Spectral method for ACF side lobes suppression for long pseudo-random binary sequences

Аuthors

Lyalin K. S.^*, Khasanov M. K.^**, Meleshin Y. M.^***, Kuzmin I. A.^****

National Research University of Electronic Technology "MIET", 1, Shokin Square, Zelenograd, Moscow, 124498, Russia

*e-mail: ksl@miee.ru
**e-mail: khaes@yandex.ru
***e-mail: i@imym.ru
****e-mail: kuzmin.ilya.al@gmail.com

Abstract

Signals with internal modulation are employed in aviation and space remote sensing systems to obtain a high-quality radar image. This approach allows reaching high image resolution preserving the high signal to noise ratio. The main disadvantage of this approach is sufficient growth of side lobe level. This drawback occurs in a radar image as bright strips that spread out of point or group targets along the range axis. This effect degrades sufficiently the ability to sense the image by human eye and automatically process the image. A typical dynamic range of radar targets brightness is 90 dB, wherein the best side lobe level, which has been achieved in this work is −35 dB by using of phase-shift keying signal of base 2000 and with matched range filtering.

One way to suppress side lobes is unmatched or quasi-optimal filtering, in which the core of the matched filter is changed to achieve desired optimum characteristics, with some loss of effective signal energy. In this work, a method for obtaining the coefficients of a quasi-optimal filter is proposed. The filter allows completely suppress the side lobes in the survey band by pushing them out of the visible radar image range.

The method uses a simple approach of transforming the desired form of the signal response fr om time to frequency domain, wh ere it is possible to find the relation between the input and desired signals, performing the “reciprocal convolution” which corresponds to the quasi-optimal filter impulse response. The main issue here is the difference between the amount of points in the input and output signals, as the output has to be the bigger length of impulse response. We solve the issue by adding additional points to the input signal, which result in a noise-like signal at the output signal’s edges. By selecting the appropriate amount of the “dummy” points we are able to establish the sidelobe-clean region in the output signal.

The article presents theoretical details as well as simulated and practical results.

Keywords:

phase-shift keying, autocorrelation function, side lobes, quasi-optimal filtering, side lobes suppression, Fast Fourier Transform, radar image, synthetic aperture radar, airborne and space radio photography

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