Background Turbo Codes Turbo Encoder Turbo Decoder

S-72.3410 **Turbo** **Codes** 5✬✩S-72.3410 **Turbo** **Codes** 7✬✩Log Likelihood RatioIterative DecodingThe estimates are usually communicated from one decoder to theother in the form of log likelihood ratio (LLR).Instead of elements from {0, 1}, it is often more convenient toconsider elements from {−1, 1}. The LLR is defined asL(b i ) = ln( ) P(bi = +1).P(b i = −1)The sign of the LLR can be used as the hard decision of the estimate;the absolute value gives the reliability of the estimate.A scheme for iterative decoding of turbo codes is shown in[MF, Fig. 7.11]. LLR estimates in an example case([MF, Example 7.3]) are presented in [MF. Tables 7.8–7.14].A priori information: Information provided by the other encoder.(In the beginning P(b i = +1) = P(b i = −1) = 0.5 so the a priori LLRis 0 for the first encoder in the first iteration.)Extrinsic information: The component of the generated reliabilityvalue that depends on redundant information introduced by theconsidered constituent code.✫✪✫✪c○ Patric Östergårdc○ Patric ÖstergårdS-72.3410 **Turbo** **Codes** 6✬✩S-72.3410 **Turbo** **Codes** 8✬✩BCJR AlgorithmInterleavers for **Turbo** **Codes**The BCJR algorithm (Bahl-Cocke-Jelinek-Raviv) determines anestimate for a given sequence element by observing the outputsequence of a discrete memoryless channel that corresponds to agiven input sequence.The BCJR algorithm is the core part of iterative decoding of turbocodes. The BCJR and other related algorithm are considered indetail in the course S-72.3280 Advanced Radio Transmission Methods(from 2008: S-72.3281 Advanced Transmission Methods).Interleavers, which permute positions in a stream of symbols, areuseful for several purposes in coding, for example, to• combat burst errors by distributing adjacent symbols (andthereby errors) among many words;• obtain statistically independent sequences of symbols forturbo encoders.Major types are block, convolutional, random, and linear interleavers.✫✪✫✪c○ Patric Östergårdc○ Patric Östergård