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

[-pull@shoot]

On Thu, 05 May 2005 17:23:48 +0200, -pull@shoot wrote:

>
>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