With the maturity and commercialization of 5G/5G-A technology, research on 6G technology has been on the agenda. As a more advanced next-generation mobile communication technology, 6G technology has stronger service capabilities and has gradually evolved from a simple communication service to a complex service integrating communication, sensing, and computing power. Additionally, in the future, the application scenarios of 6G technology will be more extensive, and be applied to smart healthcare, smart transportation, smart cities, smart factories, and other fields. In these application scenarios, communication services and sensing services should be included at the same time. Therefore, in the future, 6G networks should have both communication and sensing capabilities. In short, in the future 6G communication network, communication and sensing functions will be highly integrated, and gradually evolve into communication and sensing integration. With the continuous development of new services, the demand for high-speed data transmission and low-latency communication is growing day by day, but the current wireless frequency band cannot meet this demand. Thus, it is necessary to explore higher-frequency bands, and meanwhile the terahertz bands with rich spectrum resources are suitable for the application scenarios of ultra-high-speed communication transmission and can meet the needs of current and future communication networks. Therefore, we conduct a generation and transmission analysis on multi-order quadrature amplitude modulation and linear frequency modulation (MQAM-LFM) signals of optical carrier terahertz with integrated sensing and communication (ISAC). Additionally, a single intensity modulator is adopted to generate a terahertz ISAC signal, which can achieve high-speed communication and high-precision perception and provide references for the development of 6G communication and sensing integration technology in the future.

We employ MATLAB and VPI Transmission simulation softwares. First, a certain number of pseudo-random binary sequences are generated by MATLAB, and the constellation diagram is utilized to map the higher-order vector signal. Then this signal is modulated to the chirp carrier, the digital signal is converted into an analog signal via the digital to analog converter (DAC), and then the Mach-Zehnder modulator (MZM) is driven. Specifically, the MZM works at the orthogonal bias point, and the phase difference of the upper and lower arm drive signals is 90°. After passing through the modulator, a single-sideband (SSB) ISAC optical signal is generated, and then a certain distance is transmitted in the single-mode optical fiber. Meanwhile, the ISAC signal in the terahertz band is generated by beating with another light source.

The proposed ISAC signal can achieve high-speed communication and high-precision perception. The ISAC signal of SSB can be generated by employing a single MZM, which can reduce the influence of fiber nonlinear effects [Fig. 2(b)]. The integrated signal generated by this scheme is more correlated, the perception results can be adopted to assist with communication synchronization, and the fuzzy function plot is closer to the pushpin shape, which leads to better perception performance (Fig. 4). The SSB-integrated signal generated by this scheme has a suppression of about 26 dB of the lower sideband signal (Fig. 5). Experiments show that the proposed scheme can successfully implement the communication function (Figs. 6 and 7). After theoretical derivation, the ISAC signal can detect the maximum distance of 16.88 m (Fig. 8). Research on the sensing performance of the integrated signal shows that the sensing performance of the integrated signal is better than that of the LFM signal with smaller perception error (Figs. 10 and 11). The study of communication and perception performance boundary analysis indicates that the system can achieve a communication rate of up to 40 Gbit/s and a sensing resolution of 1.3 cm (Table 3). In the further study of communication and perception performance, when the roll down factor is 0.313, the overall performance of the system is the best and the spectrum utilization can reach 2.76 b/(s·Hz) (Fig. 12).

We employ a single MZM to successfully generate ISAC signals in the terahertz band, which adopts high-order vector modulation and linear frequency modulation signals, and thus has a high communication rate and sensing accuracy. Theoretical analysis reveals that the ambiguity function of the MQAM-LFM integrated signal is closer to the pushpin shape. Simulation experiments demonstrate that the communication quality of the integrated signal is inferior to that of the traditional QAM signals due to the nonlinear effects during the modulation and transmission process. However, in the target measurement experiments at different distances, the perception performance of the integrated signal is better than that of the LFM signal with a smaller ranging error. In the further study of communication and perception performance, when the roll down factor is 0.313, the overall performance of the system can reach the optimum, and the spectrum utilization rate of the system is 2.76 b/(s·Hz), with a perceptual resolution of 1.62 cm. The above experiments show that the system can achieve a communication rate of up to 40 Gbit/s and a sensing accuracy of 1.3 cm.