Improvement of Hybrid Codes for MIMO radar using multi-pulse waveforms and signal processing

Abstract

According to literature, MIMO radars often use orthogonal waveforms on their different channels to achieve a wide angular beam and so-called colored transmission. The generation of orthogonal signals is very difficult and true orthogonality cannot be achieved in practice. The Circulating Codes provide a simplified alternative to orthogonal signals, by transmitting the same waveform, but slightly shifted in time from channel to channel. By applying digital beamforming on transmit through signal processing on receive, a performance similar to orthogonal signals can be achieved. The Circulating Codes can also be used with a spatial code along the antenna elements, to improve the auto-correlation properties of the signal and thus the range resolution of the radar. These codes are called Hybrid or Delft Codes.

In this thesis, the Circulating Codes, as well as Hybrid Codes are revisited and analyzed again. An implementation for simulations has been created in Matlab to examine their behavior in more detail. To improve the range resolution of the Hybrid Codes, this thesis proposes the use of so-called Golay pairs as spatial codes. Each Golay pair consists of two codes, which have the property that the sum of their auto-correlation functions produces one strong peak and zero sidelobes. Since the application of these codes requires the transmission of two pulses, the phase shift due to the displacement of a moving target in between of the pulses has an impact on the result. This thesis focuses on the mitigation or correction of this phase shift, by the use of different methods and presents two possible solutions which perform well in simulations.

Finally, some of the techniques are applied in practical measurements with an available radar system. The results of these measurements show that the single-pulse techniques of the Circulating and Hybrid Codes perform well with the system, while the multi-pulse techniques with Golay pairs suffer from much higher sidelobes than expected from the simulations. First of all, this shows that the system has certain issues that need to be improved. Secondly, it shows that Hybrid Codes with Golay pairs are sensitive to disturbances and phase shifts. Therefore, very clean signals are required that might be difficult to generate in practice.