Sub-terahertz communication in 6G

6G connectivity will be able to operate in a wide range of frequency bands. This includes current frequencies in the sub-1 GHz range, 3.5-6 GHZ mid-band range and millimeter-wave range, as well as new frequencies in the 7-15 GHz centimetric range and 90-300 GHz sub-THz range.

Sub-THz can unleash vast amounts of new spectrum and thereby provide enormous data rates. However, due to the limited range of such sub-THz transmissions, operation in these frequency bands would be limited to very specific scenarios where extreme data rates or low latency in local areas are required.

Examples where sub-THz communication could be required include professional high-resolution holographic communication in factories and hospitals, wireless connectivity of compute units in data centers, or indoor immersive communication.

Nevertheless, lower frequency bands will remain extremely important and the essential spectrum for coverage, capacity, and mobility in the 6G era.

Learn more about 6G spectrum

Sub-THz deployments are expected to begin in the lower edge of the sub-THz range. This is owing to the propagation characteristics of lower sub-THz frequencies, and also because it will likely take time to reach required maturity in the equipment ecosystem for higher bands.

Combining those facts, two bands within the sub-THz range are of interest: the W and D bands. The ITU-R allocations in these bands are depicted in the figure below.

Communication in the sub-THz range is different from the 5G New Radio (NR) frequency bands for many reasons and 5G NR solutions cannot always be directly applied to these significantly higher frequencies.

Challenging propagation conditions, extreme data rates, limited availability of commercial radio frequency components, and small coverage areas are all examples of fundamental differences to NR impacting the detailed design of a sub-THz system.

Deploying a cellular system, including beamforming and mobility, at sub-THz frequencies is an area of research that presents challenges.

Ericsson and Intel have developed a concept for sub-THz communication to better understand the feasibility and performance of such a system. The concept focuses on two important aspects:

• extensive use of beamforming, and
• efficient processing to achieve very high data rates or super low latency or both.

Although the concept focuses on sub-THz and is different compared to NR in several aspects, some of the solutions may be applicable for future radio access also in lower frequency bands.

A list of areas covered by the concept include:

• Transmission scheme and time-frequency structure
• Transport-channel processing
• Physical-layer control signaling
• Scheduling and HARQ
• Mobility and beam management
• Power control and timing advance
• Initial access

• DFTS-OFDM is used as its low peak-to-average properties ease the burden on the radio frequency power amplifier and supports good power efficiency. Consequently, details on reference signals, control channels and modulation schemes are changed compared to NR. The lack of frequency-domain multiplexing, enabled by the OFDM scheme used in lower frequency bands, is less important in the sub-THz range. Transmissions can start at any DFTS-OFDM symbol, which provides extremely low latency given the short duration of each DFTS-OFDM symbol of around 600 ns.
• Parallel processing is key to handle the extreme data rates, in the order of 100 Gbit/s and above, and the description is carefully developed to enable this. As an example, the mapping of code blocks to DFTS-OFDM symbols is different from NR to allow for per-symbol processing.

• Beamforming is an essential aspect when operating in the sub-THz range to achieve the desired array gain. Analog beamforming is a likely choice for implementation. Beam-management procedures are defined to select the transmit and receive beam both at the user equipment (UE) and base station as depicted in the figure. A much larger number of beams in narrower formations is expected when operating in sub-THz frequencies compared to the case of NR in millimeter wave frequencies. This means that the beam alignment between the base station and user equipment becomes increasingly important for sub-THz deployments. Refined procedures for cell search and random access are other consequences of the increased number of beams. Line-of-sight channels are expected to be common and hence rank-2 transmission are always used.

Sub-THz communication, one of many technical building blocks of a future 6G system, offers enormous potential to address extreme data demands in certain scenarios. The sub-THz concept described above should be seen as one step towards the goal of complete 6G specifications and deployed 6G networks.