This paper addresses a novel scheme for variable rate error correction coding with interleavered puncturing serially concatenated convolutional code. In order to obtain a variable coding rate, the bits of the outer coder are perforated with a given puncturing pattern, and randomly interleaved. The effect of interleavered puncturing on the overall coding performance is analyzed, and the upper bound to the bit error probability of the proposed coder is derived. Moreover, to evaluate the effectiveness of the proposed scheme some simulation results are presented with the iterative decoding procedure, in which the channel models of Rayleigh fading and additive white Gaussian noises are assumed.

In 1995, 8 kb/s CS-ACELP coder of G.729 is standardized by ITU-T SG15 and it has been reported that the speech quality of G.729 is better than or equal to that of 32 kb/s ADPCM (G.726). However G.729 is the fixed rate speech coder, and it does not consider the property of voice activity in mutual conversation. If we use the voice activity, we can reduce the average bit rate in half without any degradations of the speech quality. In this paper, we propose an efficient variable rate algorithm for G.729. The variable rate algorithm consists of two main subjects, the rate determination algorithm and the design of sub rate coders. For the robust VAD algorithm, we combine the energy-thresholding method, the phonetic segmentation method by integration of various feature parameters obtained through the analysis procedure, and the variable hangover period method. Through the analysis of noise features, the 1 kb/s sub rate coder is designed for coding the background noise signal. Also, we design the 4 kb/s sub rate coder for the unvoiced parts. The performance of the variable rate algorithm is evaluated by the comparison of speech quality and average bit rate with G.729. Subjective quality test is also done by MOS test. Conclusively, it is verified that the proposed variable rate CS-ACELP coder produces the same speech quality as G.729, at the average bit rate of 4.4 kb/s.