Description Of Modulation Types - Comtech EF Data SNM-1001L Installation And Operation Manual

SNM-1001L Satellite Modem
Introduction
If the modem is so equipped, the optional Reed-Solomon PCB first processes the data.
The data is then sent to the scrambler for energy dispersal, and then to the differential
encoder. The differential encoder is a 2-bit encoder, which allows for resolution of two of
the four ambiguity states of the QPSK or OQPSK demodulator.
The data is sent to the convolutional encoder for encoding the baseband data. The code
rates 1/2, 3/4, 7/8, and 2/3 are based on the symbol rate range of 2.4 kbps to 2.5 Mbps.
For Viterbi codes, the convolutional encoder encodes the data at 1/2 rate. If the selected
code rate is 3/4, then 2 of every 6 symbols are punctured. For 3 bits in, there are 4
symbols out.
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For example, the 7/8 rate puts out 8 symbols for every 7 bits in. In {O}QPSK
mode, the data is split into two separate data streams to drive the I and Q channels
of the modulator.
The baseband processing for the SNM-1001L does not use the digital modulation.
Instead, the baseband Nyquist filtered I and Q signals pass through D/A converters to
drive an analog vector modulator.
The local oscillator input to the vector modulator is a single loop synthesizer
incorporating a Direct Digital Synthesis (DDS) chip to accommodate 100 Hz steps over
the range of 950 to 1750 MHz. The modulator output then passes through amplifiers and
AGC incorporating programmable output level control and switched low pass filters for
harmonic attenuation.
1.6.3

Description of Modulation Types

The modulation types for the modem include BPSK, QPSK, Offset QPSK, or 8-PSK.
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 order of modulation is represented by mPSK, where "m" relates to the number of
discrete phase angles. Refer to the following list for a brief description of the modulation
types.
•
BPSK: 2 discrete phase angles represent the 2 possible states of a symbol.
•
Offset QPSK: 4 discrete phase angles represent the 4 possible states of a symbol.
For Sequential codes, the convolutional encoder generates the parity bits from the
input data stream, which allows for error correction at the far end of the link. The
rate of the encoder may be 1/2, 3/4, and 7/8.
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