Future wireless connectivity is envisioned to accommodate functionalities far beyond broadband data transmission, taking into account environmental parameters and usage scenario characteristics. In this landscape, the concept of integrated sensing and communication (ISAC) introduces a new beyond communications paradigm shift, where communication nodes can be employed to sense the shape of surrounding obstacles, map the environment, detect the presence of objects, focus power, localize users and track mobility. In this article, the roadmap for interactive, immersive and intelligent connectivity is presented with respect to different usage scenarios, involving mobility and environmental awareness. It is analyzed how the ISAC opportunity can be transformed into a powerful 6G enabler for an intelligent beyond-communications network capable of sensing the world. In this context, ISAC is envisioned to emerge from the synthesis of three visionary pillars: sense-to-communicate, communicate-to-sense, and multifunctional ISAC connectivity.

Terahertz (THz) wireless transmission is a promising technology for 6G networks and beyond. Yet, its performance can be severely challenged by nonlinear distortions from photonic and electronic front-ends. This letter experimentally demonstrates a neural-network-based nonlinear equalizer (NN-NLEQ) at the receiver digital signal processing (Rx-DSP) stage to mitigate transceiver nonlinearities in probabilistically shaped (PCS) photonic THz links. We employ a hybrid CNN-BiLSTM architecture, identified via Neural Architecture Search (NAS) using the AutoKeras framework, and train it with a specialized entropy-regularized loss function that preserves soft demapping information and eliminates the 'jail window' pattern common in traditional MSE-trained NN equalizers. Experimental results for two different setups, using lens-coupled (PD1) and waveguide-coupled (PD2) photodiode transmitters, demonstrate bit-error ratio (BER) reductions exceeding one order of magnitude, peak achievable information rates (AIRs) of 4.85 and 3.87 bit/symbol at 16 GBd, and peak net data rates up to 139.5 Gbit/s at 32 GBd. As the NN-NLEQ exhibits robust performance across varying hardware configurations, shaping entropies, and symbol rates up to 32 GBd, it establishes itself as an effective, model-agnostic solution for nonlinearity mitigation in ultra-high-capacity THz wireless systems.

We analyze the weather-related link availabilities of high-capacity THz-wireless transmission systems, focusing on fiber-integrated point-to-point links in the lower THz frequency range around 300 GHz. First, we discuss latest component technologies for electronic and photonic THz generation, showing that there is a good basis for the implementation of wideband THz-wireless links. Then, we review the application of a THz-wireless fiber extender and identify challenges for its integration into future 6G optical networks, being mainly the linear optical-to-THz conversion and the weather-dependent THz link loss. Both aspects will be addressed in the following: We describe the concept and implementation of a fiber-integrated THz outdoor unit prototype based on electronic THz components and provide link budget calculations for a data rate of 100 Gbit/s. Then, we estimate the THz link loss from theoretical models and real weather data, showing that high link availabilities above 99.999% are possible for link distances of 500 m with state-of-the-art components. Estimates for a longer distance of 1 km are given as well. Finally, we describe our outdoor testbed built with our prototypes and present measured data on the link attenuation over 500 m in comparison to the theoretical expectations, achieving a good correlation.

The increasing capacity demand of optical communication systems has driven research on space-division multiplexing (SDM) using few-mode fibers (FMFs). While FMFs provide high spatial efficiency, their practical deployment is limited by the rapidly growing complexity of multiple-input multiple-output (MIMO) digital signal processing (DSP). Partitioned-MIMO architectures reduce DSP complexity by exploiting the existence of mode groups (MGs) in FMFs, where intra-group coupling is strong and inter-group coupling is weak. In this work, we experimentally investigate a partitioned MIMO architecture with mode-group specific interface quantization for a 15-mode FMF transmission system, which would otherwise require a full 30 × 30 MIMO equalizer. The equalizer is decomposed into five MG-specific sub-equalizers with low-resolution, MG-dependent inter-partition signal exchange to compensate residual inter-group coupling. Using 24.5-GBd dual polarization QPSK transmission over a 58.9-km graded-index FMF, we evaluate the impact of interface quantization on the achievable signal-to-noise ratio (SNR). The results show that weak inter-group coupling can be omitted entirely, while stronger neighboring MG interactions require only three to four bits to limit SNR penalties to below 1.7 dB relative to a uniform five-bit reference. This approach significantly reduces inter-partition interface throughput while preserving equalization performance, demonstrating the scalability of partitioned-MIMO architectures for high-capacity SDM systems.

