Speech compression and error correction for mobile communication

Abstract

The speech coded [25] data is forward error protected by using adaptive channel coding technique. The main objective of the adaptive channel [16, 17] coder is to select the channel encoder according to the noise level of the channel. In this thesis we propose a joint coding algorithm using which we can increase the capacity of the wireless system and improve the S/N ratio of the signal received at the receiver. The adaptive channel coder is used for error detection newlineand correction and further it also protects the data. The type of the channel encoding scheme newlinemainly depends on the signal to noise ratio of the channel to be used, signal to noise ratio at the newlineinput of the receiver and the data rate to be transmitted. In practical systems the maximal signal power and the bandwidth of the channel are restricted and noise power spectral density is also fixed for a particular operating environment. The only practical alternative to reduce the probability of the error is to use the channel coding technique [36]. The Channel coding [40] is a viable method to reduce information rate through the channel and increase reliability. This is to improve the capacity of a channel by adding carefully designed redundant information to the data being transmitted through the channel. In the present work adaptive channel coders used. The adaptive channel coding technique designed in the present work will provide good speech quality over a wide range of channel conditions. The two types of the channel coder used in adaptive channel coder are Cyclic coder [39] and Convolutional coder [43]. The channel coding techniquechosen mainly depends on the S/N of the channel through which we want to transmit the data. Inthis adaptive algorithm the first step is to check for the noise level in the channel. Depending on the noise level of the channel the algorithm designed automatically selects one of the channel coding techniques. The adaptation of the source coding rate and the channel coding rate can occur within a speech frame.