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SDM-300L3 Satellite Modem
Introduction
If the modem is so equipped, the optional overhead or 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 SDM-300L3 is the same as for the SDM-300L1 and
SDM-300A modulator except that the digital modulation is not used. 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, OQPSK, 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.
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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.
BPSK: 2 discrete phase angles represent the 2 possible states of a symbol.
1–7
Revision 1
MN/SDM300L3.IOM
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