Abstract: This invention relates to a time division multiple access (TDMA) trunked radio system and it relates to a frame structure for communication between fixed, mobile and portable radios in such a system. The radio system comprises: a base station and a plurality of remote radio units for communicating over time slots of a time divided radio channel, the base station having means for transmitting a synchronization signal for defining the timing of the time slots, and each of the remote units having a power amplifier with linearization means, characterized in that the channel comprises predetermined linearization time slots (120, FIG. 2) for transmission of training signals, interspersed between other time slots and each remote radio unit comprises means for generating a training signal during a linearization time slot and linearizing its power amplifier during that time slot in advance of and independent of transmission of signals during one of said other time slots.