Re: Satellite Bandwidth Questions

[William D Ivancic <[e-mail]>]

At 11:49 AM 12/29/98 -0500, Chris Metz wrote:
>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.
>

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:   [phone]
FAX:     [phone]
Email:   [e-mail]
         [e-mail]		
*********************************************