Narrowband digital FM modem with burst capability for PMR applications with asynchronous blockwise control channel monitoring

The author describes a narrowband quaternary FM modem employing a Nyquist baseband pulse for PMR application. The PMR network requires 8 kbit/s data rate in a 12.5 kHz channel spacing and 60 dB adjacent channel attenuation. Novel solutions are presented to enable a reliable detection of short blocks, and hence to achieve the burst capability. A simple and robust open-loop structure is derived for the blockwise estimation of the DC offset and the signal amplitude. A block equalizer is proposed to eliminate the irreducible error rate caused by the acquisition behavior of the decision feedback equalizer, which is necessary to resolve the intersymbol interference due to the FM high pass characteristic. Simulation results are presented which show that the modem with the blockwise detection achieves almost the same bit error performance as in the steady-state