Femto Search and Rejects

3G femtocells are an interesting topic but I haven't had much time yet to take a look at the details of how mobility management works in practice. There's lots of activity in 3GPP to standardize mobility management around femtocells (or Home NodeBs how they are called there) in Release 8 and beyond. However, there are already already femtos on the market today and they have to work together with pre-Release 8 mobiles. So I've had two fundamental questions: How can mobiles find the femtos when they are on the 3G macro layer and how does the femto get rid of users which do not belong to the subscriber group, i.e. everyone except the owner and his/her family and friends?

Then this book, "Femtocells – Technologies and Deployment" by Jie Zhan and Guillaume de la Roche came my way. I haven't had time yet to go through it in detail but it looks highly interesting and informative and I could answer my questions with it within minutes:

Cell-Reselection to a Femto: To make a cell reselection to another 3G cell, it needs to be part of the neighbor cell list of the cell. As there could be many femtos inside one macro network and provisioning them automatically might not be a straight forward approach, one option is to select a couple of Primary Scrambling Codes and declare them as neighbors in every macrocell or at least on those macrocells in which femtos are located. This works even if there are many femtos inside the coverage area of a macrocell as not all of the femots are overlapping and hence the PSCs can be reused. If the femtos scan their surroundings when they start up they can help to avoid the PSC overlapping issue.

How to get rid of non-femto subscribers: The femto deployments I have heard of so far are closed-subscriber-group femtos, i.e. only registered people have access. But since todays mobiles know nothing of femtos how can you ensure only those remain in the cell that are supposed to be there? The book gives this as one of the potential solutions: For the femtos a certain range of location area codes (LACs) are reserved. If a non-femto subscriber mobile finds the cell and tries to perform a location update it gets a location update reject with cause code #15 (no suitable cells in location area). The mobile then goes back to the macro layer and puts the LAC in the forbidden LAC list on the SIM. The 3GPP UMTS RRC spec says It's only removed when the mobile is switched-off or after a significant amount of time has passed (12-24h). A bit of a disadvantage here: If the user of femto-A passes femto-B during the course of a day, the mobile will try to register with femto-B and will be rejected. In case the LAC was the same as that of femto-A the mobile will not try to reselect to femto-A until the forbidden LAC list is cleared. In other words, the user comes home and the mobile will not use the femto.

Agreed, there's much much more to the topic, those where just my two most burning questions concerning femtos.

4 thoughts on “Femto Search and Rejects”

  1. So this isn’t a case of a bar buying a femto and then everyone getting coverage, we’re in the archaic period of having to register our mobile phones with a particular femto and it’ll never work with any other femto.

    Am I getting that right?

  2. @Stefan : It depends what level of access control is set-up in the femto. By default the femto will be configured for open-access with priority to pre-authorized mobiles. But a femto-owner can disable open access through O&M or simple http access as in WiFI-access router configuration

  3. Hi Stefan,

    there are three different modes femtos can be used that are described in the specifications. The first is the fully open mode and such a femto can be used by all subscribers. Great for public places. The opposite is the Closed Subscriber Group or CSG cell. As the name implies thats for private use, i.e. for your appartment and you dont want to share with your neighbors. And then theres the mode in between that allows everyone access but the owner has priority.


  4. Hi Martin,

    Regarding your questions:

    >> How can mobiles find the femtos when they are on the 3G macro layer

    The Femto’s will be more of detected cells rather than monitored cells. The network will give an option to find and use detected cells.

    If the Femto’s become part of Monitored cells then there will be a problem because there can be a long list of monitored cells.

    Due to the SON feature, each Femto will try and pick a unique code so as to avoid interference and this can change if a new Femto comes up or the Femto restarts (or crash 😉

    >> how does the femto get rid of users which do not belong to the subscriber group, i.e. everyone except the owner and his/her family and friends?

    I think the simplest solution is to force the UE to send IMSI in UE Identity duing RRC Connection Request for Registration and once you check if the IMSI is not part of the allowed IMSI’s then you send RRC Connection Reject with not allowed to camp on or some similar cause.

    I am sure there are better approaches that are being used but the above solutions are simple and practical.

    Agreed about the Forbidden LAC problem. Havent looked into it.

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