Photomixers, which convert optical signals into high-frequency electrical signals, are promising sources and detectors for terahertz (THz) wireless communications due to their broad tunability, high bandwidth, and easy integration with fiber-optic networks. Photodiode (PD)-based THz emitters are already the state-of-the-art for highest data rate THz wireless links. Photonic THz receivers, such as photoconductive antennas (PCAs), have the same benefits of high THz bandwidth and potentially the same very low phase-noise as PD emitters. However, PCAs have not yet demonstrated competitive receiver performance compared to electronic mixers. This limitation arises from the restricted conversion gain and intermediate frequency (IF) bandwidth of the top-illuminated PCAs used in current systems. In this work, we present a novel photomixing heterodyne THz receiver based on waveguide-integrated (win) PCAs, which offers a 25 dB increase in conversion gain due to benefits arising from the optical waveguide coupling. We design and optimize a high-frequency package for the win-PCAs, achieving a record 3- and 6-dB IF bandwidth of 25 and 40 GHz, respectively. With this receiver, we now attain gross data rates of up to 84 Gbit/s, which is a new record for photonic wireless links with PCA receivers. At the same time, we demonstrate the ultra-broadband operation capabilities of the win-PCA, enabling data transmission at carrier frequencies from 100 to 600 GHz with the same receiver.

Photonic transmitters are a promising approach for future wireless backhauling due to their high bandwidth and compatibility with fiber-optical networks. Additionally, photonic integration allows to create complex structures, e.g. antenna arrays, on a low-footprint chip or chiplets. So far, however, mainly fixed point-to-point links are demonstrated, and point-to-multipoint concepts come with increased effort, e.g. controlling numerous phase shifters to drive optical phased arrays. Our beam-switching approach demonstrates a multi-directional sub-THz transmitter with low operation effort. In a proof-of-concept experiment, we demonstrate 6.4 Gbps net data rate across an angular coverage of 85° in the W-band, specifically at 100 GHz.

We present a fully photonic, ultra-broadband mm-wave and THz wireless link, covering a wide carrier frequency range from 100 to 600 GHz. It employs a PIN photodiode as the emitter and a waveguide-integrated photoconductive antenna (PCA) as the receiver. To the best of our knowledge, this represents the first wireless link capable of transmitting across a 500 GHz frequency range with a single transmitter and receiver. Additionally, we achieve a new record gross data rate of 90 Gbit/s for PCA-based wireless links at a carrier frequency of 120 GHz. These accomplishments are made possible by substantial enhancements in conversion gain and intermediate frequency (IF) bandwidth of the heterodyne receiver, as well as meticulous optimization of operating conditions of the photomixers.

We review recent developments in high-capacity THz wireless transmission, focusing on point-to-point links in the lower THz frequency range around 300 GHz. Available technologies and challenges for integration into future 6G optical networks are discussed.

Terahertz (THz) wireless communication is a key enabler for 6G networks, offering ultra-high data rates and broad bandwidth scalability. Photonic generation of THz signals using photodiodes has emerged as a promising approach, yet a systematic performance comparison between different architectures remains largely unexplored. This paper presents an experimental investigation of two distinct THz transmitter architectures. One is based on a WR3.4 waveguide-coupled photodiode, while the other employs a quasi-optically coupled photodiode with a silicon lens. Both architectures are evaluated using probabilistic constellation shaping with high-order quadrature amplitude modulation (QAM). We evaluate output power, bit error rate (BER), and system linearity across both photodiode-based architectures under standardized conditions. The results reveal key trade-offs between transmitter design choices, nonlinearities, and achievable performance, offering critical insights for optimizing next-generation THz wireless systems.

