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Application of a Concatenated Coding System with Convolutional Codes and Reed-Solomon Codes for MPEG Advanced Audio Coding
Reliable delivery of audio bitstream is vital to ensure the acceptable audio quality perceived by 3G network customers. In other words, an audio coding scheme that is employed must be fairly robust over the error prone channels. Various error resilience techniques can be utilized for the purpose. Due to the fact that some parts of the audio bitstream are less sensitive to transmission errors than others, the Unequal Error Protection (UEP) is used to reduce the redundancy introduced by error resilience requirements. The current UEP scheme with convolutional codes and multi-stage interleaving has an unfortunate tendency to generate burst errors at the decoder output as the noise level is increased. A concatenated system combining Reed-Solomon codes with convolutional codes in UEP scheme is investigated for MPEG Advanced Audio Coding (AAC). Under severe channel conditions with random bit error rates of up to 5x10-02, the proposed scheme achieved more than 50% improvement in residual bit error rate over the original scheme at a bitrate of 64kb/s and sampling frequency of 48 kHz. Under burst error conditions with burst error length of up to 4 ms, the proposed scheme achieved more than 65% improvement in bit error rate over the original scheme. The average percentage overhead incurred by using the concatenated system is about 3.5% of the original UEP scheme. Further improvements are made by decreasing the puncturing rate of convolutional codes. However, this can only be adopted when high protection is needed in extremely noisy conditions (e.g., channel BER significantly exceeds 1.00e-02) since it incurs increased overheads.
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