Asymptotic analysis of superorthogonal turbo codes

We examine a low-rate turbo coding scheme based on superorthogonal convolutional encoders (SOCEs). The low-rate coding is suitable for code-division multiple-access (CDMA) applications. We use the property that the component encoders are equivalent to conventional convolutional encoders to analyze the asymptotic performance. We analyze the iterative decoding performance that can be achieved when both the code length and the number of iterations tend to infinity and present a bound on the iterative limit of the code construction. It is shown by asymptotic analysis, that the rate 1/7,1/15, and 1/31 codes with component encoders of memory 3,4, and 5 have iterative limits below -0.65, -0.88, and -0.95 dB, respectively. Simulations for codes with large permutors (interleavers) confirm these asymptotic results. The construction is general and can be done for codes of lower rates as well.