[amsat-bb] Re: requesting help on a RF link solution (imaginary, ka-band link!)

[Achim Vollhardt]

Hi Samundra,

don't see which path loss you assumed, but for 1 AU (149 mill km), I 
calculate 286 dB @32 GHz.

Also 3000K seems much to bad.. assume 300K.

So let's add up for 1 AU:

40 dBm (10W) +2x56.4 dBi -286 = -133.2 dBm at RX input

1 Gbit/s assumes 1 GHz bandwidth. 300K is roughly -174 dBm/Hz.
1 GHz = 60dB worse = -116 dBm.

So you end up with -133.2 - (-116) = -17.4 Eb/No.. not good.

If you add 20dB you end up at 2.6 dB Eb/No. Not brilliant, but close. 
20dB means 0.1 AU instead of 1 AU = 14.9 Mill km.

I think your result was 4.6E9 m (!), not km.. which is 4.6 mill km. 
Should be it, if getting back to your assumed 3000K RX.


73s Achim, DH2VA



> Date: Tue, 27 Oct 2009 06:03:30 -0400
> From: Samudra Haque <samudra.haque at gmail.com>
> Subject: [amsat-bb] requesting help on a RF link solution (imaginary
> 	ka-band	link!)
> To: Amsat-bb <amsat-bb at amsat.org>
> Message-ID:
> 	<d8c724880910270303k9a3b9cch123289eebcaebf90 at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
> 
> Hi, amsat-bb
> 
> CQ any satellite link budget expert !
> 
> I'm trying to do a calculation on my own based upon published specs
> for the NASA MRO Ka-band experiment, but am getting some unexpected
> results for a Ka-band simplex link with Temp=3000K (hypothetical),
> operating with a Signal to Noise ratio (unitless) figure of 1.171
> (representing 4.5 dB eb/no with a data rate of 1 Gbps and a bandwidth
> of 2.4x109 Hz)
> 
> Question : is 1 gbps not 1x109 bps ?
> 
> Question : if both antennas are 3m parabolic (both are the same type)
> with 56.4 dBi boresight gain, what would you think the furthest
> distance the link can perform with SNR of 1.171. I have actually used
> a padding of 3 dB Eb/No in my link budget, so am not worried about any
> further signal loss at first (ok, I should be ..) For the exercise, I
> am choosing a 10 Watt estimated output on an arbitrary basis.
> 
> So:
> 
> P_t = 10W
> 
> G_t = 56.4 dBi = G_r , can we assume the same gain for TX and RX on a
> parabolic dish ?
> 
> T = 3000K at receiver
> 
> SNR = 1.171 required
> 
> f=32.2 GHz
> 
> B = 2.4E9 Hz, (bpsk, ldpc code 0.5)
> 
> DR = 1E9 bps
> 
> So, I am puzzled why this link budget says the range with these
> parameters is equal to 4.644 x 109 Km -- that seems to be a long
> distance ! What am I not able to conceptualize.
> 
> 
> BTW, I know if I send this out, the answer will come to me soon
> thereafter, but for education, I would like to know where the problem
> in my understanding lies !
> 
> 
> Samudra N3RDX
> 
> 
> ------------------------------