 49.3.2 Frame multiplex structure coverage techniques employed by cellular mobile systems. How- ever, in a building the cells will be used on each floor consequently When a call is being initiated there is no synchronisation between the cell structure has three dimensions to it and allowance must be the handset and base, and both parts will be scanning across the forty made for signals passing through the floors as well horizontally radio channels. In order for a call to be set up and maintained through the walls of the building. CT2/CAI employs three multiplex structures during the course of a To ensure good and even coverage both antennae and radiating call. cables may be used to provide service in a large building complex Multiplex 1 is used during the normal handset to base communi- as in Figures 49.2 and 49.3 (Holmes and Swain, 1990). cation phase when synchronisation and call set up have been A typical example for CT2/CAI coverage from an antenna is achieved and the call is in progress. The structure is shown in Figure shown in Figure 49.4 (Swain, 1984) where measured signal levels 49.5 (Evans, 1990) where it can be seen that two channels are and computed bit error ratios are indicated. supported. Line unit Radiating cable Coverage zone Handset Figure 49.3 Wireless PBX system using radiating cable 49/6 Cordless telecommunications 19 2 Mean signal strength dBuV emf 3 Mean BER to the nearest power of 10 10 20 30 40 signal strength dBuV emf source power 1mW Scale building construction steel partitioning (111) coverage area Figure 49.4 Typical example for CT2/CIA coverage with an antenna The B-channel conveys the 32kbit/s speech signal (or perhaps If the handset needs to set up a call then a different frame structure, 32kbit/s data) and the base and handset identities are conveyed multiplex 3, is initially used by the handset in order to synchronise through the D-channel. While D-channel errors are detected and with the base unit. This is important because in multiplex 1 and 2 corrected by requesting a re-trans