This paper presents the design and implementation of an agile sub-terahertz (sub-THz) broadband time-domain photonic channel sounder, capable of characterizing wireless propagation channels within the 100 GHz to 500 GHz frequency range. The sounder employs a hybrid architecture that combines photonic carrier generation through heterodyne mixing of continuous-wave lasers locked to a frequency comb, along with electronic components for broadband sounding sequence synthesis and digitization. This design enables precise and tunable carrier frequency generation, overcoming the limitations of traditional electronic-only systems. Experimental evaluations highlight the system's high temporal resolution, phase stability, and broadband performance, including over-the-air calibration and monostatic channel measurements across three sub-THz bands (160 GHz, 300 GHz, and 450 GHz). The results establish the feasibility of the time-domain photonic channel sounder as a powerful and versatile tool for measuring various, widely scattered, and spread sub-THz bands.

We propose partitioned MIMO equalization with low-resolution, quantized interface between sub-equalizers for few-mode fiber systems. Experiments up to DP-64QAM yield OSNR-penalties <0.2 dB for 2…4-bit interface resolutions compared to high-resolution MIMO, depending on the cardinality. (c) 2025 The Authors

We demonstrate 5-fold partitioned MIMO equalization with low-resolution inter-partition interface for DP-QPSK 15-mode fiber transmission. The interface resolution is adapted to the coupling between mode groups. Amplitude quantization between 1-4 bits facilitates interface throughput reduction by 49% while keeping the SNR degradation < 1.6 dB. (c) 2025 The Author(s)

Terahertz wireless communications is an increasingly interesting research topic due to the high demand for un-allocated channels and high data rates. Photonic solutions have shown great potential in this field. However, most photonics assisted THz links so far have employed optoelectronics only on the transmit side. Thus, the full potential of photonic THz communication has not been utilized yet. Here, we introduce optoelectronics also on the receive side by using a photoconductive antenna based heterodyne THz detector. This allows down-conversion of data signals from the W-, D-, and THz-band to the baseband using a laser beat signal as local oscillator. Using electromagnetic modeling, we designed passive radio frequency structures and a receiver package to handle high intermediate frequency output signals. In a homodyne spectroscopic setup, the receiver shows a frequency response superior to state-of-the-art photoconductive antennas due to an improved photoconductive material. In a heterodyne testbed, the receiver exhibits a large intermediate frequency bandwidth of 11 GHz and a conversion gain of -47 dB. This enabled us to employ the receiver in a fully photonic wireless link at sub-terahertz and terahertz frequencies together with a PIN photodiode emitter. We achieved error-free transmission of 4-QAM signals with gross data rates up to 12 Gbit/s at carrier frequencies up to 320 GHz. This work shows the huge potential of optoelectronic receivers for THz wireless communications and enables the exploration of full photonic THz links.

Terahertz wireless communications is an increasingly interesting research topic due to the high demand for un-allocated channels and high data rates. Photonic solutions have shown great potential in this field. However, most photonics assisted THz links so far have employed optoelectronics only on the transmit side. Thus, the full potential of photonic THz communication has not been utilized yet. Here, we introduce optoelectronics also on the receive side by using a photoconductive antenna based heterodyne THz detector. This allows down-conversion of data signals from the W-, D-, and THz-band to the baseband using a laser beat signal as local oscillator. Using electromagnetic modeling, we designed passive radio frequency structures and a receiver package to handle high intermediate frequency output signals. In a homodyne spectroscopic setup, the receiver shows a frequency response superior to state-of-the-art photoconductive antennas due to an improved photoconductive material. In a heterodyne testbed, the receiver exhibits a large intermediate frequency bandwidth of 11 GHz and a conversion gain of −47 dB. This enabled us to employ the receiver in a fully photonic wireless link at sub-terahertz and terahertz frequencies together with a PIN photodiode emitter. We achieved error-free transmission of 4-QAM signals with gross data rates up to 12 Gbit/s at carrier frequencies up to 320 GHz. This work shows the huge potential of optoelectronic receivers for THz wireless communications and enables the exploration of full photonic THz links.

