roader carrier bandwidth, there is still some fading in a GSM radio path. To combat this remaining Rayleigh fading, GSM can employ a form of frequency diversity by using slow frequency hopping. The combination of frequency hopping and bit interleaving spreads the errors over enough time slots and reduces the number of consecutive bit errors. Strings of five or more errors in a row cause the GSM error-correction facility to lose its ability to correct errors. There is a half-rate GSM in which a voice link uses only 1 time slot out of 16 instead of 1 in 8. Half-rate GSM may be twice as efficient, but it is not a commercial success. It does not sound nearly as good as full-rate GSM, and the processing equipment for its 6500-bps speech coding is larger and more expensive than the 13-kbps coding used in full-rate GSM. In GSM, the transmit and receive time slots are deliberately staggered so that each voice channel transmits on one time slot and receives on a different time slot. This al- lows a tremendous cost saving in mobile telephone design, since the time separation of transmit and receive modes means that only one radio circuit is needed. Not only can a GSM phone use a single radio circuit for transmit and receive, the phone still has six other time slots available to make measurements of other GSM channels in the handover determination. 2 7.4 GSM Signaling Each GSM cell has two signaling channels. There is a broadcast-control channel (BCCH) that the mobile telephone tunes to when it is not on a call. The cell broadcast- control channel (CBCH) is another signaling stream that sends 80-character short messages and shares the same time slot as the BCCH. 2The base station does not have extra slots available, since the BTS is serving multiple user ter- minals simultaneously, so it has to be able to transmit and receive all eight time slots for each eight-channel GSM carrier. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All ri