Why Terahertz Communication is the Future of Wireless Networks

Terahertz communication

Terahertz communication is an emerging and fast-growing wireless technology category that operates within the terahertz (THz) frequency range. Frequencies between 0.3 and 30 THz are typically classified as belonging to this range, bridging the gap between microwaves and infrareds in the electromagnetic spectrum. As we increasingly demand for faster and more effective wireless communications, terahertz communication is expected to be one of the key future systems such as next generation of 6G networks.

What is Terahertz Communication?

Terahertz communication employs extremely high-frequency waves known as terahertz waves for wireless communications. These waves have higher frequencies than those used for Wi-Fi and phones but lower frequencies than those for remote controls based on Infrared light.

Frequency Range

• Terahertz Frequency Band: This band covers a range of 0.3 – 30 THz.
• Electromagnetic Spectrum: This includes all types of electromagnetic radiation such as radio waves, X-rays, gamma rays etc. Microwaves lie below terahertz waves while infrared rays come after them.

Why Terahertz Communication?

High Data Rates: Waves in the terahertz range can carry many more data signals compared to their low-frequency counterparts reaching up to trillions bits per second (Tbps). Hence, they find applications where large amount of data would be necessary like

Current and Future Applications

Current and Future Applications

• 6G Networks: The future 6G networks are projected to employ terahertz communication, which will support ultra-fast wireless connections. This will facilitate advanced technologies such as virtual reality (VR), augmented reality (AR) and Internet of Things (IoT).
• Imaging and Sensing: Terahertz waves can penetrate some materials making them suitable for imaging and sensing applications. For instance, they could be used in security screening to detect concealed objects or in medical imaging to reveal the structure of soft tissues without the harmful effects of x-rays.
• Terahertz waves can also go through nonmetallic materials, allowing for non-destructive testing of products and materials.
• Space and Satellite Communication: Terahertz communication is capable of providing high-speed data transfer between spacecrafts and earth thereby enhancing space mission capabilities. Terahertz frequencies are applicable to satellite-to-satellite communication as well as satellite-to-ground links hence resulting in higher data rates while minimizing latency.

Challenges

• Atmospheric Absorption: Water vapor in the air absorbs terahertz waves, which can limit their range. This makes it difficult to use them over long distances or in humid conditions.
• Technology Development: Creating devices that can efficiently generate, detect and process terahertz waves is technically difficult. Overcoming these barriers calls for better materials and technologies as per the researchers.

Technological Advances

• Sources and Detectors: There are ongoing innovations towards creating efficient sources and detectors of terahertz waves. These involve new types of lasers as well as sensors that can handle high frequencies and data rates.
• Antennas and Modulators: Furthermore, current research also focuses on designing small-sized antennas and modulators that work at terahertz frequencies.

The Origin and Evolution of THz Communications

The roots of terahertz (THz) communications can be traced back to 1999-2000. In 1999, Lee et al. demonstrated an “impulse” radio concept using a photoconductive switch triggered by a mode-locked Ti: Sapphire laser. This approach was later employed in audio-signal transmission via indirect and direct modulation. Simultaneously, Nagatsuma et al. presented the photonic generation of “continuous” sub-terahertz waves at 120 GHz by using a photodiode triggered by a mode-locked diode laser and applied it to video-signal transmission in 2000. Throughput had reached 10 Gbit/s by 2002 which meant a significant improvement in wireless communication speeds.

Semiconductor electronics technologies especially the use of InP-HEMT transistor technology have propelled since then the development of this 120-GHz band wireless link. These advances addressed issues of propagation distance, stability, size, cost and usability among others. On January 30th, 2014 Japan’s Ministry of Internal Affairs and Communications (MIC) performed an amendment to radio regulations making the first industry allocation of over-100-GHz carrier frequencies to broadcasting services for the 120-GHz wireless link in order to broadcast service via its national frequency planff.. Data rates are now up to 20 Gbit/s with QPSK modulation.

