AbstractThe interest into PMR trunking systems is due to the increase of spectral efficiency that can be obtained sharing frequencies between more users. The British DTI protocol specification MPT 1327, intended to be used on this kind of systems, has been recently released, and it is now being accepted as a standard on some european countries.This protocol is based on the MPT 1317 signaling format specification, designed to work at frequencies of 50–500 MHz, and for this reason, it presents some problems when working with RF frequencies of 900 MHz and moving vehicles.The problem comes out from the low bit rate used (1200 bit/s, and the block length (64 bit). If we consider a vehicle that is running at a speed of 100 km/h, we will have a mean of two fades into each information block, while the code (BCH 63, 48) has only a good capacity of error detection/correction for single error bursts.The paper, proposes a solution to this problem, that consists on raising the bit rate to 2400 bit/s, using indirect PSK modulation, with coherect demodulation, instead of FFSK with incoherent demodulation, as it is usual. With this method, we obtain the same BER. Added to this bit rate increase, that allows us to lower the mean number of fades into each block to one, we apply time diversity [2], sending two times each block into the forward control channel, and sending also two times each return control channel or traffic channel message. This diversity system can be implemented bitwise, using channel information to select the most likely value of each bit [4], or block to block, using the error detection capabilities of the code [6].The system has been simulated for 900 MHz and 100 km/h, and measured into a laboratory test setup, using a fading simulator.The results show that proposed method gives the same quality working at 900 MHz that the classic approach working at 150 MHz, showing an improvement of 6 dB over the standard approach working at 900 MH