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SLM-7650 Satellite Modem
Theory of Operation
4.4.3.3.5
Buffering
There are two primary purposes for a receive FIFO buffer in a satellite modem. One is to
compensate for periodic changes in the receive data rate caused by the satellite movement
(Doppler shift) and the other is to provide a controlled reset to compensate for small
differences in master timing standards at two ends of the satellite link (Plesiochronous
timing). The same receive buffer is used to compensate for the Doppler shift and for
plesiochronous timing.
The receive buffer can be clocked by a number of sources. The internal system clock
(INT), the recovered clock from the satellite (RX_Sat), the TX terrestrial clock from the
modulator (TX_Terr), or the External Master Clock (MC) from the Data I/O.
When the receive data is clocked out of the modem using the receive recovered satellite
clock (RX_Sat) the buffer is not active. The receive data is being clocked in and out of
the buffer with the same clock. When the buffer clock selection is anything other than
RX_Sat the buffer is active and must be properly configured. Also, anytime that the
receive buffer is active in the modem, that end of the satellite link is considered the
source of a master clock.
In a satellite link with one master clock, the operator will typically clock the data in and
out of the modem with the transmit clock source (TX_Terr). If there are two master
clocks in the link then the receive buffer must operate as a Plesiochronous/Doppler
buffer. The buffer must be sized to compensate for the Doppler shift plus the additional
size required to extend the time between inevitable slips caused by the Plesiochronous
timing. Plesiochronous operation is typical in international Public Switched
Telecommunications/Telephone Network (PSTN). The PSTN users will inevitably suffer
SYNC losses or periodic pattern slips because of the two master clocks are being used in
the network.
The receive buffer will periodically fill and empty as the received signal changes in
frequency. The buffer size must be set large enough to avoid an over or under fill. An
overly large buffer will unnecessarily increase the data delay. The buffer size should be
large enough to allow the buffer to start at any time, as the satellite is moving in the
inclined orbit. The receive buffer size is determined by how many days are desired
between the clock slips and the amount of Doppler that must be considered.
The amount of Doppler shift that will be present in the link will vary based on the
particular satellite that is in the link. Communication satellites in geosynchronous earth
orbit (GEO) appear to float in a repeated figure eight pattern every 24 hours. The size of
the figure eight is controlled through the use of station keeping propulsion. As these the
satellites reach the end of their operational life, station-keeping fuel is used up and the
satellites will be removed from orbit. In a move to extend the useful life of some
satellite, operators are using an operating technique called inclined orbit. This procedure
allows the satellite to drift in a 'north/south' direction while maintaining a strict 'east/west'
position. However, the side affects of this operating mode are increased Doppler shift
and required antenna tracking systems on large antennas.
4–33
Revision 4
MN/SLM7650.IOM
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