The key-bits in turbo codes and their protections

The sensitivities of the bits of low weight codewords in turbo codes to noises are discussed firstly and the result is that the lower weight the higher sensitivity. Theoretical analysis shows that through the protections on such key-bits which have high sensitivities; the minimal weight of the codes can be improved apparently. The simulations also give out the supported results: The BER could be highly improved at high Eb/N0 for code rates both 1/3 and 1/2.