RE: Satellite Bandwidth Questions

["Christopher M. Hudson" <[e-mail]>]

Chris,

There probably is a good book but, I don't know one that would give you the
answer quickly.

For the real quick answer, you can use the response from David Fritz (Though
I think you would be using QPSK not BPSK modulation):

------- Fritz, David A. [[e-mail]] --------
If you simply want to get in the ballpark, using Binary Phase Shift Keying,
a common type of modulation for satellite communications, you get roughly
one bps per Hz of bandwidth allocation.  So, if a user is allocated 1 MHz of
bandwidth and they use BPSK, then they can send roughly 1 Mbps.
-----------------------------

P.S.  While composing this the excellent response from Chris Schram arrived.
Hopefully my response will add some more insight.



Here is a more in depth but still *quick* answer in the form of an example
using a typical, geosynchronous, bent-pipe, US domestic satellite.
(Contrary to my footer, the bulk of my satellite experience is with US
commercial satellites). This example is valid for both C- and Ku-band.  One
last caveat, this is a just an example.  I am purposely avoiding the myriad
of permutations that already exist and are increasing with time.


To define the terms used above:
Geosynchronous: the satellite is stationed above the equator and orbits the
earth once every 24 hours.  So, from earth, the satellite appears to remain
in the same spot all the time.
Bent-pipe:  the satellite does nothing more than receive at the uplink
frequency, translate to the downlink frequency, amplify, then retransmit the
signal back to earth.
US domestic satellite (often called a US domsat) - a satellite that receives
and transmits signals from (usually) the contiguous 48 states and often a
bit more (i.e. Hawaii or Alaska).


The quick answer is:

Bandwidth = (Data rate)  *  ( 1 / Modulation factor )  *  ( 1 / FEC )  *
 Any additional coding factors)

Where
Modulation factor =
   = 1 for BPSK  (binary phase shift keying)
   = 2 for QPSK  (quadrature phase shift keying)
FEC (Forward error correction) is typically 1/2, or 3/4 and sometimes 7/8.
Additional coding could be Reed-Solomon where numbers like 218:201 are
common  (i.e. 218 bytes are transmitted for 201 bytes of *real* data).

An example:  64 kbps, QPSK, 3/4	 rate Viterbi, with Reed-Solomon outer
coding.
BW = 46.3 kHz  =  64 kbit/s  *  ( 1 / 2 )  * ( 1  /  [3/4]  )   *  (  218 /
201  )
HOWEVER, modems are not perfect (filter rolloffs, etc.) and the carrier
coming out of the modem is not exacting 46.3 kHz wide. On top of this,
satellite providers put guard band between carriers to help insure they
don't interfere with each other.  Combining both of these, a
spreading/spacing factor of 1.3 to 1.4 is usually used.  So, the bandwidth
allocated on the satellite for this 64 kbps example is more like 62.5 kHz
(1.35 spacing).

Various modulation and coding schemes are used to squeeze more data bits
into a fixed bandwidth and power.  David Fritz's example of computer modems
is good.  They are limited to less than 4 kHz bandwidth and cannot exceed
fixed power levels.

Contrary to the telephone system, with satellites one can tradeoff available
bandwidth versus available power.

A slight digression.  A typical US domsat has 500 MHz of bandwidth. The 500
MHz is divided up into transponders, often each of 36 MHz.  Each transponder
has its own (power limited) amplifier.

For our example, the 64 kbps signal uses 62.5 kHz of bandwidth which is
0.17% of 36 MHz.  Consequently, the signal will be allocated 0.17% of the
available power.  Depending upon the size and sensitivity of the receiving
earth station, this may or may not be strong enough for the satellite link
to work properly. The 64 kbps signal may need to use the power equivalence
of, say, 100 kHz.  The power can be used, if available, but the 64 kbps
signal will now be treated as if it is occupying 100 kHz.

I hope this helped.

Chris
-------------------------------
Christopher M. Hudson
INTELSAT
[e-mail]
Tel: [phone]


-----Original Message-----
From:	[e-mail] [mailto:[e-mail]] On
Behalf Of Chris Metz
Sent:	Tuesday, December 29, 1998 11:49 AM
To:	[e-mail]
Subject:	Satellite Bandwidth Questions

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: [e-mail]
phone: [phone]
pager: 800-365-4578



Original Recipient: HUDSOC.BMAIL @ INTELSAT