An economic method of providing mobile communication for users with a small quantity of mobiles and light traffic is a common base station or community repeater system. Although still referred to as private mobile radio, the base station is shared by several users who pay a fixed subscription for the service irrespective of airtime used. The station is controlled by radio using tone-controlled talk-through or what is sometimes called reverse frequency trigger (not normally permitted on a true PMR system). Figure 15.4 shows the layout of a single-station CBS system. Each participant’s office contains a fixed transmitter/receiver operating on the same frequencies as a mobile and equipped with CTCSS. Different CTCSS tones are assigned to each user to ensure privacy. All office stations must use a directional antenna so that they access only the base station to which they subscribe.
When a user transmits, from either the office station or a mobile, the base station, on receipt of a signal containing a valid CTCSS tone,Base transmitter and receiver
Despatcher’s VHF transmitter/ receiver
User A User B Figure 15.4 Common base station systementers a talk-through mode. The caller’s CTCSS tone is retransmitted and, in turn, opens the mute on all that user’s mobiles allowing communication via talk-through. Mobiles are equipped with ‘busy’ lamps to indicate whenever the system is engaged, and transmission time-out timers to prevent excessively long calls excluding other users.