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Femto Cells

A Femto Cell is traditionally considered to be a customer-owned base station. Typically situated in a house or small business, a Femto Cell, also referred to as a Small Cell, is able to provide targeted network coverage, which significantly increases the bandwidth available to mobiles within its local area. Figure 1 illustrates a typical Femto Cell deployment, where it can be seen that connectivity to the mobile service provider’s network is based on a broadband Internet connection. This is typically DSL (Digital Subscriber Line) or Cable technology, which is not only cost effective for the service provider but also provides suitable data rates for transporting traffic from the customer site to the network.

Figure 1 Typical Femto Cell Architecture

The Femto Cell concept is applicable to 2G, 3G, 4G and 5G architectures. As can be seen in Figure 1, a gateway device is typically located in the mobile service provider’s network which acts as a concentration point for connectivity from the many customer sites which need to be supported.

The Benefits
The introduction of Femto Cells by a service provider may be able to solve some of the challenges they currently face, in addition to enhancing their offering to subscribers. The benefits of Femto Cells include:

  • Increased Capacity – the installation of Femto Cells not only leads to increased network coverage, but also enables service providers to migrate traffic away from existing access networks, thereby increasing network capacity.
  • Higher Bandwidth / Higher Revenue – as a result of improved radio coverage and superior wireless broadband performance, subscribers will be able to access a greater range of mobile multimedia services thereby potentially driving up ARPU (Average Revenue Per User).
  • Lower Backhaul Costs – as the Femto Cells are connected to the core network via existing broadband technologies, the service provider is able to significantly reduce backhaul costs which have been traditionally based upon expensive leased line connections.
  • Existing Mobiles – as Femto Cells are able to support 3GPP based radio access technologies, subscribers will be able to exploit the Femto Cell network by using their existing mobiles. This may further benefit the service provider as they will not have to launch new mobile devices.

Figure 2 Femto Cell Benefits

The Challenges
Although the technology behind Femto Cells has advanced quickly, a number of challenges remain. These include, but are not limited to:

  • Radio Interference – even though the output power of a Femto Cell is typically in the region of 10mW to 100mW, interference problems are still possible. These will exist not only between the Femto Cells themselves but between the Femto Cells and the existing 2G/3G/4G/5G macro access network.
  • Security – the use of the public Internet to tunnel the communication between the Femto Cell and the service provider’s core network will require the support for robust security in order to protect both the confidentiality and integrity of the subscriber.
  • Quality of Service – to date, most residential broadband connections do not support any guaranteed quality of service, which may cause problems when real time applications such as voice and video are deployed across them.
  • System Selection/Mobility – in addition to the obvious issues of supporting real time handovers between the Femto Cells and the existing 2G/3G/4G/5G access networks, management systems must also address access control, thereby limiting operation to only those Femto Cells the subscriber is authorized to use.

Figure 3 Femto Cell Challenges

5G Small Cells

For 5G, the use of Small Cells is a key feature of 5G radio coverage. This can be largely attributed to the high operating frequencies used in 5G. That is, due to the requirement for large radio channels in order to fulfil capacity and data rate requirements, 5G networks typically operate at much higher frequency (between 6 and 100GHz, although lower frequencies will also be used). Radio propagation characteristics at these high frequencies mean that the range of any given cell is reduced, potentially down to hundreds of metres or less. As such, cell densification occurs, with multiple small cells in a small geographic region providing the overall 5G RAN coverage.

Note that this approach is different to the aforementioned Femto Cells, which are considered to be deployed in the customer premises. Although 5G Small Cells can also be deployed within the customer premises, they also form an integral part of the service provider’s overall 5G NG-RAN deployment.

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