Small Cell Classification and Terminology

There seems to be an increasingly vague set of terms and differing scope being used when describing Small Cells. This leads to confusion when comparing forecast numbers, examining how operators are deploying them and ensuring conversations relate to the same baseline. I've even heard it said that vendors often have to begin initial customer meetings by taking several minutes to set the ground rules.

Here we look at not only what a common set of terminology could be, but some of the underlying reasons behind the subtle differences.

Residential

The definition of a femtocell was quite well scoped by the Femto Forum several years ago. They listed the seven characteristics below:

• Operating in licensed spectrum
• Using mobile technology
• Generating coverage and capacity
• Over Internet-grade backhaul
• Cost effective
• Fully managed by licensed operators
• Self-organising and self-managing

The original femtocells were targeted at residential use, especially where there was poor coverage. By definition, that meant little interference with the outdoor mobile network. Millions of 3G femtocells have been installed in homes and small offices around the world, restoring high quality 5 bar mobile service in otherwise underserved locations.

Not all of the industry believed in the case for residential femtocells, with some of the larger vendors concerned that this was a low margin commodity business which didn't offer the same rate of return on their investments as traditional macrocell solutions. With the cost of femtocell hardware dropping below the $100 price point, it would take many tens millions of units to begin to match existing revenues and profits. This has left the market open to several smaller companies who make the FAPs (Femtocell Access Points), working in partnership with large and credible systems integrators such as Alcatel-Lucent, NEC, Nokia-Siemens and Cisco. Huawei did also enter the market but withdrew after determining it wasn't commercially attractive.

This class of device, which continues to ship in millions per year, is now named the residential small cell. It may optionally include Wi-Fi or be integrated into a router/modem or set-top box, but almost all have been shipped as standalone units. Much smaller form factors have been launched, such as Vodafone Sure Signal (size and shape of a electrical mains plug) and NSN's FAPr-hsp 5120 which fits through a postal letterbox slot. Many have a low power consumption of less than 5 Watts, and so can be powered by USB.

Enterprise

Businesses come in large and small shapes and sizes, complicated by the fact that some bigger businesses have several smaller regional offices and home workers as well as their corporate HQ. Campus environments would also fall under this scope, from large educational establishments (Universities) to the corporate HQs of Vodafone, Microsoft, Google and the like.

A sensible way to partition this sector is by floor space and purpose. A large multi-floor office building would have many more employees actively using their smartphones than an industrial warehouse, so capacity rather than coverage would be more relevant. What this leads to is a much greater traffic density per square foot, because more people are making more calls and using more data within the same confined space.

There are perhaps four different levels of capability to consider for the enterprise:

SOHO

The SOHO (Small Office/Home Office) environment catering for everything from a single home worker to perhaps 10 employees sharing the same small office. These can be adequately covered by the same residential small cell class of product, although a dedicated fixed internet connection may be considered to enhance quality of service.

Medium Office

Larger offices, perhaps up to around 100,000 square feet, would be looking for several more powerful small cells. There's a tradeoff between using more small cells vs fewer higher powered/higher capacity ones. A higher RF transmit power may be useful to reach further corners of the buildings. Several may be installed, with self-configuring ability that allowed seamless handover as you move through the building. One per floor may be a good initial consideration.

Large Office

As you move up into the larger office blocks, say more than 100,000 square feet, a more sophisticated solution is required. Groups of tens of small cells are required to provide the capacity and coverage throughout, and if not co-ordinated carefully this can lead to high levels of signalling overhead and potential for dropped calls or poor end-user experience. The two main technical approaches to this are

a) to install a local controller, such as SpiderCloud's Services Node, Contela's Femto Service Controller or Nokia-Siemens FlexiZone controller.
b) To use peer-to-peer networking between the small cells, which then co-operate as a mesh or grid system, such as Ubiquisys ActiveSON.

One benefit of a central controller architecture is that all the small cells can be co-ordinated to appear to smartphones as if they are the same single cellsite. This avoids the handover process between smartphones and cells which is handled internally by the small cell local controller, reducing signalling overhead, increasing battery life and improving call continuity rates.

