This paper presents the results of tests of 20-Mb/s frequency-shift-keying digital transmission on the 4-GHz TD radio system. On the basis of the test results, the performance of the 20-Mb/s system is projected to be satisfactory to support long-haul digital services. The system employs a 20-Mb/s terminal that multiplexes 12 signals at the first level of digital signal hierarchy (DS-1) (1.5 Mb/s) into a 10-Mbaud, 4-level signal to be transmitted by the Bell System standard 4-GHz (TD) FM microwave radio system. The maximum distance of a digital regeneration span in normal operation is limited by the intermodulation noise to approximately 10 typical hops of TD radio. The 20-Mb/s TD radio system uses the standard frequency-diversity protection switching system, which was designed for analog message service. A fundamental system trade-off is, therefore, the choice of switch threshold: long periods of error-free transmission interrupted by infrequent error bursts due to switch transients versus an occasional low background error rate with less frequent switch transients. We concluded that the protection switch threshold of a 1500-message-circuit channel is suitable for 20-Mb/s TD radio channels.
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