Caution overhead fibre

Generally terrestrial base stations each cover a cell and the cells overlap to provide (hopefully) seamless coverage of the target area. The target might be set as a proportion of the total land area, but that usually includes places with challenging topography and few users which are very expensive to cover. So the target is more often a high proportion of population.

Cells get deployed with varying sizes depending on the population density and the maximum range of the radio technology. In dense urban areas cell radius may be 500m or smaller for high capacity; in rural areas 5km or more for minimum cost per user. In towns the higher frequencies, up to 4.2GHz, can be used; but in rural areas 1800MHz or lower, down to 600 MHz, supports larger cells. Coverage planning uses sophisticated graphical information systems and propagation models but despite the science behind the approach many users complain about “not-spots” in supposedly-covered rural and urban areas.

Increasing user data rate everywhere leads to pressure on low- and mid-band spectrum. One approach could be to make rural cells smaller so they can re-use the low frequencies; or smaller still and use high band. More cells need more sites and more backhaul which increases network costs geometrically, and the low user density means that there may be not enough revenue to justify the investment. Looking ahead to 6G one objective should be to fix the coverage problem properly so everyone gets service all the time wherever they are. That would need more spectrum at lower frequencies to be economic, but the spectrum warehouse is empty.

Perhaps we are starting from the wrong end of the problem? If one focuses on where the users actually are, in those areas that are expensive to cover they are, most of the time, highly localised. They are at home, or travelling (walking/cycling/driving) along roads. If the users are “things” rather than people, by and large the bandwidth needs are significantly smaller though things can have a wider distribution (think: crop humidity sensors), so existing infra may already provide coverage¹.

In many rural areas there is already a lot of communications infrastructure. Looking at the UK specifically², in 2017/18 85% of households had a landline telephone, a decline from 95% in 2000³.

In 2021, 97% of UK households had Internet access, of which a high proportion would be via a fixed broadband connection. So there is a fixed telecoms connection to a very high proportion of premises. An increasing proportion of these connections – 21% in 2021 according to Ofcom – use Fibre To The Premises (FTTP), and this is increasing rapidly even in rural areas.

Deployed FTTP systems can support 1 Gbit/s. Where the final connection is not by fibre a high proportion of systems use fibre to the cabinet. Small yellow plastic labels (see header image above) are becoming a familiar sight on telephone poles throughout the country.

These statistics are given not because the wireless focus should be on homes especially but because they show that local communications infrastructure is all-pervasive throughout the landmass, and much of it will bring fibre right to the “edge” in the near future.  On its way to the premises the fibre cables run mainly along roads.  Usually there is power nearby, but even if not renewable energy can harvested using solar panels which are a common sight on traffic warning signs, with lithium batteries for storage.

This suggests an alternative approach to rural coverage. Use very small cells, with FTTP backhaul and possibly radio over fibre. Transmit power would be low.  Base station equipment – essentially picocells – would be mounted on existing telecom or lighting poles; though sometimes new poles might be needed. The physical layer would no longer have to support long range operation and techniques such as cell-free. Massive MIMO could come into their own. Pressure on the low frequency bands could be much reduced and these perhaps be used mainly for supporting lower data rate “things” communications to more distributed devices. 

The circuitry needed in the terminal could be much simpler when the radio looks much more like a Wi-Fi device than today’s multi-band FDD CA MIMO modems. Total radiated power levels from both handsets and infra could be much lower. The network coverage map could essentially start to look like a road map, with long corridors of high bandwidth coverage along all highways that have telecoms infrastructure along them, also covering maybe a couple of hundred meters each side. This would be overlaid on the existing “thin” coverage to pick up the small number of users who are actually not on the highways or in adjacent buildings. Interestingly, radio network planning would be much more like fixed network planning, which for over a century has had to focus on the microscopic details of where people and buildings are located and what infrastructure is in place.

Taking a “bottom up” rather than “top down” approach to wireless coverage and capacity would be a paradigm shift. Now we have the technology and tools to investigate such an approach.

This isn’t just an engineering and technology problem – we need equally to look closely at physical and economic geography – where people, things, and existing infrastructure are or travel; and consider radical approaches to regulation, not just of radio spectrum but of the fixed local infrastructure. The tools to do this in the form of Geographical Information Systems and detailed map databases such as the Ordnance Survey, and machine learning to extract and process data, are all available.

Approaches that work in the UK should be applicable in other countries with similar economic geography. In the past the UK has led in the development of short range wireless – from the first digital cordless phones through a many start-ups developing innovative pico- and femto-cell solutions for 2G through 5G.  And we have companies that are rolling out rural fibre to compete with Openreach so we have expertise in building fixed networks.