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Recent advances in digital hardware and processing power have enabled the realization of novel digital radars that are superior to radars commercially available today in flexibility and performance. This opens up new opportunities in radar waveform design, modulation, and high-performance signal processing that were previously unfeasible. Probably the most known digital radar modulation concept is OFDM, which has attracted much interest in radar research in the last two decades. The goal of this thesis is to explore new possibilities for OFDM radar waveform design and high-performance digital processing to make future digital radars even more powerful and robust. To this end, the developed novel waveform and signal processing concepts take advantage of the high degree of flexibility and diversity provided by OFDM to overcome previous limitations in radar performance, to enable new MIMO concepts, to enhance radar imaging performance, and to enable robust radar sensing in coexistence with other radars.

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., publishedVersion

Recent publications show that the potential of using orthogonal frequency division multiplexing waveforms is radar signals. Since the range resolution is proportional to the RF bandwidth, the major obstacle that obstructs the practical use in automotive and other low-cost radars is the requirement to sample the received signal at sampling rates that span the whole RF signal bandwidth requiring ADCs with sampling rates in the order of GHz. This paper presents a method to achieve the high range resolution induced by a large RF bandwidth, but with a much lower baseband bandwidth, consequently requiring a much slower ADC while at the same time delivering a velocity profile for each subcarrier. In addition, the processing scheme induces a range migration compensation, independent of the number of targets. This is achieved with barely increased computational effort. The scheme is verified with simulations and measurements at 77 GHz., acceptedVersion

Orthogonal frequency-division multiplexing (OFDM) is a promising candidate for future digital-centric radar systems. One major limitation for practical implementations is the required sampling rate in the order of GHz, which can be solved by using a stepped-carrier technique in combination with OFDM signals of smaller bandwidth, theoretically resulting in the same resolution as a conventional OFDM scheme. However, a signal source capable of generating such a carrier has to fulfill several constraints. In this paper, the effects of deterministic and random inaccuracies of the carrier generation as well as effects of phase noise are described and evaluated. A transient model that describes the switching between carriers is given and all effects are verified using a radar demonstrator at 77 GHz., acceptedVersion