130x1g2 - CCSDS

TM SYNCHRONIZATION AND CHANNEL CODING—SUMMARY OF CONCEPT AND RATIONALEPerformance with Non-Ideal Tracking Loops — Any decoder’s performance degradeswhen there are small errors in tracking and detecting the received symbols. However, withTurbo codes, there is also a possibility to improve the receiver’s tracking performance byfeeding back soft information from the decoding process to assist the receiver’s trackingloops. Preliminary assessments (see reference [20]) of potential improvements areencouraging.Residual Error Correction — In applications requiring extremely low error rates, the errorrate of a Turbo code in the error floor region may be unacceptable despite best efforts tolower it. The solution may be to add an outer code to work in conjunction with the Turbocode as the inner code. The outer code would ideally be a binary code such as a BCH coderather than a nonbinary Reed-Solomon code. Because of the sparseness of errors on the errorfloor (typically a handful of bit errors per block), the outer code could have a very high coderateand would shift the required E b /N 0 by just a tiny amount. However, an outer code willprovide very little benefit at signal-to-noise ratios below the error floor region, because inthis region there are frequently codewords for which the Turbo decoding algorithm fails toconverge and the resulting number of bit errors is beyond the error correction capability ofany reasonable outer code. Unfortunately, these errors due to non-convergence of thedecoding algorithm do not completely disappear in the error floor region, where they aresimilarly immune to being corrected by a reasonable outer code. Thus even in the error floorregion, an outer code is only effective in fixing the dominant error events, but the rarerevents in this region (due to non-convergence) may still exceed the desired error rate if theyare not rare enough.Detecting Turbo Decoding Errors with an Outer CRC Code — Turbo decoders (likeViterbi decoders) are complete decoders, in that they always produce a decoded sequence.Currently these decoders do not detect and mark unreliable sequences, though in principlethey could be modified to do so. Alternatively, a separate error detection code, such as acyclic redundancy check (CRC) code, can be concatenated as an outer code with an innerTurbo code, in order to flag unreliable decoded sequences. If is defined by the redundancyof the error detection code (CRC), the = 16 CRC code used for the CCSDS standard detectsevery possible error sequence e with the lowest weights |e| = 1, 2, or 3. An undetectedcodeword error occurs whenever the error pattern e of the sequence decoded by the Turbocode equals one of the nonzero codewords of the CRC code. The CRC/Turbo codecombination will produce a typical undetected error probability (in the case of a Turbodecoder error) of about 2 - = 2 -16 ≈1.5×10 -5 . This value must be multiplied by the probabilityof a codeword error to obtain the (unconditional) undetected error probability.Lowering the Turbo Code’s Error Floor — Even without using an outer BCH code, it hasbeen possible to design good Turbo codes that lower the error floor to possibly insignificantlevels (e.g., 10 –9 BER). Such performance may be sufficiently good for space applications toobviate the need for an outer error-correcting code. In that case, a simpler outer code (such asa CRC code) may still be desirable for error detection only.CCSDS 130.1-G-2 Page D-5 November 2012