Terahertz communications has been foreseen as a key enabler to the sixth generation (6G) of wireless communications systems. However, the design of spectrally-efficient waveforms and modulation schemes is an ongoing challenge in this regime. In this article, we propose and experimentally demonstrate the transmission of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M -ary carrierless amplitude and phase (CAP) modulated signals using a 253 GHz photomixing-based terahertz wireless communications system combined with the optical transmission over a 10-km standard single mode fiber (SSMF). Experimental results show that the CAP modulation technique has a capability to support the high-speed transmission of terahertz signals over a wide range of data rates from 4 Gbit/s (BER <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&gt;10^{-3}$</tex-math></inline-formula> ) to 96 Gbit/s (BER <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&gt;10^{-1}$</tex-math></inline-formula> ), based solely on optical intensity modulation at the transmitter side and terahertz envelope detection at the receiver side. Consequently, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M -ary CAP can potentially be adopted as a low-complexity waveform and modulation contender to simplify the transceivers architectures for 6G terahertz communications systems, without sacrificing the high throughput targeted by these systems.

Carrierless Amplitude and Phase Modulation for Terahertz Communications

Compared to the generations up to 4G, whose main focus was on broadband and coverage aspects, 5G has expanded the scope of wireless cellular systems towards embracing two new types of connectivity: massive machine-type communication (mMTC) and ultra-reliable low-latency communications (URLLC). This paper will discuss the possible evolution of these two types of connectivity within the umbrella of 6G wireless systems. The paper consists of three parts. The first part deals with the connectivity for a massive number of devices. While mMTC research in 5G was predominantly focused on the problem of uncoordinated access in the uplink for a large number of devices, the traffic patterns in 6G may become more symmetric, leading to closed-loop massive connectivity. One of the drivers for this is distributed learning/inference. The second part of the paper will discuss the evolution of wireless connectivity for critical services. While latency and reliability are tightly coupled in 5G, 6G will support a variety of safety critical control applications with different types of timing requirements, as evidenced by the emergence of metrics related to information freshness and information value. Additionally, ensuring ultra-high reliability for safety critical control applications requires modeling and estimation of the tail statistics of the wireless channel, queue length, and delay. The fulfillment of these stringent requirements calls for the development of novel AI-based techniques, incorporating optimization theory, explainable AI, generative AI and digital twins. The third part will analyze the coexistence of massive connectivity and critical services. We will consider scenarios in which a massive number of devices need to support traffic patterns of mixed criticality. This will be followed by a discussion about the management of wireless resources shared by services with different criticality.

Wireless 6G Connectivity for Massive Number of Devices and Critical Services

In this paper, we demonstrate the communication capabilities of light-fidelity (LiFi) systems based on high-brightness and high-bandwidth integrated laser-based sources in a surface mount device (SMD) packaging platform. The laser-based source is able to deliver 450 lumens of white light illumination and the resultant light brightness is over <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\bf \text{1000}~cd/mm^{2}$</tex-math></inline-formula> . It is demonstrated that a wavelength division multiplexing (WDM) LiFi system with ten parallel channels is able to deliver over 100 Gbps data rate with the assistance of Volterra filter-based nonlinear equalisers. In addition, an aggregated transmission data rate of 4.8 Gbps has been achieved over a link distance of 500 m with the same type of SMD light source. This work demonstrates the scalability of LiFi systems that employ laser-based light sources, particularly in their capacity to enable high-speed short range, as well as long-range data transmission.

100 Gbps Indoor Access and 4.8 Gbps Outdoor Point-to-Point LiFi Transmission Systems Using Laser-Based Light Sources

A New Family of Nyquist Pulses with Improved Performance

A Schottky barrier diode (SBD) based on GaAs thin-film substrate is designed and manufactured. The effective method for extracting intrinsic and parasitic parameters is discussed in detail through the development of an equivalent model, which facilitates the correction of diode measured results and performance optimization. Then, an innovative high-frequency equivalent model of suspended microstrip line (SML) is proposed first and an equivalent conductivity is introduced. Based on the corrected intrinsic parameters of SBD as well as the SML equivalent conductivity, a subharmonic mixer chip is designed, manufactured, and measured. A minimum dual-sideband (DSB) conversion loss (CL) of 8.23 dB can be obtained in upconversion measurement, as well as a radio frequency (RF) bandwidth up to 80 GHz. In addition, a minimum single-sideband (SSB) CL of 10.2 dB can be obtained in the downconversion measurement. Furthermore, an intermediate frequency (IF) bandwidth above 20 GHz can be obtained at the fixed local oscillator (LO) frequency of 260 GHz. The excellent up/downconversion measured results indicate that the mixer chip has important applications in terahertz communication and imaging systems.

A 500-GHz+ GaAs-Based Subharmonic Mixer Utilizing Terahertz Thin-Film Monolithic Integrated Circuit Technology

Terahertz and optical wireless communications (OWC) technologies have been considered as promising enablers that have potential to contribute equally to the realization of a sixth-generation (6G) wireless future. Experimental demonstrations show that, although inherently different, both technology types can support multi-kilometer Tbit/s wireless data transmission, which is a key goal set by almost all 6G roadmaps. Although both technology types can contribute equally to the realization of 6G roadmaps, to date, the development of these technologies for communications and non-communications applications has been ongoing independently. This is mainly because the distant similarity between both technology types masks their competitive performance. This article complements efforts made by academia and industry to explore the potentials and challenges of terahertz and OWC technologies as key enablers for 6G applications. Specifically, we weigh up the pros and cons of both technology types based on selected key metrics. The purpose of these comparisons is to highlight application areas in which strengths of these technologies could potentially be integrated.

Optical and Terahertz Wireless Technologies: the Race to 6G Communications

We propose and experimentally demonstrate a Nyquist pulse that can improve the error rate of a 311 GHz photonic-terahertz communications system by more than an order of magnitude at a normalized timing-jitter of 22.5% and 1.44 Gbit/s bit rate.

Mitigating the Timing-Jitter in Terahertz Communications via Nyquist Pulse Shaping

We experimentally demonstrate a highly-precise equalization technique based on a Volterra nonlinear filter for highly nonlinear terahertz communications systems. The technique is demonstrated with a 253 GHz photonics-based terahertz system with 10-km radio-over-fiber link.

Mitigating the Nonlinearity of Radio-over-Fiber Terahertz Systems

Underwater optical wireless communication (UOWC) systems can provide high-speed underwater data transmission links. Here we investigate the impact of previously overlooked wavy water surface on the bit-error-rate (BER) of non-line-of-sight (NLOS) UOWC systems, providing more accurate performance for investigating and developing UOWC systems.

Investigation of wavy surface impact on non-line-of-sight underwater optical wireless communication

The emergence of 6G wireless networks brings a lot of potential in advanced wireless applications, but also faces plenty of challenges. This work aims to address the challenges posed by the emerging 6G networks, specifically in the terahertz band (252-325 GHz). The goal is to meet the demanding requirements of high data rates, low latency, and increased reliability. To overcome this, this work proposes an ML model for enhanced signal processing that incorporates domain knowledge of the terahertz field into the traditional ML architecture. The proposed model is demonstrated to be able to increase linearity of the received signal by 68.3% compared to the Volterra filtering method, and a linearity increase of 23.9% compared to a benchmark ResNet model without domain knowledge.

Improving Signal Quality in Terahertz Communications with Neural Networks

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