Digital modems for land mobile radio

The paper surveys the digital modulation methods available for use in land mobile radio systems. The modulation methods are compared on the basis of their relative tolerances to additive white Gaussian noise, and brief consideration is also given to the equipment complexities involved. The emphasis of the paper is on basic techniques and the essential mechanisms by which they operate, with a view to clarifying the factors that contribute to a good tolerance to noise. Prominence is given to constant-envelope signals, such as FSK, PSK and the various developments of these. Consideration is also given to the introduction of correlation or dependence between neighbouring signal elements, by means of partial-response or correlative-coding techniques, or else by means of convolutional codes, these being effective methods of improving the tolerance to additive noise. Coherent, incoherent and noncoherent detectors are discussed, together with threshold-level and maximum-likelihood detection. However, the topics of carrier-phase and element-timing synchronisation are not considered here, nor are anydetails given of the detection processes. Finally, a selection is made of the techniques that are potentially the most cost-effective for land mobile radio systems.