A Frequency-Division MIMO FMCW Radar System Based on Delta-Sigma Modulated Transmitters

In this paper, we present a hardware-efficient method, which allows to implement multiple-input multiple-output (MIMO) radars with a separation of the transmit (TX) signals in the frequency domain. The proposed architecture uses binary phase-shift keying (BPSK) modulators within the TX paths of a multi-channel frequency-modulated continuous-wave (FMCW) radar. These BPSK modulators are driven by 1-bit sequences, which are generated using Delta Sigma-modulators to shift the quantization noise towards high frequencies. Thus, neither multi-bit digital-to-analog converters, nor vector modulators or phase shifters are required for the proposed approach. Since the FMCW principle relies on the evaluation of beat frequencies within a narrow frequency band, the shaped quantization noise is outside the frequencies of interest and can thus be filtered out. The chosen hardware setup is optimized for integration into monolithic microwave integrated circuits (MMICs), which is demonstrated by a prototype system based on 77-GHz chips. Several MMICs are combined to realize a radar frontend with six TX and eight receive channels resulting in 48 MIMO channels. A signal-processing approach is derived and a method to reduce ambiguities is presented. Measurements demonstrate a range resolution of 15 cm and a 3-dB beamwidth of 3 degrees with position standard deviations better than 1 mm. Furthermore, a measurement example with a target placed outside the unambiguous range of the radar shows an 8-dB power reduction of the unwanted signal from the target at the ambiguous distance. This improvement is achieved by choosing nonidentical frequency spacings for the TX signals.