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

Seamless Mobility Across Femtocell-Macrocell Boundaries

Mobility in Standby Mode
A mobile device in standby mode changes the cell that it is camped on as it moves across cell boundaries. This process is often helped by parameters broadcast by the cell sites.

It is desirable for the mobile device to switch to its femtocell when the signal received from the femtocell is strong enough to support reliable service. This needs to occur even when the macrocell network can still provide reliable service, because switching to the femtocell will improve the mobile user experience and ensure that he/she can instantly take advantage of any subscribed flat-rate femtozone calling plan.

In either CDMA or UMTS, the mobile device will switch to a new cell on the same radio channel based on continuous measurement of pilot signals from neighboring cells. But mobile operators in most markets use multiple radio channels and switching to a cell on a different radio channel has more stringent requirements. To increase battery standby time the mobile device scans other radio channels only when the signal-to-noise ratio of the current cell is lower than a certain threshold. In UMTS systems, this threshold is determined by a system parameter called SIntersearch. Setting SIntersearch to a higher value can force all mobile devices to perform inter-frequency searches under more circumstances, thus increasing their battery drainage. On the other hand, if SIntersearch is set to a lower value, the inter-frequency scans required to detect the femtocell may not be triggered if the signal received from the macrocell base station is strong and a mobile device may never switch to the femtocell. Thus operator must optimize how they set SIntersearch. UMTS also includes a feature known as Hierarchical Cell Structures (HCS) in which femtocells can be given a higher priority than macrocells. HCS can be used to accelerate the selection of the femtocell, but it does not increase the frequency of inter-frequency searches.

CDMA2000 femtocells solve this problem by including a special transmitter, called a beacon, which makes the femtocell’s presence known on all macrocell radio channels except the one used by the femtocell. Beacon approaches are also being considered for UMTS femtocells. The beacon is a special signal that consists essentially of a low-power pilot signal along with a broadcast signal that forces mobile devices to camp on the femtocell radio channel. In UMTS this is done by broadcasting appropriate cell reselection parameters while in CDMA a command is sent to all mobile devices or only to those devices authorized to use the femtocell.

Since most CDMA2000 devices support both CDMA2000 1x and EV-DO, a mobile device needs to camp on the femtocell in both 1x and EV-DO systems simultaneously. Airvana’s CDMA femtocells implement a unique beacon solution to allow a device entering femtocell coverage area to attach and remain attached to both systems.

The overall femtocell user experience will be best when the beacon range is somewhat smaller than the service range most of the time and occasionally equal to the service range, as illustrated in Figure 1. The operation of the beacon is optimized through detailed simulations and lab/field testing to ensure that target mobile devices attach to the femtocell as quickly as possible and avoid unnecessary interference to other mobile devices that are being served by the macrocell base station.[source]

Figure 2 Illustration of beacon and service coverage areas.


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