Link-Level Evaluations of Low Rate Code Extensions of Turbo Codes Using Shortened Code Techniques

In this paper we consider methods of applying shortened code techniques to turbo codes (Berrou, 1993) in order to construct codes with a lower code rate than the original turbo codes. As long as the difference of the code rates is not so large, the methods can provide some coding gain compared with bit repetition schemes. For turbo codes, we can consider two types of shortened codes that we term the "outer method" and "inner method." For the UTRA turbo code, which has a code rate of 1/3, simulations show that the performance gain is about 0.2 dB for the outer method and about 0.4 dB for the inner method, and the performance is degraded at most by about 0.1 dB even compared with a dedicated turbo code with a rate of 1/5 for rates of between 1/9 and 1/7. The complexities of the decoder for the methods are also considered. The complexities slightly increase compared with the bit repetition schemes based on the same code formats but are similar to the dedicated code.