Subject: Satellite Bandwidth Questions

From: Chris Metz <[email-masked]>
Date: Tue, 29 Dec 1998 11:49:12 -0500

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.

Chris Metz
Consulting Systems Engineer
Cisco Systems
email: [email-masked]
phone: 212-714-4207
pager: 800-365-4578

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From: Craig Partridge <[email-masked]>
Date: Tue, 29 Dec 1998 09:11:29 -0800

In message <[email-masked]>, Chris Metz writ

>apologies for the "simpleton" question.

Hi Chris:

Unfortunately the answer is "it depends."

Basically you're converting a pulse of some power into some number of bits
(and yes, you can send more than one bit per pulse).

I'm not an expert in the particular signalling techniques for satellite
(I'm barely competent in the fiber optic domain).  But someone on this
list can surely give you more details.

Craig

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From: Andreas Voigt <[email-masked]>
Date: Tue, 29 Dec 1998 19:41:55 +0100

> Consulting Systems Engineer

Hi Chris,

how are you doing? I hope everything is going well.

Concerning the Satellite application, unfortunately, you can't specify a
velocity in the range of several kbps to several hundreds of Mbps directly
while regarding the frequency band.

It depends, more or less, on the Bandwidth you have available on the satellite
transponder, which does not make any difference to throughput by using C-, Ku-
or Ka-Band. Concerning cost and availability, it does, but that is not the
question here. If the satellite is not a regenerative one like some Txps of
ACTS or the Txps of the Skyplex System of EUTELSAT, Up- and Downlink are the
same Signal (hopefully :-) ), so Up- and Downlink are considered to have the
same velocity.

For example:

You have 20 MHz available and you are using plain QPSK Modulation with Viterbi
1/2 Encoding. Then you will be able, depending on the correct Linkbudget, to
transmit and receive a 20 Mbps Carrier, on which you can play with the TCP
enhancements to increase throughput. It does not make a difference if this
carrier is distributed in C-, Ku- or Ka-Band.

So, i.e., the more bandwidth you have, the more maximum theoretical throughput
you get. The transponders with the highest bandwidth of GEO satellites on the
world are the Ka-Band ones of ACTS, but how ever, tests can be carried out on
smaller ones in Ku- or C-Band as well. Regard also the fact of the Ka-Band
availability problem in the linkbudget due to rain in comparison with Ku- and
ecspecially C-Band!

I hope I have been able to help you with this question.

Kind regards,

Andreas Voigt
GMD - German National Research Center for Information Technology
Dep. Networking - Satellite Communications

Kind regrad

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From: Lloyd Wood <[email-masked]>
Date: Tue, 29 Dec 1998 20:42:27 +0000 (GMT)

On Tue, 29 Dec 1998, Chris Metz wrote:

> good book on satellite basics

I tend to recommend Maral and Bousquet's "Satellite Communications
Systems: Systems, Techniques and Technology", third edition, for an
overview of communications from a satellite-application viewpoint, but
then that's the introduction I'm most familiar with.

Bruce Elbert's books are also okay introductions, but in terms of
filling in physical-to-data-link mapping choices (constellations in
symbol space, error rates, coding design etc. - the various things
that determine how the available bandwidth is used to give bps and
throughput)  I'd suggest getting large doses of theory from Simon
Haykin's "Digital Communications" and "Communication Systems". 

cheers,

L.

and satellites are just an implementation that people get overexcited about.

<[email-masked]>PGP<http://www.ee.surrey.ac.uk/Personal/L.Wood/>

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From: William D Ivancic <[email-masked]>
Date: Wed, 30 Dec 1998 10:47:18 -0500

>apologies for the "simpleton" question.

I'll try to answer this without teaching a complete course in RF
communications, modulation and coding.  

The Uplink and Downlink GHz is the RF modulated carrier.  For all practical
purposes, it has nothing to do with the transmitted or recieved data rates.
 The antenna gain is proportional to the square of the frequency.
Therefore, for a fixed size antenna, a higher frequency will give you
higher gain from the antenna.  One can use the antenna to offset the RF
amplifier requirements.  Other factors do come into play, however.  For the
same size antenna, a higher frequency results in a narrower beam resulting
in spot beams which may then require onboard switching and routing.   --
nothing is easy --  

Satellite transponders come in many types (RF carrier, bandwidth and
power).  36 MHz is a common bandwidth using in satellite transponders.
Satellite often carry multiple transponders at multiple Bands.  For example
check out this Intelsat Site

http://www.intelsat.com/cmc/info/i707maps.htm

For this example, each 36 MHz channel can be further subdivided in code,
time, or frequency (CDMA [code-division-multiple-access], TDMA, or FDMA).
Each subdivision will carry a specific modulated signal that is capable of
a specific data throughput (your bits per second).  

At the high-end commercial systems, a 72 MHz transponder with 8-PSK
modulated  concatenated convolutional and Reed-Solomon coded 
link can provide OC3 (155 Mbps) throughput.  This is possible because for
every three coded data bits, one symbol is being transmitted.  Thus, we can
get 155 Mbps plus the coding bits through 72 MHz of RF spectrum.

There are many good satellite books available.  One I use is Bernard
Sklar's "Digital Communications Fundamentals and Applications".

I hope this helped.

*********************************************
William D. Ivancic
NASA Lewis Research Center
21000 Brookpark Rd.   MS 54-8
Cleveland, Ohio  44135
USA
Phone:   1 216 433 3494
FAX:     1 216 433 8705
Email:   [email-masked]
         [email-masked]		
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