A spectrum efficient speech processing method for land mobile radio systems

A method employing a novel combination of speech coding and channel modulation to provide improved spectrum efficiency for land mobile radio systems is described. The method employs a variant of the adaptive subband coding method and multilevel modulation which permits up to 4 TDM (time-division-multiplexed) voice channels to be carried by a single 25 kHz FM channel and more than 10 voice channels if an efficient linear modulation is employed. Details of the implementation show that the complexity required is modest. Comparison with current analog FM radio in terms of practically achievable performance in a mobile has shown that while the induced audible defects due to channel errors are quite different the overall performance is very comparable.<
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