There are several "flavours" of femtocell for different 2G and 3G technologies. All must comply with international standards because they operate on licenced spectrum. These also ensures different vendors equipment work well together.
Most of the attention and interest is about the 3G UMTS variety (the 3G variant of GSM), with the first baseline femtocell standards was completed in the 3GPP Release 8 baseline and formally released at the end of March 2009. We'll ignore GSM or CDMA radio technology for now.
In the 3GPP committees, the focus is not just on 3.5G UMTS/HSPA standards, but work also continues in parallel for 4G LTE femtocells. Mainstream LTE standardisation itself is a key foundation of Release 8, and the idea is that femtocell issues will be incorporated into Release 9 in this emerging standard from the start rather than having to be added as an afterthought.
So who’s working on these femtocell standards?
There are two main organisations working on these standards, with typically the same organisations participating and sometimes even the same people attending both.
- The Small Cell Forum (formerly Femto Forum), a non-profit organisation sponsored by the industry with an objective to promote femtocells and make them successful. Their work continues behind closed doors, and only members of the Forum have access to working documents and technical papers. Their output feeds through to...
- The 3GPP (3rd Generation Partnership Program), a worldwide standards organisation formed in 1998, sponsored by regional standards bodies such as ETSI, ARIB, CCSA, ATIS, TTA and TTC. All papers submitted at meetings, minutes and agreements are published and freely downloadable from their website. The agreed standards are adopted by regulatory organisations around the world, mandating network operators and equipment manufacturers to abide by them. These standards may adopt/reference or incorporate existing standards from other organisations, rather than re-inventing a new version - a much quicker, cheaper and less risky approach. These foundation elements come from...
- Other standards organisations, such as the Broadband Forum (who created TR-069 standard for remote device management of DSL modems and other broadband devices), IETF (who specify many of the Internet Protocols such as IP, IPsec etc).
The process typically involves some discussion within the Femto Forum meetings which then result in technical proposals to the 3GPP. These and other inputs are discussed at the 3GPP working group meetings and decisions reached through consensus. Contentious issues can be put to a vote. It is only the 3GPP specifications which are legally adopted and implemented worldwide.
Which interfaces are being standardised?
There are two main interfaces to be formalised, and these are dealt with at different 3GPP committees:
Radio Interface: There are two aspects here:
- Between the handset and the femtocell
- Between the macrocell basestation and the femtocell. For example, it’s already been agreed that each macrocell can broadcast some additional information for use by femtocells, such as frequencies or scrambling codes to be used by femtocell.
This does include optimisations to the handset, although the strategy is for femtocells to be compatible with existing standard 3G handsets - newer handsets which incorporate these optimsations may perform better, for example handover between macrocell and femtocell, battery life and displaying the name of the femtocell itself.
Femtocell to Mobile Network: Named the Iu-h interface, the choice of protocols and data stream format will be decided here.This includes wider architectural issues such as orchestrating handover between the femtocell and the handset. Again, some of these specifications may affect the handset as well as the femtocell itself.
Which 3GPP committees are working on these standards?
The 3GPP organisation is structured into several working groups, each of which may also have sub-committees. The main working groups dealing with femtocell standards are:
Radio Access Network Working Group 2 (RAN2):
- Management of neighbour cell information
- Restrictions in handover in one or both directions
- Frequency reuse within overlapping/hierarchical cell layout
Radio Access Network Working Group 3 (RAN3):
- Interface between femtocells and femtocell gateway
- Access control, managing unwanted access
Radio Access Network Working Group 4 (RAN4):
- Identifing any new or changed requirements for femtocells
- Identifing relevent deployment scenarios
- Investigate RF related aspects such as interference and performance
- Investigating if frequency accuracy specifications can be relaxed for the home environment
What's been agreed already?
Standards tend to evolve by agreeing the high level principles first, then subsequently filling in the details. Major agreements so far include:
- A technical study report into the feasibility of 3G femtocell standardisation. Started in July 2007, this latest revision is from July 2008 and identifies the various standards affected. The main conclusion is that femtocells are technically feasible and would not disrupt the existing mobile network. Incidentally, the document also states in Section 4 that "No impact to terminal specifications is foreseen." - something that now appears to have changed.
- Iu-h interface. It has been agreed that there will be one common interface between femtocells and femtocell gateways, identifying the key components in the femtocell system architecture. Previously, there had been several different competing architectures and protocols, each promoted by different groups of vendors. The Iu-h interface allows femtocells to be interoperable, so that operators can source their products from different vendors. Whilst the standard should permit this, there will need to be some interoperability testing between vendors to confirm that they have interpreted the standards in the same way.
