Femtocells: Why small cells are a big idea

Computerworld - 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.