In part one of this miniseries on femtocells I've been looking at the benefits for mobile operators and part two covered the question why users would put a femtocell into their home. This final part looks at the technical background and hurdles and gives a conclusion.
In practice it is extremely important to integrate femtocells with DSL or cable modems for several reasons. First, femtocells are installed by the user and such an approach therefore ensures that the installation is easy and is done properly.
Additionally, an integrated device is the only way to ensure quality of service for the femtocell since data traffic generated by 3G voice calls must be prioritized on the fixed line link over any other traffic. If a femtocell was attached to an already existing DSL or cable router which already serves other users, uplink data traffic of these users could severely impact 3G voice calls since ordinary DSL or cable routers do not have quality of service (QoS) features to ensure that traffic from the femtocell is prioritized. This behavior can already be observed in practice today in other situations. If an ordinary DSL or cable router is used for a VoIP call in addition to a simultaneous file upload, voice quality is usually very bad due to the packet delay and insufficient bandwidth availability caused by the file transfer.
Thus, a mobile operator deploying femtocells ideally owns DSL or cable access as well or is at least partnering with a company owning such assets. This way a single fixed line gateway could be deployed with Wifi for PCs and other devices and a femto radio module for 3G mobile devices. The single phone per user idea also benefits from such an approach since owning or partnering for DSL or cable access removes the competition between fixed and wireless voice. This also ensures that a femtocell is only used in locations where the mobile operator has licenses to operate femtocells since they use licensed 3G frequency bands.
In practice it can be observed today that a number of mobile operators are taking this route already by either buying DSL access provider companies or at least partnering with them (e.g. Vodafone/Arcor or O2/Telefonica in Germany). It’s unlikely that this is done specifically to roll out 3G femtocells at a later stage but it seems that such companies have understood that it is vital for the future of a telecommunication company to have both wireless and fixed assets in order to stand a chance to be more than a mere bit-pipe for services running over the network. On a side note it is interesting to see the trend of splitting up fixed and mobile access into separate companies several years ago seems to revert now and pains of separation are now followed by pains of re-unification.
Another technical aspect concerning femtocells is interference. In 3G networks, cells usually all transmit on the same frequency and interference is managed by having enough space between them and by adjusting output power and antenna angles. Most 3G operators have at least two frequencies they can use so femtos could for example use the mostly unused second frequency. However, there is still an issue with interference between femtocells of users which live in the same apartment building and have thus installed their equipment close to each other. Left on its own this will result in lower capacity of each cell and might impact quality of service.
When looking at the arguments presented above, femtocells are not likely to be an immediate and outright success. A number of hardware evolutions will probably be needed before form factor, usability and quality of service are adequate. This is likely to take a couple of years. Also, mobile operators need to continue their path of buying or partnering with companies owning fixed line DSL or cable access. This will surely also not happen overnight. However, there is currently still enough capacity available in the macro layer of the network so femtocells are not immediately needed to reduce the load on the network. Therefore, the major immediate benefit of femtocells is improving in-house coverage especially in rural regions, which thus remains a niche market for now, since 2G and 3G coverage and capacity for urban users is usually sufficient for in-house coverage. As such the story of femtocells might parallel the evolution of UMA (Universal Mobile Access) which has similar goals but a completely different concept. That’s a story for another day however…
As always, comments are welcome.