The terahertz band (0.3 THz to 10 THz) is the next frontier in wireless communications for its ability to unlock significantly wider segments of unused bandwidth. Though radio channels above 100 GHz are little known, several high-speed terahertz communication links have been demonstrated in recent years.
There is not much talk about terahertz communications because the industry is currently preoccupied with mmWave frequency bands (30 to 300 GHz) to offer multi-gigabit-per-second (Gbps) data rates for 5G mobile devices. Moreover, terahertz communication takes the problem that mmWave bands are currently facing to a whole new level: propagation loss caused by a tremendous amount of signal attenuation due to molecule absorption of electromagnetic waves.
Like mmWave communications, terahertz bands can be used as mobile backhaul for transferring large bandwidth signals between base stations. Another venue for fiber or copper replacement is point-to-point links in rural environments and macro-cell communications.
More importantly, terahertz bands can be employed in close-in communications, also known as whisper radio applications. That includes wiring harnesses in circuit boards and vehicles, nanosensors, and wireless personal area networks (PANs).
Then, there are applications like high-resolution spectroscopy and imaging and communication studies that require short-range communications in the form of massive bandwidth channels with zero error rate in crucial areas like coding, redundancy, and frequency diversity.
Figure 1: Terahertz communications will reinvent the wireless building blocks like RF front-ends, interconnects, transceivers, and antennas. Source: Research Gate
The above-mentioned use cases provide a glimpse of how terahertz communications can be a game changer by offering even lower latency than fiber networks. Additionally, terahertz bands can complement mmWave communications in the commercial realization of indoor wireless networks, position localization studies and gigabyte Wi-Fi support of internet of things (IoT) applications.
Terahertz Trials Underway
So, what is next for terahertz band communications? For a start, mmWave communication is accelerating wireless broadband deployment by removing barriers to small cell infrastructure deployment and investment. Once that happens, terahertz communications can significantly complement mmWave in indoor environments such as a hallway, meeting rooms, cubical offices, laboratories, and open areas.
Take, for instance, the propagation measurements in the 140 GHz D-band conducted by Aalto University at a shopping mall. The transmitter (TX) and receiver (VX) were placed at 200 m using a channel sounder system and omnidirectional path losses at 140 GHz were compared with the 28 GHz wireless channel. The slope and variations of the path loss data of the two bands were quite similar.
Likewise, NYU Wireless, a research program at New York University working on mmWave and terahertz communications, formulated a 140 GHz channel sounding system that carried out both long-distance propagation measurements with angular and delay spread and short-range dynamic channel measurements for Doppler and rapidly-fading characterization, respectively [Figure 2].
Figure 2: NYU Wireless has been investigating transmission properties of the terahertz band for both short- and long-range communications. Source: NYU
That shows how wireless communication research and rulemakings above 100 GHz is underway for both indoor and outdoor channels for various TX and RX antenna configurations and polarizations at multiple frequencies. These fundamental experiments on channel modeling at terahertz bands are mostly focused on propagation measurements, directional path losses, and penetration losses.
For example, the research team at NYU Wireless has carried out penetration measurements at the 140 GHz band for various building materials. That includes drywall, clear glass, and glass doors. Here, the average penetration loss of clear glass has been 3.2 dB/cm at 28 GHz, while it amounted to 14 dB/cm at 140 GHz, showing how the penetration loss increases with frequencies.
Prototyping Terahertz Communications
The above section underscores the need to thoroughly understand the propagation models of terahertz bands and then build real-world prototypes for these communication systems. A prototype can demonstrate the viability of a communication channel that simulations alone cannot.
Engineers, for instance, can employ these prototypes in real-time and over-the-air (OTA) setups in a variety of scenarios. Here, two things are fundamental in creating prototypes for new technologies like terahertz communications: flexible hardware and software that allow engineers to iterate and optimize designs quickly.
Figure 3: An SDR-based mmWave prototype at NYU Wireless is reminiscent of the infrastructure required for investigating terahertz-band propagation characteristics. Source: National Instruments
A terahertz communication system processing a multi-GHz channel will demand greater computational capacity in the digital domain where the baseband subsystem encompasses tasks like physical layer (PHY) channel coding. Similarly, the analog domain mandates improvements in analog-to-digital (ADC) and digital-to-analog (DAC) parts to efficiently capture higher frequency signals.
At a time when broadband and high-gain components are not available, a prototype can facilitate the key building blocks of the terahertz communication systems. That includes high-bandwidth baseband processing subsystem, bandwidth-filtered immediate frequency (IF) stage, local oscillator (LO) module, and radio heads to cover multiple frequency spectrums.
These prototype systems can also incorporate third-party RF front-ends to explore new frequency bands like 140 GHz. Next, a modular platform based on software-defined radio (SDR) technology can accommodate a variety of applications ranging from channel sounding to prototyping real-time, two-way communications.
Following mmWave Path
In 2016, National Instruments (NI) unveiled a transceiver system for mmWave communications that has evolved over the years and can offer the basic building blocks to also serve terahertz communication channels. It is based on modular hardware and software components that significantly save development time and allow maximum system reuse.
Figure 4: A view of the prototype system that uses multiple FPGAs to perform different processing tasks while employing software in a configurable manner. Source: National Instruments
Start with the baseband subsystem that is built around computationally intensive FPGAs and that can facilitate the massive amount of data flow back and forth [Figure 4]. Next, radio heads can facilitate the connections with IF modules; they also allow design teams to integrate their own or third-party RF subsystems to prototype different frequency bands.
There are also high-speed ADCs and DACs, as well as LO and IF modules that can be assembled in various configurations for tasks such as channel sounding and communications link prototyping. The transceiver system can synchronize with multiple transmit and receiver streams using the LO reference signals.
For the mmWave communication prototype, this PXI-based system has delivered PHY source code using the LabVIEW system design software. The flexible software environment working hand-in-hand with high-performance processing modules is crucial in the developer’s quest to build and test new technologies and algorithms for new communications protocols.
Terahertz Next in Line
The 5G dynamo is opening the floodgates of wireless bandwidth, and once mmWave bands move toward wider adoption and proliferation for 5G and other communication use cases, terahertz bands like 140 GHz are inevitably going to be the next in line.
That may not be very far away amid the speed of wireless technology advancements, so industry players need to start preparing for wireless communications above 100 GHz. And, while commercial components are not yet available, the SDR-based communication prototype systems seem to be the best bet for conducting the propagation and signal loss measurements in terahertz bands.
For more on wireless system prototypes for high-bandwidth applications, visit National Instruments.