Metro Femtocells and Distributed Antenna Systems (DAS) Compared

 There’s been a lot of hype recently proposing femtocells as a low cost means of rolling out mobile network capacity (specifically for LTE, the new 4th Generation radio standard). Femtocells could be a low cost way of providing hotspot capacity in public areas. But in some situations, it will compete with existing techniques. Here we compare and contrast femtocells with DAS.

What is DAS

Distributed Antenna Systems (DAS) are a commonly used technique to provide excellent coverage and capacity throughout large buildings or campus areas. They are commonly deployed in large office buildings, shopping centres and conference centres. Radio power levels are much reduced for both transmitters and mobile devices, because an antenna is usually within line of sight and only a short distance away. This benefits call quality, data rates/response and battery life.

The formal definition of DAS?

Like every good new invention, the technology has its own not-for-profit organisation – the DAS Forum , although this seems to be very US centric and not very active. Their official definition of DAS is “a network of spatially separated antenna nodes connected to a common source via a transport medium that provides wireless service within a geographic area or structure. DAS antenna elevations are generally at or below the clutter level and node installations are compact”.

How does DAS work in Plain English?

What this means in plain English is that the RF signals to and from the mobile operators cellsite basestation are piped through a system of multiple antennas. There are a number of different types of DAS each with their own characteristics :

• Passive DAS – where RF signals are combined using passive components such as filters, splitters and couplers.  Great for multiple bands and small to medium size locations
• Active DAS – RF signals are converted and distributed over fibre.  Great for larger installations but more costly and have to be dedicated to bands e.g. GSM900, GSM1800, UMTS2100.
• Hybrid – combination of the above techniques

DAS can be employed purely within a large building (In-building DAS) or across a large urban area (Street Level DAS).  Street Level DAS can provide a very efficient solution for large urban regeneration projects which require dense coverage. They can also be provided in other busy areas such as Metros, Airports or Railway Stations.

Pros and Cons

The benefits of this approach include:

• Operator equipment is located in one place – simpler maintenance and upgrade procedures
• Supports multiple network operators, allowing sharing of costs and resources
• RF coverage can be tailored to meet the needs of specific buildings and use cases
• Easily upgraded to handle new frequencies, transmission technologies, capacity
• Allows capacity uplift by offloading in-building traffic from Macro layer
• Supports rollout of bandwidth hungry, low latency applications

But the downsides include:

• High capital investment – only justified for large airports, businesses, campuses, centres
• Complexity - Needs specialist RF expertise to design and maintain

Large Business Premises

DAS is commonly used today by operators for equipping large business premises. Where a business signs up exclusively with one operator, large amounts of capacity and good coverage are required and the investment to provide this is justified. It’s cheaper than trying to penetrate the building from several high capacity outdoor macrocells, which could have interference issues in the wider area.

Shared Buildings

A separate situation arises for large shared offices, shopping centres, airports and the like where large numbers of mobile users can be expected from all national networks. The business case for the DAS is largely driven by volume,  but some building owners now deploy DAS as a ‘hygiene factor’ to encourage more people on site. 

Shared Access Approach

One commercial approach is where property owners deal with an intermediary who installs a common DAS system and arranges the commercial and technical aspects with each individual operator. Typical companies in this field include Shared Access, who have both the technical and commercial expertise and an enlightened approach on financing the solution.

In a deployment, each network operator would install their own high capacity cellsites (separate 2G and 3G equipment) in a central location. The RF output from all is combined and distributed throughout the building via the DAS. The system can support a wide variety of technologies and frequencies, including GSM, CDMA, WCDMA, HSPA, LTE, WiFi and even PMR.

DAS versus Metro-Femto

With the recent hype around the Metro-Femto concept, these DAS systems may be more easily upgraded to provide LTE capability than installing separate LTE femtocells from each operator around the buildings.  Operators would only need to install one central LTE basestation each, rather than many independent LTE femtos. This depends to some extent on what spectrum is used for LTE – additional spectrum may require additional active components.

Outside these areas, where individual operators need to provide coverage in dense urban areas/hotspots, the metro-femto concept could be more advantageous. Where femtocells are very low cost, and are self-organising, there are likely to be situations where these are more attractive solutions.

DAS versus Domestic/Small Business Femtocell

The business case for femtocells in small and medium businesses remains however. DAS systems are only viable for large premises with 1000’s of employees or users, whereas femtocells can be viable for single employee businesses through to hundreds.

There is some overlap between femtocells and single repeaters in SME's but whilst repeaters installations have been de-skilled to a large degree, operators look at them carefully from a commercial perspective before proceeding. Today, businesses have to be spending several thousands of dollars a month before operators are interested in specially engineered solutions - something which could change dramatically with the lower price point and TCO of femtocells.

Thanks to Graham Steele of Shared Access for enlightening me on the practicalities and possibilities of DAS .