Safran CORTEX CRT Quantum - Pre Detection Diversity Combining

COMMAND RANGING & TELEMETRY UNIT CORTEX
CRT QUANTUM USER'S MANUAL
Ref. DTU 100042
Is.Rev. 5.17
Date: Dec.
03, 2021Sept. 30, 2021
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Page 54
3.3.5.1.2. Pre Detection Diversity Combining
The combining system is based on a combiner block that:
 Aligns the phases of the two signals using a second-order PLL.
 Weights them using the IF level measurement in the predetection filter.
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:
 Video mode: PM, FM, coherent AM, non-coherent AM demodulations,
 PCM mode: PCM/PM, PCM/BPSK, PCM/QPSK or PCM/OQPSK.
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.
Sub Carrier
Demodulators
Frame
Synchronizer
(only with video demodulation)
Pre detection filter
Channel A
IF input
Pre Detection
Diversity
Combining
Channel B
IF input
Viterbi decoder
Differential decoder
Pre detection filter
PM,FM
BPSK,QPSK
demodulator
Turbo decoder
RS decoder
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%.