o ramp up and down. The data structure has a length of 255 symbols or 510 modulation bits. It consists of a start sequence that is followed by 216 bits of scrambled data, a sequence of 52 bits of what is termed a training sequence. A further 216 bits of scrambled data follow and then the stream is completed by a stop sequence. The training sequence in the middle of the data is required to allow the receiver to adjust its equalizer for optimum reception of the whole message. 1 multiframe 1.02 S 1812345 = 18 TDMA frames 1 TDMA frame 56.667 ms 4 time slots 1 timeslot 14.167 ms = 510 modulation 216 bits 216 bits bit durations Start 1 modulation bit duration Training 27.78 uS Figure 9.3 TETRA data structure 246 Newnes Guide to Radio and Communications Technology The data is modulated onto the carrier using differential quaternary phase shift keying. This modulation method shifts the phase of the RF carrier in steps of /4 or +3/4 depending upon the data to be transmitted. Once generated the RF signal is filtered to remove any sidebands that extend out beyond the allotted bandwidth. These are generated by the sharp transitions in the digital data. A form of filter with a root raised cosine response and a roll-off factor of 0.35 is used. Similarly the incoming signal is filtered in the same way to aid recovery of the data. Beyond this TETRA uses error tolerant modulation and encoding formats. The data is prepared with redundant information that can be used to provide error detection and correction. The transmitter of each mobile station is only active during the timeslot that the system assigns it to use. As a result the data is transmitted in bursts. The fact that the transmitter is only active for part of the time has the advantage that the drain on the battery of the mobile station is not as great as if the transmitter were radiating a signal continuously. The base station, however, normally radiates continuously as it has many mobile stations to service. One important feature of TETRA is that the call set-up time is 