6. Wireless alarm system techniques (part 1 of 2)..

[-pull@shoot]

Its time to give some Complementary and more detailed technical
information on the Wireless Alarm System "Data communication link
techniques" in order to clarify Radio Frequency Interference (RFI)
susceptibility of such devices.

NOTE: To avoid the huge size of this post, a second part follows.

Burst and Synchronisms:
Sensors operate and send on an "asynchronous" burst communication
principle.
The receiver receives within hiss bandpass all signals belonging and
not belonging to the system, some are disturbed due to transmission
time collisions, and some are clean and come true unaltered to the
receiver when they are "alone" on the air.
Don't forget, wireless alarm system frequencies are shared by other
devices of the same and/or different nature (car locks, door openers,
++).

Code Validity Checking:
The receiver demodulated audio signal end uses a decoder to select the
wanted signals out by checking the content of the signal. This content
contains system selective codes that identify themselves allowing
rejecting other signals not part of the concerned system.

Signal requirements:
The principle of wireless allows correct decoding only when the
received signals are strong enough and above the required receiver
signal noise level, clean and alone on the air.
You certainly know that when two signals transmit together on the same
frequency a Radio Frequency (RF) collision occurs and causes the
receiver to receive a cluttered of undecodeable signal. I don't like
to use the word interference here, see other discussions held by me,
but in fact that is what it is on a small scale when collisions arise.
If all systems transmitting on that wireless frequency respect the
burst principle those collisions will be of a limited duration due to
the shortness of the bursts and have no consequences.

Frequency assignment:
The wireless alarm system frequencies assigned by the FCC (+other) are
based on the burst principle and low power (<1 milliWatt) and by
consequence the short range nature allowing multiple and different
types of utilization's. All wireless alarm system frequencies are
shared.

Asynchrony burst rate limitations:
How faster the burst repetition rate is how faster the receiver will
receive the informative signal.
However, the repetition of the burst rate must be limited (beside the
FCC requirements) by the possibility and number of other systems in
the vicinity that may cause jamming when they operate on the same
frequency or close to it, in other words, the collision (interference)
probability increases with rate.

Influences and quality of the receiver:
How more selective the receiver is how better he will reject signals
close to the frequency that's evident.
How better the Intermodulation rejection level is, how higher the
input stage and IF saturation level is,.. how less prone the system is
to receive unwanted or cluttered signals.

Battery power limitations:
Here again a sensor limitation that affects burst rate (explained
previously).
The length of the burst transmission is important for battery life.
How faster the information in the burst, the shorter the burst and how
longer battery life. Look to FCC limitations on modulation bandpass.
On the other hand, regular interval burst transmission also limit
battery live. Some sensors provide regular time depending burst in
order to check eventual link connection problems.
All above is a tradeoff between batteries live and features.

Frequency band limitations:
I have already mentioned frequency allocation but didn't talk about
the allocated frequency band to each assigned frequency.
Here again see FCC. Usually this band is restricted to a low range and
is so narrow that it is covered in a whole with the receiver bandpass.
Besides the fact that it allows some inherent frequency shift of the
transmitters and receivers, it jeopardizes the interference capability
and use of what is called frequency hopping. Frequency hopping, spread
spectrum or/and multi frequency utilization, is used in military and
other applications to communicate still in a reliable manner when one
or several frequencies are interfered. This needs frequencies in
different frequency bands far away of each other. It needs also the
receiver tuned to each specific frequency and here the required $$$
amount goes bingo.

Sensor location problems:
When the sensor / receiver path is altered by conductive objects, the
signal at the receiver may be to week to be decoded.
Normally, if nothing moves around those reflections stay constant and
an initial selection of the locations may avoid any problem.
The reality is different, think about cars and other conductive
(metallic?) object located in the vicinity. Only a few tends of and
inch may be enough to alter the RF path reflections who my cause and
affect communication link reliability.
In order to circumvent in some sense this problem, frequency hopping
is used. This frequency changes the reflection paths and decreases the
possibility of communication loss.
Just note that how higher the basic frequency, how more path
alteration occurs on small conductive items displacement.
It's good to have a signal level indication for initial sensor
location setup realizing that all path alteration problems will not be
measurable on the long run.

Paul