One of the hot topics at this years Mobile World Congress was that of offloading data traffic from the mobile networks. Wi-Fi was most commonly mentioned in this context, but femtocells were also prominent. Together with picocells and microcells, this approach is described as “small cells”. It's based on the principal that to get the highest traffic capacity, you have to use more and more smaller radius cells regardless of the radio technology used.
This month, we’re going to be looking in more detail at various offload schemes. To set the context, let’s first classify the different categories of where small cells can (and cannot) be used. We’ll then review and compare the various technical options available today to meet these different needs.
Trusted indoor locations. These are the one or two places where most people spend a lot of their time and either own or have a direct relationship with the building owner. Your own home is the most common of these, but I’d extend it to include your office and perhaps the homes (or offices) of family and friends.
When accessing data services from these locations, you can do so securely on Wi-Fi by being given the security keyword or a femtocell by being added to the whitelist of authorised users. Usage is typically free (or is covered by the owners internet account), although data through a femtocell may also count towards the user’s (not the femtocell owner’s) own mobile data plan.
Untrusted indoor locations. Typically relate to public areas where you might sit down. Transport hubs (e.g. railway, airport, metro, motorway service stations), pubs/restaurants, conference centres spring to mind. Those public indoor areas where you remain standing are much less likely to provide service – shops and other retailers may not want to encourage the public to hang around inside using their phones.
Wi-Fi is becoming more widely available in many such areas in different ways:
- Completely open access for anyone in the building, no password or other identification required.
- Closed access requiring a password that is issued on request, sometimes for a fee and sometimes in exchange for your validated email address.
- Commercial access through a service provider, either using online credit card or voucher
- Commercial access through a roaming agreement with a Wi-Fi access aggregator (such as TheCloud, Boingo, iPass etc.) Some of these systems are seamless and automatically register and authenticate your mobile device.
Outdoor stationary locations: These are likely to be specific hotspot areas where users congregate outdoors and don’t move much. Some town centres, especially in warmer climes; some outdoor shopping arcades may fit this category. Users would typically not be moving when accessing their data service to lookup some information, check their messages etc., so could be served by a similar solution to untrusted indoor.
Outdoor slow moving locations. An increasing amount of data usage is occuring when on the move. This includes the more obvious facebook or email user walking along the street, but also the regular automated status and email checks performed by many data applications including push notifications.
Walking down many streets these days can show a plethora of Wi-Fi hotspots, which by the time you’ve registered for use on, are already out of range. The solution here needs to be unobtrusive and efficient on battery life, security and cost.
Outdoor fast moving locations. Listed here for completeness, the speed of the user rules out the use of small cells in most cases. There are two main reasons: the user would be moving quickly between coverage areas of different cells, requiring very frequent handover between them. This leads to a risk of handover failure, not enough time for the device to scan the environment thoroughly enough to identify and measure the best cells to handover to, and a short time to achieve the handover itself. The other reason is that faster moving devices take more radio resources from the cell which has to cope with a wider doppler effect of fast moving traffic. This makes the signal more difficult to decode, requiring higher levels of signal processing. So you are unlikely to find Wi-Fi or femtocells being of any use for roadusers, except when they are stationary – which may be a good thing for road safety.
Outdoor rural sparsely populated. Although femtocells have been discussed for use in some rural applications, such as villages, using cells with some 1-2km radius, this isn’t so much about data offload as providing basic service in these locations. The range of Wi-Fi is inappropriate for wide area coverage too. So again, I’d rule this out.
Transit systems. Airplanes, ships and trains (including underground metros) are also places where people sit down, want to use their phones for data services. Installing equipment on the train or aircraft itself sharing a common transmission link back to the network is initially more about providing basic service where not possible before. But for some applications (especially trains), this may become more important to avoid the disruption caused by large groups of fast moving users.
The categories where data offload can be considered are the following:
|Indoor Trusted||Home or office|
|Indoor Untrusted||transport hubs, conference centres, pubs, restaurants|
|Outdoor Stationary||Town centres, outdoor shopping centres|
|Outdoor Slow moving||Pedestrian streets|
|Transit Systems||Trains, planes and ships|
The common theme across many of these environments is that the users are not moving quickly (at least in relation to the base station), typically sitting down and able to view the screen.
Trust is very important because it allows security to be setup for commonly used locations rather than rely on ad-hoc, often unencrypted and insecure arrangements.