Theory Of Modulation Types; Description; Qpsk Encoding/Modulation - Comtech EF Data SDM-9000 Installation And Operation Manual

SDM-9000 Satellite Modem

5.2.4 Theory of Modulation Types

5.2.4.1 Description

The modulation types for the modem include:
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•
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The PSK data transmission encoding method uses the phase modulation technique. This
method varies the phase angle of the carrier wave to represent a different bit value for the
receiver. The higher levels of modulation are required for an operating range that has a
limited bandwidth.
The QAM method uses a combination of differential phase shifts and amplitudes totaling
16 different states to represent different bit values.
The order of modulation is represented by mPSK or mQAM, where m relates to the
number of discrete modulation states. The following table is a brief description of the
modulation types:
QPSK
8PSK
16QAM

5.2.4.2 QPSK Encoding/Modulation

The modulator converts transmitted baseband data into a modulated QPSK carrier.
Using vector analysis of the constellation pattern, QPSK represents two symbols with the
carrier phase angle at 45°, 135°, 225°, or 315°. The 1/1, 1/2, 3/4, and 7/8 rates encoded at
the convolutional encoder (k = 7) provide the desired bit rates as follows:
Code Rate
Rev.4
QPSK
8PSK (optional)
16QAM (optional)
4 discrete phase angles represent the 4 possible combinations of
2 symbols.
8 discrete phase angles represent 8 possible combinations of 3
symbols generated by the encoder.
16 discrete modulation states represent 16 possible combinations
of 4 symbols generated by the encoder.
Symbols/Bit
1/1 1
1/2 2
3/4 1.33
7/8 1.143
Bits/Hz
2
1
1.5
1.75
Theory of Operation
5–9