On the performance of turbo codes

The performance of the turbo code is sensitive to its code structure, which is made up of code rate, constraint length, tap connection, block size, interleaving pattern and number of decoding iterations. In this paper, mitigation techniques that can lower the error floor are adopted in both encoder and decoder. Optimum tap connections are listed for codes with constraint lengths 3, 4, and 5. Performance variations achieved by changing different parameters are investigated. Simulation results show that (1) using 10 decoding iterations is adequate; (2) the S-random interleaving provides the best performance among the interleavers investigated; (3) using codes with constraint lengths greater than 3 does not buy any additional coding gain for short block size (in the vicinity of 100); (4) for block size greater than 500, no significant improvement is noticed by increasing the constraint length from 4 to 5 in the range of bit error rate (BER) <10^-6; and (5) the punctured rate 1/2 code structure degrades the performance by only 0.5 to 0.7 dB in comparison with the classic rate 1/3 turbo code for constraint lengths 3, 4, and 5