Tuesday 29 September 2009

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:

Hope someone finds all this info useful :)


Gustav Holst said...

This is very useful. I have been reading a lot about femtocells lately. :)

I have an unrelated question. In 36.101 the E-UTRA chart says that Band 6 (830 - 840/875 - 885 MHz) is not applicable. Why is that, and what does that mean?

Thanks :)

Zahid Ghadialy said...

Hi Gustav,

Which version of the Specs and what is the table number?

Gustav Holst said...


I'm looking at 36.101 v9.0.0 (June 2009), table 5.5-1. It's very puzzling!

Thanks :)

Zahid Ghadialy said...

Hi Gustav,

Very interesting. Since I generally refer to Release-8 version of specs, i did not see this statement in Release 8 specs. The answer to your question is present in 36.800 :). I quote it here from section 6.5:

The frequency arrangement of Band 6 and Extended LTE 800 (Band 19) is as follows.

- Band 6: 830-840MHz(UL), 875-885MHz(DL)
- Band 19: 830-845MHz(UL), 875-890MHz(DL)

In Japan, the commercial service in Band VI for UTRA has already been operated, so the Band VI remains handled as ever. On the other hand, there are no plans for the commercial service in Band 6 to be operated at least in Japan due to use Band 19 for E-UTRA. In this case, it is expected that the maintenances for Band 6 need to be continued in spite of not be in use and the misinterpretation that Band 6 is applicable like Band VI in Japan occurs. In order to prevent these problems from occurring, two solutions can be imagined that Band 6 is deleted or "Band 6 is not applicable" is added in 3GPP specification where the descriptions on Band 6 are not maintained any longer. The former is seemingly-simple, but it is difficult to predict the risk of influencing other specifications because there is no precedent for this. So, it is assumed that the latter is reasonable. Therefore the following two items are proposed.

Proposal 1 : The descriptions on Band 6 are not updated for the WI TR and the corresponding 3GPP specifications.

Proposal 2 : "Band 6 is not applicable" is added in TS36.101, TS36.104, TS36.141, and TS36.521-1 as shown in Table 6.5-1.

As you can see Proposal 2 has been accepted.

Gustav Holst said...

Thank you so much for finding that! That's strange to me. Why would they designate it as unapplicable for just one country? I guess the standardization process is more opaque than expected. :)

Thank you again!