User d ata tlm packets r s codeblock trailer r s

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Unformatted text preview: LER UNCODED D ATA ASM. (a) TRANSFER FRAME FRM HDR TRANSFER FRAME + SYNC USER D ATA (TLM PACKETS) ASM. TRAILER TRANSFER FRAME CONVOLUTIONA L ENCODER OUTPU T (b) TRANSFER FRAME REED-SOLOMON CODEBLOCK + SYNC FRM HDR ASM. USER D ATA (TLM PACKETS) R-S CODEBLOCK TRAILER R-S PARITY 1 to 5 times 223x8 bits (c) TRANSFER FRAME REED-SOLOMON CODEBLOCK + SYNC FRM HDR ASM. USER D ATA (TLM PACKETS) R-S CODEBLOCK TRAILER R-S PARITY 1 to 5 times 223x8 bits CONVOLUTIONA L ENCODER OUTPU T (d) TRANSFER FRAME TURBO CODEBLOCK + SYNC FRM HDR USER D ATA (TLM PACKETS) ASM. TRAILER TURBO ENCODED D ATA (e) Typically randomized (a) Uncoded transmission (b) Convolutional code only (c) Reed-Solomon code only (d) Concatenated Reed-Solomon and convolutional (e) Turbo code Figure 2-2: Telemetry Data Structures CCSDS 130.1-G-1 Page 2-5 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE 3 3.1 TM SYNCHRONIZATION AND CHANNEL CODING OVERVIEW This section describes the CCSDS TM Synchronization and Channel Coding systems, and is divided into the four main coding methods: Convolutional Code, Reed-Solomon Code, Concatenated Code, and Turbo Codes. 3.2 INTRODUCTION Channel coding 1 is a method by which data can be sent from a source to a destination by processing data so that distinct messages are easily distinguishable from one another. This allows reconstruction of the data with low error probability. In spacecraft, the data source is usually digital, with the data represented as a string of zeroes and ones. A channel encoder (or simply ‘encoder’) is then a device that takes this string of binary data and produces a modulating waveform as output. If the channel code is chosen correctly for the particular channel in question, then a properly designed decoder will be able to reconstruct the original binary data even if the waveforms have been corrupted by channel noise. If the characteristics of the channel are well understood, and an appropriate coding scheme is chosen, then channel coding provides higher overall data throughput at the same overall quality (bit error rate) as uncoded transmission - but with less energy expended per information bit. Equivalently, channel coding allows a lower overall bit error rate than the uncoded system using the same energy per information bit. There are other benefits that may be expected from coding. First, the resulting ‘clean’ channel can benefit the transmission of compressed data. The purpose of data compression schemes is to map a large amount of data into a smaller number of bits. Adaptive compressors will continually send information to direct a ground decompressor how to treat the data that follows. An error in these bits could result in improper handling of subsequent data. Consequently, compressed data is generally far more sensitive to communication errors than uncompressed data. The combination of efficient low error rate channel coding and sophisticated adaptive data compression can result in significant improvement in overall performance (reference [6]). Second, a low bit error rate is also required when adaptive (or self-identified) telemetry is used. Adaptive telemetry is much like adaptive data compression in that information on how various ground processors should treat the transmitted data is included as part of the data. An error in these instructions could cause improper handling of subsequent data and the possible loss of much information. 1 The method is called ‘channel’ coding because it is adapted to the statistical behavior of the channel and it applies to the overall transmitted data stream, not to specific sources only. CCSDS 130.1-G-1 Page 3-1 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE Third, low error probability telemetry may allow a certain amount of unattended mission operations. This is principally because the operations systems will know that any anomalies detected in the downlink data are extremely likely to be real and not caused by channel errors. Thus, operators may not be required to try to distinguish erroneous data from genuine spacecraft anomalies. In a typical space channel, the principal signal degradations are due to the loss of signal energy with distance, and to the thermal noise in the receiving system. The codes described in reference [3] can usually provide good communication over this channel. 3.3 RECOMMENDED CODES If interagency cross support requires one agency to decode the telemetry of another, then the codes recommended in reference [3] should be used. The recommended codes consist of: a constraint length 7, rate 1/2 convolutional code, and various punctured versions of it; (255,223) and (255,239) Reed-Solomon codes and arbitrary shortenings of them; codes formed by concatenating any of the recommended Reed-Solomon codes with any of the recommended convolutional codes; and a series of turbo codes of different rates and block sizes. A block diagram of the recommended coding system using concatenated codes appears in figure 3-1. A block diagram of the recommended coding system using turbo codes appears in figure 3-2. * REED-SOLOMON ENCODER AND INTERLEAVER * REED-SOLOMON DECODER AND DE-INTERLEAVER NRZ-L TO -M CONVERSION (IF USED) * NRZ-M TO -L CONVERSION (IF USED) OUTER CODE * SHORT CONSTRAINT LENGTH CONVOLU...
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