Early marketing promotion of femtocells highlighted their compatibility with existing 3G mobile phones. Unlike home access points which use WiFi, femtocells would not require the user to change their phone, and choose from a limited range of capable devices limited range of capable devices.

But is this still the case?

In recent months, we’ve seen growing recognition of the fact that femtocells will work better when used with a phone that is femtocell aware. This doesn’t discount the use of existing phones, but improves the user experience and reduces problems.

Specifically, Dean Bubley of Disruptive Wireless published a report in June 2008 (still available for sale), which identified a number of concerns. He has also recently suggested  that the industry players accept the need for femto-aware handsets, so that femtocells and femto-aware handsets conforming to the 3GPP Release 8 standard due out later this year would form the baseline commercial offer.

Let’s consider separately how it affects:

• Those with femtocells
• Those without.

Handsets for owners with femtocells

The key requirement is for the handset to switch over to use the femtocell when in range. The owner will want to take advantage of the superior call quality, reduced call rates and longer battery life. Where indoor coverage is poor, handover to the outdoor cellular system is essential to avoid call dropout.

Phones operate differently depending on whether they are actively on a call or not.

Not on a call

In idle mode, the phone will wake up periodically, try to listen to the last cellsite it communicated with, specifically to find out if there are any messages waiting for it (i.e. any incoming calls or text messages). Most of the time there won’t be and the handset will go back to sleep again.

Where the last known cellsite can’t be found, the handset will then go into a cell reselection process, scanning for suitable cellsites. Complex algorithms are used, but generally the strongest signal will be selected. If the new cellsite is in a different zone (for paging purposes), then the phone will notify the network of this change.

Call in progress

If the phone is actively involved in a call, it will be continuously measuring signal strength of the current and neighbouring cells and transmission quality data. Each cellsite broadcasts a neighbour list of potential alternative sites in the vicinity, directing the mobile to the frequencies and/or scrambling codes to lookout for. The recorded measurements are sent to the network and combined with similar measurements from the active cellsite. Complex algorithms then determine when and where to handover the call, directing the cellsites and handset to make the switch in a co-ordinated manner.

Closed Subscriber Groups

In a poor coverage area, a femtocell user entering their house may rapidly lose signal strength, requiring a rapid handover to the femtocell. With large numbers of femtocells deployed, it isn’t feasible to list all potential femtocells as neighbour candidates at each outdoor cellsite. Therefore, unless the handset has prior knowledge of the femtocell and the need to check for it, then it won’t be able to make this handover.

With this in mind, the femtocell standards have created a new concept of Closed Subscriber Groups (CSG), which is a list of one or more femtocells that include the femtocell(s) which an individual user might use.

In this respect, it’s unlikely that standard 3G handsets will support in-call handover into femtocells. Instead, calls will continue on the outdoor system which may be acceptable where poor coverage was not the primary reason for the femtocell.

Handsets for owners without femtocells

Operators will be keen to avoid any impact on their network which causes problems for existing users, who may have no interest in femtocells. Studies have been conducted showing minimal impact where even large numbers of femtocells are deployed in dense urban environments. Their low power, combined with poor penetration of 3G signals through external walls, limits their effect.

A concern is that where handsets do detect femtocells, the handset may attempt to register through it. Depending on the configuration of the femtocell, this may either be allowed (in which case the user will benefit from stronger signal, better voice quality and longer battery life due to lower transmit power) or rejected. On receipt of the rejection message, the handset will ignore that cellsite (at least until it is next switched on).

One scenario is that users walking down a street may try to register with many femtocells in turn, causing unnecessary signalling traffic and battery drain. I would not expect this to be such a major problem because the cellphone is unlikely to search for alternative cells whilst still able to receive an adequate signal from the outdoor cellsite.

There have been reports that Virgin Mobile users in the US, which shares the Sprint network, have been prevented from making or receiving calls when their phones are in the vicinity of a Sprint femtocell. It is likely this is a configuration problem which has already been fixed, but shows the potential consequences.

A two-tier solution

We’ve said before that femtocells are designed to be compatible with and support any existing 3G phone. Later phones which include some of the optimisations will work better than older ones. So whilst it’s not essential to upgrade your phone to one which includes 3GPP Release 8 optimisations when you start using a femtocell, it will be beneficial.

The standards require only software, rather than hardware, changes. Therefore the production cost of including them is virtually zero on a per unit basis. The investment to design and test the software, then support it in future releases can be amortized over large volumes of product. There are many demands on features to be included in handsets, so the timescale to incorporate them will depend on forecast demand for these features and whether the buying public will differentiate their choice on this basis. Will “Joe Public” insist that femtocell optimisation is the next “must have” feature for their phone, in place of Bluetooth, Camera, MP3, VoIP or other improvements? Probably the form factor and colour choice will have more bearing.

Pre-configured to ensure success

There is a high cost of failure, where femtocells do not work with the customers handset. This is expensive in terms of technical support costs to identify and resolve the problem. It also affects the overall customer experience and therefore can cause customer dissatisfaction, poor reputation, loss of service takeup and churn.

Some operators are therefore said to be considering bundling new preconfigured handsets with the femtocell. Whilst this increases the overall package cost, it would help attract customers from other networks.

Market Positioning

Femtocell vendors (and mobile operators) need to ensure differentiation between Dual mode WiFi/Cellular and other alternatives. If femtocells are perceived to be restricted for use with only a limited set of (upmarket/expensive) smartphones and luxury models, then the competitive choice for dual-mode WiFi handsets may appear more attractive.

This may be one more reason why operators are cautious and waiting for further standardisation work to complete alongside extensive field trials, before committing to major rollout programs of this exciting new technology.

In the meantime, it does open up a gap for more Dual-Mode WiFi system sales and give this existing, proven technology more time to establish a stronger foothold.

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Barlow Keener said:

You hit the nail on the head with a femtocell software load for devices on femtocells in this blog. If there are "flat rate" plans for calling from home - like the Airave (or even the UMA T-Mobile plan), how does the user know if she is on the femtocell or the macro-tower? Will the phone buzz, beep, or does the user have to look at the screen - no easy answer here of course. When the user receives the monthly bill with 1000 extra minutes on the network that should be on the femtocell's flat rate plan then the issue will start to be resolved.

September 23, 2008 | url

Thinkfemtocell said:

Barlow Dual Mode WiFi/UMA services like T-Mobile's hotspot@home have an icon indicator on the display which shows when you are operating in WiFi mode. Sprint's Airave injects a beep-tone when making calls, so you know at the start that they are through the femtocell. Future phones are likely to have an indicator and would also show the name of your femtocell in place of the operator's name. With flat rate bundled voice call plans becoming cheaper/more minutes all the time, it may be less of a problem if some of your minutes are made outdoors. Perhaps you're more likely to notice that you aren't using the femtocell because the voice quality will go down - especially if you got the femtocell because of poor indoor coverage in the first place.

September 25, 2008 | url