Femtocell Radio Technology

In this section:
Interference Management in Femtocells
- Interference Avoidance Through Frequency Planning
- Interference Mitigation in Co-Channel Femtocell Deployments
Seamless Mobility Across Femtocell-Macrocell Boundaries
Mobility in Standby Mode & Access Control
- Handoffs at Femtocell Coverage Boundaries

Handoffs at Femtocell Coverage Boundaries
When a user on a call moves in or out of femtocell coverage, a handoff procedure is executed to maintain call continuity. Such hand-in (from macrocell to femtocell) or hand-out (from femtocell to macrocell) procedures greatly enhance femtocell experience as users enjoy seamless call continuity when they cross network boundaries.

Hand-out can be supported with relative ease without any changes to the existing macrocell network or to mobile devices. This is achieved by making the femtocell network behave just like a macrocell network towards the macrocell Mobile Switching Center (MSC), essentially making the legacy equipment think that it is handling a handoff between two macrocell networks. Except when the femtocell and macrocell networks are sharing a single available radio channel, hand-out is generally performed to a macrocell radio channel that is different from the femtocell radio channel. In UMTS systems, when UMTS macrocell coverage is not available, a voice call can also be handed out to the ubiquitous GSM system.

Hand-in on the other hand is more difficult because there is no simple mechanism for the macrocell network to determine the identity of the target femtocell from the measurement reports sent by the mobile device approaching the femtocell.

In CDMA2000 systems, the measurement report contains the strength of pilot signals seen by the mobile device and uses the so-called PN offset to identify the target cell. However, target femtocells cannot be identified without ambiguity based solely on PN offset report because out of the available 512 distinct PN offsets only a small number will be allocated to femtocells and these are re-used amongst them. Thus with possibly hundreds of femtocells per macrocell it is not possible to uniquely identify the handoff target.

CDMA femtocells can solve this problem by measuring the RL signal of the approaching mobile device. First, using cdma2000 1x signaling protocols the Base Station Controller (BSC) in the macrocell network triggers the handoff and forwards the mobile device’s RL scrambling code information and the target PN offset to the Femtocell Convergence Server (FCS) via the MSC. Based on this and other available information, the FCS then requests a subset of the femtocells it is serving to listen for the mobile device based on the RL scrambling code. All femtocells with the same target PN offset then report back the mobile device’s RL signal quality together with the femtocell’s FL pilot transmission power level. Based on the reports from the various femtocells, the FCS determines the correct target femtocell and signals the macrocell BSC to order the mobile device to handoff to the femtocell.

The same femtocell identification issue occurs in UMTS femtocells. A scalable hand-in solution requires changes to existing standards, thus can work with only future devices that will be compliant with these new standards. A proposed 3GPPstandard solution is based on “autonomous gaps”, which allows the mobile device in a call to break from the call for a sufficiently long period of time to allow it to search for a femtocell on a different radio channel and decode its broadcast channel. Based on the cell identity and the measurements, the macrocell network can trigger a hand-in to the femtocell.[source]


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