Ultra-high bandwidth, negligible latency and seamless communication are envisioned as milestones that will revolutionize the way by which societies create, distribute and consume information. The remarkable expansion of wireless data traffic has advocated the investigation of suitable regimes in the radio spectrum to satisfy users’ escalating requirements and allow the exploitation of massive capacity and massive connectivity. To this end, the Terahertz (THz) frequency band (0.1-10 THz) has received noticeable attention in the research community as an ideal choice for scenarios involving high-speed transmission. As such, in this work, we present an up-to-date review paper to analyze key concepts associated with the THz system architecture. THz generation methods are first addressed by highlighting the recent progress in the devices technology. Moreover, the recently proposed channel models available for propagation at THz band frequencies are introduced. A comprehensive comparison is then presented between the THz wireless communication and its other contenders. In addition, several applications of THz communication are discussed taking into account various scales. Further, we highlight the milestones achieved regarding THz standardization activities. Finally, a future outlook is provided by presenting and envisaging several potential use cases and attempts to guide the deployment of the THz frequency band.

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Terahertz Band: The Last Piece of RF Spectrum Puzzle for Communication Systems

Optical wireless communication (OWC) is an excellent complementary solution to its radio frequency (RF) counterpart. OWC technologies have been demonstrated to be able to support high traffic generated by massive connectivity of the Internet of Things (IoT) and upcoming 5th generation (5G) wireless communication systems. As the characteristics of OWC and RF are complementary, a combined application is regarded as a promising approach to support 5G and beyond communication systems. Hybrid RF/optical and optical/optical wireless systems offer an excellent solution for recovering the limitations of individual systems as well as for providing positive features of each of the technologies. An RF/optical wireless hybrid system consists both RF and optical-based wireless technologies, whereas an optical/optical wireless hybrid system consists two or more types of OWC technologies. The co-deployment of wireless systems can improve system performance in terms of throughput, reliability, and energy efficiency of individual networks. This study surveys the state of the art and key research directions regarding optical wireless hybrid networks, being the first extensive survey dedicated to this topic. We provide a technology overview of existing literature on optical wireless hybrid networks, such as RF/optical and optical/optical systems. We consider the RF-based macrocell, small cell, wireless fidelity, and Bluetooth, as well as optical-based visible light communication, light fidelity, optical camera communication, and free-space optical communication technologies for different combinations of hybrid systems. Moreover, we consider underwater acoustic communication for hybrid acoustic/optical systems. The opportunities brought by hybrid systems are presented in detail. We outline important challenges that need to be addressed for successful deployment of optical wireless hybrid network systems for 5G and IoT paradigms.

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Optical Wireless Hybrid Networks: Trends, Opportunities, Challenges, and Research Directions

Radio-over-fiber technology provides a simpler pathway for the distribution of wireless signals in broadband wireless networks via optical means. The technology has evolved over the last 30 years, with many challenges already overcome and with many more to address. This paper provides an overview of the technology evolutionary path of RoF with a review of its past, evaluation of its present, and a discussion of its challenges into the future.

Evolution of Radio-Over-Fiber Technology

Ultra-high bandwidth, negligible latency and seamless communication for devices and applications are envisioned as major milestones that will revolutionize the way by which societies create, distribute and consume information. The remarkable expansion of wireless data traffic that we are witnessing recently has advocated the investigation of suitable regimes in the radio spectrum to satisfy users' escalating requirements and allow the development and exploitation of both massive capacity and massive connectivity of heterogeneous infrastructures. To this end, the Terahertz (THz) frequency band (0.1-10 THz) has received noticeable attention in the research community as an ideal choice for scenarios involving high-speed transmission. Particularly, with the evolution of technologies and devices, advancements in THz communication is bridging the gap between the millimeter wave (mmW) and optical frequency ranges. Moreover, the IEEE 802.15 suite of standards has been issued to shape regulatory frameworks that will enable innovation and provide a complete solution that crosses between wired and wireless boundaries at 100 Gbps. Nonetheless, despite the expediting progress witnessed in THz wireless research, the THz band is still considered one of the least probed frequency bands. As such, in this work, we present an up-to-date review paper to analyze the fundamental elements and mechanisms associated with the THz system architecture.

Client-side optics are facing an ever-increasing upgrading pace, driven by upcoming 5G related services and datacenter applications. The demand for a single lane data rate is soon approaching 200 Gbps. To meet such high-speed requirement, all segments of traditional intensity modulation direct detection (IM/DD) technologies are being challenged. The characteristics of electrical and optoelectronic components and the performance of modulation, coding, and digital signal processing (DSP) techniques are being stretched to their limits. In this context, we witnessed technological breakthroughs in several aspects, including development of broadband devices, novel modulation formats and coding, and high-performance DSP algorithms for the past few years. A great momentum has been accumulated to overcome the aforementioned challenges. In this article, we focus on IM/DD transmissions, and provide an overview of recent research and development efforts on key enabling technologies for 200 Gbps per lane and beyond. Our recent demonstrations of 200 Gbps short-reach transmissions with 4-level pulse amplitude modulation (PAM) and discrete multitone signals are also presented as examples to show the system requirements in terms of device characteristics and DSP performance. Apart from digital coherent technologies and advanced direct detection systems, such as Stokes–vector and Kramers–Kronig schemes, we expect high-speed IM/DD systems will remain advantageous in terms of system cost, power consumption, and footprint for short reach applications in the short- to mid- term perspective.

