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Unformatted text preview: dictors as guides, it was possible to design the turbo codes in the Recommended Standard (reference [3]) so as to lower the error floor to possibly insignificant levels (e.g., as low as 10-9 bit error rate). CUTOFF RATE THRESHOLD LOW SNR REGION 10 HIGH SNR REGION 0 k=BLOCK SIZE SIMULATION 10 -1 K=3, k=1000 RATE= 1/3 CODE 10 -2 -5 10 -6 10 -7 K=5, k=4096 RATE=1/4 CODE RATE 1/3 10 ANALYTICAL UPPER BOUND CAPACITY 10 -4 RATE 1/4 BER 10 -3 LOW INPUT WEIGHTS, ERROR FLOOR 10 -8 10 -9 ALL INPUT WEIGTHS -1 0 1 2 3 4 Eb/No (dB) Figure 7-14: Illustration of Turbo Code Error Floor CCSDS 130.1-G-1 Page 7-14 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE 8 IMPORTANT ANCILLARY ASPECTS OF THE CODING SYSTEM 8.1 GENERAL The preceding four sections have described how one would encode and decode each of the recommended codes, and their corresponding performance, under ideal circumstances. CCSDS Recommended Standards (references [3] and [2]) also impose certain ancillary conditions on the coding system in order to approach this ideal performance in a practical system. Chief among these ancillary requirements addressed in Recommended Standards (references [3] or [2]) are the following: a) the coded output of all codes (or of uncoded data) must be sufficiently random to ensure proper receiver operation; b) there must be a method for synchronizing the received data with the codeblock 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 affect performance. 8.2 RANDOMIZATION OF THE CODED OUTPUT 8.2.1 GENERAL Randomization of the data stream provides three useful functions. It aids in achieving: – signal acquisition; – bit synchronization; – ambiguity resolution for convolutional decoder operation. 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 nonpunctured (7,1/2) convolutional code contains an inverter on one of its outputs, which assures a sufficient symbol transition density when this code is used with BPSK modulation. Although this inverter may be sufficient for proper operation of the bit synchronizer, it does not guarantee that the receiver and decoder will work correctly. In contrast, when the CCSDS 130.1-G-1 Page 8-1 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE recommended Reed-Solomon code is used alone, or the data is uncoded, there may be no symbol transitions, e.g., if all-zero data is sent. While alternate symbol inversions solve the symbol synchronization problem for the case of convolutional codes with BPSK modulation, it is desirable to offer a universal solution for all three issues and any of the recommended codes. The pseudo-randomizer defined in section 7 of reference [3] gives such a solution. This randomizer adds (modulo-2) a pseudo-random sequence to the coded symbols. The result is a maximally random sequence of 0s and 1s regardless of the transition density characteristic of the particular code’s output. The pseudorandomizer is likely to solve the issues that can arise from non-random data for all combinations of CCSDS-recommended modulation and coding. 8.2.2 DESCRIPTION OF THE RECOMMENDED PSEUDO-RANDOMIZER The method for ensuring sufficient transitions is to exclusive-OR each bit of the Codeblock or Transfer Frame with a standard pseudo-random sequence. If the Pseudo-Randomizer is used, on the sending end it is applied to the Codeblock or Transfer Frame after turbo encoding or RS encoding (if either is used), but before convolutional encoding (if used). On the receiving end, it is applied to derandomize the data after convolutional decoding (if used) and codeblock synchronization but before Reed-Solomon decoding or turbo decoding (if either is used). The configuration at the sending end is shown in figure 8-1. ATTACHED SYNC MARKER TRANSFER FRAME, R-S CODEBLOCK, OR TURBO CODEBLOCK PSEUDO-RANDOM SEQUENCE GENERATOR Randomized output to modulator or convolutional encoder (if used) Figure 8-1: Block Diagram of the Recommended Pseudo-Randomizer The Attached Sync Marker (ASM) is already optimally configured for synchronization purposes and it is therefore used for synchronizing the Pseudo-Randomizer. The pseudorandom sequence is applied starting with the first bit of the Codeblock or Transfer Frame. On the sending end, the Codeblock or Transfer Frame is randomized by exclusive-ORing the first bit of the Codeblock or Transfer Frame with the first bit of the pseudo-random sequence, followed by the second bit of the Codeblock or Transfer Frame with the second bit of the pseudo-random sequen...
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This document was uploaded on 03/06/2014.

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