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This method relies on the fact that frame sync is not

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Unformatted text preview: an be acquired using a sync marker defined in the information bit domain rather than the encoded symbol domain, and detected after Viterbi decoding. This method relies on the fact that frame sync is not required for successful operation of the Viterbi decoder but is necessary for decoding the Reed-Solomon code. The Viterbi decoder is capable of finding its own ‘node sync’ with or without the aid of known sync markers in the data stream. The Reed-Solomon decoder has no effective method (other than trial and error) for determining frame sync on its own, and so it must be presented with externally synchronized codeblocks. It is irrelevant to the performance of the RS decoder whether this synchronization is determined from the channel symbols or from Viterbi decoded bits. In a similar way, the turbo decoder relies on being handed externally synchronized codeblocks, but a bit-domain approach does not work effectively for turbo decoders, because each constituent convolutional decoder is too weak by itself to detect a reasonable size marker reliably, and because the powerful combined turbo decoding operation needs to know the codeblock boundaries before it can iterate between permuted and unpermuted data domains. Therefore, turbo code applications need to use channel-symbol-domain frame sync methods as specified in the Recommended Standard (reference [3]). Note that, for equivalent performance, channel-symbol-domain frame synchronization requires longer sync markers and faster processing (at the channel symbol rate rather than the Viterbi decoded bit rate). CCSDS 130.1-G-1 Page 8-6 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE 8.4 CERTIFICATION OF THE DECODED DATA (FRAME INTEGRITY CHECKS) 8.4.1 GENERAL The CCSDS applications are packet-oriented, which means that data are collected and transmitted in frames. With all coding options, and also for uncoded data, it is important to have a reliable indication whether the decoded data is correct. A frame integrity check can be used at the receiver side to validate the received frame or, when suitable, for requiring retransmission in case of check failure. As with the problem of randomizing the coded output, a universal solution to this data validation problem exists in the form of a cyclic redundancy check (CRC) code, as specified in the TM Space Data Link Protocol Blue Book (reference [2]). 8.4.2 DESCRIPTION OF THE RECOMMENDED CRC CODE Generally speaking, a binary CRC code is an (N,k) code obtained by shortening a cyclic code, capable of detecting the following error patterns: a) all error bursts of length N–k or less; b) a fraction of error bursts of length equal to N–k+1; this fraction equals 1–2–(N–k–1); c) a fraction of error bursts of length greater than N–k+1; this fraction equals 1–2–(N–k); d) all error patterns containing dmin – 1 (or fewer) errors, dmin being the minimum distance of the CRC code; e) all error patterns with an odd number of errors if the generator polynomial G(D) for the code has an even number of nonzero coefficients. The circuits for coding and syndrome computation are simple feedback shift registers with r = N–k cells. In the CRC code used for the CCSDS TM Space Data Link Protocol Recommended Standard, 16 parity check bits are added to every information frame consisting of (N–16) information bits, according to the following generator polynomial: G(D) = D16+D12+D5+1 Thus the rate of this CRC code is (N–16)/N. The CRC circuit in the Recommended Standard (reference [2]) is preset to an all ‘1’ state prior to encoding; this is a peculiarity of the CCSDS CRC, which implies that the 16 parity check bits are inverted with respect to the usual CRC encoding (N–16) all ‘0’ information bits generate 16 all ‘1’ parity check bits). CCSDS 130.1-G-1 Page 8-7 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE Serial concatenation of the CRC and a turbo code with nominal rate 1/3 is shown in figure 8-3. u cs CRC p c1 C1 INTERLEAVER N p c2 C2 Figure 8-3: Turbo-CRC Encoder The information frame is encoded by the CRC before entering the turbo encoder, and the CRC syndrome is used to check the integrity of the decoded frame produced by the turbo decoder at the receiver side. 8.4.3 USAGE CIRCUMSTANCES FOR THE RECOMMENDED CRC CODE The recommended CRC code is included in the Telemetry Frame and consists of 16 check bits computed from the remainder of the frame contents. This code can reliably detect incorrect frames with an undetected error rate of around 2–15≈10–5. This CRC code achieves approximately the same undetected error rate for any of the recommended telemetry channel codes. A much lower undetected error rate is achieved when the RS code with E = 16 is used, either by itself or concatenated with an inner convolutional code. In this case, the undetected error rate of the RS decoder is on the order of 1/E!≈10–13, which is many orders of magnitude better than the validation offered by the CRC code. Thus, the error de...
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