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After locating the asm in the received data stream

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Unformatted text preview: ce, and so on. On the receiving end, the original Codeblock or Transfer Frame is reconstructed using the same pseudo-random sequence. After locating the ASM in the received data stream, the pseudo-random sequence is exclusive-ORed with the data bits CCSDS 130.1-G-1 Page 8-2 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE immediately following the ASM. The pseudo-random sequence is applied by exclusive-ORing the first bit following the ASM with the first bit of the pseudo-random sequence, followed by the second bit of the data stream with the second bit of the pseudo-random sequence, and so on. The pseudo-random sequence used in the CCSDS standard is generated by using the following polynomial: h(x) = x8+x7+x5+x3+1 This sequence begins at the first bit of the Codeblock or Transfer Frame and repeats after 255 bits, continuing repeatedly until the end of the Codeblock or Transfer Frame. The sequence generator is initialized to the all-ones state at the start of each Codeblock or Transfer Frame. The first 40 bits of the pseudo-random sequence from the generator are shown below; the leftmost bit is the first bit of the sequence to be exclusive-ORed with the first bit of the Codeblock or Transfer Frame; the second bit of the sequence is exclusive-ORed with the second bit of the Codeblock or Transfer Frame, and so on. 1111 1111 0100 1000 0000 1110 1100 0000 1001 1010 ... 8.2.3 USAGE CIRCUMSTANCES FOR THE RECOMMENDED PSEUDORANDOMIZER The Recommended Standard (reference [3]) does not always require the use of the universal solution provided by the pseudo-randomizer. As we have seen, its use would be superfluous in the case of convolutional coding with alternate symbol inversions and BPSK modulation. Less conclusively, turbo codes might inherently provide a sufficient coded symbol transition density due to their recursive convolutional encoding of non-zero data headers at the beginning of each data block. Other codes might obtain sufficient transitions if their input information bits are guaranteed to be sufficiently random. I&T project personnel may prefer un-randomized data so that during testing, they can read the binary data that they are familiar with. One answer is to implement the recommended pseudo-randomizer but make it switchable so that during early testing it can be turned off. While the recommended pseudo-randomizer is not strictly required, the system engineer must take all necessary steps to ensure that the coded symbols have sufficient transition density. Several projects have encountered unexpected problems with their telemetry links because this pseudo-randomizer was not used and sufficient randomness was not ensured by other means and properly verified. These problems are traced to a lack of randomization at the data or modulation symbol level. In many communication system designs, the receiver, bit/symbol synchronizer and convolutional decoder all have specific requirements that are met by using randomized data. Details may change depending on modulation type, data format (NRZ-L vs. Bi Phase L) and signal to noise ratio. If the implementer can adequately prove that a symbol stream with the proper randomness and balance of 1s and 0s can be achieved without the use of the recommended pseudo-randomizer to 1) ensure a high probability of receiver acquisition and lock in the presence of data, 2) eliminate DC offset CCSDS 130.1-G-1 Page 8-3 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE problems in PM systems, 3) ensure sufficient bit transition density to maintain bit (or symbol) synchronization, and 4) to handle special coding implementations (i.e., data that is multiplexed into multiple convolutional encoders), then the recommended PseudoRandomizer may be omitted. The presence or absence of Pseudo-Randomization is fixed for a physical channel and is managed (i.e., its presence or absence is not signaled in the telemetry but must be known a priori) by the ground system. 8.3 8.3.1 CODEBLOCK SYNCHRONIZATION GENERAL Each of the recommended codes requires a method for aligning the sequence of received code symbols with the boundaries of its codeblocks (or code symbol periods in the case of convolutional codes). Otherwise, the decoder would fail because it would be applying the correct decoding algorithm to an incorrect subset of received code symbols. The synchronization requirements are different for each of the recommended codes, as described in the next four subsections. 8.3.2 SYNCHRONIZATION FOR CONVOLUTIONAL CODES For 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 the convolutional codeword is indefinitely long, the only requirement for proper synchronization is to correctly establish the identity of the starting symbol of any group of n symbols produced in one bit time. This procedure is commonly called ‘node synchronization’. For the recommended rate-1/2 non-punctured convolutional code, as well as the entire series of recommended punctured convolutional codes derived from the rat...
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