Any invertible mapping may be applied to the binary

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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 [31]). 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 [3] and [2]) 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 [3] and [2]). 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 [3]) 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 GLOSSARY 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 errors. 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 the da...
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