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
■ 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.
In a femto cell, the back haul
connection is your high speed Internet
broadband connection. (See Figure 2.)
That would be a cable TV data service