Beyond 100 Gb/s Optoelectronic Terahertz Communications: Key Technologies and Directions

Nowadays, many wireless communication applications and services demand high capacity and bandwidth. Experts expect that in next few years, communications markets need to use high data rates up to 40 Gbps. For meeting the requirements of high data rate, bands up to 3 THz hold promise as they can achieve data rates up to a few tens of Gbps. The bands ranging from 275 to 3,000 GHz are known as terahertz (THz) bands. THz bands are not assigned yet globally for dedicated services and communication applications. The implementation of THz communication requires various kinds of evaluations and effort in order to assess different aspects related to channel characterizations, THz system designs, propagation, algorithms and techniques for signal processing, platform configurations, in addition to security and privacy issues. The evaluation and assessment should also consider the statistical parameters and measurements of the channels. This chapter introduces the key aspects of THz channel specifications and related considerations. In addition, a comparison between mm-waves and THz channel characteristics is made. The chapter also discusses the theoretical concepts of THz channel characterizations, limitations, requirements, and methodologies for measurements including a comparison between different channel analyses. Moreover, the chapter provides a brief description of THz channel characterization in nano communications.

The ever-growing number of interconnected devices and the emergence of high-bandwidth technologies such as holographic communications in mixed reality are driving an exponential growth in the demand for terabits-per-second (Tbps) data rates with sub-milliseconds latency. For this reason, the sixth-generation (6G) wireless communications are moving towards the terahertz (THz) frequency band because of its wider spectral bandwidth availability to enable higher channel capacity and data rates according to the Shannon-Hartley theorem. However, the efficiency of wireless THz signal transmission is reduced by the increased path loss and atmospheric attenuation due to water vapor absorption at THz frequencies, which significantly decreases the signal-to-noise ratio and the spectral efficiency of THz wireless communications. The realization of THz band communications is further impeded by the limited power output of existing compact THz sources due to the extremely low conversion efficiencies. Therefore, it is crucial to develop broadband THz integrated circuits with waveguiding and beamforming functionalities for supporting high data-rate on-chip communications and to provide sufficient gains to counteract the path loss and limited power of THz wireless transmissions. This thesis aims to improve the spectral efficiency of 6G wireless communications via the spatiotemporal shaping of THz waves using silicon photonics. Spatiotemporal light shaping is the basis of nearly all THz photonic devices, ranging from THz wavefront manipulation using metasurfaces to THz planar waveguiding using photonic crystal waveguides. Silicon photonics is a promising material platform for THz wave shaping due to the low-loss propagation of THz waves in high-resistivity silicon and the prospect of a monolithic integrated circuit due to the complementary metal-oxide semiconductor (CMOS) fabrication process compatibility. Silicon metasurfaces allow the wavefront modulation of incident THz radiation by exploiting the generalized Snell’s law of refraction and reflection to beamform highly directional THz waves in a dynamic multi-user communication environment for enhancing the spectral efficiency. Likewise, broadband silicon photonic crystal waveguides allow the in-plane and lossless transport of THz waves to support on-chip high data-rate THz communications that can also seamlessly integrate multiple THz photonic devices in a THz integrated circuit. In addition, silicon electron emitters and silicon gratings potentially allow the on-chip generation of highly intense and broadband THz radiation to counteract the significant THz attenuation. In this thesis, spatiotemporal wave shaping techniques using silicon photonics for enhancing the spectral efficiency of THz band communications is presented. First, a self-adaptive and deep reinforcement learning model for THz beamforming is proposed, which can beamform multiple highly directional THz radiation beam in real-time to serve a dynamically changing multi-user communication environment and improve the signal-to-noise ratio of THz wireless transmission while potentially reducing the communication latency. Second, a broadband THz topological silicon waveguide designed based on valley photonic crystals is proposed for the robust and lossless transport of broadband THz waves for the construction of THz integrated circuits with broadband channel capacity for enabling higher data-rate on-chip communication. Last, the versatility of free-electron radiation from silicon gratings is investigated for the generation of highly intense spatiotemporal wave packets with user-designed group velocities and tunable intensity profile, with the potential to realize integrated high-power THz sources. The deep learning beamforming model, the broadband topological waveguide designs, and the free-electron radiation intensity shaping techniques presented in this thesis extends beyond the THz regime and are scalable across the entire electromagnetic spectrum. The CMOS process compatibility of silicon metasurfaces, silicon planar waveguides, along with silicon electron emitters and silicon gratings offers the opportunity to realize a broadband THz photonic integrated circuit with waveguiding, wave generation and beamforming functionalities for enhancing the spectral efficiency of THz communications in 6G wireless networks.

