RANGE SIDE LOBE SUPPRESSION IN PHASE CODED RADAR APPLICATIONS.PDF

Abstract

In this thesis, we develop and investigate signal-processing algorithms for phasecoded Doppler radar. Different types of phase-coded sequences are assumed to be transmitted and subsequently received by a radar. The central issue is the range side lobe problem faced by such radars. These side lobes impede the range resolution capability and may sometime result in false alarms or masking of weak targets. Certain binary codes have desirable side lobe levels in this context. These specially include the well known pseudo noise (PN) sequence and not very commonly used Golay (complementary) codes. A comparison of the performance of these sequences is made with the other phase coding sequences used in ranging applications which include Barker, Kasami and Gold sequence. The performance comparison criteria include the correlation side lobe suppression properties, Doppler tolerance and noise rejection capability of these sequences. PN sequence is suitable for continuous and pulse radars, whereas the complementary Golay code pair leads to good solution in various pulsed radar topologies. However, using these sequences in radar applications require certain considerations. The foremost being selection of an appropriate transmission scheme. Development of relevant (optimal/suboptimal) signal processing algorithms follows. The ranging problem is essentially base-band detection and time of arrival iv estimation of phase-coded signals with unknown arrival time. Its solution is based on Neyman-Pearson criterion. Presence of Doppler shift converts ranging into a two dimensional detection problem. The net signal-processing algorithms are investigated, which constitute pulse integration onto the correct Doppler frequency through DFT/FFT followed by correlation with the phase coding sequence. The developed algorithms are essentially generalized likelihood ratio test (GLRT) detectors. Wherein, maximum likelihood estimate (MLE) of the delay corresponds to the target range and that of the Doppler shift to its speed. All the algorithms are demonstrated by computer simulations. In case of Golay code, the complementary code pair is transmitted on two consecutive phase coded pulses which have a fixed delay in their transmission. At the receiver, the two codes are processed separately and then added to achieve improved range resolution by exploiting the zero side lobe property of the complementary Golay code pair. Another suggestion in ranging applications is to use bi-carrier signal in which the two codes remain non-interacting on two closely separated orthogonal carriers. The solution exploits the orthogonality of the two carriers of the bi-carrier signal to separate the two codes. In case the pulse Doppler radar, the spacing between the two carriers of the bi-carrier signal also caters for the Doppler shift. A major contribution in the thesis is the mathematical analysis of the detection performance of the various discussed algorithms. The analytical detection performance of each developed algorithm is then validated by performing Monte-Carlo simulations for different signal to noise ratio (SNR) values. For non-orthogonal Doppler frequencies and non-integer delays, performance is evaluated using Monte Carlo simulations which degrades slightly. Computational complexity of each developed algorithm is worked out