As far as I recall, 'femto' in terms of unit means '10^-15'. You would notice this represent very, very small number. From this, you can guess 'femtoCell' is 'very, very small cell', 'very, very small eNB'.
When you say 'eNB' you would imagine such a large towers with three antenna spaced at every 120 degree, but the femtoCell is very small size compared to the normal eNB (they usually call this 'Macro Cell').
How small it would be ? It would be just a little bit larger than your cable modem or WLAN Access point you would have at your home.
The motivation to femtoCell is to develop a small sized eNB that can be installed within a house like cable modem or WLAN Access point.
Does femtoCell has same capability as normal eNB(Of course, RF power would be much small... I mean the capability except RF power) ?
- Yes... original intention is 'femtoCell has full eNB capability as normal one'.
- Transmission Power should be much lower than normal (Macro) eNB since it is designed to cover much smaller area.
- Reciever sensitivty and dynamic range may be smaller than eNB due to small coverage
- Number of subscribers (UE) that can support is smaller than eNB. (I think in most case a Femto Cell would support less than 10 UEs even though some specification shows it can cover up to 20 or more)
Why they want to introduce this femtoCell into the network ? We can easily think of several motivation
- i) in High data mobile network, usually a cell radius is very small and to guarantee the signal quality at cell boundary is crucial to achieve such high data rate communication. To achieve this kind of condition, they need huge cost and time to determined the location and install the normal cell tower. But if they can put a lot of small scale eNB at home like WLAN Access point, they
the signal quality with very low cost.
- ii) If we can get the high quality signal with femtoCell even at shadowy/cell boundary area, they can reduce the transmission power of the normal eNB which in turn can save a lot of mony in terms of engergy consumption of system operator. (Of course, you should pay some of this electricity charge in stead of system operator if you have a femtoCell at your home).
How a Femto cell can be connected to other network components (like other Macro eNB or MME and other Femto cell) ?
- Up to Rel 8/9, there seems to be no clear specification of these connectivity, but in Rel 10/11 3GPP start defining these connection interface and finally it seems that a Femto cell is using almost same 'connection interface' as a Macro eNB.
Isn't there be anything to be concerned ? or anything that may cause any side effect ?
- i) Most cricial issue would be "Who would install those femtoCell if I want to have it in my house ?". Would the network operator send an engineer to my home to install this ? or I have to install it myself ? If I have to install it myself ? How can I configure it ? I am not a trained people to configure this kind of thing. Just plug in the power would be OK ? I don't think
the network operator would send you the people to install it since it would be costly. You might have to install it by yourself. Then what about the configuration of the cell ? That is why they are talking a lot of SON(Self Organizing Network). Basic idea is that once you just plug in the power, the femtoCell automatically configure itself to provide service.
- ii) If femtoCell works like a normal eNB, it should have its own Physicall Cell ID so we may need a huge pool of Cell ID, but LTE cellID is only 504. How we can handle this kind of cell ID saturation ?
- iii) Usually a Femto cell is owned privately. So the owner would not want to allow everybody to get access to the cells. He/She would may want to set of specific users to get access to the cell. How to handle this kind of situation. The concept of CSG would handle this issue.
- If you are arount a Femto cell but you are not allowed to get access to it. In this case, the signal from the Femto cell can be a strong interference source to you and you will be a serious interference source to the Femto cell. This would be the most critical issues which everybody is looking for solutions to. ICIC/eICIC can
be a strong candiate for one of the solutions.
- iv) On the aspect of mobile phone maker, would this femtoCell introduction can bring in any technical complication ? Yes, it is highly possible and we don't know now exactly what kind of complication it would introduce.
For some further details in terms of business model and some technical issues, refer to following links.