Terahertz (THz) communication is one of the remarkable topics in the field of communication. Terahertz communication, which is one of the promising topics in meeting the rapidly increasing number of devices and the need for data speed and channel capacity, aims to increase the wireless communication frequency band up to terahertz level. When the terahertz frequency is used the wavelength decreases significantly, which causes electromagnetic waves to be more affected by the channel effects. There must be a clear line of sight (LOS) between the transmitter and the receiver in THz communication systems and the electromagnetic wave sent must be highly directive. The use of intelligent reflective surfaces in the terahertz band can provide advantages in the reflected wave being more directive. In this study, different Intelligent Reflective Surface designs in the literature are implemented and a performance comparison is presented in terms of beam characteristics.

Focused THz wave from a spintronic terahertz Fresnel Zone Plate emitter

High modulation depth fiber-based modulators working at terahertz (THz) wavelength are of paramount importance for the application of THz communication technology by taking advantages of the low loss, high interference resistance and strong adaptability THz fiber network. Here, we demonstrate a THz modulator based on side-polished THz fiber combined with GaAs film/Au-nanoparticles (Au-Nps). The Au-Nps and the remaining thickness of the side-polishing are careful designed to achieve target resonance wavelengths. The side-polishing region of fiber provides a flat platform for integrating nanostructures to enhanced light-matter interactions which greatly enhances the modulation performance. The experimental results show that this new fiber modulator can achieve the THz wave amplitude modulation under a relatively low irradiation of an external 808 nm laser, with a tunable frequency range of 0.366-1.093 THz and a maximum modulation depth of 99.24%. Besides, resonance frequency can be adjusted by changing the remaining thickness of the device. Moreover, the area of this modulator is only 9 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> which can be easily integrated into THz systems. Therefore, we believe our work will contribute to the developments of high-performance fiber-based THz modulator for applications in THz communication system.

Enabling terahertz communications will alleviate the spectrum limitation of current wireless systems. This article presents an indoor multiuser terahertz system with array-of-subarrays architecture to accommodate hardware constraints as well as channel characteristics in the terahertz band. Specifically, the difference between terahertz and millimeter-wave communications is first clarified. Then the advantage of the array-of-subarrays structure is explained through comparison with the fully connected structure in both spectral and energy efficiency. Furthermore, a distance-aware multi-carrier scheme with the array-of-subarrays structure is introduced for wideband terahertz communications. Finally, the associated open issues and future research directions are discussed.

Chapter 2 - High data rates in mobile communication

The advent of wireless communications technology, human connection and collaborations now span countries, continents, and even planets. Despite these advances, however, today's wireless technology still experiences limitations. In the past two decades, terahertz (THz) communications have been researched in depth, and it has become clear that THz communications will be part of expanding the capabilities of wireless technology. Although there was once a substantial gap in knowledge as well as transmission and reception capabilities at THz frequencies, this gap has been steadily closing. Some challenges, still remain; namely, the need to characterize and design for near-field propagation as well as the ability to take advantage of spatial diversity despite the primarily line-of-sight channel. In this proposal, these challenges begin to be addressed. The sub-THz MIMO channel is characterized using a novel channel-sounding implementation, and then a practical physical layer implementation to take advantage of the spatial diversity of the MIMO channel is presented. Then, a research roadmap for enabling broadband, mobile THz communications in the near-field followed by experimental results that characterize the near-field propagation characteristics with both Gaussian and non-diffracting Bessel beams. The performance of Bessel beams are further explored for sub-THz communications in the presence of blockage and shown to be a viable option for broadband, near-field communications. Other wavefront engineering techniques to enable line-of-sight MIMO using orbital angular momentum are discussed from a physical-layer-security perspective, and a novel successive-interference-cancellation eavesdropping attack is proposed along with a potential counter measure. Finally, concluding remarks are given. --Author's abstract

