The 6G communication network is envisioned to provide intelligent interconnection among people, machines, and things, requiring not only high-speed wireless communications but also various sensing functions. Integrating radar ranging and imaging, wireless communications, and spectrum sensing into a single system—a joint radar, wireless communications, and spectrum sensing (JRCSS) system—is a cost-effective solution. However, conventional electronic approaches face challenges in generating broadband high-frequency complex waveforms and meeting high data rate and high-resolution requirements. Microwave photonics, combining microwave engineering and photonic technology, offers advantages such as large bandwidth, high frequency, low loss, wide tunability, and electromagnetic interference immunity, making it a promising solution for 6G. Previous research has explored microwave photonics for individual functions (radar, communication, spectrum sensing) and some dual-function systems. This paper addresses the gap by presenting a unified system integrating all three functions.

Existing research in microwave photonics has focused on high data rate millimeter-wave or terahertz signal generation and low-loss, long-distance wireless signal distribution. All-optical microwave photonic radar implementations have been reported, along with efforts to integrate broadband linearly frequency-modulated (LFM) waveform generation with echo de-chirping in the optical domain for high-resolution imaging. Microwave photonics-powered spectrum sensing uses frequency-to-power, frequency-to-space, or frequency-to-time mapping (FTTM), with FTTM, often employing fiber dispersion or optical signal sweeping, enabling multi-frequency measurements. Studies on multi-function systems such as joint communication and radar (JCR) and joint radar and spectrum sensing systems have shown the benefits of microwave photonics. These systems have used various signal multiplexing techniques (frequency-division, time-division) or waveform sharing (using signals like QPSK, ASK-LFM, QPSK-LFM) to integrate functionalities, but a fully integrated JRCSS system for 6G was lacking until this work.

The proposed JRCSS system uses a continuous-wave optical signal split into two paths. One path employs a dual-parallel Mach-Zehnder modulator (DP-MZM) to generate an optical signal carrying both amplitude-shift keying (ASK) communication and negatively chirped LFM signals using carrier-suppressed tandem single-sideband (CS-TSSB) modulation. For spectrum sensing, a portion of this optical signal is further modulated by the signal under test (SUT) using a Mach-Zehnder modulator (MZM) and then interacts with a pump wave via stimulated Brillouin scattering (SBS) in a nonlinear medium (NM), implementing frequency-to-time mapping (FTTM). The time of the resulting optical pulses indicates the SUT frequency. For radar and communication, the other portion of the optical signal from the DP-MZM is photodetected to generate an electrical ASK-LFM signal. This signal is split: one part is transmitted, the reflected echo is received, mixed with a local oscillator signal for de-chirping, and then processed for ranging and imaging; the other part undergoes self-mixing for communication signal extraction. The experiment used specific components (laser diode, DP-MZM, AWGs, photodetectors, amplifiers, antennas, mixers, oscilloscope), carefully configuring signal generation, modulation, and processing techniques to achieve the three integrated functionalities.

The experimental results demonstrated the successful integration of radar, communication, and spectrum sensing. Radar ranging showed a measurement error within ±4 cm, while two-dimensional imaging achieved a resolution of 25 × 24.7 mm. High-speed communication was demonstrated at 2 Gbaud (2 Gbps data rate using 2ASK modulation). Spectrum sensing achieved a frequency measurement error within ±10 MHz over a 6 GHz bandwidth. The system demonstrated time-frequency analysis capabilities, accurately identifying various signals (LFM, NLFM, step-frequency signals). The impact of the ASK baud rate on image quality and ranging accuracy was assessed. The frequency resolution of the spectrum sensing was shown to be dependent on the sampling rate, with a resolution of 80 MHz achievable at a sufficiently high sampling rate. System tunability, allowing adjustment of center frequency and bandwidth through parameters such as the LFM signal and ASK signal characteristics, was also explored. The inclusion of an additional DP-MZM extended the spectrum sensing measurement range.

The integrated JRCSS system successfully addresses the challenge of combining radar ranging and imaging, wireless communications, and spectrum sensing within a single, efficient system. The use of microwave photonics enables superior performance compared to electronic-only systems by overcoming bandwidth limitations, thereby achieving simultaneous operation with high accuracy and data rates. The results demonstrate the feasibility of this approach for 6G applications where integrated sensing and communication is crucial. The system's tunability enhances its adaptability to different application requirements. The achieved performance parameters (ranging accuracy, image resolution, data rate, spectrum sensing accuracy) are comparable to or better than many existing single- or dual-function systems reported in the literature. This work significantly contributes to the development of advanced 6G systems.

This paper demonstrates a novel microwave photonics-based approach for seamlessly integrating radar, communication, and spectrum sensing. The system successfully achieved high performance in all three functions simultaneously. Future work should focus on optimizing component selection, miniaturizing the system using photonic integrated circuits (PICs) to reduce cost and complexity, and exploring more sophisticated modulation techniques to further enhance data rates and system performance. The findings contribute significantly to the development of advanced, multi-functional 6G systems.

The current system uses discrete components, resulting in higher cost compared to fully integrated solutions. The uneven frequency response of some components impacted the communication signal quality, particularly at higher frequencies. While the system demonstrates good performance, further optimization of the system design and component selection could further improve performance metrics. The relatively long nonlinear medium used in the experiments could be replaced with shorter highly nonlinear fiber to improve the compactness of the system. The use of a more complex modulation scheme may require higher signal-to-noise ratio for reliable operation.