Pre Detection Diversity Combining - Safran CORTEX CRT Quantum User Manual

3.3.5.1.2. Pre Detection Diversity Combining

The combining system is based on a combiner block that:
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
The combined signal is fed to an IF Receiver that operates the demodulation of the signal.
Pre Detection Diversity combining is available for these types of modulation schemes:


The advantage of this scheme is that the carrier acquisition can take advantage of the diversity gain, since the
combiner operation is totally independent from the carrier lock status of the IF Receiver. Moreover, since the
signal levels are measured in the predetection filters before the demodulator (non-coherent measures), their
estimations are less precise, but they can be done rapidly. Then, the combiner is able to react swiftly to signal
fades.
Channel A
IF input
Pre detection filter
Channel B
IF input
Pre detection filter
Figure 27: Pre detection Diversity Combining : functional block diagram
The Pre Detection Filters filter the two IF signals before combining with bandwidths from 5 kHz to 13 MHz. They
must be chosen as low as possible to maximize the signal-to-noise ratio at the input of the combiner block.
Nevertheless, their value must be high enough to ensure that the signal falls always in their bandwidth, even in
the presence of Doppler.
A spectral display of DCU output is available permanently.
The Predetection PLL bandwidth has to be set equal or less that the IFR PLL bandwidth divided by 10. This
ensures that the predetection PLL does not perturbate the operation of the receiver PLL.
In video mode, its value is given in Hertz and goes from 1 Hz to 30 kHz.
In PCM mode, its value is given in percentage of the symbol rate and goes from 0.001% to 0.3%.
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COMMAND RANGING & TELEMETRY UNIT CORTEX
CRT QUANTUM USER'S MANUAL
Aligns the phases of the two signals using a second-order PLL.
Weights them using the IF level measurement in the predetection filter.
Video mode: PM, FM, coherent AM, non-coherent AM demodulations,
PCM mode: PCM/PM, PCM/BPSK, PCM/QPSK or PCM/OQPSK.
Pre Detection
Diversity
Combining
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PM,FM
Sub Carrier
BPSK,QPSK
Demodulators
demodulator
(only with video demodulation)
Safran Data Systems
.
Ref. DTU 100042
Is.Rev. 5.17
Date: Dec.
03, 2021Sept. 30, 2021
Viterbi decoder
Frame
Differential decoder
Synchronizer
Turbo decoder
RS decoder
Page 54