability of the beamformer.

The effectiveness of increasing the number of time-taps in space-time adaptive processors (STAP) as a method of increasing the number of interferers the beamformer can simultaneously cancel in an environment containing a mix of narrowband, partialband and broadband interference is studied. An alternative scheme to free up degrees of freedom in the beamformer is proposed based on frequency-domain excision to pre-filter narrowband interference before it reaches the beamformer. This approach frees up degrees of freedom in the beamformer, which would otherwise be consumed by the narrowband interference, for use in cancelling partialband and broadband interference. To enable the excision filters to identify narrowband interference while allowing partialband and broadband interference to pass through, a novel scheme is presented which produces a frequency mask that varies on a per-bin basis. Eigenanalysis of the beamformer's covariance matrix is used to explore the ability of frequency-domain excision to desensitise the beamformer to narrowband interference while bit error rate (BER) simulations demonstrate the enhanced interference protection the scheme affords an LPI communications link.

Mismatches between the gain and phase responses of the radio-frequency (RF) front-end channels in broadband adaptive beamformers can limit their cancellation performance against higher power partialband and broadband interference. This performance limitation arises from the decorrelation experienced by the received signals across the array due to the interchannel mismatches. In STAP systems this performance limitation may be mitigated by increasing the number of taps per channel in the STAP. However, the computational complexity of adding additional time-taps tends to be high in STAP beamformers. Two new methods of interchannel mismatch compensation are proposed based on efficient frequency-domain methods. Simulations using software models and experiments using a hardware STAP system demonstrate the proposed techniques' ability to improve the cancellation performance where interchannel mismatches are limiting cancellation performance.

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