Indoor Wireless: Follow the Use Case

When we talk about cellular coverage, primarily LTE today, much attention is focused on the challenge of covering rural areas, and providing dense coverage to populated urban areas such as major city centres. But what of the indoor spaces where many of us rely on cellular connectivity?

Some reports estimate that the percentage of cellular data traffic generated to and from mobile devices which are indoors is around 70% of all cellular data traffic, making this an interesting problem. Even if 70% seems a bit high, it still remains a challenge for operators to provide adequate capacity and coverage for mobile devices indoors, exacerbated by 3 main factors:

1. Device density

In a given indoor space such as an office building or retail store, there are likely to be large numbers of people in a small space, at least compared to the spread of people actively using cellular data outdoors in many areas. The more devices in a given area attached to a single cell, the less time available for each to transmit and receive data if they were all to attempt to at the same time.

2. Application usage

Although many do use applications such as streaming video outdoors with cellular data, these applications are frequently used indoors as users are more likely to be sitting, perhaps eating or talking in a group where watching video is a more common activity than when walking or driving.

3. Material penetration

Although the majority of cellular networks use frequencies which are well-suited to material penetration such as 700 MHz, there is still a loss of signal strength and quality involved when attempting to provide indoor coverage in many areas. The outside of the building, insulation, air conditioning ducts, doors and internal walls all contribute to indoor ‘not-spots’ where cellular coverage is weak or non-existent.

The route taken to solve these issues depends on which of them is considered more important. 1 and 2 suggest a problem with capacity – that is, the capability of the network to adequately serve a large number of devices in a small area, using high-bandwidth applications with an expectation of good service.

Issue 3, though it has the potential to impact 1 and 2 by degrading signal quality, is best considered a separate problem, that of coverage. By splitting the issues between capacity and coverage, it becomes easier to align the most common solutions in the industry today with the problems they are trying to solve.

The answer whether capacity or coverage is more important depends heavily on the purpose of the building that is to be covered. For example, a hospital may prioritise coverage, whilst a retail space prioritises capacity as they are serving two very different use cases.

To solve indoor cellular capacity, the usual solution is a small cell. Operating under many names including femtocell, these devices connect to the wired network inside a building to provide connectivity back to the core of the cellular network, and provide cellular connectivity to an area of varying size depending on the small cell in use inside the building.

Although these devices have received a lot of press over the years and have even been given or sold at subsidised prices to users in poor signal areas in many countries, they have never reached their full potential due to a combination of cost, complexity and difficulty marketing the solution to consumers and enterprise IT managers. When deployed correctly however, they can be quite effective, although spectrum planning and management is a challenge.

Whereas small cells contribute to improving both cellular capacity and coverage in one device, a Distributed Antenna System (DAS) is often the go-to option for solving issue 3, that of indoor cellular coverage.

These systems typically use an array of cabling and antennas distributed around a building, sending and receiving signals either repeated from a cell outdoors or from an indoor small cell, expanding coverage throughout the indoor space – but not providing significant extra capacity, as either none or a small number of additional cells are added to the cellular network, so all devices within the cell are still competing for its limited radio resources. This can even exacerbate capacity issues if the cell is grown large enough to cover a greater number of devices.

An interesting hybrid has developed in recent years, combining the best aspects of both solutions: hybrid DAS. In these systems, a number of small cells are connected to a single large or several small DAS systems throughout the building, intending to provide a good balance of coverage and capacity.

The issue that all indoor cellular systems have is cost. With the rapid and continuing increase in the deployment of WiFi networks, particularly indoors in office buildings, retail locations and other managed spaces, do we need to still be concerned about indoor cellular coverage?

And if so, is it worth the price?

The answer really comes down to use case. Returning to the example of the hospital compared to the retail space (think of a local mall), these two buildings and their inhabitants have drastically different needs and concerns.

For the hospital, reliable, consistent coverage is key. Capacity is not likely to be as much of an issue as the network is primarily intended for mission-critical comparatively low-bandwidth communication, such as announcing codes or paging doctors and other critical staff to emergency areas. In this case, the ubiquitous coverage provided by a solid DAS system for indoor areas and existing cellular coverage outdoors is a good solution.

For the retail space, used as a social gathering place rather than mission-critical, life or death nerve-centre, watching videos with friends in the food court or looking up a video review of the latest game is a more pressing use case than always-on communications. In this case, WiFi is likely to be a more cost-effective option providing higher capacity – although a small cell or hybrid DAS system would also potentially meet the capacity need, WiFi provides a better roadmap for higher capacity.

Whichever technology is used to improve indoor wireless network coverage and capacity, spectrum planning and management remains a key issue, becoming all the more important as the number of cells in a given area increases and the network continues to densify.