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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 pseudorandom sequence. After locating the ASM in the
received data stream, the pseudorandom sequence is exclusiveORed with the data bits CCSDS 130.1G1 Page 82 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE immediately following the ASM. The pseudorandom sequence is applied by exclusiveORing
the first bit following the ASM with the first bit of the pseudorandom sequence, followed by
the second bit of the data stream with the second bit of the pseudorandom sequence, and so on.
The pseudorandom 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 allones state at the start of each Codeblock or Transfer Frame.
The first 40 bits of the pseudorandom sequence from the generator are shown below; the
leftmost bit is the first bit of the sequence to be exclusiveORed with the first bit of the
Codeblock or Transfer Frame; the second bit of the sequence is exclusiveORed 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 pseudorandomizer. 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 nonzero 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
unrandomized data so that during testing, they can read the binary data that they are familiar
with. One answer is to implement the recommended pseudorandomizer but make it
switchable so that during early testing it can be turned off.
While the recommended pseudorandomizer 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 pseudorandomizer 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 (NRZL 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 pseudorandomizer to 1) ensure a high
probability of receiver acquisition and lock in the presence of data, 2) eliminate DC offset CCSDS 130.1G1 Page 83 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 PseudoRandomization 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 rate1/2 nonpunctured convolutional code, as well as the entire series of
recommended punctured convolutional codes derived from the rat...
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