channel (FACCH) Speech Coding on the Radio Link The transmission of speech is, at the moment, the most important service of a mobile cellular system. The GSM speech codec (coder and decoder), which will transform the analog signal (voice) into a digital representation, has to meet the following criteria: It must have good speech quality, at least as good as the quality obtained with previous cellular systems. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction to GSM Introduction to GSM Reduce the redundancy in the sounds of the voice. This reduction is essential due to the limited capacity of transmission of a radio channel. The speech codec must not be very complex because complexity is equivalent to high costs. The final choice for the GSM speech codec is a codec named Regular Pulse Excitation Long-Term Prediction (RPE-LTP). This codec uses the information from previous samples (this information does not change very quickly) in order to predict the current sample. The speech signal is divided into blocks of 20 ms; these blocks are then passed to the speech codec, which has a rate of 13 Kbps, in order to obtain blocks of 260 bits. Channel Coding Channel coding adds redundancy bits to the original information in order to detect and correct, if possible, errors that occurred during the transmission. The channel coding is performed using two codes: a block code and a con- volutional code. The block code corresponds to the block code defined in the GSM Recommendations 05.03. The block code receives an input block of 240 bits and adds four zero tail bits at the end of the input block. The output of the block code is consequently a block of 244 bits. A convolutional code adds redundancy bits in order to protect the information. A convolutional encoder contains memory. This property differentiates a convolutional code from a block code. 