Improving the speech quality of cellular mobile systems under heavy fading

Digital cellular systems experience heavy fading for short intervals during which the encoded bit stream may be severely corrupted. Requirements exist to detect the corrupted data slots and replace the corresponding information in the decoded speech stream by predicted speech based on previously corrected received data. The paper reports on error detection and speech regeneration techniques which achieve considerable quality improvements over those proposed for North American cellular use in IS-54, yet are compatible with it. Error detection is enhanced by setting a maximum likelihood threshold for Viterbi decoding and rejecting frames where the decoded sequence exceeds the threshold. Rejected frames are regenerated using the most recent correctly received speech coding parameters