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How femtocells work

We'll explain the details for a 3G UMTS mobile phone system here, because this is the most common technology being used for small cells today. Other mobile phone systems would operate in a very similar way.

Let's start with the original residential femtocell architecture where there are already many millions of units installed.

Residential Femtocell Architecture

The femtocell appears to the standard 3G phone as just another cellsite from the host mobile operator, and can be used by almost any 3G phone including (where permitted) roamers visiting from other countries.

The mobile operators telephone switch (MSC) and data switch (SGSN) also communicate to the femtocell gateway in the same way as for other mobile calls. Therefore, all services including phone numbers, call diversion, voicemail etc. all operate in exactly the same way and appear the same to the end user.

The connection between the femtocell and the femtocell gateway uses secure IP encryption (IPsec), which avoids interception and there is also authentication of the femtocell itself to ensure it is a valid access point.

The figure below illustrates the system architecture and context for femtocell operation.

Femtocell Architecture Overview

Inside the femtocell are the complete workings of a mobile phone basestation. Additional functions are also included such as some of the RNC (Radio Network Controller) processing, which would normally reside at the mobile switching centre. Some femtocells also include core network element so that data sessions can be managed locally without needing to flow back through the operators switching centres.

The key functions are integrated onto a single chip, such as the BCM61670 from Broadcom or TCI6630 from Texas Instruments. These and other chip manufacturers document the different components in more detail in their reference designs. In addition to these highly integrated chips, a radio frontend (such as from Maxim) and a highly accurate frequency reference crystal oscillator (such as from Rakon) are also required.

The extra capabilities of a femtocell demand it to be self-installing and self-configuring. This requires considerable extra software which scans the environment to determine the available frequencies, power level and/or scrambling codes to be used. This is a continuous process to adapt to changing radio conditions, for example if the french windows are opened in a room containing the femtocell.

Within the operators network, femtocell gateways aggregate large numbers of femtocell connections (typically 100,000 to 300,000) which are first securely connected through high capacity IP security firewalls.

Enterprise Small Cells

The same technology and architecture is also used for small to medium businesses. Typically the small cells have higher capacity and slightly higher RF power to give a larger range. Some enterprise small cell vendors have developed solutions where small cells co-operate in clusters to provide seamless service. For larger enterprises, a small cell controller may be used to provide additional local services including direct connection to the enterprise network.

Metrocells and Rural Small Cells

Public areas, both inside and out, may use specially designed metrocells which are also based on the same architecture. These can also take advantage of the same small cell/femtocell gateway, sharing its use between residential, enterprise and metrocell installations. These products must be weather proof and vandal proof, operating in sometimes harsh unsupervised environments with wide temperature fluctuations. Metrocells are installed by the network operator themselves, and a broadband IP connection (called the backhaul) is also required.

Read further to learn about which radio technologies are used for small cells.

Interested to learn more? Want to get fully up to speed on femtocells?
Why not buy our book on the subject.

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Comments   

#1 Anurag said: 
how handover happens to the 3G network from the femto cell when people go out from home to the road while talking...
0 Quote 2011-04-10 09:34
 
#2 ThinkFemtocell said: 
@Anurag: Handover when leaving a femtocell works almost exactly as when moving between outdoor cellsites. First, the femtocell sniffs the environment to identify what other outdoor cellsites are available and what frequencies they are using. This is broadcast to all mobile phones by the femtocell, so that they can measure the signal strength and quality of nearby cellsites. When this is much better than the signal from the femtocell (such as when you walk outdoors), the system then co-ordinates a hard handover between the femtocell to the RNC, with the connection anchored in the central voice (MSC) or data (GSN) switch. Most of the functionality which controls the handover exists in the RNC (for outdoor cellsites) and in the femtocell - these both communicate with each other using standard messages/signal ling protocols. Handover from 3G to 2G GSM when moving outdoors is also possible using the same mechanisms.
0 Quote 2011-04-13 22:45
 
#3 SAMET YILDIRIM said: 
hi,ı wonder that what the most significial harm of FEMTOCELL..
0 Quote 2011-05-01 12:29
 
#4 ThinkFemtocell said: 
@Samet: Why should they be harmful? They emit less RF power than many Wi-Fi hotspots, and mean that your own mobile phone whispers at very low RF levels across a few metres, rather than needing to power up and penetrate walls/several miles to the nearest outdoor basestation. If many femtocells are deployed in densely populated areas, the transmissions all add up to a much lower total. So I'd argue that these should reduce concerns about harm than increase them. This viewpoint is validated by a report commissioned by the Femto Forum into the subject (available from the website), and I've not seen any studies that refute it.
0 Quote 2011-05-03 20:26
 
#5 samet said: 
In fact,FEMTOCELL is perfect technology for people to connect to each other at fast and qualitied... THANK you for replay.
0 Quote 2011-05-10 10:42
 
#6 Mahin said: 
this is really an interesting technology.. but i dint understand the concept of femto gateway..i mean where it is located n is it necessary to have a gateway in the city where we want to use femto cell...???
0 Quote 2011-06-09 07:32
 
#7 rahi said: 
i'm telecomm student doing final yr project on femtocells and its tech benefits. does any one know how to throughput analysis and outage probability using matlab?
0 Quote 2011-06-27 06:54
 
#8 ThinkFemtocell said: 
@rahi: I haven't come across this analysis being made public yet. It should make for an interesting research project. At last week's conference, operators made it clear that femtocells had to provide at least as good performance as the macro network and (if correctly engineered) that this was certainly achievable.
0 Quote 2011-06-27 09:01
 
#9 CASTRO said: 
Can I use the femtocell to build a private gsm network ??
0 Quote 2012-01-08 11:47
 
#10 ThinkFemtocell said: 
@CASTRO
Yes, you could do this - provided you had legal rights to use the frequencies. For example, many cruise and commercial ships have installed their own. Calls made to others onboard are directly connected, whilst international calls are sent via satellite. Another example is for smaller/remote islands. ip.access has plenty of experience in providing systems for these applications, with companies like Quortus (a ThinkFemtocell Sponsor) providing small core network to support it.

You can also look at open source projects, like OpenBTS project, which I've described here
http://www.thinksmallcell.com/Technology/build-your-own-opensource-femtocell.html
0 Quote 2012-01-11 19:32
 
#11 Abrar said: 
thnks for the fig and explanation..wa s looking for it for my seminar
0 Quote 2012-03-31 08:26
 
#12 Taylor Li said: 
When handover occurs, is it possible to record the cellphone's ID through customized programming within the firmware
Second,Does the femncell still work under high speed? such as a 60 km/h speed (imagine a car pass by a outdoor femnocell)
0 Quote 2012-09-28 07:08
 
#13 ThinkSmallCell said: 
@Taylor: A small cell does know which cell phones it has handled and could store those, but these aren't the telephone number (MSISDN), they are the SIM card number. In some cases, the small cells record how many unique SIM card numbers it has handled in a given day, to give an idea of the volume of usage.

The time it takes for a car to pass through the coverage footprint of a small cell/femtocell means that it isn't a good idea to handin and out during the transit. Instead, networks are optimised to deter fast moving vehicles from being handed down to the small cells. Additionally, extra baseband RF processing is required to cope with faster moving mobiles and this may exceed the capability of the smaller units. Far better to leave these on the macrocell layer if possible.
+1 Quote 2012-11-07 14:07
 

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