Multi-channel Waveform Agile Radar

Abstract

Recent advancements in Multiple-Input Multiple-Output (MIMO) radar techniques has created a paradigm shift in the overall radar technology to increase degrees of freedom in multi-function radar capabilities. The underlying principle of MIMO transmissions is to transmit independent waveforms from each antenna-element which do not interfere with other transmitted signals and establish wide illuminations which create multiple received signals back-scattered from the same target under observation to provide more information on the target aspects. One other principle is called colored transmission, by simultaneously radiating specific waveforms from each antenna-element/sub-array in different directions to achieve ‘space-time coding’. This can be explained as colored spatial distribution (multiple coded beams to probe the radar environment) instead of the white spatial distribution (single wide beam), such that the transmitted signals are now function of time as well as space. Recently, a novel multi-channel waveform agile demonstrator namely ASTAP (Advanced Space-Time Adaptive Processing) radar system has been designed and developed in Microwave Sensing, Signals and Systems (MS3) group. It consists of eight transmit channels with a single receive channel and hence can also be called as co-located Multiple-Input Single-Output (MISO) radar. The ASTAP radar system is capable of generating and transmitting independent waveforms via multi-channel Arbitrary Waveform Generator(AWG) simultaneously . However, the transmission of different coded waveforms with a radar system such as ASTAP demonstrator has some major challenges to be addressed first. These challenges include the impact of AWG on digital-domain generation of waveforms including limited-bits quantization errors, time- and phase-skew ( and/or jitter), influence of high-frequency up-conversion hardware components such as RF mixers (as non-linear devices) and antenna dispersion effects. First novelty of this thesis work is to investigate the influence of these system imperfections on waveform transmit ambiguity functions, waveform orthogonality in MISO transmissions, received signals separation and beamforming process for the synthesis of target azimuth distributions. It follows that an end-to-end system-level calibration to compensate these system imperfections on transmit side for different waveforms also serves as the second novelty of this thesis work and has been demonstrated with ASTAP radar system. For system performance analysis and beamforming applications, Over-the-Air (OTA) channel measurements have been done to compensate the ASTAP hardware distortions significantly and obtain the near-ideal waveform responses. Furthermore, it is investigated that to what extent it is possible to separate signals corresponding to each transmit channel from the composite received signal in a single receive channel. This study is extended further to generate and transmit two orthogonal beams occupying the same frequency band simultaneously and the corresponding transmit radiation patterns are recovered from the composite received signal matched filtering. Finally, conclusions along with future aspects and recommendations have been discussed.