THE PERFORMANCE OF RATE-COMPATIBLE PUNCTURED CONVOLUTIONAL-CODES FOR DIGITAL MOBILE RADIO

In many source coding schemes, the effect of a channel error varies significantly from one bit to another. Hence, to make the best available use of the limited channel coding redundancy, unequal error protection is needed. In this paper, we study the unequal error protection capabilities of convolutional codes belonging to the family of rate-compatible punctured convolutional codes (RCPC codes). The performance of these codes is analyzed and simulated for the fast fading Rice and Rayleigh channels with differentially coherent four-phase modulation (4-DPSK). To mitigate the the effect of fading, interleavers are designed for these unequal error protection codes, with the interleaving performed over one or two blocks of 256 channel bits. These codes are decoded by means of the Viterbi algorithm using both soft symbol decisions as well as channel state information. For reference, we have also presented the performance of these codes on a Gaussian channel with coherent binary phase-shift keying (2-CPSK). A number of examples are provided to show that it is possible to accommodate widely different error protection levels within short information blocks. Unequal error protection codes for a subband speech coder are studied in detail. The paper contains a detailed study of the effect of the code and channel parameters such as the encoder memory, the code rate, interleaver depth, fading bandwidth, and the contrasting performance of hard and soft decisions on the received symbols. The study clearly highlights the need for soft decisions and channel state information to extract the maximum benefit from Viterbi decoding on a Rayleigh fading channel. We also give a simple closed-form expression for the bit error probability for 4-DPSK in the presence of correlated Rayleigh fading. © 1990 IEEE