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Technical FAQ
What is the connection between bit rate and
bandwidth?
The bit rate of a circuit is measured in
bits per second and is known as the “Information rate”.
Some satellite circuits use an engineering overhead (information rate *
16/15 for IBS / SMS and a fixed 96kbps for IDR). The resulting data rate
is known as the “Composite rate”.
Satellite links all use forward error correction (FEC). Some links use two
error correction systems (e.g. Viterbi plus Reed Solomon). Each FEC
technique reformats the data and adds an overhead. The output rate from
the FEC is known as the “Transmission rate” and is equal to the
composite rate multiplied by the inverse of the FEC code rate.
The data is carried on a RF signal which is modulated normally as a PSK
signal. Depending on what modulation system is used, the resulting
spectrum will carry a number of bits per Hz. This is known as the
“Modulation index” Typical modulation indexes are:
BPSK : 1
QPSK : 0.5
8PSK : 0.333
16QAM : 0.25
By multiplying the transmission rate by the
modulation index, you arrive at the “Symbol rate”.
The occupied bandwidth depends upon the filtering used in the modulator,
the majority of modulators expand the bandwidth to 1.2 x symbol rate.
So if we were to take a typical 38Mbps, QPSK, DVB carrier operating with
¾ Viterbi and 188/204 Reed Solomon FEC, the symbol rate would be:
38.014706 * 204/188 * 4/3 *0.5 = 27.5 Msps
And the corresponding bandwidth would be 27.5 x 1.2 = 33 MHz
Single vs. dual synthesizer transceivers
Manufacturers produce two models of C-Band
transceivers, one has a single synthesizer and the other (slightly more
expensive variety) has two synthesizers.
Intelsat satellites traditionally have a fixed translation frequency of
2225 MHz between the uplink and the downlink. If the service is intended
solely for use on such a satellite, then a transceiver with a single
synthesizer will be suitable as this type of transceiver will have a fixed
2225 MHz offset between the transmit and the receive frequencies.
If the satellite does not have a fixed 2225 MHz offset or if the
transponders uplink and downlink footprint does not permit the use of
this, then a dual synthesizer transceiver will be required.
A transceiver with a dual synthesizer permits full independent tuning of
transmit and receive frequencies.
Why is space segment on inclined orbit
satellites so cheap?
The Inclined orbit is used for two
purposes:
(a) To allow extended geographical coverage
(b) To extend the life of the satellite as it's station keeping motors run
low on fuel.
There are three problems with operating to
inclined orbit satellites:
(a) You probably only have about a year of service left before the
satellite finally dies.
(b) You will suffer a large Doppler shift (not a major problem if the
Router is at the earth station, but could cause you problems re-timing
onto an existing backhaul).
(c) You will need add tracking to your antenna (typ. +$20k for a 2.4m).
Since you are talking about quite a large bandwidth, you could probably
justify the cost of the tracking system against the savings in space
segment costs.
Don't worry, the technique is widely used and providing you take care of
the above issues you should have no problems.
Glossary
of terms
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