Quality enhancement of coded and corrupted speeches in GSM mobile systems using residual redundancy

In GSM system, speech is coded in frames. Here the redundancy is characterized mainly by the statistical correlation among certain temporally neighboring coefficients or bits. In our investigations the redundancy among bits is first utilized to realize a source-controlled channel decoding [2], so that the bit error rate can be further reduced. Then based on the redundancy among coefficients an improved criterion for frame error detection is developed for GSM full rate speech transmission. As a result, both subjective and objective performance gains can be obtained, especially in a noisy transmission environment. There is often residual redundancy remaining in coded speech data, even if a powerful speech codec (e.g. the full rate coder used in GSM mobile communications) is employed. By using such redundancy together with the information provided by the channel decoder, such as soft output (L-value), the number of channel bits inverted by the decoder, or a cyclic redundancy check, the bit error rate can be further reduced and a more graceful degradation of speech quality can be achieved, especially under bad channel conditions. In this paper, we report on the study with regard to this aspect for GSM full rate speech transmission and error concealment. The algorithms developed can be easily implemented with a currently available DSP designed for GSM mobile phones. 2. GSM FULL RATE SPEECH AND CHANNEL CODING The GSM full rate speech and channel coding scheme are briefly explained here. Stress is placed on what is relevant for this work. The GSM speech coding is done in 3 steps (see Figure 1).