6G    

 

 

 

6G Radio Spectrum

At least for now (early 2021), it seems that teraHeartz frequency (0.3 ~ 1 THz) is taken as the strongest candiates. However the debate over the best frequency for 6G will continue and we may end up with multiple segements of spectrum sitting far apart from each other being used for 6G. It may proceed as it has gone through in 5G. In 5G/NR, mmWave was taken as the major spectrum for 5G and this was a strongest trigger for 5G at early discussion. And then at the phase of formalizing 3GPP, another segment (FR1) sitting far away from mmWave(FR2) was introduced. Finally in real deployment, at least as of now (Oct 2021) FR1 seems to be the dominent spectrum for 5G which is far away from the initial 5G dream.

6G Spectrum Candidate

For now (as of writing this, May 2021), the strongest candidates for 6G spectrum seems to be between 0.3 THz(300 GHz) and 1 THz. This frequency would be an interesting region which is right above the maximum radio frequency and right below the lowest spectrum of light. But electronics, test equipment etc are not fully prepared for the research in this are for communication purpose and majority of the research on communication in this epctrum is being done in a few spectrums lower than this are as highlighed in the figure below.

Reference : ETSI, NYU Wireless

Following is a range of candidate spectrum magnified across the range of 100-300 Ghz.

Source : Rohde & Schwarz

Following figure shows various 6G spectrum candidates and potential applications for the spectrum segments.

Source : Wireless Keynote; Onward to 6G! Jeffrey Andrews PhD, UT

 

Following figure shows the potential communication windows within the range of the 6G candidate spectrum. The Windows are the area where the specific attenuation is relatively lower than surrounding spikes.

Source : 6G Wireless Systems: Vision,Requirements, Challenges,Insights, and Opportunities

 

Source : Terahertz Communication: The Opportunities of Wireless Technology Beyond 5G

 

Following is the frequency range and the frequency candidate for 6G defined by Hera-X deliverable document.

Source : Deliverable D2.1 Towards Tbps Communications in 6G: Use Cases and Gap Analysis

 

Source : Deliverable D2.1 Towards Tbps Communications in 6G: Use Cases and Gap Analysis

 

Source : Keynote 1: The road to 6G and what 5G still has in store (PIMRC 2021)

Possible Candiates in sub-mmWave ?

We all know that there are much available resources (bandwidth) in the range of  sub tera-hertz and tera-hertz However, we all know that the utilization of the such a high frequency spectrum is extremely challenging manily based on the experience in 5G.

As the industry use FR1 as a major anchor point and use FR2 on necessicity bases, it seems to (likely to) follow similar approaches (i.e, using a low frequency anchor point and use high frequency (sub terahertz) on necessicity bases). As a strong candidates for the low frequency anchor point, a spectrum of low frequency often called as FR3 in many documents or presentation are being proposed as follows.

Source : Rohde & Schwarz

How much bandwidth we need for 6G ?

The answer to this question would vary depending on many factors like modulation scheme, spectrum efficiency (how many bits can be carried by 1 Hz frequency spectrum).

The rough estimation done by Hera-X is as follows. According to this, even with 64QAM we would need around 20GHz BW. It would be challenging to achieve this bandwidth in single carrier... so likely to require multiple carrier (carrier aggregation) to achieve this.

Source : Deliverable D2.1 Towards Tbps Communications in 6G: Use Cases and Gap Analysis

Are we sure of going with Terahertz for 6G ?

When the 6G discussion started a couple of years ago and suggested Terahertz to be a major spectrum, everybody seems to accept the idea (at least no strong resistance against the idea as far as I remember), but as time goes on the debate over the teraheartz idea started heating up. As of now (Oct 2021), I see roughly three categories of opinions as summarized below.

Optimistic : I think we will see Terahertz as main spectrum for 6G. I know there are a lot of obstacles to overcome for now but they will be removed by the time we deploy 6G. We had similar pessimism about mmWave when we first talked about utilizing mmWave for main spectrum of 5G, but now we see the mmWave being used in real deployment.

Pessimistic : mmWave being used for 5G ? I don't think mmWave is really working as we expected and I don't see any clear vision that the mmWave issues will be cleared in near future. Considering this situation, I think it is premature to talk about Terahertz range spectrum in cellular communication system.

Neutral : I agree that it will be challenging (probably less likely) to see Terahertz being the major spectrum for 6G, but I don't think the terahertz will go completely out of the scope of 6G. Even though it would be less likely for Terahertz to be used for UE (e.g, mobile phone) with mobility, there would still be possibility to be utilized for some part of cellular system like wireless backhaul.

Practical / Realistic : We have to admit that we haven't accumulated enough experience and haven't achieved the desired performance even in 5G FR2, so it would be more realistic/practical options to start with lower frequency rather than directly jumping to sub-Thz/Thz spectrum. The frequently mentioned spectrum in more recent discussions (late 2022 and later) is upper mid band or often called as FR3 which is in the range of 7~24 Ghz.

Why Terahertz ?

This may be supporting 'Optimistic' opinions mentioned above. Why we are considering Terahertz as a major spectrum for 6G ?

Some of the advantage of Terzhertz spectrum can be listed as follows (Ref[5])

  • Wide bandwidth Availability : from tens and up to hundreds of GHz of contiguous bandwidth,
  • Super short symbol duration : picosecond-level symbol duration,
  • Huge number of Antenna in small dimension : integration of thousands of sub-millimeter-long antennas,
  • Easy Coexistece : ease of coexistence with other regulated and standardized spectrum

NOTE : In another perspective, some of these advantage can be challenges in real implementation of the technology.

  • Wide bandwidth Availability : from tens and up to hundreds of GHz of contiguous bandwidth ==> this requires various electronic components (especially DAC/ADC) that can handle super wide bandwidth.
  • Super short symbol duration : picosecond-level symbol duration ==> this requires extremely high performance of baseband processing power.
  • Huge number of Antenna in small dimension : integration of thousands of sub-millimeter-long antennas ==> this implies that it is hard to establish the communication with small number of antenna elements. With the huge number of antenna elements, more more advanced technlogy of beam management (beam forming, beam scan, beam selection, beam measurement etc)

Reference :

[1] ETSI GR mWT 022 V1.1.1 (2021-04) : Analysis of Spectrum, License Schemes and Network Scenarios in the RF bands above 174,8 GHz

[2] ETSI GR mWT 018 V1.1.1 (2019-08) : Analysis of Spectrum, License Schemes and Network Scenarios in the W-band

[3] ETSI GR mWT 008 V1.1.1 (2018-08) : millimetre Wave Transmission (mWT); Analysis of Spectrum, License Schemes and Network Scenarios in the D-band

[4] Hera-X : Deliverable D2.1 Towards Tbps Communications in 6G: Use Cases and Gap Analysis

[5] Terahertz Band Communication: An Old Problem Revisited and Research Directions for the Next Decade

[6] 6G Wireless Systems: Vision,Requirements, Challenges,Insights, and Opportunities - 2021

[7] 6G Spectrum Considerations - NEXT G ALLIANCE