A novel technique which combines a speech waveform encodingprocess with multi-level modulation to realise a robust, spectrallyefficient land mobile radio channel is described. The speech codingmethod is based upon a variant of adaptive bit allocation sub-bandcoding in which samples of the most energetic of eleven sub-bands aretransmitted as discrete, multi-level symbols, together with some binaryand additional multi-level symbols representing the overheads needed forreconstruction. Using this method, excellent audio quality has beenachieved at symbol rates lower than 3 kS/sec under both simulated andreal land mobile channel conditions. Description of how the method wasused to provide a 4:1 spectrum efficiency over a 25 kHz spaced radiochannel using TDMA in conjunction with multi-level (constant envelope)FSK, or more than 10:1 using linear modulation, are presented. At 4:1with FM, the speech quality under strong signal conditions is verysimilar to standard FM and the coverage about 3 dB better for the samechannel power
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