Comtech EF Data SNM-1010L - Manual (Page 174)

SNM-1010L Data/Control Modem Revision 0
Theory of Operation MN/SNM1010L.OM
6.3.2.2 QPSK Encoding
The modulator converts transmitted baseband data into a modulated QPSK carrier at the
following parameters:
• • 4.8 kbit/s to 2.5 Mbit/s (1/2 rate)
• • 7.2 kbit/s to 3.75 Mbit/s (3/4 rate)
• • 8.4 kbit/s to 4.375 Mbit/s (7/8 rate)
Using vector analysis of the constellation pattern, QPSK represents a symbol with the
carrier phase angle at 45
°, 135°, 225°, or 315°. The 1/2, 3/4, and 7/8 rates encoded at the
convolutional encoder provide the desired input/output bit rates.
Code Rate Symbols/Bit Bits/Hz
1/1 1 2
1/2 2 1
3/4 1.5
1.33
7/8 1.143 1.75
6.3.2.3 OQPSK Encoding
The modulator PCB converts the transmitted baseband data into a modulated OQPSK
carrier within the same parameters as QPSK.
The OQPSK modulation is mainly different from QPSK by offsetting the I and Q channel
modulation signals. This offset prevents the RF envelope from going through zero. Under
certain conditions, this may allow less back –off in the High Power Amplifier (HPA)
system. The 1/1, 1/2, 3/4, and 7/8 rates encoded at the Convolutional encoder provide the
desired input/output bit rates.
Code Rate Symbols/Bit Bits/Hz
1/1 1 2
1/2 2 1
3/4 1.333
1.5
7/8 1.143 1.75
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