f 456 bits is finally obtained. An interleaver rearranges a group of bits in a particular way. It is used in combination with FEC codes in order to improve the performance of the error correction mechanisms. The interleaving decreases the possibility of losing whole bursts during the transmission, by dispersing the errors. Because the errors less concentrated, it is then easier to correct them. This is shown in Figure 6-32. A burst in GSM transmits 2 blocks of 57 data bits each. Therefore, the 456 bits corresponding to the output of the channel coder fit into four bursts (4 X 114 = 456). The 456 bits are divided into 8 blocks of 57 bits. The first block of 57 bits contains the bit numbers (0, 8, 16, 448), the second one contains the bit numbers (1, 9, 17, 449), and so on. The last block of 57 bits contains the bit numbers (7, 15, 455). The first 4 blocks of 57 bits are placed in the even-numbered bits of 4 bursts. The other 4 blocks of 57 bits are placed in the odd-numbered bits of the same 4 bursts. Therefore, the interleaving depth of the GSM interleaving for control channels is four and 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. Radio and MS-PCUSN Interfaces Chapter 6 Figure 6-32 11011011101011010 Interleaving spreads the errors. Read In Input Output Corruption 1001011011011110 Errors are Spread 1X110X11XX01X01 a new data block starts every four bursts. The interleaver for control chan- nels is called a block rectangular interleaver. Interleaving Speech Channels The block of 456 bits, obtained after the channel coding, is then divided in eight blocks of 57 bits in the same way as it is explained in the previous paragraph. However, these eight blocks of 57 bits are distributed differently. The first four blocks of 57 bits are placed in the even-numbered bits of four consecutive bursts. The other four blocks of 57 bits are placed in th