57 lines
1.9 KiB
XML
57 lines
1.9 KiB
XML
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<?xml version="1.0"?>
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<block>
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<name>Pseudo (De)randomiser with Softbits (CCSDS)</name>
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<key>ccsds_randomiser_softbits</key>
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<category>CCSDS/Telemetry</category>
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<import>import ccsds</import>
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<make>ccsds.randomiser_softbits($poly,$seed)</make>
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<param>
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<name>Generating Polynimoal</name>
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<key>poly</key>
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<value>0x95</value>
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<type>int</type>
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</param>
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<param>
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<name>Seed</name>
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<key>seed</key>
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<value>0xFF</value>
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<type>int</type>
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</param>
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<check>$poly >= 0</check>
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<check>$poly < 256</check>
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<check>$seed >= 0</check>
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<check>$seed < 256</check>
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<sink>
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<name>in</name>
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<type>message</type>
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</sink>
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<source>
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<name>out</name>
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<type>message</type>
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</source>
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<doc>
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Pseudo (De)randomiser
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Combines the input frame with a pseudo random sequence by XOR operation. A one
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in the PRN sequence will invert the softbit, a zero will copy it untouched. The
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pseudo random sequence is generated by an 8-bit linear feedback shift register
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(LFSR) as defined in ECSS.
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The pseudo random sequence can be modified by the generating polygon as well as
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shifted by choosing a different initial seed state.
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poly: Polynomial that defines the network of the 8 shift registers. The MSB
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encodes whether the last shift register (which is used for the output) should be
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used to generate the new input while the LSB encodes whether the first shift
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register (that holds the input bit of the last step should be used to generate
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the output. Or in ECSS definition: The MSB holds the coefficient for x^0 up to
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the LSB which holds coefficient x^7. x^8 is not encoded as it is always set (due
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to the fact that this is a maximum length 8 bit shift register). The default
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value is 0x95 which corresponds to the randomiser defined in ECSS with the
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polynomial representation x^8+x^7+x^5+x^3+x^0.
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seed: Initial state of the shift registers. MSB corresponds to the last register
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(which will be the output on the next step). Default to all ones as defined in
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ECSS.
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</doc>
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</block>
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