Principles of Adaptive Array Processing

In this lecture we present the principles of adaptive beamforming, the problem of estimating the adaptive weights and several associated practical problems, like preserving low sidelobe patterns, using subarrays and GSLC configurations. We explain the detection problem with adaptive arrays and the methods for angle estimation. Finally the methods for resolution enhancement (super-resolution methods) are presented. 1.0 INTRODUCTION – WHY DO WE NEED ADAPTIVE BEAMFORMING By adaptive beamforming we mean a data dependent modification of the antenna pattern. We are talking here about two types of data: the training data from which we calculate the adaptive weightings and the primary data with which detection and parameter estimation (angle, range, Doppler estimation) is performed. In this section we consider spatial pattern forming only. However, our description is more general to allow the extension to space-time adaptive processing to be developed in the following lectures. First, one may ask if adaptive beamforming against spatial interference sources is necessary at all, if we can create an antenna with very low sidelobes. A low sidelobe antenna is a fixed spatial filter that does not need training and hence has no adaptation problems. However, the overall reduction of the sidelobes implies an increase of the antenna beamwidth. This effect is not present if single nulls in the directions of the interference are formed. Often the detection and estimation of a target in the vicinity of the interference is of interest (from a jamming point of view co-locating the jammer and the target to be protected is most efficient). Therefore the increased beamwidth would be counter-productive. Figure 1 shows the significant loss of radar visibility with a -40 dB low sidelobe antenna (the 902 element generic array of Lecture 1 with –40 dB Taylor weighting was chosen). Not only is the sidelobe level insufficient to suppress the jamming power, but the broad main beam also makes the antenna blind in the whole sectors with closely spaced jammers. The remarkable property of adaptive antennas is that they can suppress jammers on the skirt of the main beam such that the radar can look through between two neighbouring jammers. Adaptive antennas can only suppress interference that was measured in the training data. To be protected against surprise interference or insufficiently measured interference a compromise solution using a moderate low sidelobe antenna with simultaneous adaptive beamforming is often sought. Nickel, U. (2006) Principles of Adaptive Array Processing. In Advanced Radar Signal and Data Processing (pp. 5-1 – 5-20). Educational Notes RTO-EN-SET-086, Paper 5. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.asp. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 01 SEP 2006 2. REPORT TYPE N/A 3. DATES COVERED 4. TITLE AND SUBTITLE Principles of Adaptive Array Processing 5a. CONTRACT NUMBER