ital 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. Radio and MS-PCUSN Interfaces Radio and MS-PCUSN Interfaces differentiates a convolutional code from a block code. A convolutional code can be defined by three variables: n, k, and K. The value n corresponds to the number of bits at the output of the encoder, k to the number of bits at the input of the block, and K to the memory of the encoder. The ratio, r, of the code is defined as r = k / n. Let's consider a convolutional code with the following values: k is equal to 1, n to 2, and K to 5. This convolutional code uses a rate of r = 1/2 and a delay of K = 5, which means that it will add a redundant bit for each input bit. The convolutional code uses 5 consecutive bits in order to compute the redundancy bit. As the convolutional code is a half-rate convolutional code, a block of 488 bits is generated. These 488 bits are punctured in order to produce a block of 456 bits. The block of 456 bits produced by the convolutional code is then passed to the interleaver. Before applying the channel coding, the 260 bits of a GSM speech frame are divided in three different classes according to their function and importance. The most important class is the class la contain- ing 50 bits. The class Ib is next in importance, which contains 132 bits. The class II is the least important, which contains the remaining 78 bits. The different classes are coded differently. First of all, the class Ia bits are block coded. Three parity bits, used for error detection, are added to the 50 class Ia bits. The resultant 53 bits are added to the class Ib bits. Four zero bits are added to this block of 185 bits (50 + 3 + 132). A convolutional code, with r = 1/2 and K = 5, is then applied, obtaining an output block of 378 bits. The class II bits are added, without any protection, to the output block of the convolutional coder. An output block o