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.
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.
Following is the frequency range and the frequency candidate for 6G defined by Hera-X deliverable document.
Source : Keynote 1: The road to 6G and what 5G still has in store (PIMRC 2021)
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.
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.
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)
NOTE : In another perspective, some of these advantage can be challenges in real implementation of the technology.