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200 Gbps/Lane IM/DD Technologies for Short Reach Optical Interconnects

We experimentally demonstrate the photonics-aided wireless transmission of 4 × 4 multiple-input multiple-output probabilistic shaping 64-ary quadrature-amplitude-modulation (PS 64QAM) millimeter-wave signals at D-band (110–170 GHz) over 3.1-m distance with a total bit rate of 1.056 Tb/s and a bit-error ratio under 4 × 10−2. The employment of advanced digital-signal-processing techniques, including probabilistic shaping, the Nyquist shaping, and look-up-table predistortion, significantly improves the transmission capacity and distance as well as the system performance. To the best of our knowledge, this is the first time to realize >1-Tb/s millimeter-wave signal wireless delivery.

1-Tb/s Millimeter-Wave Signal Wireless Delivery at D-Band

We propose and experimentally demonstrate a photonics-aided 2 × 2 multiple-input multiple-output (MIMO) wireless Terahertz-wave (THz-wave) signal transmission system, which realizes 6 × 20-Gb/s six-channel polarization division multiplexing quadrature-phase-shift-keying THz-wave signal delivery over 10-km wireline single-mode fiber- link and 142-cm wireless 2 × 2 MIMO link with a bit-error ratio under the hard-decision forward-error-correction threshold of 3.8 × 10−3. Our employed multi-carrier frequencies are located within the range of 375 GHz to 500 GHz. To the best of our knowledge, this is the first experimental demonstration of 2 × 2 MIMO wireless transmission of multi-channel THz-wave signal. Here, it is worth noting that our wireless 2 × 2 MIMO link, which offers point-to-point straight transmission and brings neither interference nor gain, is different from the traditional MIMO link defined in the field of wireless communications.

120 Gb/s Wireless Terahertz-Wave Signal Delivery by 375 GHz-500 GHz Multi-Carrier in a 2 × 2 MIMO System

The increased bandwidth demand has motivated the exploration of fiber-wireless integration (FWI) for future broadband 5G+ cellular communication networks. FWI offers ultra-wideband (UWB) wireless delivery with low interference, which will be prospective for 5G/5G+ mobile communication wireless access, military application, disaster emergency communication, broadband communication at home, and so on. As an effective carrier, millimeter-wave (mm-wave) frequencies between 30 GHz and 300 GHz are a new frontier for FWI that offers the promise of orders of magnitude greater bandwidths. In this paper, we summarize all kinds of enabling technologies for FWI, including the photonic vector mm-wave generation scheme, the integration of various multi-dimensional multiplexing techniques, radio-frequency-transparent (RF-transparent) photonic demodulation technology for fiber-wireless-fiber network, and low-complexity high-efficiency digital signal processing (DSP). Based on DSP for UWB high-spectrum-efficiency signal coherent detection, we have made great progress in the field of the mm-wave-band (from Q- to D-band) broadband signal generation and long-distance transmission. These experimental results show that FWI with large-capacity, long-distance, and high-spectrum-efficiency has important scientific and practical significance for the development of the future 5G+ wireless communication.

Tutorial: Broadband fiber-wireless integration for 5G+ communication

Nonlinearities induced by the electrical amplifiers and the optoelectronic devices can be detrimental effects in visible light communication (VLC) systems. In this paper, clustering algorithm based perception decision (CAPD) is proposed to mitigate the nonlinear distortion in a VLC system. Aided by CAPD nonlinear compensation, we experimentally demonstrate a multiband CAP modulated VLC system consisting of a red light-emitting diode as a transmitter and a p-i-n photodiode based differential receiver. The system performances including the Q factor, bit error rate (BER), and computational complexity are thoroughly investigated when using a pure linear blind equalization scheme (modified cascaded multimodulus algorithm, M-CMMA) and when using hybrid linear and nonlinear equalizers (M-CMMA + Volterra series based nonlinear equalizer). The experiment results show that compared to pure linear equalizer case, the measured BER can be enhanced up to 1e−6, correspondingly the Q factor of each subband can be improved for around 1.6–2.5 dB by employing CAPD. The CAPD method can outperform the Volterra series based nonlinear equalizer with a lower BER value (at least 10% reduction) and relatively lower complexity. To the best of our knowledge, this is the first time that the clustering algorithm in machine learning is successfully applied to VLC systems.

Nonlinear Compensation of Multi-CAP VLC System Employing Clustering Algorithm Based Perception Decision

We experimentally demonstrated single-lane EML-based IM/DD 106-Gbaud PAM-4 and PS-PAM-8 signals transmission over 1-km NZDSF based on Pre-EQ and clipping technique. 260-Gb/s PS-PAM-8 signal transmission can be achieved.

Wen Zhou

Papers

1-Tb/s Millimeter-Wave Signal Wireless Delivery at D-Band

120 Gb/s Wireless Terahertz-Wave Signal Delivery by 375 GHz-500 GHz Multi-Carrier in a 2 × 2 MIMO System

Tutorial: Broadband fiber-wireless integration for 5G+ communication

Nonlinear Compensation of Multi-CAP VLC System Employing Clustering Algorithm Based Perception Decision

Demonstration of 260-Gb/s Single-Lane EML-Based PS-PAM-8 IM/DD for Datacenter Interconnects