Data transmission performance of 18 kbps non-coherent GMSK in the land mobile environment

Data is presented showing the BER performance of 18 kbps GMSK using non-coherent demodulation. Multilevel decision data detection is used for data recovery in the radio designed for use at 851-870 MHz with a 25 kHz channel separation. A 0.2 Gaussian FIR and a 7-pole analog premodulation filter are compared for frequency response, transmit eye pattern, and modulated spectrum. Performance is compared for two receivers with different IF selectivities provided by crystal filters having 8-poles (10.7 kHz bandwidth) and 10-poles (13.6 kHz bandwidth). Received eye patterns are shown to compare distortion from the receiver bandpass filters. Static BER data is compared along with sensitivity degradation from frequency offset. Receiver susceptibility to adjacent-channel interference is also presented.