130x1g2 - CCSDS

TM SYNCHRONIZATION AND CHANNEL CODING—SUMMARY OF CONCEPT AND RATIONALE9 IMPORTANT ANCILLARY ASPECTS OF THE CODING SYSTEM9.1 GENERALThe preceding four sections have described how one would encode and decode each of therecommended codes, and their corresponding performance, under ideal circumstances.CCSDS Recommended Standards (references [3] and [2]) also impose certain ancillaryconditions on the coding system in order to approach this ideal performance in a practicalsystem. Chief among these ancillary requirements addressed in RecommendedStandards (references [3] or [2]) are the following:a) the coded output of all codes (or of uncoded data) must be sufficiently random toensure proper receiver operation;b) there must be a method for synchronizing the received data with the codeword orcodeblock boundaries;c) there must be a way to certify the validity of decoded data with high certainty.There are a couple of additional ancillary issues associated with the recommended codes:a) some of the recommended codes are ‘transparent’ to inversion of the received data,and some are not;b) 1:1 remappings of the information or coded bits may be permitted but may affectperformance.9.2 RANDOMIZATION OF THE CODED OUTPUT9.2.1 GENERALRandomization of the data stream provides some useful functions. It aids in achieving:– signal acquisition;– bit synchronization;– proper decoding;– ambiguity resolution for convolutional decoder operation;– absence of incorrect frame synchronization for RS and LDPC codes;– reduction of spurious frequencies and compliance with power density masks.Receiver acquisition performance is often impaired by short periodic data patterns.Randomizing the data avoids this.In order to acquire and maintain symbol synchronization with the coded symbol boundaries,a bit synchronizer requires a sufficient symbol transition density. The recommended non-CCSDS 130.1-G-2 Page 9-1 November 2012