- Closed Subscriber Groups. Not to be confused with Closed User Groups (an existing standard feature), a CSG defines a set of one or more femtocells which could be located on a campus or business environment. When a mobile phone has switched to using a femtocell within a CSG, it will prefer to keep on the same CSG rather than revert back to the external mobile network. CSG's may be restricted to individuals/groups (closed access) or open for general use by the public. CSG's will have their own name which will display on newer handsets which have the capability to do so.
- Using the TR-069 device management standard. Original developed by the DSL Forum (now renamed the Broadband Forum) to remotely manage DSL modems in customer premises, this widely deployed standard is to be reused for femtocells.
- Using IPsec to encrypt and secure the data traffic between the femtocell and femto gateway. An obvious choice perhaps (this also being used within web browsers to protect sessions involving eCommerce, Internet Banking, remote VPN access to office systems etc.), but there was some debate during 2008 involving alternatives before a consensus was reached.
- Several radio parameters are agreed. These include the maximum transmission power of 0dB (100mW) - although femtocells dynamically reduce power to the minimum required; femtocells will support handsets moving at speeds up to 30km/h (pretty fast when walking around the home, but perhaps achievable on a bike in a university campus); macrocells can continuously broadcast additional information about femtocell parameters (so handsets know where to look for them); femtocells are compatible with existing 3G mobile phones, but later models which include new standards optmisations will be able to display the name of the femtocell zone, handover to/from the femtocell more easily hence drop fewer calls, and have longer battery life because they won't be continuously scanning for other cellsites as much. The frequency accuracy of femtocells is relaxed to 250 part per billion, which should help reduce the costs.
What standards documents will be produced?
The overall 3GPP standard specifications involve a large number of different documents addressing all aspects from radio interface, protocol messages, testing and performance. Each year, many work packages come up with changes that affect the same sets of specifications. The output of each different project is a long change request with many minor updates to the different documents.
A typical 3GPP work package first produces a technical study report, then one or more Stage 1, 2 and 3 documents which go into more technical detail.
These changes are then incorporated into the main set of standards documents and formally approved. 3GPP Release 8 was the first release to include femtocells and was initially developed by December 2008. Some further refinements, such as detailing the parameters for remote device management were added during Q1 2009. As with any new standard, an ongoing process of clarification and refinement will continue in subsequent releases.
3GPP Release 9 is expected to include a full detailed specification for femtocells using a SIP based architecture (rather than Iu), and should be completed during 2009.
Where can I find out more?
The documents from each meeting of the 3GPP sub-committees are published on the 3GPP website. The report document is worth looking at first, it will refer to the topics covered, documents referenced and decisions reached. Typically a draft copy of the report is published a few days after each meeting with the final version agreed at the subsequent one.
If you find trawling through many detailed technical standards discussion documents isn't as exciting as watching paint dry, keep an eye on ThinkFemtocell.com for a precis of significant developments and decisions.
Specific Standards Documents
You can download and view any of the 3GPP standards documents directly from their website
A list of the affected standards documents is shown below:
22.220 - Service requirements for Home Node B (HNB) and Home eNode B (HeNB)
23.830 - Architecture aspects of Home Node B (HNB) / Home enhanced Node B (HeNB)
23.832 - IMS aspects of architecture for Home Node B (HNB)
25.367 - Mobility procedures for Home Node B (HNB); Overall description; Stage 2
25.467 - UTRAN architecture for 3G Home Node B (HNB); Stage 2
25.468 - UTRAN Iuh Interface RANAP User Adaption (RUA) signalling
25.469 - UTRAN Iuh interface Home Node B (HNB) Application Part (HNBAP) signalling
25.820 - 3G Home Node B (HNB) study item Technical Report
25.967 - FDD Home Node B (HNB) RF Requirements
32.581 - Telecommunications management; Home Node B (HNB) Operations, Administration, Maintenance and Provisioning (OAM&P); Concepts and requirements for Type 1 interface HNB to HNB Management System (HMS)
32.582 - Telecommunications management; Home Node B (HNB) Operations, Administration, Maintenance and Provisioning (OAM&P); Information model for Type 1 interface HNB to HNB Management System (HMS)
32.583 - Telecommunications management; Home Node B (HNB) Operations, Administration, Maintenance and Provisioning (OAM&P); Procedure flows for Type 1 interface HNB to HNB Management System (HMS)
32.821 - Telecommunication management; Study of Self-Organizing Networks (SON) related OAM Interfaces for Home Node B (HNB)
33.820 - Security of Home Node B (HNB) / Home evolved Node B (HeNB)