Acta Optica Sinica, Volume. 45, Issue 8, 0806002(2025)

Design and Analysis of Optically Loaded Terahertz Communication and Sensing System Based on OFDM-MSK-LFM

Qi Feng and Feng Zhao*
Author Affiliations
  • School of Electronic Engineering, Xi’an University of Posts & Telecommunications, Xi’an 710121, Shaanxi, China
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    Objective

    In the upcoming sixth-generation (6G) mobile communication systems, new intelligent applications and services have emerged, such as autonomous driving and machine-type communication. Currently, the separate design of sensing and communication cannot meet the simultaneous requirements of high data transmission rates and high-precision sensing. Therefore, the integrated sensing and communication (ISAC) system has become one of the key technologies for 6G. ISAC technology combines communication and sensing functions, resolving the spectrum conflict between these systems and effectively alleviating the scarcity of spectrum resources. In recent years, the development and application of terahertz (THz) technology have promoted the implementation of ISAC. The abundant spectrum resources in the THz frequency band meet the growing demand for high-speed communication, and the short wavelength of THz waves enables fine spatial resolution, making them suitable for applications in imaging, localization, and environmental sensing. THz ISAC technology can provide real-time precise mapping and localization capabilities while maintaining high-speed communication links, presenting great potential for intelligent applications. Currently, THz signals are mainly generated by electronic and optical methods. The communication rate of THz signals generated via electronic technology is constrained by the microwave signal prior to the frequency multiplication, and its performance is limited by the bandwidth of electronic devices. In contrast, THz signals generated by optical methods directly leverage the large bandwidth advantage of THz communication, effectively overcoming the bandwidth bottleneck of electronic devices. This significantly simplifies the system structure and offers advantages such as high sensitivity and low power consumption. Orthogonal frequency-division multiplex (OFDM) is increasingly utilized in ISAC systems due to its excellent anti-multipath performance and high delay-Doppler resolution. By integrating minimum-shift keying (MSK) and linear frequency modulation (LFM) technologies, the fuzzy function can be further optimized, yielding better sensing performance. Therefore, we propose an optically loaded THz ISAC system based on OFDM-MSK-LFM, which provides a reference model for the fusion of communication and sensing functions in 6G.

    Methods

    The ISAC signal is generated in MATLAB, where a certain number of pseudo-random binary sequences are mapped to MSK signals. Subsequently, series-parallel conversion and inverse Fourier transform are applied to perform OFDM modulation. The signal is then modulated onto an LFM carrier. The ISAC signal is converted into an analog signal and drives the I/Q modulator, which is modulated onto the continuous-wave optical source generated by an external cavity laser. The optical signal is coupled with another local oscillator light source, and finally, a THz signal is generated by the photodetector. A portion of the terahertz signal is sent to the communication receiver, where the communication signal is recovered after de-chirping, Fourier transform, and channel equalization. The remaining signal is reflected by the target and received by the sensing receiver. The range and velocity of the target are recovered after estimating the sensing channel from both the echo signal and the original signal.

    Results and Discussions

    The proposed ISAC system achieves both high-rate communication and high-precision sensing. The fuzzy function of the ISAC signal is thumbtack-shaped, indicating excellent sensing performance (Fig. 8). The peak sidelobe ratio increases with the LFM bandwidth (Fig. 9), enhancing the anti-interference ability of ISAC. In the close-range static dual-target simulation test, the ranging error is kept within 5 mm (Fig. 10). In the high-speed dynamic dual-target test, relatively high precision for both range and velocity measurements is achieved (Fig. 11). After transmitting the 10 GBaud ISAC signal over a 25 m wireless link, the bit error rate reaches the threshold for hard decision forward error correction [Fig. 13(b)]. However, the bit error rate of the ISAC signal deteriorates as the LFM bandwidth ratio increases [Figs. 13(c) and (d)]. The partial sequence transmission algorithm can suppress the peak-to-average power ratio (PAPR) by up to 3.30 dB. As NFFT increases, the system’s PAPR also increases, while the theoretical maximum measurement error decreases. A balance is achieved when NFFT is 256, with simulated distance and speed errors of 0.4 cm and 10.83 m/s, respectively. The complementary cumulative distribution function for PAPR of >8 dB is 0.123.

    Conclusions

    In this paper, we propose an optical carrier ISAC system based on OFDM-MSK-LFM. Compared to traditional OFDM, MSK and LFM suppress Doppler sidelobes, concentrating energy towards the main lobe, thus improving the anti-interference performance and high-speed target detection capabilities of the ISAC signal. Due to the nonlinear effects of LFM, the bit error rate (BER) of the integrated signal exacerbates as the LFM bandwidth ratio increases. The adopted multi-carrier modulation enhances the overall sensing performance of the system but also increases the PAPR. The PTS algorithm, while adding some computational complexity, effectively suppresses the PAPR. The proposed system allows for flexible adjustment of the ISAC signal’s bandwidth and NFFT according to system requirements, enabling optimization of the detection range, sensing resolution, communication rate, and PAPR. This provides a valuable reference for the future development of 6G ISAC technology.

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    Qi Feng, Feng Zhao. Design and Analysis of Optically Loaded Terahertz Communication and Sensing System Based on OFDM-MSK-LFM[J]. Acta Optica Sinica, 2025, 45(8): 0806002

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    Paper Information

    Category: Fiber Optics and Optical Communications

    Received: Dec. 30, 2024

    Accepted: Feb. 10, 2025

    Published Online: Apr. 27, 2025

    The Author Email: Zhao Feng (hfengzhao@xupt.edu.cn)

    DOI:10.3788/AOS241959

    CSTR:32393.14.AOS241959

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