The objective of 6G and beyond is providing connectivity of ‘everything, anywhere and at any time’. Huge amount of data has to be processed for achieving this objective, necessitating employing Deep Learning techniques

THz spectrum is the most unexplored part of the radio spectrum. It is in between RF and optical spectrum. 10^11 Hz (0.1 THz or 100GHz) to 10^13 Hz (10 THz) can be considered as THz band. 1 THz is 10^12 Hz.

In 2008, Japan’s NTT used a 120 GHz wireless link in a 1 km transmission trial to provide live TV coverage of the 2008 Beijing Olympics. In 2014, Japan allocated 116-134 GHz (18GHz band) to accommodate such service. In 2020, for broadcasting the Tokyo Olympics, this band was proposed to be used. But because of the disturbance due to Covid-19, this could not be done. Amateur Service (ARS), Amateur Satellite Service (ARSS), Earth Exploration Satellite Service (EESS), Inter Satellite Service (ISS), Radio Location Service, Radio Navigation Service (RNS), Radio Navigation Satellite Service (RNSS) and Space Research Service (SRS) which are active services (transmitting and receiving) use 95- 275 GHz range of spectrum.

6G technology is on the anvil and a lot of research activities are going worldwide on this technology. 6G KPIs are; Data Rate: peak data rate is 1 Tbit/s and Experience data rate 10-100 Gbps, Sensing Accuracy: 1mm, Latency: 0.1 msec, Energy Efficiency: 1Tb/J, Spectral efficiency: 60b/s/Hz and Reliability: very high (99.9999%).

These KPIs can't be achieved using legacy frequency bands. The enabling inter disciplinary Terahertz technologies to achieve these KPIs are; Terahertz Integrated Sensing and Communications, Terahertz MIMO (Multi Input Multi Output), Terahertz Intelligent Reflecting Surfaces (IRS) and Machine/Deep Learning.

Some of the applications of Terahertz spectrum are Backhaul (instead of Optical Fibre), Small Cell sites, Vehicle to Vehicle communications & Radar Sensing, Positioning and Tracking.

The objective of 6G and beyond is providing connectivity of ‘everything, anywhere and at any time’. Huge amount of data has to be processed for achieving this objective, necessitating employing Deep Learning techniques.

The advantages of THz frequency band are High bandwidth, as bandwidth is proportional to carrier frequency: High Data rate per user, as data rate is proportional to cube of carrier frequency: Use of spatial multiplexing which enables multiple independent beams carrying different data and independently aimed: High receive power as as eceive power increases quadratically with carrier frequency: Low power consumption as power consumption decreases with higher data rates which are achievable at THz frequencies.

But one concern with THz band is high atmospheric loss (caused by oxygen, water vapour, rain, fog, ozone and carbon dioxide) and high free space propagation loss. In Atmospheric attenuation, peak points and valley points are available across the THz band and carrier frequencies have to be selected at valley points to achieve low atmospheric attenuation. High Free Space Propagation loss can be compensated by using high gain antennas.

6G technology: Key challenges

1. InP HBT (Indium phosphide Hetero Junction Bipolar Transistor), InP MOS HEMT (Indium phosphide Metal Oxide Semiconductor High Electron Mobility Transistor) and SiGe HBT (Silicon Germanium Hetero Junction Bipolar Transistor) semiconductor technologies are to be used.

2. Integrated transceiver design

3. Achieving Terabit per second in reality

4. Realisation of sub millisecond latency

5. Overcoming the barriers of backward compatibility.

THz signal generation can be Electronics based (solid state/ vacuum tube) or Optics based (Continuous wave or pulse). In electronics based signal generation, low frequency signal is generated and frequency multiplication is done using Schottky diodes to get THz frequency. THz signal detection also can also be Electronics based or Optics based.

Research issues in THz band

1. THz communications and sensor system requires miniaturised and reconfigurable components to reduce power consumption, cost, size and weight

2. Below 100 nm, copper interconnects have major limitations like signal distortion, attenuation, cross talk, power dissipation per unit area. Waveguide is an alternative but their dimensions can't be reduced beyond a certain limit due to cut-off wavelength conditions.

3. As per Moore's law, transistor density doubles every 18 months. But miniaturisation is a problem in interconnects because of which there is a limitation on chip size.

6G: THz Test Bed

The project on ‘6G: THz Test Bed with Orbital Angular Momentum (OAM) Multiplexing’ was awarded to a consortium of SAMEER (Society for Applied Microwave Electronics Engineering & Research under Department of Electronics), IIT Chennai, IIT Patna and IIT Guwahati in the program launch of Bharat 6G Alliance (B6G A) on 11.07.2023. The test bed will be developed at SAMEER Kolkata. The task is to develop and demonstrate 6G THz communication link at 270GHz. SAMEER will develop the major hardware, IITChennai will develop Transceivers, IIT Patna will develop IRSs (Intelligent Reflecting Surfaces) and IIT Guwahati will develop Chip sets for this project. Government has allocated funds for this project through the Telecom Technology Development Fund (TTDF). This test bed will provide academic institutions, industries, startups etc. a platform to test and validate evolving 6G technology in THz band.

OAM multiplexing is a physical layer method of multiplexing signals carried on electromagnetic waves using the orbital angular momentum of the electromagnetic waves to distinguish between the different orthogonal signals.

Generating THz OAM beams can bring multi fold benefits related to improving the bandwidth, channel capacity and spectral efficiency.

IRS (or RIS (Reflecting Intelligent Surface) or RMTS (Reflective Metallic Surface)) enhances the coverage of the THz OAM beams by enabling non line of sight components by reshaping the planar wavefront of the incident wave into the helical wavefront to be redirected towards the direction of interest.

IRS assisted OAM antenna at THz frequencies can solve the limited bandwidth issue of IRS structures, reduce divergence of OAM and improve spectral efficiency. The orthogonality among different OAM modes enables exchanging data simultaneously through several streams at the same frequency compared to sending a single beam in a traditional antenna. There will be no interference because of orthogonality among them. Till now in the experiments four modes are tried.

Simultaneously Transmit and Reflect Surfaces (STARS) in THz band

This is another promising technique In THz communications. IRSs can only reflect the incident signal, leading to half-space coverage. It necessitates the transmitter and receiver to be on the same side of IRSs. As a result multiple IRSs are required to realise full space coverage. STARS can simultaneously transmit and receive the incident signal into both sides of the surface resulting in the full space coverage.

Way forward

The huge differences between 6G and 5G technologies are Dynamic Digital Twins and Virtual worlds, Wireless in Data Center, Zero Energy Devices, Swarms of robots or drones and Bio-sensors and AI which are promised by 6G technology. TIG (Technology Innovations Group)-6G, has recommended extensive research in mm wave and Terahertz communications. Sensing in the Sub THz band (100 GHz to 300 GHz) is also envisaged.

Let us hope that Indian academic institutions and industries work on 6G THz technology to catch up with the world in time.

(The author is Former Advisor, DOT, Government of India, Bangalore.)