06 May 2025
Luke Kehoe, Industry Analyst, Ookla
As much as 80% of all mobile data usage originates from indoor environments like homes, offices and shops. However, mobile networks were initially designed with an ‘outside-in’ approach—relying on outdoor towers to deliver coverage, with the expectation that the signal would reach indoors without being specifically optimised to do so. 
This strategy helped minimise deployment costs and was based on the assumption that indoor connectivity could be provided by low-band spectrum layered over the macro mobile network, with higher data rate demands met by home broadband and public Wi-Fi networks indoors.
Consumers have come to rely on mobile data to serve their indoor browsing needs and expect performance parity as they move around from home, work, the shops, and everywhere in between. Even where Wi-Fi and related features like VoWiFi are available and sufficiently fast, in-building mobile coverage remains critical for last resort access to basic telephony features like calling and texting to ensure reliable access to emergency service networks. Indeed, in many advanced European markets, operators and regulators prioritise routing 112 emergency calls over mobile networks using VoLTE rather than Wi-Fi, as VoLTE offers greater reliability and quality of service through dedicated voice packet routing on mobile networks.
But if indoor connectivity is so important, why is it still so lacklustre? While there is no one easy answer, there are a few clear contributing factors.
Network Design
One of the biggest barriers to good indoor connectivity lies in how networks are designed, and this challenge is becoming more common with the deployment of 5G. The trend towards higher frequency spectrum for 5G (e.g. 3.5 GHz mid-band) limits the ability of the existing mobile network site grid to provide high-speed mobile coverage deep indoors. This is due to the more constrained propagation characteristics of this spectrum. Simply put, the signals that mid-band 5G networks rely on struggle to penetrate the materials in their path when the user is indoors.
Lower frequency signals do not face this problem to the same extent, but their utility has become more limited over time. While the lower frequency spectrum (e.g. 800/900 MHz with 3G/4G and 700 MHz more recently with 5G) traditionally used to provide in-building mobile coverage previously sufficed, the significant increase in the density of devices and the intensity of their data traffic demands mean these frequencies alone are unable to support the higher performance attributes often expected with 5G, particularly in dense urban settings.
Because of this, the traditional approach of outside-in network design, where signals are transmitted from the macro coverage layer of a lattice or monopole-based high site into a cluster of buildings, is no longer fit for purpose in the absence of investment in network densification if demands for reliably fast connectivity indoors are to be met.
Building Design
Network design is not the only contributing factor to the profile of signal propagation. While it is true that the signals typically used for 5G networks struggle to travel through buildings, some materials present a bigger challenge than others.
The use of modern insulation materials in new-build and retrofitted developments is posing a significant challenge for mobile operators. Take low-E glass, for example – a type of energy-efficient glass with a microscopic coating designed to reduce energy consumption, which is becoming a commonplace alternative to double glazing. Low-E glass has a significant negative impact on radio signal propagation, and with its growing use in retail and office buildings, the indoor connectivity problem is set to worsen, especially with the use of higher frequency bands.
As these kinds of construction materials – those that significantly increase signal attenuation and effectively turn buildings into Faraday cages – become more widely used, network design and building design must go hand-in-hand. Otherwise, the ability of 5G signals to penetrate newer buildings will continue to be diminished.
Network Sunsets
The sunset of legacy network technologies like 2G (in markets such as Switzerland and the US) and 3G (in most developed markets) has introduced further challenges as operators seek to preserve indoor coverage levels while upgrading equipment and repurposing frequencies.
The process of improving network performance and optimising long-term operating costs with technology sunsets is not as simple as removing and replacing outdated equipment. Operators need to ensure legacy end user devices are upgraded to take advantage of 4G and 5G networks and that older mobile sites are refreshed with modern radio equipment to ensure there is full continuity in coverage levels.
Analysis of Speedtest Intelligence data has revealed a concerning trend of increased time spent on 2G networks or with no service at all in several advanced markets where operators have been slower to repurpose spectrum employed by legacy technologies upon sunsetting 3G. This has manifested in increased reports of dropped calls and other mobile connectivity issues, particularly in areas where decommissioned 3G coverage has yet to be fully replaced by 4G or 5G networks.
Policy Oversight
Governments and regulators around the world have historically focused headline policy goals on achieving outdoor population coverage targets. This model has overlooked the importance of indoor mobile coverage, contributing to poor outcomes throughout in-building environments and a lack of public data on the extent of indoor coverage gaps. Some countries, like Ireland and Germany, have made progress by mandating minimum coverage levels at buildings and infrastructure of national importance as part of spectrum licence conditions. In the Irish context, for example, this includes a requirement to provide a minimum 30 Mbps service across key infrastructure sites like train stations and hospitals, as well as community hubs and tourist locations.
These types of progressive policies, as well as those being adopted by city governments to increase building access for mobile sites through amendments to planning and zoning conditions on future renewals and large-scale commercial and residential developments, can play a positive role in stimulating better indoor coverage outcomes by re-aligning deployment incentives and removing obstacles.
What’s the solution?
While consumers expect consistently high-performing in-building mobile performance, the path to get there is not a simple one. There is no one-stop solution to the indoor connectivity problem.
That said, the neutral host model is emerging as a key solution to improve in-building mobile outcomes, providing multi-operator access to promote fair competition and share deployment costs, typically based on small cell solutions like the Ericsson Radio Dot. Freshwave (UK) and Proptivity (Sweden) are early examples of neutral host specialists leading the charge in this space.
While the scaling up of small cell deployments at the street and building level, enabled by the neutral host model, is key to improving indoor performance, there are other factors at play. Operators must prioritise repurposing the spectrum in the wake of 3G sunsetting, and building developers and the planning system should take better account of the accommodations needed to host radio equipment. But if indoor connectivity is truly to see a material improvement, these changes should be underpinned by progressive regulatory policies that measure indoor coverage levels and provide better incentives to improve in-building mobile outcomes and remove barriers to deployment.
