RE: satellite bandwidth questions

["William A. Kissick" <[e-mail]>]

Hello Chris:

I believe you are asking about how many bits per second (bit rate) one can
achieve for a given bandwidth (channel width, not uplink or downlink
frequencies as in your table). The ratio of bit rate to channel width is
called spectral efficiency. Sorry, there is no simple answer to your simple
question. Look for any book with the approximate title "Satellite
Communications."

The information capacity of any communication system operting over a noisy
channel is limited. This limit is given by the Shannon-Hartley law which
gives the information capacity in terms of the channel width in Hertz and
the received signal to noise ratio (for Gaussian noise). In digital terms,
regardless of the bandwidth used, the Eb/N0 (energy per bit to noise density
ratio) cannot go below -1.6 dB. This is the Shannon limit below which NO
information gets through.

At higher (more realistic and typical for satellite channels) values of
Eb/N0, like 5 to 25 dB, the theoretical spectral efficiency is about 16
bits/Hz. (A channel width of 100 kHz would theoretically support 1,600
kB/s). In practice, links using PSK (phase shift keying) do not achieve
capacities anywhere near the Shannon limit because the theory assumes zero
bit errors. For a practical BER (bit error rate) of about 10**-10, in a QPSK
(quadrature phase shift keying) the spectral efficiency will be about 2
bits/Hertz. So, a 100 kHz channel will support about 200 kB/s.

Now, you should also know that satellite channels fade, so FEC may be added
during times of rain attenuation. This will reduce the information bit rate
to one-half, for example, to tolerate a 3-dB fade and keep the BER the same
in the same bandwidth.

Regards,
Bill

>Hi-
>I am studying TCP over Satellite considerations and have reviewed
>draft-ietf-tcpsat-stand-mech-06.txt. I live in a "bits per second" world in
>terms of bandwidth I would like to understand how bps relates to the terms
>used to describe satellite bandwidth. So is there a reference somewhere or
>good book on satellite basics that can fill in the following terms:

>Band     Uplink (GHZ)    Downlink (GHz)    Uplink (bps)    Downlink (bps)

>C            6              4                 ??               ??

>Ku           14             12                ??               ??

>Ka           30             20                ??               ??


>I suppose I am asking how does one convert MHz and GHz into conventional
>bandwidth (bps) terminology. This for clearing this up for me and my
>apologies for the "simpleton" question.



>Consulting Systems Engineer
>Cisco Systems
>email: [e-mail]
>phone: [phone]
>pager: 800-365-4578  

-----------------------------------------------------------
  Dr. William A. Kissick
  Institute for Telecommunication Sciences
  National Telecommunications and Information Administration
  325 Broadway
  Boulder, Colorado 80303

  Phone: [phone]
  FAX:   [phone]
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