130x1g2 - CCSDS

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TM SYNCHRONIZATION AND CHANNEL CODING—SUMMARY OF CONCEPT AND RATIONALEother means and properly verified. These problems are traced to a lack of randomization atthe data or modulation symbol level. In many communication system designs, the receiver,bit/symbol synchronizer and convolutional decoder all have specific requirements that aremet by using randomized data. Details may change depending on modulation type, dataformat (NRZ-L vs. Bi Phase L) and signal to noise ratio. If the implementer can adequatelyprove that a symbol stream with the proper randomness and balance of 1s and 0s can beachieved without the use of the recommended pseudo-randomizer to 1) ensure a highprobability of receiver acquisition and lock in the presence of data, 2) eliminate DC offsetproblems in PM systems, 3) ensure sufficient bit transition density to maintain bit (orsymbol) synchronization, and 4) handle special coding implementations (i.e., data that ismultiplexed into multiple convolutional encoders), then the recommended Pseudo-Randomizer may be omitted.The presence or absence of Pseudo-Randomization is fixed for a physical channel and ismanaged (i.e., its presence or absence is not signaled in the telemetry but must be known apriori) by the ground system.9.3 FRAME SYNCHRONIZATION9.3.1 GENERALEach of the recommended codes requires a method for aligning the sequence of receivedcode symbols with the boundaries of its codewords or codeblocks (or code symbol periods inthe case of convolutional codes). Otherwise, the decoder would fail because it would beapplying the correct decoding algorithm to an incorrect subset of received code symbols. Thesynchronization requirements are different for each of the recommended codes, as describedin the next four subsections.9.3.2 SYNCHRONIZATION FOR CONVOLUTIONAL CODESFor a rate 1/n convolutional code, the encoding rule, and hence the decoding rule, are ‘timeinvariant’in that the same rule is applied at each bit time. Thus even though theconvolutional codeword is indefinitely long, the only requirement for proper synchronizationis to correctly establish the identity of the starting symbol of any group of n symbolsproduced in one bit time. This procedure is commonly called ‘node synchronization’. For therecommended rate-1/2 non-punctured convolutional code, as well as the entire series ofrecommended punctured convolutional codes derived from the rate-1/2 code, nodesynchronization is a relatively simple matter of distinguishing between two possible ‘phases’of the received symbol stream. This can be accomplished with or without the aid of framesynchronization markers in the data. For example, the Viterbi decoder may determine thecorrect phase by monitoring the rate of growth of its own internal metrics. Some usefultechniques for node synchronization are described in reference [13]. Alternatively, for therecommended rate-1/2 convolutional code, node synchronization and frame synchronizationcan be established simultaneously by locating the (52-symbol invariant part of the)convolutionally encoded synchronization marker within the received symbol stream.<strong>CCSDS</strong> 130.1-G-2 Page 9-7 November 2012
TM SYNCHRONIZATION AND CHANNEL CODING—SUMMARY OF CONCEPT AND RATIONALE9.3.3 SYNCHRONIZATION FOR REED-SOLOMON CODESA Reed-Solomon decoder will only decode properly if the starting symbol of each codewordis identified; i.e., the decoder requires accurate codeword synchronization. If interleaving isused, further resolution is necessary to determine the starting symbol of each codeblock(interleaved set of I codewords), or else the de-interleaver will fail to work properly.The recommended method for synchronizing the codeblock is to look for an attachedsynchronization marker of 32 bits. This procedure is commonly called ‘framesynchronization’, because, in the absence of Reed-Solomon coding, the same 32-bitsynchronization marker is attached directly to the Transfer Frame and is used to locate thestart of the frame. When Reed-Solomon coding is used, the 32-bit marker is attached to thebeginning of the Reed-Solomon codeblock and is used in the same way to identify thestarting symbol of a codeblock. In this case, the synchronization procedure is properly called‘codeblock synchronization’, but the term ‘frame synchronization’ is often usedindiscriminately to cover both cases.It is important to note that the codeblock synchronization marker is not encoded by the Reed-Solomon encoder. Thus even though the same 32-bit marker is attached to the same block ofinformation bits, whether they occur in an uncoded Transfer Frame or as the data bits in asystematic Reed-Solomon codeblock, the Reed-Solomon coding cannot be considered atotally separate layer that follows the attachment of the marker to the Transfer Frame. If thecoding layer should receive a Transfer Frame with frame synchronization marker alreadyattached, it must detach the marker, encode the Transfer Frame only, and reattach the markerto the encoded codeblock.9.3.4 SYNCHRONIZATION FOR CONCATENATED CODESSynchronization for concatenated codes requires finding proper alignment with theboundaries of both constituent codes. The Recommended Standard (reference [3]) requiresthat the same 32-bit synchronization marker be attached to the recommended Reed-Solomoncode, regardless of whether it is concatenated with an inner convolutional code. At thereceiving end, the two levels of synchronization can be established by first nodesynchronizingthe inner convolutional code, and then locating the 32-bit synchronizationmarker after convolutionally decoding. Alternatively, when the inner code is therecommended rate-1/2 convolutional code, node synchronization and frame synchronizationcan be established simultaneously by locating the (52-symbol invariant part of the)convolutionally encoded synchronization marker within the received symbol stream.9.3.5 SYNCHRONIZATION FOR TURBO CODESCodeword synchronization is necessary for proper decoding of Turbo codewords.Synchronization of the Turbo codewords is achieved by using an attached sync marker. Thecode symbols comprising the sync marker for the Turbo code are attached directly to the<strong>CCSDS</strong> 130.1-G-2 Page 9-8 November 2012
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