THz communication system architecture

Terahertz Transceivers

• Electronic Devices: THz waves are located between MM-wave and optical waves, and it is possible to obtain them with electronic and photonics means. Lately, new approaches in nano-fabrication have made it possible to create such semiconductor devices as GaAs and InP electronics together with Si-based technologies that can function in the THz band. A unique development is Silicon architectures that are innovative and effectively design and build the oscillators, antenna, and amplifier of THz on one microchip thus solving the problem of THz wave generation and having a large range of THz wavelengths.
• Photonics-Based Devices: These devices utilize lasers, modulators and photodiodes to produce and control THz waves in the systems. The combination of MF with THz emission circuits improves data transfer rates. Photonics-based techniques can cope with multi-carrier THz channels because the techniques can switch between carriers, due to which they are suitable for hybrid networks.
• Antenna Arrays: THz frequencies enable the placement of extensive antenna arrays on terminal tools and devices. Thus, diversity and directivity gains by Multiple Input Multiple Output (MIMO) methods can be achieved. In THz communication systems, seven tunes array such as 1024 × 1024 has been deployed to enhance the communication distance.
• Graphene-Based Components: Such materials used at lower frequencies are ineffective at high THz frequencies resulting to high loss. A complex like graphene which is atomic in thickness, can be tuned and possess high kinetic inductance does present a solution. Graphene plasmonics is applied for the generation, modulation and detection of Terahertz waves and open the possibilities to build new devices such as subwavelength waveguides, nanoantennas and light concentrators.

Terahertz Channel Model

• Channel Characteristics: The THz frequency band has high atmospheric absorption as a result of water vapor, and large free-space path loss and therefore, the channel is very selective of frequency. This restricts the distance over which the signal can be transmitted and also calls for proper carrier selection depending on the application in question.
• Modeling Approaches: Today’s channel models are chiefly constrained to indoor settings. These are the path loss and ray tracing to model the THz channel. One of them extends from 275 to 325 GHz and uses multiple ray tracing calculations to obtain mean values based on statistical analysis; however, it does not compare channel statistics such as correlation functions or power, delay profiles.
• Scenario-Specific Models: Other models for such applications as kiosk environments are also exist. These models give the characteristics of the channels for close-proximity communication systems in THz frequencies, which are useful to design the transceiver systems. Some of the recent works that have proposed statistical models for the sub-THz band D2D scatter channels and for kiosk cases with the help of 3D ray tracing simulators.

Summary of THz Band Technologies (source: repository.kaust.edu.sa/)

Patents in Terahertz communication

Zhejiang University ZJU – Tunable Terahertz Pulse Emitter

The patent CN108712214B addresses the lack of spectrum resources primarily due to the constantly growing demand for data connection. In traditional schemes of terahertz pulse generation like ultra short optical pulse irradiation and photoconductive antennas, the transfer efficiency is low because very high optical peak power is needed. Also, current terahertz pulse emission systems have poor controllability for some crucial pulse parameters like middle frequency, pulse repetition frequency, and pulse width to meet present different application requirements.

To address these issues, the patent presents a tunable mostly band terahertz pulse wireless communication emitter that contains a frequency comb generation module and a program calling optical processor. The frequency comb generation module provides a stable and tunable frequency comb which its variability can be programmed by the programmable optical processor to form several channels. All the channels include a single-wavelength light local oscillator and a multi-wavelength light carrier signal so that the generation of terahertz pulses can be adopted with high accuracy. Baseband signals are modulated on to the multi-wavelength light carrier signals while the photodetectors convert into multiband terahertz pulses. This will enable the achievement of better control and tuning of certain vital parameters that include the center frequency, pulse recurrence frequency, and pulse width of the terahertz pulses making the communication system very efficient and adaptable.

