A modem design for single-channel (64-kb/s) point-to-point digital radio applications is presented and discussed. Emphasis is placed on a number of dedicated techniques and algorithms adopted to treat all the problems peculiar to single-channel spectrum-efficient transmission. A number of problems concerning Gaussian and impulse noise, both influencing a very-low-capacity radio system which adopts multilevel quadrature amplitude modulation (QAM) and coherent demodulation, are investigated. Viable solutions are found using complex baseband signal processing for modulation and demodulation. The extensive use of software-programmable digital signal processors (DSPs), already adopted on telephone channel modems, is demonstrated to be an optimum solution for radio applications also. Advantages in terms of design flexibility, compactness, and performance are obtained mainly when a very high bandwidth efficiency is required. Some preliminary experimental results are presented.
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