A modem design for single-channel (64-kb/s) point-to-point digitalradio applications is presented and discussed. Emphasis is placed on anumber of dedicated techniques and algorithms adopted to treat all theproblems peculiar to single-channel spectrum-efficient transmission. Anumber of problems concerning Gaussian and impulse noise, bothinfluencing a very-low-capacity radio system which adopts multilevelquadrature amplitude modulation (QAM) and coherent demodulation, areinvestigated. Viable solutions are found using complex baseband signalprocessing for modulation and demodulation. The extensive use ofsoftware-programmable digital signal processors (DSPs), already adoptedon telephone channel modems, is demonstrated to be an optimum solutionfor radio applications also. Advantages in terms of design flexibility,compactness, and performance are obtained mainly when a very highbandwidth efficiency is required. Some preliminary experimental resultsare presented
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