130x1g2 - CCSDS

TM SYNCHRONIZATION AND CHANNEL CODING—SUMMARY OF CONCEPT AND RATIONALEpunctured (7,1/2) convolutional code contains an inverter on one of its outputs, whichassures 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 doesnot guarantee that the receiver and decoder will work correctly. In contrast, when therecommended Reed-Solomon code is used alone, or the data is uncoded, there may be nosymbol transitions, e.g., if all-‘zero’ data is sent.While alternate symbol inversions solve the symbol synchronization problem for the case ofconvolutional codes with BPSK modulation, it is desirable to offer a universal solution for allthree issues and any of the recommended codes. The pseudo-randomizer defined in section 7of reference [3] gives such a solution. This randomizer adds (modulo-2) a pseudo-randomsequence to the coded symbols.9.2.2 RANDOMIZATION AND DE-RANDOMIZATIONA PN data sequence used for randomization is commonly written in terms of binary values,‘1’ and ‘0’. It can also be written using the symbols ‘1’ and ‘-1’ or other symbols.Randomization of data (different from scrambling) is done by applying the first bit of therandomization pattern to the first bit of the data, applying the second bit of the randomizationpattern to the second bit of the data, and so on. Generally, when the bit in the randomizationpattern is a ‘0’, the data is left unchanged and when the bit of the randomization pattern is a‘1’, the data value is inverted.By ‘inverted’ is meant that, if the original data bit is a ‘0’, it is changed to a ‘1’ and, if it isoriginally a ‘1’, it is changed to a ‘0’. This is the same as applying an exclusive-OR to the bitpairs. At the receiver, when a soft decision representation is used for the data, inversion isstill a valid concept, but the representation of the data must be taken into account. Exclusive-OR no longer applies.For example, with 3-bit soft decision, data and its inverse is represented as:Binary Binary Sign-MagSign-Mag2’scomp2’scompnormal inverse normal inverse normal inverseStrong 1 111 000 011 111 011 100110 001 010 110 010 101101 010 001 101 001 110Weak 1 100 011 000 100 000 111Weak 0 011 100 100 000 111 000010 101 101 001 110 001001 110 110 010 101 010Strong 0 000 111 111 011 100 011CCSDS 130.1-G-2 Page 9-2 November 2012