Large Tower Blocks

Within the largest buildings, sport stadiums and very crowded areas, say over 500,000 square feet, DAS (Distributed Antenna Systems) are likely to retain their place. With installation costs of many $100,000's, it's more difficult to justify the expense on anything other than very large buildings. These systems distribute the RF signal through dark fibre to multiple radio heads and can accommodate signals from multiple network operators within the same system. This is particularly relevant in large high traffic multi-tenanted indoor buildings which are not tied to any particular network operator.

Indoor Public Access

Residential and some Enterprise systems are often restricted for use by the owners/employees only and are installed by (or at the request of) the building owner. The costs of the installation are borne either directly by the resident (e.g. buying a femtocell in a retail shop) or attributed to revenues they will generate (e.g. given free to high value customers or business account holders).

There are many public areas where traffic is generated from the entire customer base, and where the operator wants to improve service to their users by increasing network capacity and/or improving coverage.

Indoor traffic levels continue to grow because users are consuming data intensive content such as video when sitting down. The cost of delivering this from an outdoor macrocell can be high, because the signal has to penetrate indoors and may use a less efficient modulation rate, taking up a higher percentage of the overall macrocell resources.

Indoor public access small cells are similar to Enterprise in many ways, but the precise location, configuration and format are carefully pre-planned by the operator's own network planners. They may have to work closely with building owners to determine and agree the exact sites, and even the colour or format of the device to be deployed.

Unlike residential and enterprise environments, indoor public access are considered to be a more unsupervised environment. The equipment needs to be more physically robust, so as to avoid or deter any casual vandalism (e.g. pulling out the plug). These units may have quite high performance demands – capable not only of handling more users, but more transient activity as people walk past/through the areas.

Urban Metrocells and Picocells

Small cells are also needed outdoors, where there has been a lot of attention around installing them on streetlamps and on the side of buildings. Some operators believe it better to locate their small cells outdoors and penetrate into multiple buildings, while others see benefits from indoor installation. Both techniques will be used.

The Metrocell term has been used by several operators and vendors keen to differentiate from earlier more expensive/larger products that were adapted from larger macrocell designs. Essentially this was about cost, with contrasting views about the suitability of upsized femtocell designs versus downsized macrocell ones. Some vendors strongly avoid the use of the term Metrocell and prefer the term Picocell, with the industry beginning to use these words interchangeably.

It's important to recognise that both Metrocells and Picocells can be found indoor and outdoor, targeted for public access.

For a device located outdoors and close to street level, the physical attributes step up a notch. It has to cope with weather, including rain, and regular steep temperature cycles not usually found indoors. Unsupervised locations mean it has to be robust and vandal-resistant.

Outdoor urban small cells will be connected using wireline (DSL) or fibre where possible, but at least 50% of units are expected to require some form of wireless backhaul. This has led to a growing range of new and upgraded products which fit alongside the urban small cell to connect to the nearest hub/aggregation point.

Rural

The low cost of a small cell combined with newly introduced low cost satellite backhaul is making standalone remote rural small cells financially attractive. For example, equipment from Altobridge has been deployed in several African and Asian countries using this concept, each serving anything up to 1,500 people within a 10 kilometer range.

This is true not only for developing countries, but remote areas of developed countries which would otherwise be underserved. Softbank, Japan, is one such operator who has deployed many rural small cells in remote communities.

Rural small cells have a wider coverage range than other small cells of as much as 2-4km. Simple lightweight mast poles may be used to improve the range. The service provided is more concerned with coverage than capacity, where a small hamlet or village can be served by a single site. The low power consumption of the small cell and satellite backhaul allow these to be solar and/or wind powered, operating completely "off grid" for months without maintenance visits.

Some different views of small cell segmentation...

John Donovon, CTO of AT&T, presented their summary in January 2013. They don't seem to differentiate much between indoor/outdoor or enterprise/public areas. They introduce a new term – Multi-Standard Metrocell – which handles all three radio interfaces.

EE UK have said they don't include indoor residential or enterprise femtocells within the scope of Small Cells. That's not to say they don't offer/use them, just that the network planning team prefer to restrict the scope of small cells to what they directly deploy.

China Mobile uniquely uses the term nanocell, meaning a 3G or LTE public access small cell.