The joint radar-communication (RadCom) concept has been continuously gaining interest due to the possibility of integrating radar sensing and communication functionalities in the same radio frequency hardware platform. Besides a number of challenges in terms of hardware design and signal processing, the choice of suitable modulation schemes plays a significant role in driving the performance of RadCom systems. In this sense, this article presents an overview of state-of-the-art modulation schemes for RadCom systems, namely, chirp sequence, phase-modulated continuous wave, orthogonal frequency-division multiplexing, and orthogonal chirp-division multiplexing. For each of them, a detailed system model is outlined, and parameters for quantifying both radar and communication performances are presented. Finally, a comparative analysis of the aforementioned RadCom modulation schemes is carried out to illustrate the presented discussion.

Automotive radar sensors play a vital role in the current development of autonomous driving. Their ability to detect objects even under adverse conditions makes them indispensable for environment-sensing tasks in autonomous vehicles. As their functional operation must be validated in-place, a fully integrated test system is required. Radar Target Simulators (RTS) are capable of executing end-of-line, over-the-air validation tests by looping back a received and afterward modified radar signal and have been incorporated into existing Vehicle-in-the-Loop (ViL) test beds before. However, the currently available ViL test beds and the RTS systems that they consist of lack the ability to generate authentic radar echoes with respect to their complexity. The paper at hand reviews the current development stage of the research as well as commercial ViL and RTS systems. Furthermore, the concept and implementation of a new test setup for the rapid prototyping and validation of ADAS functions is presented. This represents the first-ever integrated radar validation test system to comprise multiple angle-resolved radar target channels, each capable of generating multiple radar echoes. A measurement campaign that supports this claim has been conducted.

Orthogonal chirp-division multiplexing (OCDM) has been recently proposed as an alternative scheme to orthogonal frequency-division multiplexing (OFDM) for data communication. Among the reasons for that are its superior robustness, e.g., to frequency selectivity of channels and intersymbol interference. Furthermore, channel estimation in the discrete-Fresnel domain has been investigated for both communication and sensing, without, however, considering the effects of frequency shifts. This article investigates the discrete-Fresnel domain channel estimation in OCDM-based radar systems as an alternative to the correlation-based processing from the conventional OCDM (C-OCDM) radar-communication (RadCom) system, which yields high, transmit-data-dependent sidelobe level. In this context, a mathematical formulation for the investigated channel estimation approach is presented. In addition, extensions to multiuser/multiple-input–multiple-output (MIMO) and to a sector-modulated OCDM (SM-OCDM) RadCom operation are introduced. Finally, the performance of the proposed schemes is analyzed based on simulation and measurement results for an emulated automotive scenario. In summary, both OCDM-based radar and SM-OCDM RadCom systems yield improved peak-to-average power ratio and comparable or improved radar performance in terms of sidelobe level and Doppler shift robustness to their C-OCDM and OFDM counterparts for practical range and velocity values. In addition, equal communication performance robustness to C-OCDM, which outperforms OFDM, is experienced in the SM-OCDM case.

In the context of all-digital radar systems, phase-modulated continuous wave (PMCW) based on pseudorandom binary sequences (PRBSs) appears to be a prominent candidate modulation scheme for applications such as autonomous driving. Among the reasons for its candidacy are its simplified transmitter architecture and lower linearity requirements (e.g., compared to orthogonal-frequency division multiplexing radars), as well as its high velocity unambiguity and multiple-input multiple-output operation capability, all of which are characteristic of digital radars. For appropriate operation of a PMCW radar, choosing a PRBS whose periodic autocorrelation function (PACF) has low sidelobes and high robustness to Doppler shifts is paramount. In this sense, this article performs an analysis of Doppler shift tolerance of the PACFs of typically adopted PRBSs in PMCW radar systems supported by simulation and measurement results. To accurately measure the Doppler-shift-induced degradation of PACFs, peak power loss ratio (PPLR), peak sidelobe level ratio (PSLR), and integrated-sidelobe level ratio (ISLR) were used as metrics. Furthermore, to account for effects on targets whose ranges are not multiples of the range resolution, oversampled PACFs are analyzed.

Phase-modulated continuous wave (PMCW) has been widely regarded as a promising modulation scheme for radar systems, e.g., in highly automated driving (HAD) applications. Although the so-called outer coding can efficiently enable the multiple-input–multiple-output (MIMO) operation of PMCW-based radar systems, the yielded processing gain in this multiplexing approach may be reduced at increasing Doppler shifts. In this context, this letter introduces a normalized Doppler shift parameter that enables predicting the Doppler-shift-induced degradation of the processing gain in a MIMO-PMCW radar system. Finally, simulation and measurement results confirm the usefulness of the introduced parameter in desigining MIMO-PMCW radars.

This letter proposes a joint channel estimation and Schmidl & Cox synchronization for the orthogonal chirp-division multiplexing (OCDM) scheme. First, it is proven that a symbol composed of two identical halves in the Fresnel domain has two identical halves in the time domain, which is required by the considered synchronization technique. In sequel, a Fresnel-domain pilot symbol that enables joint channel estimation and synchronization is proposed, allowing overhead reduction and hence higher data rates. Finally, measurement and simulation results validate the obtained findings for both passband and baseband OCDM schemes.