3GPP Femto Specifications

The post on Femtospots a couple of days ago had some good feedback and one reader pointed me to TS 22.220 where 3GPP currently lays the ground for an end-to-end femtocell architecture, or Home NodeB architecture in 3GPP talk. Thanks for that, quite an insightful document! Here's a link to the document after the latest 3GPP meeting (December 2008) which hasn't yet made it to the official specification server. While still being a somewhat early draft today, it nevertheless gives some interesting insight into which directions operators want to go with femtos.

I've had a look at the contributors to the document and from the operator side, T-Mobile, Vodafone, Softbank, SK Telecom, and NTT-Docomo seem to be the most enthusiastic ones. On the vendor side, I've seen input from RIM, ETRI, Qualcomm, NEC, Alcatel, Huawei, Nortel and Marvel. The lists are not exhaustive but show that there is a lot of interest in the topic.

Here are the some of the highlights of the document:

Open and Closed Operation

3GPP TS 22.220 is a requirements specification so it will serve as a guideline for future stage 2 and stage 3 documents which will contain the implementation details for those requirements. So while trying to stay realistic, the document tries to explore the topic in as wide a range as possible and to keep as many options open as possible. Three operating modes are specified for femtos / Home NodeBs (UMTS) or Home eNodeBs (LTE) and I use the terms interchangeably below: The first one is called open, which means all UEs (user equipment in 3GPP talk or mobile devices) of an operator are allowed to use the cell. The second mode is called Closed Subscriber Group (CSG), which means only selected UEs, for example those belonging to a household, are allowed to use the cell. The third mode is called hybrid and combines the first two. I imagine that in hybrid mode, CSG users might potentially get higher priority and access to the local network.

Local IP Access

Speaking of local network access, the requirements specification also contains a chapter on allowing the UE access the the users home network. No specifics are mentioned yet as to how this should be implemented in practice or what kind of services could be used over such a connection. I expect that the 'how' will be clarified in stage 2 and stage 3 documents while the 'what' will be left for other standards bodies to clarify. The document says that both operator and users will have a say which users are part of the CSG and which users will be allowed to have access to local resources.

Local IP Access to the Internet

A so far empty chapter is present for how to connect to the Internet via the local network therefore bypassing the operators core network. I can hardly wait to see if this chapter will be filled with text or removed in later versions of the document.

MBMS and Mobile TV

Some parties also seem keen to use the Home NodeB for mobile TV and would like to see MBMS specified for femtos.


Further, there seem to be operators or vendors who would like to have some parts or all of an IMS implemented in the femto in a transparent way for the UE to potentially bypass the circuit switched network. I don't quite yet fully get the concept and purpose of this feature but I am sure some more text will be added to this chapter as the document evolves.

The Achilles Heel : Pre-Release 8 UEs

In my opinion the biggest overall issue for femtos used in closed subscriber group (CSG) mode is how to prevent mobiles not belonging to the CSG trying to reselect to the femto. For future 3GPP Release 8/9 compliant UEs, things can be standardized to avoid unnecessary cell reselections and signalling. TS 22.220 gives some general guidance on how that could be done by adding femto related information on the broadcast channel of the cell. For today's UEs, however, any solution has to work with what is already in place. 3GPP TR 25.280 gives a number of potential solutions in Chapter 6.2. Personally I think the Equivalent PLMN solution has a lot of merrit, but no definite recommendation of how to solve this is given yet.

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