Adaptive noise processing of a virtual beam formed radar

An adaptive filter specifically designed to process noise phase-shift keyed waveforms is demonstrated to replicate the least squares estimate of the scattering scene to within the error of the least squares estimate. Thus, the clutter (or multiplicative noise) is statistically no worse than the clutter produced by solving the least squares problem while the computational complexity is greatly reduced. The filter is derived using the statistical properties of the noise waveform, and is, therefore, tuned for this waveform. One benefit of the filter presented here over other adaptive filters is the lack of a step-size parameter. The ideal choice of the step-size parameter requires knowledge of the SNR. Without a priori knowledge of the radar cross section, the step-size parameter is misestimated causing traditional adaptive filters to converge slowly, or worse, diverge. This new adaptive noise filter (ANF) has been demonstrated in a simulated Multiple Input, Multiple Output (MIMO) architecture employing a set of mutually orthogonal quadraphase noise waveforms (QPN). The transmit and receive elements are collocated to form a single dense array. The orthogonality of the waveforms results in a transmitted beam which is unfocused, covering a wide region. The ANF is used to simultaneously isolate the return by transmit element and to range compress the return. Next, virtual beam forming (VBF) processing is applied to realize both the transmit and the receive antenna gain and beam steering. This gain is compared with the result of a traditional electronically steered array (ESA) employing a single waveform and verified to be identical [1]. The signal to interference noise ratio (SIR) is quantified in the range dimension as a function of the number of waveforms employed, the number of chips processed, and size of the range swath. It was found that the SIR in the VBF output is not adversely dependent on the number of simultaneous transmitters. The number of simultaneous transmitters and, thus, the illuminated region can, therefore, be increased constrained by neither additive nor multiplicative noise.