Mobile signal boosters improve mobile coverage inside buildings by taking usable signal from the mobile network outside the building, amplifying it and redistributing it to areas where indoor coverage is poor.

They are commonly used where building materials, location or layout prevent the outdoor mobile signal from reaching people and connected devices reliably indoors.

Why Is Mobile Signal Poor Inside Buildings?

Your mobile phone connects wirelessly to a nearby mobile network base station.

Even where outdoor coverage is good, the signal can weaken significantly as it travels into a building. Common causes include:

• reinforced concrete;
• steel cladding and metal roofs;
• modern insulation and energy-efficient glazing;
• basements and underground areas;
• large or complex building layouts;
• distance from the nearest mobile mast;
• surrounding buildings, hills or other obstructions.

The result can be dropped calls, poor voice quality, slow mobile data or areas with no reliable indoor coverage.

How Does a Mobile Signal Booster Improve Coverage?

A typical off-air mobile signal booster system has three main parts:

1. Donor antenna

The donor antenna receives the existing mobile signal from outside the building.

It is normally positioned where the strongest and cleanest usable signal can be obtained, often on the roof or exterior of the building.

The quality of this donor signal is important because a signal booster amplifies an existing mobile signal — it does not create a mobile network signal where none is available.

2. Mobile signal booster

The donor signal is passed to the booster or network unit.

The system processes and amplifies the appropriate mobile network frequencies before passing the improved signal to the coverage side of the system.

CEL-FI systems use Nextivity's IntelliBoost technology to control and optimise this process while protecting the surrounding mobile network.

3. Coverage or server antennas

The amplified signal is distributed inside the building through one or more coverage antennas.

These antennas are positioned in the areas requiring improved mobile coverage.

The same system also carries signals from mobile devices inside the building back towards the mobile network, allowing normal two-way voice and data communication.

Can a Mobile Signal Booster Work if There Is No Signal Indoors?

Yes. Even if you have no mobile signal indoors, a signal booster can usually improve reception as long as there is a mobile signal available outside or at roof level.

A correctly designed system receives the available outdoor signal, boosts it and distributes it throughout the areas of the building requiring coverage.

This can improve both signal strength and reception quality, helping provide reliable mobile connectivity in buildings where mobile phones struggle to receive a signal indoors.

One Network or Multiple Mobile Networks?

Some CEL-FI systems are designed to improve coverage for a single mobile network, while others can support multiple mobile network operators simultaneously, depending on the model and configuration.

For a business, the right choice depends on which networks need improving, the signal available for each operator and the size and complexity of the building.

Single-network solutions are generally cheaper than multiple-network options, so it helps to establish which networks actually require improvement.

If you use a virtual mobile network (MVNO), such as giffgaff, Tesco Mobile or Sky Mobile, see our Networks page to identify the main UK network it uses.

How Are Larger Buildings Covered?

A small building may only require a relatively simple booster and antenna configuration.

Larger sites can require multiple coverage points distributed throughout the building. Enterprise CEL-FI systems such as the QUATRA range are designed to distribute mobile signal across larger, multi-floor and more complex environments using multiple coverage units positioned where improved service is required.

The system can therefore be designed around the building, the available donor signal and the areas requiring coverage rather than expecting a single booster or antenna to cover the entire site.

What Determines How Well a Signal Booster Will Work?

Several factors affect the performance of an in-building mobile signal solution:

• strength and quality of the available outdoor signal;
• mobile network and frequency bands available at the site;
• location and type of donor antenna;
• building size and construction;
• number of floors;
• areas requiring coverage;
• placement and number of internal antennas;
• separation between donor and coverage antennas;
• and the CEL-FI system selected.

There is therefore no meaningful single coverage figure that applies to every building.

When Is a Site Survey Needed?

For an off-air mobile signal system, performance starts with the signal available at the site, so the first step is understanding the existing signal strength and quality.

For smaller and relatively straightforward commercial requirements, we may be able to recommend a suitable solution using information provided about the building and existing mobile coverage.

For larger, multi-floor or more complex environments, a site survey allows existing mobile signal conditions to be measured properly before the system is designed.

A survey can establish:

• which mobile networks are available;
• signal strength and quality around the site;
• suitable donor antenna positions;
• which areas require improved coverage;
• likely antenna locations;
• and the most appropriate CEL-FI system architecture.

This reduces the risk of specifying equipment that cannot deliver the required result.

Are Mobile Signal Boosters Legal in the UK?

Certain mobile signal repeaters can be installed and used in the UK without an individual Ofcom licence where the equipment and its use meet the applicable licence-exemption requirements.

Ofcom's technical requirements for licence-exempt mobile phone repeaters are set out in UK Interface Requirement IR2102.

For more information about the requirements for legal use in the UK, read our Ofcom compliance guide.

CEL-FI Commercial Mobile Signal Solutions

Frequency Signal Solutions is the specialist commercial mobile signal solutions division of Frequency 3G Telecom Ltd, the UK and Ireland Master Distributor for Nextivity CEL-FI products.

We help organisations determine the right approach for improving mobile coverage in environments including offices, warehouses, factories, healthcare facilities, hotels, retail sites, data centres and public buildings.

Depending on the requirement, this can include consultation, site survey, system design, CEL-FI specification, professional installation through trusted installation partners, testing, remote monitoring and ongoing support.

Discuss Your Mobile Signal Requirement

Need Better Mobile Coverage in Your Building?

Whether you need to improve reception in a small office or provide coverage throughout a large commercial building, Frequency Signal Solutions can help assess your requirements and recommend an appropriate solution.

Contact us for an indicative quotation or learn more about our three-stage process.

Looking for a Self-Install Mobile Signal Booster?

For a home, home office or smaller premises where you are looking for a CEL-FI system that you can purchase and install yourself, visit Mobile Signal Boosters.

Visit Mobile Signal Boosters