One of the problems with lots of small cells, whether they are Wi-Fi or Femtocells, is that your handset/smartphone has to spend a lot of time scanning for them, often picking up those which it isn’t allowed to connect to. A feature of the already developed standards will allow newer handsets to do this better.
Scanning for Wi-Fi hotspots
A side-effect of this is the behaviour of my iPhone, which I typically set to search for Wi-Fi hotspots that can be used where available and ask if I want to connect to them. As I walk along the street (or sometimes just stop in the station on the train), a list of potential Wi-Fi hotspots pops up (which I usually decline). I’ve now set the option to stop it prompting me, but I know the iPhone continues to lookout for hotspots in the background.
What I’ve found so good about this feature is that for many of the common places I go that do have Wi-Fi – the coffee shop, pub, home, office etc. – the password is remembered and reused automatically. I get fast data speeds and probably lower battery drain because of the lower transmission power required when near a hotspot.
What you notice though is that with Wi-Fi enabled throughout the day, the overall battery drain is increased. This is probably because the iPhone needs to have two radios in use, scanning both Wi-Fi and 2G/3G cellular networks in parallel and transmitting to them when needed.
You also find there are many Wi-Fi hotspots (most of the privately owned ones these days) which prevent access without a password. Even with the correct settings on my iPhone, there is still some wasted effort in detecting and discarding those hotspots that are inaccessible.
Scanning for Femtocells
Handsets trying to connect to femtocells operate in a similar way. Today’s handsets don’t treat femtocells any differently from other 3G cells and so will try to connect by default. For those femtocells that aren’t open access, a reject message is sent. There’s no quick way for the handset to find out that it can’t connect until it tries. Your handset doesn’t remember that it was denied access previously and will try again each time you pass by. This means there’s an overhead for visitors coming near to your femtocell that may cause some battery drain that wouldn’t otherwise happen.
But there is a solution
The industry has come up with a solution to this, and it’s one feature that will be built into femtocell aware handsets. The feature was standardized last year in 3GPP Release 8, so depending on market demand it could potentially start appearing within handsets anytime now. My guess is that until femtocells have been deployed in much larger numbers (there are perhaps 10,000 handsets for each femtocell in the world today), it won’t make it into the software development plans for handset vendors. [Let me know if you disagree/know better].
Femto aware handset features should improve battery life, shorten the time taken to detect the presence of and connect to femtocells, and improve handover of calls as you move into or out of the femtocell coverage area (including femto to femto handover).
A preferred list of femtocells
As with my iPhone for Wi-Fi hotspots, the standard allows for a 3G handset to remember a list of femtocells it’s allowed to use and make better decisions about which cells to connect (or not connect) to.
The feature has been standardized in 3GPP2 for CDMA femtocells too. The Preferred User Zone List (PUZL) can be automatically learned by the handset over time as you use more femtocells in different places. The network operator can also update or prioritise this list. This means that your handset will ignore those femtocells it has found but been rejected by.
Not all access is open or closed
Wi-Fi hotspots tend to be either blocked or allow a connection (often using a passphrase for security). All users get the same treatement/priority. Femtocells allow a third option, called the hybrid mode. This allows anyone to access it but gives priority to a defined subset of users (the Closed Subscriber Group).
Ideally, all femtocells would be open access and allow any network user to connect through them. As with Wi-Fi hotspots, owners want to restrict access. Today’s femtocells and handsets provide this functionality, but femto-aware handsets will optimize the performance at some point in the future. Hybrid mode retains some benefit for femtocell owners who are prepared to share spare capacity but don't want to degrade their own service.
Lots of potential for clever innovation
There's bound to be lots of smarter ways of doing or improving this. For example, this US patent application (which looks like it originated from Airvana staff) proposes downloading a (long) list of possible femtocells and their GPS co-ordinates. Assuming your device knows where it is, it can then seek out suitable femtocells to connect to. This patent application from Motorola put your handset on best behaviour - if its rejected by a femtocell it changes to a different frequency to avoid interfering with it.
Surely, this is one area to watch for further innovative ideas...