A Laser In Your Home? To Infinity And Beyond: MIMO, 6G, and Terahertz

A Laser In Your Home? To Infinity And Beyond: MIMO, 6G, and Terahertz

On one side of the Edinburgh bypass, I can get 5G, and on the other side, I can get 4G. Like it or not, we are moving into a world that will free us of those pesky cables, and allow us to connect to the Internet in a way that provides us with the bandwidth that we need to live in our digital lives.

Our demand for bandwidth seems almost endless, and we must now need to look to the future, and that future is likely to be 6G. In a possible timeline, we may see the first standards for 6G arrive by 2026, and for full deployment in 2030 (Figure 1).

The wonderful world of electromagnetic waves

As humans, we are rather limited in how we “see” our world. We “feel” heat though infrared transmission, but only “see” a very small part of the electromagnetic spectrum. Overall, it was James Clerk Maxwell, who discovered that electromagnetic waves were covered by a unifying theory. Electromagnetics waves which have different characters are all just part of an overall spectrum, and where we basically change the frequency/wavelength of our waves to produce gamma rays, X-rays, visible light rays, radio waves, and so on.

For the ultra-high high-frequency (ultra-low wavelength) waves, we start with gamma and X-rays, and then move onto ultraviolet. This moves us into the nanometer wavelength (ultraviolet), and then onto micrometer waves (infrared). Then, in a tiny little range, we have our visible light range:

Then, as we should all know, we move into radio frequencies which carry our wifi traffic, and satellite communications.

But, what’s in-between infrared and microwave radiation? Well, there’s an untapped region known as millimeter waves, and they could be the next big thing in the development of the Internet. These waves could free us from those pesky cables, and give us almost instant access to any piece of data that we want.

We love 2.45GHz and 5GHz

The roots of my research trace back to electromagnetic waves, and I still find them fascinating. My knowledge of the area goes up to around 5GHz, and then after that, I kinda lose track of how radio systems work. Why? Well at 2.45GHz and 5GHz — the two frequencies typically used by wifi — we have a wavelength of around 12cm and 6cm, respectively. This means that our dipole antennas are typically around 6cm or 3cm in height (as they have the wavelength divided by two). Somewhere in your mobile phone, you will find a wrapped antenna, and which has these dimensions.

Towards millimeter waves

But, what if we go higher in frequency? Well, our antennas become a whole lot smaller, and the range, too, also becomes much smaller. The range relates to the fact that longer wavelengths can bend around things, but smaller wavelengths get stopped by objects (especially metal ones). If we move up to 50GHz, our wavelength drops to just 6 mm — and where we define these as millimeter wavelengths. Everything we build for these millimeter systems will thus shrink in size.

MIMO

But, why use these? Well, the bandwidth of the signals that can be carried on these frequencies varies with the carrier frequency. As an estimate, we can say that the bandwidth is around one-tenth of the carrier wave. So, our 2.45GHz carrier gives us a potential bandwidth of 245Mbps. But, with IEEE 8011.11n, we can get around 500Mbps. For this, we use MIMO (Multiple In, Multiple Output), and where we can split the data stream up into different channels, and then send them all at the same time. If you purchase an IEEE 802.11n wifi access point, you will see it has multiple antennas:

But, as we increasingly demand more bandwidth with always-on systems, our limits of 500Mbps are just not going to cope with the demand. We also now demand low-latency connections, and where we get almost instant access to content. And, so we see the rise of the millimeter waves, and that will take us up to 300GHz. But, that might only allow us to move to 30Gbps. What happens after that? Well, that’s where 6G will play a role, and make use of the gap between microwaves and infrared.

The Terahertz (THz) range

Well, say hello to the terahertz (THz) range — 300 GHz to 3 THz, and “sub-terahertz” which is between 100–300 GHz. These frequencies would support a step-change in our consumption of data. With this, we could transfer gigabytes of data in less than a second. The greatest problem is that we know how to create infrared and microwave systems, the whole radio spectrum between infrared and microwave is a bit of a black hole for us, and where our traditional electronics will just stop working. With this, we push our clock speeds to the limit of their physical properties, and with clock speeds of several GHz, and have generally reached their limits. Overall, processing speeds have not quite increased due to increased clock speeds, but with multiple cores.

So, we must develop new systems, but where the communications are likely to be around line-of-sight communications. This will provide localized hotspots with ultra-high bandwidth. For this, we will turn to lasers to generate our signals. But, don’t worry that these will do damage to your body, as they will be low-powered devices and which will be well outside our vision (as they sit below infrared — which we can’t “see”).

The backend

6G will need an extensive back-haul network of interconnected devices — typically using satellite and airborne communications (see Figure 2). With these, we start to see the kind of Internet that we should have had, and where the data packets can take different routes to get to their destination.

A new world of health care

With our new PhD work, we aim to fuse a new world, and which seamlessly integrates AI and connectivity into health care. Imagine having almost instant access to every single sensor without any delay, or using the Internet as a distributed brain. Overall, intelligence can could then be integrated into every layer (Application, Control, Edge, and Sensing layers) of a cyber-physical infrastructure (Figure 5). But, this integration will bring many challenges in terms of trust and security — with attacks possible at each layer.

Conclusions

As I said, on one side of the road I get 5G, and on the other I get 4G. Hopefully, 5G will properly scale out, but, it's really just treading water before a new cyber-physical world arrives.

Reference

[1] Jiang, W., Han, B., Habibi, M. A., & Schotten, H. D. (2021). The road towards 6G: A comprehensive survey. IEEE Open Journal of the Communications Society, 2, 334–366.

[2] Siriwardhana, Y., Porambage, P., Liyanage, M., & Ylianttila, M. (2021, June). AI and 6G security: Opportunities and challenges. In 2021 Joint European Conference on Networks and Communications & 6G Summit (EuCNC/6G Summit) (pp. 616–621). IEEE.