University of Electronic Science and Technology of China – Intelligent Reflecting Surface for Terahertz Communication

The patent US11218199B2 addresses challenges in terahertz communication, particularly the severe path attenuation and molecular absorption loss that limit terahertz communication to short-range indoor scenarios. Additionally, the strong directivity and poor diffraction of terahertz waves make them susceptible to being blocked by obstacles like furniture and walls. The patent proposes a solution utilizing an Intelligent Reflecting Surface (IRS) to improve the coverage range and spectral efficiency of terahertz communication systems. The IRS can change the direction of terahertz waves by adjusting phase shifts for each reflecting element, thus enhancing the secrecy rate of the communication system. Traditional methods to control phase shifts, like the exhaustive search method, are computationally complex.

The proposed solution optimizes the secrecy rate for an IRS-aided terahertz multi-input single-output (MISO) secure communication system using a cross-entropy based method for phase search, reducing the phase search complexity of IRS elements while improving the secrecy rate. This system includes a base station (BS) with multiple antennas, an IRS with numerous reflecting elements, a single-antenna authorized user, and a single-antenna eavesdropper. The BS transmits signals to the IRS, which adjusts and reflects these signals to the authorized user, while suppressing the signal received by the eavesdropper. The cross-entropy based method iteratively updates the phase shift matrix to approach the optimal value, thereby maximizing the secrecy rate with minimal computational complexity.

University of Michigan – Photoacoustic Detection of Terahertz Pulses

The patent US9759689B2 addresses issues of terahertz communication and the major problem for terahertz communication which is the path attenuation and molecular absorption loss that restricts the terahertz communication to short-range indoor applications. Moreover, the strong directivity and poor diffraction of terahertz waves make for their vulnerability to shielding by objects of everyday life such as furniture and walls. The patent describes a solution of adding an IRS to amplify the coverage range of the TCom system and enhance the spectral efficiency of the terahertz communication link. The IRS can alter the flowing direction of the terahertz waves by providing phase shift for each reflecting element thus increasing the secretive rate of the communication system. The earlier techniques of controlling phase shifts such as the exhaustive search method are time consuming.

The given solution aims at enhancing the secrecy rate of an IRS-aided terahertz MISO system adopted for secure communication and cuts down the unreasonable phase search procedure for the IRS elements based on the cross-entropy method suitable for the phase search procedure. This system contains a BS with several antennas, a large-sized IRS with many reflecting elements, a single-antenna licensed user, and a single-antenna unauthorized user. The BS sends information signals to the IRS which, in turn, processes and re-radiates these signals to the intended user/caller while at the same time minimizing the signal that gets to the intended eavesdropper. With fewer computations, the phase shift matrix is updated by using the cross entropy based strategy, and the method ensures that the secrecy rate is maximized with the best possible value of the phase shift matrix.

LG Electronics Inc – Orbital Angular Momentum in Wireless Communication Systems

The patent US11974263B2 addresses the challenge of increasing the communication capacity and improving beamforming performance in wireless communication systems. These classic architectures have issues in the authority of resource allocating as well as the beam control, including the high frequency bandwidth such as terahertz (THz). The proposed method uses OAM to increase the spectral efficiency and to enable the spatial multiplexing. OAM leads to the creation of various orthogonal beams that can be separately encoded and sent; this substantially enhances the data transfer rate and range of the communication. However, the first major issue encompasses the management of OAM states and the possibility for them to be detected.

To address these issues, the patent suggests a technique where a base station transmits a set of information on one or more OAM states and one or more OAM state subsets. This information is received by the user equipment (UE) and the later establishes a detectable OAM state from the received OAM beams. This method employs a precoding matrix (or codebook) to apply OAM to transmit detectable OAM states and attain the highest possible multiplexing gains. The system also provides antenna array to form the right-hand circular polarized beams which arranged in the form of ring arrays and helps in transmitting and receiving of OAM states. Thus, through OAM, the capacity and the accuracy of the beamforming are improved and thus the system is appropriate for the future generation wireless communication networks.