Code Optimization for Fast Chirp FMCW Automotive MIMO Radar

Fast chirp FMCW MIMO radar with inter-chirp coding provides high emission power by utilizing simultaneous antennas transmissions. However, the coding produces high side-lobes in the Doppler and angle dimensions of the radar's ambiguity function. The high side-lobes may cause miss-detection due to masking between targets that are at similar range and have large received power difference, as is often the case in automotive scenarios. In this paper, we develop a novel code optimization method that attenuates the side-lobes of the radar's ambiguity function and hence improves the probability of detection in the case of multiple targets. We formulate an analytical expression for the ambiguity function side-lobes, which takes into account that the targets range difference migrates during the detection interval, and that the migration is proportional to the targets Doppler frequency difference. We then develop an optimization algorithm that finds the code that minimizes the ambiguity function side-lobes average energy. The optimized code attenuates the ambiguity function side-lobes at the Doppler frequencies that are near the main lobe frequency. These frequencies correspond to targets with small speed difference, for which the range difference between the targets does not change significantly during the detection interval, and hence overlapping in range can cause severe interference. We demonstrate by simulation that the optimized code achieves an improvement in the ambiguity function side-lobes attenuation and an improvement in the probability of detection compared to reference codes.