In the modern wireless communication system high data rates and more bandwidth efficiencies is required. Apertures for different inventive applications will be made in automotive, medical accommodations, industry etc. which is because of the incipient 5G requisites, which recollect 1000× increment for limit and 10× higher data rates, among others. In this paper a microstrip patch antenna is proposed for 60 GHz milimeter wave next generation of wireless connectivity in 5G communication. This propsed antenna is designed with dimanisation of 4 × 4 × 0.035 mm using rogers RO3003 substrate with dielectric constant (ɛr) = 3, loss tangent (tan δ) = 0.001. KeywordsmmWaveWireless communicationAntennaCellular communication5G technologyHigher data rates

Terahertz (THz) communication networks, combined with Intelligent Reflecting Surfaces (IRS), hold significant potential for advancing the capabilities of Sixth-Generation (6G) wireless systems. The integration of these technologies offers opportunities for ultra-fast data transmission and substantial improvements in network capacity, meeting the growing demand for higher bandwidth and faster connectivity. This study investigates the use of flying IRS-assisted Unmanned Aerial Vehicles (UAVs) within a THz communication framework, aiming to overcome challenges in environments with complex signal propagation, such as urban settings. To facilitate this integration, we introduce the Fly-IRS algorithm, designed to optimize key aspects of network performance, including user device grouping, IRS phase shifts, and UAV positioning. These factors are critical for maximizing data transmission rates, boosting network capacity, and minimizing service disruptions that affect user experience. Our approach leverages a combination of Multi-Agent Deep Reinforcement Learning (MADRL) and Particle Swarm Optimization (PSO) to effectively handle user grouping, IRS phase shift optimization, and UAV trajectory planning. By merging the strengths of both techniques, the Fly-IRS algorithm enhances system adaptability and efficiency. Simulation results demonstrate significant improvements in data rates, with the Fly-IRS algorithm delivering up to 10% better performance compared to the MADRL method alone. These findings highlight the transformative potential of integrating THz communication networks with IRS and UAV technologies, emphasizing their ability to shape future wireless communication systems. This innovation could lead to more reliable, high-capacity networks, capable of addressing the advanced requirements of 6G, offering improved service quality even in difficult environments.

Digital revolution and recent advances in telecommunications technology enable to design communication systems which operate within the regions close to the theoretical capacity limits. Ever-increasing demand for wireless communications and emerging numerous high-capacity services and applications mandate providers to employ more bandwidth-oriented solutions to meet the requirements. Trend and predictions point out that marketplace targets data rates around 10Gbps or even more within the upcoming decade. It is clear that such rates could only be achieved by employing more bandwidth with the state-of-the-art technology. Considering the fact that bands in the range of 275GHz-3000GHz, which are known as Terahertz (THz) bands, are not allocated yet for specific active services around the globe, there is an enormous potential to achieve the desired data rates. Although THz bands look promising to achieve data rates on the order of several tens of Gbps, realization of fully operational THz communications systems obliges to carry out a multidisciplinary effort including statistical propagation and channel characterizations, adaptive transceiver designs, reconfigurable platforms, advanced signal processing algorithms and techniques along with upper layer protocols equipped with various security and privacy levels. Therefore, in this study, several important statistical parameters for line-of-sight (LOS) channels are measured. High resolution frequency domain measurements are carried out at single-sweep within a span of 60GHz. Impact of antenna misalignment under LOS conditions is also investigated. By validating exponential decay of the received power in both time and frequency domain, path loss exponent is examined for different frequencies along with the frequency-dependent path loss phenomenon. Furthermore, impact of humidity is also tested under LOS scenario. Measurement results are presented along with relevant discussions and future directions are provided as well.

