Next-generation radar sensors require imaging capabilities with high angular resolution. As for a single sensor, the aperture, and thus the achievable resolution, is limited due to the constraints of the front end, radar networks consisting of multiple sensors are a possible solution. However, their incoherency usually makes joint angle estimation impossible. This article presents a network concept consisting of an orthogonal frequency-division multiplexing (OFDM) radar and repeater elements, which receive the reflections from targets and retransmit them back to the radar. Thereby, any frequency conversion from radio frequency to baseband and vice versa is omitted such that the signal remains coherent to the initial transmit signal. To distinguish the bistatic signal transmitted by the repeater from the monostatic one of the OFDM radar, the orthogonal subcarrier structure of OFDM waveforms is exploited by combining a sparse radar transmit signal with a low-frequency modulation in the repeater. This allows to evaluate the bistatic signals at the radar with standard multiple-input-multiple-output (MIMO)-OFDM signal processing, leading to separate range-Doppler images for each virtual channel. Finally, it is shown that this method offers a coherent angular estimation based on the extended aperture of the network. For this purpose, a method to establish phase coherency by a reconstruction of the modulation phase is presented. The network concept is proved with measurements at 77 GHz.

In order to simulate realistic traffic scenarios, a radar target simulator must be able to generate multiple radar targets with different directions of arrival. The presented concept is able to generate an arbitrary amount of targets with individual directions of arrival for the radar under test. By measuring the radar channel, the novel approach enables target simulators to simulate arbitrary directions of arrival, while minimizing the required hardware. The optimum setup is derived for radars with a uniform linear receive antenna array. The compensation of placement errors for automotive chirp-sequence frequency modulated continuous wave radars is demonstrated. Finally, the calibration for the setup is provided, and the performance of the presented approach is validated.INDEX TERMS Automotive radar, chirp-sequence modulation, direction of arrival, FMCW radar, radar target simulator.This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

A coherent sensor network without hardware link between the network nodes can be set up based on a single MIMO-radar and several incoherent repeater elements spatially distributed. The repeater sends back the radar signal on the same way it was impinging onto the repeater. All repeater elements have an integrated double-sideband modulator, which allows to separate the signals of different repeater elements at the radar. For angle estimation, the phases of the signals of the repeater elements are crucial, but due to an unknown initial phase of the modulation signal, they cannot be determined directly. Thus, a reconstruction of the modulation phase is necessary for network-based coherent angle estimation. In this paper, a concept for phase reconstruction in such type of networks is proposed, exploiting both sidebands of the modulated signal of the repeater elements.

Due to its noise-like signal property, digital orthogonal frequency-division multiplexing (OFDM) radars are often assumed to be robust against interference. While a lot of research has been carried out for interference between different modulation schemes, the mechanisms of interference from OFDM to OFDM radars have been barely addressed. This paper provides a thorough analysis of mutual OFDM to OFDM interference based on radar measurements using a 4x4 77 GHz multiple-input multiple-output (MIMO) OFDM radar prototype. The effects of interference are described both qualitatively and quantitatively for cyclic-prefix and stepped-carrier OFDM. Second, it is shown that conventional mitigation methods in the spectrogram are not suitable due to the random coding of cyclic-prefix OFDM. As an alternative, the application of adaptive beamforming is proposed and two realization possibilities are provided. Finally, new mitigation strategies in the modulation domain are proposed. They allow to shape interference to specific range-Doppler cells, yielding an interference-free range-velocity map for the area of interest. Additionally, the method may be used as the basis to enable simple conventional interference mitigation strategies.

Multiple sensors in a radar network can be jointly evaluated to process the bistatic Doppler information of a target obtained under different aspect angles. This enables the estimation of the target's vectorial velocity in a single snapshot. As a precursor to the joint evaluation, the monostatic and bistatic Doppler components need to be matched for each target in the range-Doppler map. In this work, a new approach for the target matching in the case of a radar-repeater network is presented. The matching is performed based on the correlation of monostatic and bistatic steering vectors extracted from the range-Doppler map for each receive channel of the radar. After validating the matched responses, the vectorial target velocity is determined. The approach is successfully tested with a 77 GHz radar and two repeaters by measurements of rotating cylinders.