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

The tiny cells created by femtocells typically lie inside larger cells served by nearby macrocell base stations. To operate such an underlay network reliably, femtocells need to avoid or strongly mitigate any interference with macrocells and provide a seamless experience to users as they roam in and out of femtocell coverage. Since existing macrocell networks and mobile devices have been designed without awareness of femtocells, these requirements must be met without requiring any changes to mobile devices.

The underlying femtocell radio technologies used in CDMA and UMTS femtocells are quite similar. Therefore, we will cover them together in this section noting differences where relevant. An important difference between these two systems to note at the outset is that in UMTS, voice and high-speed data (HSxPA) services can be delivered on the same 5 MHz wide radio channel, whereas in CDMA these services are delivered on different 1.25 MHz wide radio channels using two different air interfaces: CDMA2000 1x is used for voice services and CDMA2000 EV-DO is used for broadband data.

Interference Management in Femtocells

Femtocells implement several layers of interference management to optimize user experience across both femtocell and macrocell networks.

Interference Avoidance through Frequency Planning

A mobile operator has three basic options for allocating available frequencies in femtocell deployments. These are represented by the scenarios A, B and C in Figure 1., below.

Scenario A represents a dedicated radio channel femtocell deployment that provides separate macrocell and femtocell radio channels. This has the advantage of minimizing interference between the two networks and simplifies initial deployment of femtocells. Scenario A is typically more suitable in rural areas where the mobile operator may have unused radio channels. Scenario A is also more feasible in CDMA2000 systems where radio channels are more numerous because they have a narrower bandwidth.

Scenario C shares all available radio channels between the macrocell and femtocell networks. This has the advantage of providing more degrees of freedom to manage interference between femtocells, especially in dense urban deployments, but also requires the greatest degree of interference management to ensure minimal impact on the macrocell network from the co-channel femtocells.

Scenario B represents a compromise between scenarios A and C in which some radio channels are shared between the macrocell and femtocell networks and other radio channels are reserved for the macrocell network only. In Scenario B the macrocell can redirect the mobile devices it is serving on the shared radio channel to a dedicated macrocell radio channel when they approach a femtocell. One way of deploying scenario B is to use the shared radio channel primarily for macrocell data services (HSxPA), and for femtocell voice and data services, and leave the dedicated radio channels for macrocell voice services.

Since mobile operators may not be able to dedicate radio channels to femtocells in many of their markets, femtocells need to be designed with advanced interference mitigation techniques that allow reliable operation when femtocell and macrocell networks share the same radio channels—Scenario C. This is the scenario that we discuss next. [source]

Figure 1. Deployment Scenarios for Sharing Radio Channels Between Macrocell and Femtocell Networks.


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