Integrating sensing function is becoming a key feature in the sixth generation (6G) radio access network. In this paper, we proposed and experimentally demonstrated a novel method that integrates high-resolution radar and long-distance communication in terahertz (THz) band. The THz communication and linear frequency modulation (LFM) signals were simultaneously generated with the aid of photonics technology. To support long-distance THz communication, a pair of special lenses was utilized. A net rate of 38.1 Gbit/s was achieved after 50 m of wireless transmission. Meanwhile, a range resolution of 1.58 cm was achieved for the THz radar. Two targets at distances of 10 and 35 cm from the reference position were successfully detected, and the distance measurement error was within the range of centimeters. This is an effective method of incorporating high-resolution radar sensing and long-distance communication functions system in the THz band, which may be beneficial in several potential applications of 6G in the future.

Terahertz band (0.1 to 10 THz) with high carrier frequency and large available bandwidth has become a promising candidate to meet the 100 Gbit/s or even 1 Tbit/s data rate required by the future six-generation (6G) mobile communication networks. Compared with the all-electrical methods to generate terahertz signals, the photon-assisted technology can break the bottleneck of the bandwidth limit of the electronics devices, and generate the terahertz signal with high frequency, large bandwidth, flexible tunability and easy integration with the large capacity fiber link. In this paper, we introduce the typical methods to generate terahertz signals based on the photon-assisted technology, and review the representative achievements in different areas of terahertz communication, such as the large capacity terahertz transmission, the long distance terahertz transmission, the real-time terahertz communication, and the integrated terahertz sensing and communication. Based on the photon-assisted technology and various key techniques, devices and advanced digital signal processing (DSP) algorithms, we have obtained many great achievements in broadband terahertz communication and sensing, and the experimental setups and results have also been demonstrated in detail.

From the perspective of wireless transmission distance in the terahertz-band (THz-band), the 10-100 m, 100-1000 m, and ≥1000 m ranges are recommended for indoor communications, medium-distance communications, and long-distance communications, respectively. However, most existing photonic terahertz communication demonstrations reported in previous studies have a distance of less than 100 m, with some being even shorter at 10 m or 1 m. In this study, by combining high-gain and high-sensitivity terahertz modules, as well as signal processing technology, we have successfully demonstrated transmission over 850 m with a net rate of 50 Gbit/s at a 320 GHz carrier frequency through outdoor field experimental verification. This particular demonstration represents the most significant distance-rate product, as well as the longest wireless distance achieved so far for photonic terahertz wireless communication. The results demonstrate that the integration of terahertz photonics, electronics, and cutting-edge DSP algorithms, has the potential to enable long-distance and high-speed wireless communication for frequencies ≥300 GHz. Our work represents another significant stride towards achieving our goal of realizing the world's first photonic terahertz-wave communication over kilometer-level distances.

Terahertz (THz) photonics technique can break through the bandwidth limitation of electronic mixing method and be seamless integrated with optical access networks, which has bright prospects in future 6G broadband communication. However, due to the bottleneck of photoelectric conversion efficiency of uni-travelling photodiode (UTC-PD), the power of the generated THz-wave signal is usually only in the microwatt level, and the THz-wave wireless transmission distance is short. In this paper, we summarize the key techniques we use to extend the wireless range for photonics-aided THz-wave communication, which includes THz-wave amplifiers and antennas, heterodyne detection, probabilistic shaping (PS) technique as well as advanced digital signal processing (DSP). Meanwhile, we report our recent research including 104\200\400-m photonics-aided THz-wave wireless transmission records.

We provided a feasible solution for 800-Gb/s per carrier long-distance wavelength division multiplexing (WDM) coherent optical fibre transmission based on offline DSP. We experimentally demonstrate 5-carrier 125-GHz-spacing WDM coherent optical fibre transmission achieving 2000-km transmission distance and 800-Gb/s net bit rate per carrier. We utilized 100-Gbaud truncated probabilistically shaped (TPS) polarization-multiplexed 64QAM format in this work. In terms of advanced components, the high-speed digital-to-analog converter with 35-GHz 3 dB-bandwidth and Raman-amplified ultra-large-area fibre were utilized to achieve higher baud-rate and longer transmission distance. In terms of modulation format, we changed PS to TPS to obtain a more suitable shaping depth, and make the shaped signal better suited to the DSP process of coherent optical communication, including constant modulus algorithm (CMA) equalizer. Besides, our results show that the TPS-64QAM signal with 5-bit/symbol/polarization entropy outperforms the standard-32QAM with a same bit rate by around 1 dB sensitivity gain. Furthermore, we proposed a multiple-input multiple-output (MIMO) Volterra equalizer (VE), which consists of a 1 ^st -order 2 × 2 MIMO complex-valued widely linear (WL) equalizer and a 2 ^nd -order real-valued nonlinear equalizer. Our proposed MIMO VE can effectively resolve the impact of In-phase/quadrature (IQ) imbalance and IQ skew, and compensate linear and nonlinear impairments at the same time. We also utilized partial kernel VE to reduce the complexity by retaining part of the 2 ^nd -order kernels.

We propose and experimentally demonstrate a real-time fiber-THz-fiber 2 × 2 multiple-input multiple-output (MIMO) seamless integration system at 340–510 GHz using commercial digital coherent optics (DCO) modules for baseband signals processing, which realizes a record net rate of 103.125 Gb/s dual polarization quadrature-phase-shift-keying (DP-QPSK) signals delivery over two spans of 20 km wireline single-mode fiber-link and 3 m wireless 2 × 2 MIMO link without using THz power amplifier under 15% soft-decision forward-error-correction threshold (SD-FEC) of 1.56 × 10 ⁻² . Two kinds of THz-to-optical conversion schemes based on an integrated dual-polarization Mach-Zehnder modulator (DP-MZM) and two discrete intensity-modulators (IMs) are extensively investigated and compared. To the best of our knowledge, this is the first real-time demonstration of THz-wireless and 100 GbE fiber-optic networks seamless integration transmission, which is compatible with IEEE 802.3 and ITU-T G.798 specifications. It is a promising scheme to meet the demands of future fiber-wireless-integrations communication for low power consumption, low cost and miniaturization.