Femtocells and the Lawnmower Effect

You may have read my note last week about the femtocell Release 8 standards not supporting active call handover into a femtocell. I'd suggested this really wasn't so important, but in many cases it can be. This has been called the "lawnmower effect" - read on to learn what is meant by this.

 

Handover between mobile phone basestations

A lot of complex engineering has gone into making an active voice call continue regardless of where or how fast you move, how many other people are using the system and even when parts of the system fail. A range of different criteria are used to determine when and to which new cellsite a call should be transferred to. A set of well refined processes are used to execute those transfers and recover from problems if and when they fail - this is called handover.

We've travelled a long way from the early days of mobile phone service, when pops, clicks and dropouts were common as we walked down the street. This has led customers to expect the same high standard of service from new technologies when they are added into the system.

Handover in and out of femtocells

Today's 3G femtocells support handover when leaving the femtocell area, but not when entering. A common scenario is to handover from 3G on the femtocell back down to 2G GSM when leaving - the GSM signal would typically have better range and in-building penetration than 3G because it uses a lower frequency (900 MHz compared to 2100 MHz). [Ed Note: This may less common in USA, where 1900MHz is used for both 2G and 3G].

The Lawnmower Effect

Imagine you were making a mobile phone call while mowing your lawn (yes, I know this sounds unrealistic, but bear with me). As you move up and down your garden, you move in and out of the coverage area of your femtocell. Should the system handle this by switching your call repeatedly between the indoor and outdoor systems?

Why this might be a bigger issue

A couple of people have highlighted that femtocell coverage within their home is not 100%. You may find as you move to the bedroom/dining room/kitchen that you leave your femtocell zone, and your call needs to be handled by an outdoor cellsite from elsewhere.

Another potential issue relates to some of the location based services proposed for femtocells:

• Calls made from a femtocell may be charged differently. If your call switches in and out several times, what charge rate should be applied? How would a customer react to this?
• Several femtocell applications have been proposed which take some action as you return home. How annoying would it be to receive 20 "Honey I'm Home" text messages in the space of a few minutes! Presumably, some hysteresis would need to be applied to these applications.
• Currently, calls would drop (or potentially be of very poor quality) if users move out and then back into femtocell coverage, because the call would not return back to the femtocell - the standards don't implement this yet. Even when they do, it will require the very latest Release 9 femtocells and handsets.

The longer term solution

Some changes to the network, femtocells and handsets are likely to be adopted in the longer term. This is very much an evolving design. Features may include:

• Handsets will become aware of the Closed Subscriber Groups of which they are a member.  They will remember and associate these with the context of neighbouring cells, so they know when to look for particular femtocells. They'll think, I know that when I'm on macrocell number XYZ that there is a femtocell that I've been allowed to use before on frequency ABC and scrambling code DEF.
• Finding completely new femtocells is likely to be done through discovery, i.e. handsets in idle mode scanning and finding strong signals in an area.
• Macrocells will be updated to broadcast the frequency and primary scrambling code used by femtocells in that region.

Summary

The bottom line is that whilst existing 3G handsets will work with femtocells today, Release 9 compatible handsets and femtocells will be required for truly optimal user experience.

The learning from 3G femtocells is already being incorporated into the 4G LTE standards, which will give this next generation technology a jump start to femtocell adoption.