Nuts and Volts - February 2008

OPEN COMMUNICATION

THE LATEST IN NETWORKING AND WIRELESS TECHNOLOGIES

■ BY LOUIS E. FRENZEL W5LEF

A FEMTO IN YOUR FUTURE? What the devil is a Femto?

I SUPPOSE IT HAD TO HAPPEN SOONER OR LATER. With indoor cell phone usage as poor as it often is, it was inevitable that someone would come along to solve that problem. And here it is — the femto cell. It just could be one of the next big electronic purchases you make.

Afemto cell is a miniature cell phone basestation designed to operate inside your home to give you improved indoor cell phone coverage. Cell phones are low power UHF or microwave two-way radios designed to operate outdoors. Their typical range is a few miles at most and even shorter in most cases usually because of environmental limitations. Radio wave propagation in the UHF and microwave regions is strictly line-of-sight (LOS) and heavily impacted by obstructions like buildings, trees, mountains, and especially the walls of buildings. Reflections causing multipath signals are always a problem.

If you are using your cell phone outside, you have a much better chance of hitting the closest cell site antenna tower and getting a clean clear signal. But if you are in an area obstructed by trees or surrounded by tall buildings, cellular service could be poor to non-existent. It is even worse if you are indoors since the signals are greatly attenuated by walls, ceilings, furniture, and the like.

Many people complain about their bad to worse indoor cellular coverage, but just remember, these phones were not optimized for indoor operation. Yet, that is not how people tend to use them.

It has been estimated that over 50% of all cell phone calls originate from indoors. In fact, some even say that better than 70% of calls come from inside homes or from office buildings. Think about your own cell phone usage. Is it mostly indoors or outdoors? In any case, it is amazing that performance is as good as it is. People adapt and move their positions to optimize the signal when calling from indoors. They get near a window or move around a room until they get one or more “bars” of signal. Or, as a last resort, go outside.

This problem of poor indoor cellular coverage has been resolved by the cell carriers for years by using smaller basestations (cell sites) we call micro or pico cells. These compact basestations are designed to be installed inside a building or other public place. They may sit on top of a tall building or be mounted on the wall inside an airport.

Micro means small ( 10-6) while pico means really small

( 10-12). Now, along comes the femto cell. Femto literally means 10-15 or one thousandth of a trillionth. Even smaller. Femto cells are very small but complete low power basestations designed to be used in the home.

Figure 1 shows what one looks like. It’s like a cable TV set top box or a wireless LAN router in size. Your cell phone will talk to it rather than an outside basestation when you go to make a call. With such close range, your connection will be sure and good.

THE BACK HAUL PROBLEM

■ FIGURE 1. Femto cells made by Airvana of Chelmsford, MA. It incorporates a 3G radio transceiver, processor, Ethernet interface, and the antenna.

All basestations have to get connected to the main telephone networks somehow. This is done by their back haul connection. This connection is made in a variety of ways, depending upon where the basestation is. The most common connection is one or more T1 data lines that all telecommunications companies use. Recall that these lines carry up to 24 digital voice calls in a time division multiplexed mode at a data rate of 1.544 Mb/s. Sometimes the back haul is by a fiber optic cable or by a short range microwave point-to-point radio.