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.

Integrated sensing and communication (ISAC) has been defined as one goal for sixth-generation (6G) mobile communication systems. In this context, this article introduces a bistatic ISAC system based on orthogonal frequency-division multiplexing (OFDM). While the bistatic architecture brings advantages such as not demanding full duplex operation with respect to the monostatic one, the need for synchronizing transmitter (Tx) and receiver (Rx) is imposed. In this context, this article introduces a bistatic ISAC signal processing framework where an incoming OFDM-based ISAC signal undergoes over-the-air (OTA) synchronization based on preamble symbols and pilots. Afterward, bistatic radar processing is performed using either only pilot subcarriers or the full OFDM frame. The latter approach requires estimation of the originally transmitted frame based on communication processing and therefore error-free communication, which can be achieved via appropriate channel coding. The performance and limitations of the introduced system based on both aforementioned approaches are assessed via an analysis of the impact of residual synchronization mismatches and data decoding failures on both communication and radar performances. Finally, the performed analyses are validated by proof-of-concept measurement results.

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.

In this study, a time-domain reflectometry (TDR) system based on the orthogonal chirp-division multiplexing (OCDM) scheme is proposed for power line sensing. After the system model description, a multiple access scheme that exploits the convolution theorem of the modified discrete Fresnel transform for enabling distributed reflectometric and transferometric power line sensing is introduced. Next, the effects of the parametrization of the proposed OCDM-based TDR system on range resolution and maximum unambiguous range are assessed. A comparison with multiple access schemes based on the Hermitian symmetric orthogonal frequency-division multiplexing (HS-OFDM) is then performed, being the number of measurements obtained over time and signal-to-interference-plus-noise ratio (SINR) used as performance metrics. Numerical results show that the proposed multiple access scheme achieves the highest number of measurements over time, besides yielding the same range resolution as the HS-OFDM-based counterparts. Finally, the proposed multiple access scheme is slightly outperformed by the HS-OFDM based on code-division multiple access and by the HS-OFDM based on frequency-division multiple access at some power line modems (PLMs) consisting the distributed sensing system, although its yielded SINR values at distinct PLMs are at a same level.

To overcome the necessity of high sampling rates for common orthogonal frequency-division multiplexing (OFDM)-based radar waveforms with a good range resolution, a scheme called frequency comb OFDM radar has recently been proposed. It keeps the sampling rates at the transmitter and receiver constant while simultaneously increasing the signal bandwidth in the radar channel. This letter describes the extension of the scheme for multiple-input multiple-output (MIMO) applications where a nonequidistant subcarrier interleaving can be used to prevent a reduction of the unambiguous range that occurs in case of regular allocation to the transmit channels. The functionality is demonstrated for the first time with a 4 x 4 MIMO system in a measurement scenario with static and emulated moving targets.