Femtocells: Why small cells are a big idea

Why femtocells can improve wireless communication

Femtocells are all the rage these days; you can't flip through a wireless trade publication without finding an article about them. And for good reasons.

By way of explanation, while most industrial-grade cellular infrastructure covers a relatively wide area, femtocells are small base stations that can improve coverage in places where coverage is spotty, such as indoors and at the edge of a cellular operator's coverage area. They hearken back to the early days of Personal Communications Services (PCS), the first spectrum to be auctioned off by the Federal Communications Commission. While this spectrum was primarily used for macrocell, which are large cells and reside at the opposite end of the cell-radius coverage spectrum from femtocells, that wasn't the original intent.

A word about the coverage area of cells is in order here. Cells are the mechanism with which we make a particular chunk of spectrum available to users in the form of channels, each of which has a certain bandwidth. Imagine one big cell covering a very large area. This is how the precellular Mobile Phone System worked.

The problem with that approach was that we quickly used up the available channels. When that happened, we experienced blocking; once the system is full, additional users had to wait. Now let's imagine we have two cells instead of one. We thus have two complete systems with the ability to hand off calls as users roam between them, again subject to capacity constraints.

It isn't hard to see that, if we have a lot of cells, we can increase capacity by reusing particular channels across cells. We can also use smaller handsets that consume less power because we don't have to transmit as far. As a result, making cells smaller is a win-win situation.

Now let's suppose we make the cell very small, with a radius of coverage of a few tens of meters. Small cells are quite common; this is the idea behind Wi-Fi cells, which we call microcells, and Bluetooth cells, which are called picocells. The term nanocells is sometimes used to describe cells in micromesh industrial-control applications. Today's application of femtocells is the same as that intended for microcells in early PCS networks, a concept, known as low-power PCN (personal communications networks).

Lately, femtocell has become a catch-all term for small cell WiMax deployments in both residential settings and in public spaces. However, the term is increasingly being applied to small cells being used to provision cellular service of any form. The idea is exactly the same as Wi-Fi base stations but using different frequencies and wireless technologies.

This was the idea in early PCS discussions -- small cells with dynamic spectrum allocation on the basis of demand, priority or pricing. The issue, of course, is in planning for the overlap of these small cells, which is necessary to process handoffs. That is especially true outdoors, where suitable mounting sites can be difficult to come by. Just ask anyone deploying a metro-scale Wi-Fi system; small cells are tough, although today's spectrum management tools make the job a lot easier than it was 15 years ago. Nonetheless, femtocells are a key option and will be a very popular deployment option.

One interesting potential direction for femtocells is whether they will be the basis of future cellular systems. I've seen some very clever designs that can be used both in the home and, with a bigger power amplifier and antenna, as a commercial base station. In other words, the same basic equipment can be used in two widely differing applications -- and with stunningly low prices. To see some very interesting femtocell designs, take a look at picoChip's site. This company specializes in the highly integrated, high-performance processors required for low-cost implementations.

Femtocells are not, as is often reported in the press, a threat to Wi-Fi deployment. Wi-Fi uses unlicensed (free) spectrum and femtocells will operate on licensed (expensive) spectrum. As I noted last week, Wi-Fi will always have a cost advantage. But femtocells are a step in the right direction, shrinking cells and thus making the best use of the scare resource that the radio spectrum is, and always will be.

Craig J. Mathias is a principal at Farpoint Group, an advisory firm specializing in wireless networking and mobile computing. He can be reached at craig@farpointgroup.com.

Copyright © 2007 IDG Communications, Inc.

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