Femtocell network architecture

- an overview of the basics the femtocell handover or handoff techniques used to ensure seamless coverage.

Femtocell handover or femtocell handoff techniques need to ensure that seamless coverage is perceived by the user when moving onto or off a femtocell.

Femtocell handover is more challenging than normal macrocell cellular handover because the backhaul network is different and there is also little possibility of direct communication between the femtocell and the macrocell.

Femtocell handover basics

Femtocell handover, i.e. handover or handoff to and from femtocells is obviously an essential element of the technology. It is essential that users do not see any problems with the handover process, otherwise this would provide a basic distrust of the system and lead to their use being avoided.

There are a number of scenarios for femtocell handover:

  • Inbound:   This is where handover occurs from the macro-cell or standard cellular network to the femtocell.
  • Outbound:   This is where a handover occurs from the femtocell to the macro-cell or standard cellular network.
  • Femtocell to femtocell:   There will be situations where handover will occur between one femtocell and another close by. This will be commonplace in offices that may have a number of femtocells to give continuous coverage within a building.

These are the three ways in which femtocell handover or femtocell handoff occurs.

Inbound femtocell handover

This form of femtocell handover occurs when a cell-phone or user equipment needs to transfer from a macro-cell on the standard external network to a femtocell. This form of handover is one of the most common forms, but it is also quite challenging because the macrocell and the femtocell will have different backhaul routes, and efficient communication is required between the two as well as transfer of the backhaul route.

To effect an inbound femtocell handover the same basic principles are implemented as for a macro to macro handover. However signalling for the handover needs to be undertaken via the backhaul route (S1 interface for LTE femtocells).

When the User Equipment is in operational it will detect local cells, including femtocells (HNBs - UMTS or HeNBs - LTE). It will therefore be ready to undertake a handover when the conditions are correct.

The femtocells will have a unique physical cell identifier, ID and they will be known to the macrocell. In this way it is possible to effect a handover more efficiently.

In view of the fact that femtocell technology is being deployed with UMTS and it is part of the deployment strategy for LTE, the method for macrocells identifying femtocells within its area is well defined within Release 9 of the 3GPP standards.

Outbound femtocell handover

Outbound femtocell handover is somewhat similar to the standard macro-cell to macro-cell handover except that the direct interface between the base stations does not exist. Signalling must take place over the backhaul link to the core network for both the macrocell and the femtocell, although the femtocell routing will include the femtocell gateway.

Femtocell to femtocell handover

For the femtocell to femtocell handover, the signalling is handled entirely within the femtocell gateway - HeNB-GW via the S1 interface for LTE.

Femtocell handover is a key element of femtocell technology. For femtocell technology to be adopted, the femtocell handover must be able to be undertaken effectively. While there are significant challenges to implementing it in a way that operates efficiently, because of the different backhaul routes for the signalling, these issues have been addressed.

By Ian Poole

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