lity 47.4.2 TACS voice circuit with the capability to maintain performance in a high capacity (poor interference ratio) configuration. TACS stands for Total Access Communications System, and was Signalling between mobile and base station is at 10kbit/s, with adapted from the AMPS standard by the UK when cellular radio was Manchester encoding applied taking the bit rate to 20kbit/s. The licensed for operation from 1985. The adaptation was necessary to data is modulated onto the radio carrier by direct frequency shift suit European frequency allocations which were at 900MHz, with keying (FSK). Error control is achieved by multiple repetition (5 or 25kHz channel spacing. This meant a reduction in frequency devi- 11 times) of each signalling word, with majority voting applied at ation and signalling speed was necessary (BS, 1990). the receiver to correct errors. A BCH block code is also applied to The signalling scheme of AMPS was retained largely unchanged, detect any uncorrected errors. but some enhancements were introduced, particularly in the proce- Whilst a call is in progress, the base station transmits a low level dures for location registration, to make the standard more suitable supervisory audio tone (SAT) in the region of 6kHz. Three different for deployment in systems offering contiguous nationwide cover- SAT frequencies are used by the network, and are allocated to the age. The opportunity was also taken to introduce extra features, such base stations SO that the nearest co-channel base stations (i.e. those as signalling of charge rate information (e.g. for payphones). most likely to cause interference) have a different SAT from the TACS was originally specified to use the full 1000 channels (2) wanted base station. The mobile continuously monitors the received 25MHz) allocated to mobile services in Europe. However in the SAT and also transponds the signal back to the base station. If the UK, only 600 channels (2 X 15MHz) were released by the licensing mobile (or the base station) detects a difference be