In the upcoming 6G, the integrated sensing and communication (ISAC) system in the millimeter wave (MMW) band will have a vital role in numerous application scenarios, enhancing the convenience of our lives. The photonic-based MMW ISAC system can exploit the broad bandwidth of photonic devices, significantly improving system performance. Furthermore, the use of photonic devices enables the seamless integration of the ISAC system with the fiber-wireless network. In this paper, we proposed a simple-structured photonic-based W-band ISAC system, incorporating optical fiber transmission into the system. Additionally, we designed integrated waveforms for time-frequency-division multiplexing (TFDM), allowing for flexible tradeoffs between data rate, range resolution, and detection distance according to the requirements of application scenarios. As a proof-of-concept, a photonic-based ISAC system for the fiber-wireless network with flexible TFDM waveforms at 96.5 GHz over 10-km fiber transmission was demonstrated, achieving adaptive access rates from 15 to 60 Gbit/s after transmission over 1-m free space, and adaptive range resolutions from 1.53 to 4.39 cm. Moreover, this paper provides a detailed analysis of the causes of system distance error and corresponding solutions. In the experiment, the distance error of the proposed system can be reduced to less than 3 cm after external calibration.

The hot-carrier photothermoelectric (PTE) effect in two-dimensional materials can be used to develop room-temperature, fast, and sensitive detectors for microwave, terahertz, and far-infrared radiations. Here, we present a van der Waals semimetal PtSe ₂ terahertz detector and find a giant terahertz PTE response mediated by hot carriers at the metal-PtSe ₂ interfaces. The detector demonstrates an excellent zero-bias responsivity of 0.62 ampere per watt (A/W), a low-noise equivalent power of 19.6 picowatt per square root of hertz (pW/Hz ^1/2 ), and a fast response time of 4.5 nanosecond (ns) at 0.1 terahertz (THz). The metal doping effect and asymmetric terahertz heating play vital roles in the hot-carrier PTE response at the metal-semimetal interfaces. Moreover, scanning photocurrent mapping shows that the short-wave zero-bias photoresponse is closely localized to the metal-semimetal interfaces, further revealing the metal doping effect. The bias-dependent photocurrent indicates that the short-wave photoresponse still originates from the PTE effect and the photovoltaic response is negligible. Our results provide important guidance for developing high performance semimetal detectors.

In the impending 6G era, the integration of sensing and communication (ISAC) systems in the millimeter-wave (MMW) band will assume a pivotal role across a myriad of applications, thus enhancing everyday convenience. The photonic-based MMW ISAC system, renowned for mitigating electromagnetic interference and integrating with the fiber-wireless network, exhibits tremendous potential across diverse application domains. In intricate application contexts, the ISAC system should possess the capability to execute multitarget detection to preclude false target generation, simultaneously maintaining a balance between radar sensing and communication performance. This article presents a flexible photonic-based W -band ISAC system, designed to facilitate centralized, seamless fiber-wireless networks. It boasts the ability to detect multiple targets within the same angular sector. We scrutinize the potential causes for false target generation and the corresponding solutions in detail, validating the correctness of this theory through simulations and experiments. In addition, we incorporate subcarrier modulation (SCM) and linear frequency modulation (LFM) signals using frequency-division multiplexing (FDM). This allows us to allocate the bandwidth and power of these two signals, thus achieving a balance between sensing and communication. To demonstrate this concept, we designed a photonic-based ISAC system for the fiber-wireless network operating at 97.5 GHz over 10-km fiber transmission, which yielded access rates ranging from 15 to 60 Gbit/s after 1-m free-space transmission and range resolutions varying from 1.76 to 3.15 cm. The distance error subsequent to external calibration is under 3 cm.

Integration of sensing and communication (ISAC) systems at the terahertz (THz) band will play a crucial role across a myriad of applications in the 6G era, profoundly enhancing our daily experiences. Photonic-based THz ISAC systems harness the inherent broad bandwidth to directly generate high-frequency broadband signals, ensuring seamless integration with fiber-wireless networks and demonstrating significant potential. Current ISAC systems employ linear frequency modulation (LFM) signals for sensing struggle to directly receive and de-chirp THz echo signals, which, nonetheless, curtails sensing capabilities; moreover, no research has yet investigated the impact of varying fiber lengths on the performance of downlink sensing signal transmission and uplink echo signal return. This article presents a THz photonic-based ISAC system designed to surmount these limitations. At the radar receiver, full-photonic direct reception and de-chirping of D-band LFM signals are realized by a high-bandwidth thin-film lithium niobate Mach–Zehnder modulator (TFLN-MZM); moreover, we evaluate the impact of different fiber lengths on system performance. Our system attains an exceptional range resolution of 6 mm and a peak data rate of 116 Gbit/s at a center frequency of 134 GHz, with a calibrated mean ranging error of less than 3 mm.

The integration of fiber and millimeter-wave (MMW) technology offers a promising solution for next-generation (6G) communication. However, the experimental investigation of multi-user fiber-MMW integrated communication with diverse channels is yet to be explored, as the complex and dynamic nature of this system makes it challenging to overcome the channel distortion. In this paper, we propose and demonstrate an adaptive multi-user end-to-end framework (AMEF) for symbol-to-symbol optimization of a multi-user fiber-MMW integrated system. This framework leverages a two-tributary heterogeneous neural network (TTHnet) based multi-channel model (MCM) and a multi-user transceiver (MUT) composed of a multi-user encoder (MUE) and a multi-user decoder (MUD). The multi-layer perceptron (MLP)-based MUE and MUD are jointly optimized as an auto-encoder, facilitated by the well-trained MCM, which enables gradient back-propagation and end-to-end optimization. The weights of different users are adaptively allocated by the AMEF to ensure a balanced transmission performance. We experimentally demonstrate the effectiveness of the AMEF through a 10 km fiber-MMW integrated system for two-user communication. With the proposed method, we realize a 66 Gbps wireless-only transmission and a 49.5 Gbps fiber-wireless transmission under a soft decision-forward error correction (SD-FEC) threshold. Compared to the conventional multiband carrierless amplitude and phase (mCAP) modulation scheme, our proposed AMEF achieves significant receiver sensitivity gains exceeding 1.1 dB and 0.6 dB for the wireless-only and integrated systems, respectively. These results prove that the AMEF can provide a robust and adaptive solution for achieving high-speed and multi-user communication in next-generation radio access networks.

The E/W band millimeter-wave (MMW) emerges as a promising frequency band with many advantages such as the possession of abundant spectrum resources, low atmospheric attenuation, and good directionality. Meanwhile, full-photonic based MMW systems offer the potential to further reduce the complexity of electronic down-conversion and enhance overall system bandwidth, and allow seamless integration with the fiber-optic network. In this paper, we proposed an E/W band MMW communication architecture based on full-photonic up- and down-conversions, utilizing a frequency- and phase-locked optical two-tone generator at both the transmitting and receiving end. We successfully demonstrated a long-distance field trial of E/W-Band wireless transmission, achieving 26.8-km wireless delivery of 32-Gb/s quadrature phase shift keying (QPSK) signal, and the largest net-rate-distance-product up to 745.74 Gb/s·km per antenna per channel. The long-distance transmission performance of different modulation formats was also studied. We also achieved a 22-Gb/s QPSK transmission over a longer distance of 30.4 km, with a net-rate and distance product reaching 581.56 Gb/s·km. Moreover, this paper analyzes the complex sea surface channel and experimentally verifies the influence of the sea surface environment on the channel, which is instructive for long-distance MMW sea surface communications.