Emerging digital radar concepts such as orthogonal frequency-division multiplexing (OFDM) allow flexible signal generation. This opens up new opportunities in waveform design in a multiple-input multiple-output (MIMO) system such as introducing coding for signal multiplexing. In this article, coded MIMO OFDM waveforms are proposed and investigated that allow continuous and simultaneous wideband transmission for all transmitters of a multiple transmit and receive antenna array for spatial radar environment perception. Challenges for coded MIMO OFDM radar operation are derived, and three coded MIMO strategies are introduced and analyzed. Their potential is validated and compared to the standard subcarrier interleaving OFDM approach using simulations and measurements of an experimental 4×4 MIMO OFDM radar at 77 GHz.
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.
Flawless operation even with a high density of radar sensors in the vicinity is an essential prerequisite for radar sensors in automotive applications, safety systems, future autonomous driving, and in industry applications. Recent research shows the great potential of digitally modulated radars such as orthogonal frequency-division multiplexing (OFDM). Furthermore, novel frequency agile OFDM approaches such as stepped and sparse OFDM are superior to standard OFDM as they are able to achieve high resolution beyond the hardware-related band limits. In order to prevent possible mutual interference issues of common and novel modulation schemes, the influence of a state-of-the-art FMCW on stepped and sparse OFDM is examined in this paper. The effects of interference are investigated theoretically and through measurements using an experimental OFDM setup at 76 GHz and scenarios with up to two FMCW interferers. To prevent signal deterioration and limited detection performance due to interference, an efficient but low-cost detection and mitigation approach is presented for each of the two approaches.
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