Re: Nigel's gratuitous troll

[Nigel Orr <nigel.orr@xxxxxxx>]

At 13:18 01/09/99 +0100, you wrote:
>Well, my gut feelings tell me that I agree, but I'd be interested to
>hear some science to back them up...

Gratuitous troll indeed- hrumph.

There's not a great deal of science behind it, but I'll summarise the
boring bits...

Basically, you have a source signal, which you want to transmit to a
destination.  You need to cause some sort of perturbation or disturbance
that will propagate so, ignoring acoustics (if only I could...), you are
left with sending something electromagnetic.  Now your basic choices are to
send the signal down some sort of guide (eg cable for most practical
purposes, fibre optics or waveguide for higher frequencies), or just put it
into the air.

If you choose cable, the signal is transmitted as its own little
electromagnetic field surrounding the wires (technically, it doesn't travel
along the wires but around them).  This field can affect other nearby
cables if it is sufficiently close or strong to interfere with their field.
However, for most practical purposes, where the signals are in the same
frequency band, have similar drive capability (eg not one really weedy high
impedance signal and one low impedance one), and are at a similar level
(not 240V and 10mV!), the resulting interference will be insignificant.

If it does cause a problem, you can shield the cable (which you can think
of as 'soaking up' the unwanted field) and/or use impedance-balanced pairs,
where the wanted signal and its electrical inverse are sent down the 2
wires, which are close enough to be considered colinear (mostly), and the
interference will be equally picked up, with equal polarity, on each.  At
the other end, take the signal on one wire, subtract the signal on the
other 'et voila' 2x signal and 0x noise!

Then there's the option of just transmitting it through the air.  Now all
of your signals are in the same place as all of everyone else's signals.
You are all trying to get a good range, and good signal quality, so you all
make sure you have the highest power transmitter permitted in your band,
and the most sensitive receiver to your transmitted signal.

The problem comes when you have to grab your signal from all the unwanted
mush that everyone else is transmitting out of your control.  You can't
balance it, you can't shield it, in fact you can't do anything to it once
it has been let loose from your antenna until it arrives at the other end.
There are various techniques (almost said bodges, but this is supposed to
be informative, not argumentative!) for separating the signals which come
down to 4 commonly used.

1- Frequency division
2- Time division
3- Code division
4- Not sure what to call this one, maybe power division ;-)

The first is pretty effective- each broadcast radio station has its own
frequency, allocated regionally, with permitted bandwidths etc to make sure
they don't interfere with others.  It does need fairly strict regulation to
stop someone else using your frequency in your area, so isn't really
practical for unregulated systems (as you would want your home network etc
to be).  Receivers are fairly easy, with tight filters in analogue
circuits, or, increasingly, using DSPs.

Time division (everyone gets their own time slot to transmit) is next to
useless in wireless systems without complete regulation, so you can ignore
it.  It is used is request-response systems, with one 'master' sending
requests, and the appropriate 'slave' responding, but it's hard to
co-ordinate when it gets more complex than that...

Code division (AKA spread spectrum etc) has great potential.  Every user
can be allocated a unique 'code' from a large "bucket o' codes",
and it is
practical to unscramble their wanted signal using their code, and reject
other people's unwanted signals.  It currently needs rather a lot of
electronics to cope with that, and it is important that all the users are
received with similar power levels, but it could work very well.

Power division (which doubtless has a better name) might refer to things
like car alarms, garage door openers (and my wireless doorbell!), operating
on 418 / 433/ 900MHz, at strictly regulated powers and frequencies, where
lots of users have them, but the power is supposed to be low enough to
avoid interference.  Add a simple coding system to try to avoid false
alarms/commands and off you go.  Basically the same idea as X-10, try to
produce just enough power to cover your target area but not enough to
interfere with someone else's area, and add a little bit of coding (house
codes and unit codes) to avoid false triggering.  It sort of works, right
up until the time (like now) when someone decides to reallocate your band,
or something near it, to a PMR radio system, and all your carefully
designed low power systems stop working reliably (or, in the case of X10,
someone decides that noise on the power line is a "Bad Thing" and
encourages everyone to do their best to suppress it...

Basically, wireless transmission is like sharing the same cable with
absolutely everyone else in the world, so there will have to be concessions
and additional complexity for everything to work smoothly.  Whether the
system is digital or analogue, it still shares the same 'wireless' cable,
and still has the same problems.

For that reason, I do use wireless for necessarily portable systems (eg
radio mics) and 'for fun(?)'- like my wireless doorbell, but I'd rather
wire the rest- it doesn't take _that_ much extra effort, and saves a lot of
faffing about afterwards...

This is a fairly fast summary 'as I see it', mostly compiled as I was
having my lunch... feel free to add, subtract or ask questions, on the
group or email...

Nigel
--
Nigel Orr                  Research Associate   O   ______
Underwater Acoustics Group,              o / o    \_/(
Dept of Electrical and Electronic Engineering     (_   <   _ (
University of Newcastle Upon Tyne             \______/ \(

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