[opendtv] Re: 47 year old television signals bouncing back to earth
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 22 Dec 2011 18:27:53 -0600
Olivier Houot wrote:
>> Pluto is between 4.2E+9 Km and 7.5E+9 Km from Earth, which means that
>> in principle, this signal could be receivable just within our solar
>> system.
>
> This is the classical limit for the solar system, indeed, but it has
> expanded recently to at least as far as Eris which currently lies at
> 14.6E+9 km.
The obesity crisis even hit our solar system?
> If we had people there, they could not rely on leakage, then, but a
> focused beam from earth could retransmit broadcasts with good quality in
> digital format.
True. TF1 beamed to francophone colonists on Eris. But I doubt they would opt
for DVB-T1 or T2! The PAPR is too much of a waste of power, and the fact that
you're using a very narrow and directed beam, with parabolic transmit and
receive antennas, implies that you're not banking on a lot of multipath anyway.
> Mars Reconnaissance Orbiter is capable of 6Mbps at a 100 millions
> kilometers range with a 3 meter antenna fed with 100 w at 8 Ghz(not
> clear from nasa Website whether it is the power absorbed by the
> amplifiers or delivered to the antenna). Are you willing to compute the
> requirement for an equivalent Eris link (a bit late in the evening for
> me to attempt it, or just plain lazyness :-) ?
I don't see the bandwidth, so I can't determine what SNR they required. Anyway,
we can postulate some numbers.
Free space propagation loss to Eris, at 8 GHz, is 313.8 dB(!!). On the other
hand, antenna gain becomes easier at such high frequency.
To allow for 6 Mb/s, at 8 GHz, we can use a conservative modulation scheme
(like something with spread spectrum). Say, 0.1 b/s/Hz over a 60 MHz channel,
which means ideally you can get away with only -11.4 dB SNR (Shannon's
equation). So we'll use turbo codes, and hope for something like -10.5 dB SNR
min required.
8 GHz is .0375m wavelength.
Antenna gain = 10logbase10(eff*[piD]^2/lamda^2), where D is dish diameter in
meters. Which means that if we postulate a reasonable efficiency of 0.6, and
postulate a 30 meter dish at each end, each antenna will provide a 65.7 dB
gain. Not bad, thanks to the high frequency.
A wide band receiver, so we'll assume -90 dBm sensitivity. Can't expect too
much.
So transmitted power >= 313.8 - 65.7 - 65.7 - 10.5 - 90 dBm
Min transmitted power = 81.9 dBm = 155 KW
Reasonable, I'd say. Or you can reduce the diameters of the two dishes, and
increase the output power. Or reduce the channel bandwidth, which will increase
the marginal SNR required. And so on and so on.
Bert
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