Ofcom UK Explore Reflective Surfaces to Boost Mobile Coverage

The UK communications regulator, Ofcom, has today published an interesting new white paper that, in collaboration with Queen Mary University of London, explores whether “reflective surfaces” could be used to cost-effectively extend the coverage of wireless broadband and mobile networks. But more research is required.

The term “reflective services” is almost too generic to have much meaning beyond the obvious (you’re probably thinking of a mirror), but in this case it means surfaces that are “primarily envisaged as non-amplifying devices [unlike repeater/relay technologies] and limited to adding advantageous signal paths, referred to as multipath diversity, between wireless terminals under non-line of sight (NLOS) conditions“.

In the practical sense, such reconfigurable reflective surfaces / devices, widely referred to as a Reconfigurable Intelligent Surface (RIS) in the white paper, would be placed some distance away from the original signal source (e.g. a mobile mast) and help to extend the coverage of the wireless / mobile network.

All the modelling and simulations done for this in the paper were limited to frequencies below 6GHz (mostly reflecting 5G friendly bands), although Ofcom states that the fundamental behaviours they observed would also be relevant to higher frequencies. But use cases targeting even higher millimetre wave (mmWave) frequencies were not considered. Reflective surfaces are currently an area of active research and standardisation in this area is still in its infancy.

The driving attraction behind reflective surfaces is to provide a potentially low-cost and low-complexity solution to extend the coverage of wireless networks, but the research highlighted a number of potential technical challenges and regulatory implications that would need to be addressed before reflective surfaces can be widely deployed.

Ofcom’s Initial Considerations

While we recognise that reflective surfaces may have potential benefits, we believe that further work is required by field experts to understand the regulatory implications for their deployment, especially in environments where multiple independent networks and services coexist. This is because, on the basis of the work we have undertaken in collaboration with Queen Mary University of London, we consider there are potential technical challenges associated with the adoption of reflective surfaces. In particular:

• Reflective surfaces can produce unwanted scattering effects (technically referred to as “spurious sidelobes”) which can increase interference and degrade the performance of network terminals unless upgraded to cope with additional interference levels.

• Redirecting wireless coverage using reflective surfaces works like a zero-sum game. While using a reflective surface to enhance the coverage in one area, it can weaken the coverage in other areas or the area behind it due to shadowing. Shadowing effects are inversely proportional to the distance between a reflective surface and signal source.

• Reflective surfaces can alter propagation conditions of out-of-band frequencies, which can impose coexistence challenges and impairments to existing services and networks.

To address these challenges, the white paper recommends that further research is needed to examine the performance of reflective surfaces and to develop mitigation techniques for interference. The paper recognises that reflective surfaces have potential benefits, but further work is required by field experts to understand the regulatory implications for their deployment, especially in environments where multiple independent networks and services coexist.

Sadly, the paper doesn’t really give us much indication of how much this approach could improve the performance or coverage of a mobile network in a particular area, but hopefully that may come further down the road – assuming they can find ways of reducing its negative impacts.