So-called because it occupies part of the now redundant television band III, 174 to 225 MHz, and often referred to as private mobile radio, this is a subscriber access system. The network has almost nationwide coverage in the UK, arranged on both local and regional bases. The network permits radio communication between a fixed office station and mobiles, mobile to mobile, and limited access from mobiles to the PSTN.
The relevant Radiocommunications Agency specifications are:MPT 1323 Angle-modulated Radio Equipment for Use at Base and Mobile Stations Private Mobile Radio Service Operating in the Frequency Band 174–225 MHz.
MPT 1327 A Signalling Standard for Trunked Private Land Mobile Radio Systems.
MPT 1343 System Interface Specification for Radio Units to be Used with Commercial Trunked Networks Operating in Band III sub-band 2.
MPT 1347 Radio Interference Specification for Commercial Trunked Networks Operating in Band III sub-band 2.
MPT 1352 Test Schedule. For Approval of Radio Units to be Used with Commercial Trunked Networks Operating in Band III sub-band 2.
A new European standard for digital trunking. The system is aimed at PMR users and will employ TDMA with four users sharing a 25 kHz radio channel with the option of two users on a 12.5 kHz channel. Speech will be digitally encoded at 4.8 kbit/s and transmitted at 36 kbit/s (gross bit rate including error correction overhead) using π/4 differential quaternary phase shift keying (DQPSK).
This utilizes trunking techniques but with no infra-structure of control or switching centres; mobiles work to mobiles via a locally installed repeater. It offers great potential for short-term hire for local events. The band 933 to 935 MHz has been allocated to the service and accommodates 76 traffic and 2 control channels separated by 25 kHz. The operating procedure may be either single-frequency simplex or twofrequency semi-duplex. Mobiles remain on standby waiting to receive their selective signalling code (SSC) on one of the two control channels; mobiles with even serial numbers normally listen on control channel 26 and those with odd serial numbers on control channel 27. A mobile wishing to initiate a call first scans for a free traffic channel and, having obtained one, selectively addresses the intended recipients of the call over the relevant control channel. The recipient mobiles are then instructed automatically by the system to go to the appropriate traffic channel.
The modulation is GMSK at 4 kbit/s for addresses and 16 kbit/s for speech and data messages. The maximum transmitter erp is 4 W.