We present a novel waveguide-integrated photoconductive antenna as heterodyne receiver for THz wireless links. The receiver features 20 dB higher conversion gain and 4 GHz higher IF bandwidth compared to the best top-illuminated photoconductive heterodyne receiver demonstrated so far. With this detector, we are able to extend the current data rate records for full photonic THz links by a factor of 5.5 and 4 to 68 Gbit/s and 40 Gbit/s at 120 GHz and 300 GHz, respectively.

We analyze the system performance of a large bandwidth photonic THz transmitter based on a WR3 coupled photodiode. Employing probabilistic constellation shaping (PCS) alongside a 64-QAM base constellation, we achieve net data rates of up to 76.8 Gbit/s at 32 GBd. The system's performance is evaluated across various symbol rates and shaping entropies, with bit-error rates (BER) analyzed in relation to signal-to-noise ratio (SNR) at the receiver. (c) 2024 The Author(s)

We will present the results of a recent demonstration of a high-capacity dual link THz-wireless prototype system at a carrier frequency of 300 GHz, capable to deliver high data rates of up to 50 Gb/s for two spatially separated downlink connections using high-symbol rate single-carrier modulation and a burst-mode capable DSP. Furthermore, also a real-time implementation of the uplink connection is characterized at a data rate of 2.8 Gb/s.

We characterize the performance of high symbol rate probabilistic constellation shaping (PCS) in a THz wireless link utilizing a PIN photodiode-assisted transmitter. Using a 64-QAM base constellation we demonstrate high data rate adaptivity, achieving up to 109 Gbit/s net data rate at 32 GBd.

For terrestrial fiber-optical data communication, the digital-coherent intradyne detection method is commonly used for data recovery at the receiver. This method has several advantages compared to optical phase locked homodyne systems. For example, the handling of phase noise is shifted into the digital domain and the fabrication of the complete system and the setup for operation are easier. Furthermore, frequency acquisition can be performed much faster, which is a major advantage in the presence of fades as in GEO space to ground links. In this paper, the demonstration of the technology in optical freespace communication is shown with the TDP1 testbed.

One of cost-effective ways to increase the transmission capacity of current standard wavelength division multiplexing (WDM) transmission systems is to use a wavelength band other than the C-band to transmit in multi-band. We proposed the concept of multi-band system using wavelength conversion, which can simultaneously process signals over a wide wavelength range. All-optical wavelength conversion could be used to convert C-band WDM signals into other bands in a highly nonlinear fiber (HNLF) by four-wave mixing and allow to simultaneously transmit multiple WDM signals including other than the C-band, with only C-band transceivers. Wavelength conversion has been reported for various nonlinear waveguide materials other than HNLF. In such nonlinear materials, we noticed the possibility of wideband transmission by dispersion-tailored silicon-on-insulator (SOI) waveguides. Based on the CMOS process has high accuracy, it is expected that the chromatic dispersion fluctuation could be reduced in mass production. As a first step in the investigation of the broadness of wavelength conversion using SOI-based waveguides, we designed and fabricated dispersion-tailored 12 strip waveguides provided with an edge coupler at both ends. Each of the 12 waveguides having different widths and lengths and is connected to fibers via lensed fibers or by lenses. In order to characterize each waveguide, the pump-probe experimental setup was constructed using a tunable light source as pump and an unmodulated 96-ch C-band WDM test signal. Using this setup, we evaluate insertion loss, input power dependence, conversion bandwidth and conversion efficiency. We confirmed C-band test signal was converted to the S-band and the L-band using the same silicon waveguide with 3 dB conversion bandwidth over 100-nm. Furthermore, an increased design tolerance of at least 90 nm was confirmed for C-to-S conversion by shortening the waveguide length. It is confirmed that the wavelength converters using the nonlinear waveguide has sufficiently wide conversion bandwidth to enhance the multi-band WDM transmission system.