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Unformatted text preview: elements of the coding system depicted in figure 3-1 are used, the ‘NRZ-L to
NRZ-M conversion’ is actually just a form of 1:1 mapping applied to the binary data, not a
conversion of modulation formats, since the modulation of the data occurs after the
convolutional encoding stage. Any invertible mapping may be applied to the binary data
without apparent consequence as long as all the data bits are correct; however, performance
is affected by 1:1 mappings when errors enter the system. For example, there is a large
performance penalty if one puts an NRZ-L to NRZ-M mapping at the output of a
convolutional code (see reference ). For this reason figure 3-1 does not include an option
that allows an ‘NRZ-L to NRZ-M conversion’ block to serve as a true modulation conversion
at the output of the convolutional encoder.
The CCSDS Recommended Standards (references  and ) do not regulate whether a
user’s source data might be subjected to a 1:1 mapping (or any other form of data processing)
before being packaged as information bits in a Telemetry Transfer Frame prior to coding.
Thus any form of 1:1 mapping of the source data that precedes any of the recommended
CCSDS codes is implicitly allowed by Recommended Standards (references  and ). In
this case, the code performance curves shown in this Green Book pertain only to the error
rates for the remapped data presented to the encoder. The user has the responsibility to
determine whether these errors might propagate or multiply throughout the original source
data as a result of the 1:1 premapping. For example, the discussion following figure 5-4
mentioned two methods (row-by-row and column-by-column) for reading the source data
into the matrix used for interleaving Reed-Solomon codewords; this choice affects the
characteristics of errors in the decoded source data. CCSDS 130.1-G-1 Page 8-10 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE The ‘NRZ-L to NRZ-M conversion’ block in figure 3-1 can be viewed simply as an
implicitly permitted 1:1 remapping of the source data in the case when the Reed-Solomon
code is not used. Curiously, however, the figure also indicates that this mapping may be
placed between the two components of a concatenated code. This placement makes sense
from a performance standpoint: unlike an NRZ-L to NRZ-M mapping at the output of a
convolutional code, the same mapping applied to Reed-Solomon coded bits has only minor
effects on the code’s performance . However, in this position this remapping in fact makes
the overall code a concatenation of three codes, not two, when all three elements of the
coding system depicted in figure 3-1 are used. The Blue Book (reference ) does not clearly
state that such an arrangement is permitted. Also, figure 3-1 fails to show where the optional
‘NRZ-L to NRZ-M conversion’ block between the Reed-Solomon and convolutional codes
fits with respect to the interleaving of Reed-Solomon codewords. CCSDS 130.1-G-1 Page 8-11 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE ANNEX A
Block Encoding: A one-to-one transformation of sequences of length k of elements of a
source alphabet to sequences of length n of elements of a code alphabet, n>k.
Channel Symbol: The unit of output of the innermost encoder which is a serial representation
of bits, or binary digits, which have been encoded to protect against transmission induced
Clean Data (Bits): Data (bits) which are error free within the error detection and optional
error correction capabilities of the TM System.
Codeblock: A codeblock of an (n,k) block code is a sequence of n channel symbols which
were produced as a unit by encoding a sequence of k information symbols, and will be
decoded as a unit. Code Rate: The average ratio of the number of binary digits at the input of
an encoder to the number binary digits at its output.
Codeword: In a block code, one of the sequences in the range of the one-to-one
transformation (see Block Encoding).
Command Link Control Word: The Telecommand System Transfer Layer protocol data unit
for Telecommand reporting via the TM Transfer Frame Operational Control Field.
Concatenation: The use of two or more codes to process data sequentially with the output of
one encoder used as the input of the next.
Constraint Length: In convolutional coding, the number of consecutive input bits that are
needed to determine the value of the output symbols at any time.
Convolutional Code: As used in this document, a code in which a number of output symbols
are produced for each input information bit. Each output symbol is a linear combination of
the current input bit as well as some or all of the previous k-1 bits, where k is the constraint
length of the code.
Fill Bit(s): Additional bit(s) appended to enable a ‘data entity’ to exactly fit an integer
number of octets or symbols.
Inner Code: In a concatenated coding system, the last encoding algorithm that is applied to
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