[opendtv] Re: "Live" DTV from Mars now possible !
- From: Olivier HOUOT <olho_avatar_i@xxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sat, 14 Aug 2004 19:04:27 +0200
Thanks for the detailed answer, Bert.
But isn't that wavelength-dependant antenna size concept a little artificial ?
There may be some engineering situations where it makes sense to tie the
antenna size to the wavelength, but in the case of a space probe, i think you
try to use the bigger possible antenna within the constraints of the project
(space available in the rocket nose, weight, probe inertia versus attitude
thrusters power, etc...).
After that, given that maximum antenna size, you try to see what you can do to
improve the data link. If you increase the frequency, then you should get
higher antenna gain on both side, plus a better focused beam (i don't know to
which degree those two items are dependant on one another).
So that would definitely seem to be part of the improvement. One other
important factor being also the higher power amplifiers, which certainly does
something for SNR.
Speaking of the Shannon's capacity theorem, it is not dependant on frequency.
So what is the hypothesis when saying SNR will be influenced by this parameter
? If i consider the same bandwidth and just transpose it to a higher center
frequency then, unless the ambiant noise (natural, amplifier-generated,...) is
very different the resulting capacity should not change.
On the other hand, higher power can increase the SNR, and perhaps allow for a
less robust, but more efficient modulation scheme. Are they toying with 8PSK or
16QAM on a Mars-Earth link ?
Of course there is the small matter of what will happen to the signal in the
last 100 km of its trip, when it needs to cross the Earth's atmosphere.
Speaking of long distance data links, i will allow myself to drift even further
off-topic in the hope that some physics-oriented people have an answer to the
following question :
Quantum entanglement (or the EPR paradox) has sometimes raised hopes for
instantaneous communication because, if you produce two correlated photons and
send them in opposite directions and, at some point, perform a measurement on
one of them, the state of the other one is instantly affected, whatever the
distance between the two.
Scientists keep saying that you cannot use that to transmit information, as
there is absolutely no way to influence the final state of the photon after the
measurement.
But what happens if you have a stream of many such entangled photons ?
Quantum mechanics says that if you perform a measurement on a system you modify
its properties. So if i put a first experiment on the path of the stream, and
a second one a little further down the stream, the second yields results that
are not identical to what you would get if the first experiment was not there.
In other worlds , the second experiment knows that the first one is there. I
can get two different results by putting or removing the first experiment, and
that can be taken as 0 or 1.
Now if i have a source (at the origin of a coordinate systems) that sends two
streams of correlated photons in opposite directions, and i put the first
experiment on the stream that goes in the negative direction (say -1000 km),
it should instantly modify the properties of the corresponding wave front in
the positive path at +1000 km.
If i now put the second experiment at +1000 km plus one meter, it should be
able to detect this modification, because the measurement results will suddenly
change.
You still can't control the result of the first experiment, but it doesn't
matter because all you need to know is the fact that the experiment is being
performed or not. In that case it seems you have instantaneous communication
between two points "separated" by a distance of 2000 km, except for the one
meter propagation delay.
So what's the catch , here?
Albert.e.Manfredi wrote :
It's caused by the way antenna apertures work at capturing
energy. In essence, the free space path loss equation is
telling you that for an antenna of the same size RELATIVE TO
THE WAVELENGTH, the higher frequency antenna is physically
smaller, and therefore captures less energy (power density).
In other words, the constant here is antenna *gain*.
So for a dish antenna, double the frequency and for the same
antenna size wrt wavelength you cut the radius by 1/2 and the
area by a factor of 1/4. So it follows that you're capturing
less signal.
Of course, if you increase the size of this higher frequency
antenna, you will be increasing its gain. So you can
compensate for the loss of signal level. But you also *need*
to compensate, is the point.
So in fact, it didn't make a lot of sense to make a big
deal about the 8 GHz center frequency, I don't think. What's
to hype? That you need more antenna gain?
I think you have it in general. In the Shannon equation, one
of the variables is SNR. The way to look at this intuitively
is this:
Assume a channel of X width in Hz. If you want to pump more
and more bits/sec through that fixed width channel, what are
your options?
First, you try to cram as many symbols per second as you
can through the channel. Link them up so there's no gap
between the symbol train, make them beautifully smooth so
they don't slop over beyond your assigned bandwidth, and
you have successfully maximized the symbol rate through
the channel. (Symbols in RF channels typically use a so-
called "raised cosine" shape. Draw a cosine, and raise it
above the X axis. No sharp edges, no wasted bandwidth.)
Now you have the channel all full of these symbols, so you
can't do any better. Right?
Wrong. You can assign more and more bits to each symbol. But
to do so, as you already alluded to, you need to vary
frequency or phase and possibly also amplitude of the
symbols, to be able to distinguish, for example, a 001 symbol
from a 010 symbol. But the more fancy you get with delicate
little variations of the symbol, the more difficult it is to
receive it without errors. Because a noisy channel or noisy
receiver will make it hard to tell the difference between
small changes in amplitude or phase.
So Shannon cleverly takes this into account. Not only that,
his equation also tells us that he leaves it up to you to
decide how much to fix with error correction vs how robust
to make the symbols to begin with. He doesn't care.
And you're right about the wider the channel, the more
the noise energy. Noise levels are typically given as
nV/SQR(Hz), which says that noise power varies with
bandwidth. Which means that these wide band receivers
will have to be very high tech indeed.
So the whole thing is a delicate balancing act. Everything
is a compromise.
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