OFDMA Femtocells: A Roadmap on Interference Avoidance

Earlier, I have blogged about LTE femtocells being starting point of LTE and how LTE can be better technology than HSPA. In this months IEEE Communications magazine, there is a series of articles on Femtocells. I will try and cover some of these (unless I wander off in some other direction). The first one is titled 'OFDMA Femtocells: A Roadmap on Interference Avoidance'. At the end of this post, I have provided links to the research and the actual paper (in a legal way ;) so if you are not interested in the post and want to directly jump on the actual paper see the end of this post.

There are all kinds of statistics about the number of Femtocells worldwide. There could be upto 70million by 2012. If this happens the big problem would be the interference between Macro and Femtocells and also between Femtos. OFDMA (used in LTE and WiMAX both) Femtocells can handle the interference better than CDMA (UMTS and CDMA2000) Femtocells due to its Intracell interference avoiding properties and robustness to multipath.

So what are the main problems that the operators will face when deploying femtocells? Lets look at some of them:

• Access method: Three different approaches exist namely, Open access, Closed access and Hybrid access which is a mix of both of them. The first two approach has some problems and I have suggested a solution before ;) but the best solution may be to go for Hybrid approach where limited connectivity is available to non-subscribers of the femto.
• Time Synchronisation is another important aspect of OFDMA Femtos. To minimise multi-access interference and for successful handovers, synchronisation between all the Femtos and between Femto and Macro is a must. This should be acheived without any complicated hardware so as to keep the cost down.
• Physical Cell Idendities (PCI) could be a problem because of limited numbers
• Neighbouring cell list, which is restricted to 32 in LTE, could be a problem if too many Femtos are around
• Handovers could also be a problem if the UE keeps jumping between Femtos and macro. One solution could be the use of HCS.

Interference analysis will definitelty play an important part in the rollouts. If not properly managed, could result in dead zones within Macro. Power control Algorithms and Radio Resource Management strategy will help but effective Spectrum allocation technique is needed as well. The diagram above shows different approaches for subchannel allocation in OFDMA femtocells.


The Femtocells would need to be self-configurable and self-optimising. I tried to explain the SON concept earlier which is similar. Self-configuration comes into picture when the Femto is switched on. Once the parameters are adjusted then Self-Optimisation tries to optimise these defaults into something better and more suited to the current environment. Sensing of the environment plays an important part in this. The diagram above shows different approaches being used by different Femtocells. The cheapest approach would ofcourse be the measurement report approach where the phone is made to report the environment. The only problem being that whichever phone was used (automatically selected) will have considerable amount of its battery power used up :)

The team behind this IEEE paper has been doing some excellent research work in the field of femtocells.

There is a book that is under publication and will be available early next year. At the same time if it interests you, you can look at some of their publications including the IEEE one that has been quoted here. Here are all the necessary links:

• The Centre for Wireless Network Design
• Professor Jie Zhang homepage
• David López-Pérez, Research Fellow, homepage
• Femtocells World Summit presentation on Self-organisation of the Subchannel Assignment in OFDMA Femtocells
• OFDMA Femtocells: A Roadmap on Interference Avoidance - IEEE Communications magazine, Sep. 2009
• ICC 2009: Femtocells panel presentation slides

Hope someone finds all this info useful :)