The rapid development of various advanced wireless applications in diversified fields makes the microwave spectrum congested, thereby demanding higher frequency bands (terahertz) with high transmission data rates (tbps) for next-generation wireless systems. Terahertz frequency band will eliminate communication spectrum shortages and lead to various novel applications at terahertz radiation, such as advanced wireless sensing and next-generation medical applications, detection of explosive materials, etc. The stable terahertz communication link can play a crucial role to achieve higher data rate transmission within a short distance. Antenna is an integral, essential element in any wireless communications system, and choosing the right antenna is critical to the successful use of wireless communication links. However, almost all the terahertz antennas suffer from high attenuation losses, which drive the requirement of high gain and wideband antennas. In this chapter, the various applications at terahertz radiation, the assorted antennas for terahertz frequencies, and microstrip terahertz antennas for detection of explosive materials with novel methods that achieved high gain and good bandwidth and their designs and development challenges are discussed.KeywordsMicrowave spectrumHigh transmission data rateNext-generation wireless systemsTerahertz antennasAttenuation lossesApplications at terahertz radiationDetection of explosivesMedical applications

The nature of future wireless applications demands high data rates. Naturally dealing with ever-unpredictable wireless channel at high data rate communications is not an easy task. The idea of multi-carrier transmission has surfaced recently to be used for combating the hostility of wireless channel and providing high data rate communications. OFDM is a special form of multi-carrier transmission where all the subcarriers are orthogonal to each other. OFDM promises a high user data rate transmission capability at a reasonable complexity and precision. At high data rates, the channel distortion to the data is very significant, and it is somewhat impossible to recover the transmitted data with a simple receiver. A very complex receiver structure is needed which makes use of computationally expensive equalization and channel estimation algorithms to correctly estimate the channel, so that the estimations can be used with the received data to recover the originally transmitted data. OFDM can drastically simplify the equalization problem by turning the frequency-selective channel into a flat channel. A simple one-tap equalizer is needed to estimate the channel and recover the data. Future telecommunication systems must be spectrally efficient to support a number of high data rate users. OFDM uses the available spectrum very efficiently which is very useful for multimedia communications. For all of the above reasons, OFDM has already been accepted by many of the future generation systems [1].

The large amount of unlicensed bandwidth available in the millimeter-wave has enabled very high data rate wireless applications. The IEEE 802.15 Task Group 3c has completed standardization efforts for multi-gigabit data rate communications on both the physical (PHY) and medium access control (MAC) layers. This paper presents a performance evaluation of the PHY and MAC layers of the newly published standard. Packet error rate (PER) and PHY throughput are simulated for different modes based on a typical 60 GHz channel model. The theoretical MAC throughput is calculated for different packet sizes and modes under different link conditions. The link adaptation mechanism is used to analyze the operating range versus data rate. The results show that very high data rates can be achieved over low distances.

In the modern world, bandwidth and data rates are two most important parameters under consideration in any communication system. An optical fiber communication system is capable of transmitting the data at high bit rates and has a large bandwidth capacity. This high data rate and bandwidth capacity in an optical fiber communication system is further enhanced by using wavelength division multiplexing (WDM) in conjunction with the information transmission system, in which information from multiple sources are transferred using the same fiber, at the same time, but at different wavelengths. But the high capacity and data rates in optical WDM system is limited by nonlinear effects that occur in optical WDM system, which not only pose a limitation on the channel capacity and data rates but also degrades the performance of the data transmission system. In this paper, analysis has been done to investigate the efficiency of Four Wave Mixing (FWM) effect under the influence of different system parameters such as chromatic dispersion coefficient, channel spacing, transmission power level, effective area of fiber and fiber length in the form of input signals and output signals frequency spectrums.