Traditional Turbo-codes with binary phase-shift keying modulation assign equal noise margins to the Turbo-encoder output bits. It is shown that by using unequal power allocation (UPA) among the encoder output streams, one can improve the code performance especially for large block lengths. On the other hand, there has been a growing interest in the application of Turbo-codes for signaling over a code-division multiple-access (CDMA) channel. A usual practice in CDMA systems for matching of rate is based on repeating the encoder output bits. This feature can be used to provide UPA with a negligible increase in the complexity. Simulation results are presented showing a noticeable improvement in the bit-error-rate performance.

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Abstract-In this correspondence, we evaluate the variance of the union performance bound for a rate-1 3 turbo code over all possible interleavers of length , under the assumption of a maximum-likelihood (ML) decoder. Theoretical and simulation results for turbo codes with two-memory component codes indicate that the coefficient of variation of the bound increases with the signal-to-noise ratio and decreases with the interleaver length. Theoretical analysis for large interleaver lengths shows that the coefficient of variation asymptotically approaches a constant value. The results also demonstrate that the majority of the interleavers have performance bounds very close to the average value of the bound. This phenomenon is more palpable for larger interleaver lengths.

Traditional turbo-codes with BPSK modulation scheme, use Equal Error Protection (EEP) for the turbo-encoder output bits. In this paper, it is shown that the role of the encoder output bits is not necessarily the same in determining the code performance. Imposing Unequal Error Protection (UEP) on the output bits can result in improvement of the turbo-code performance.