Equipment Configuration and Performance Criteria for Fully Optimized Tropospheric-Scatter Systems

The "optimum" system is examined with the aid of a basic equation related to channel SNR. Expressions are then developed which show that for equal channel loading and capacity, FM systems are likely to be superior to SSB systems for normal voice channel operation. Further improvement in FM performance may be achieved with "phase-locked" receivers. Performance equations for such receivers are used in basic expressions for optimized scatter circuits to express channel capacity as a function of maximum distance for minimum power. From this equipment configurations for various performance capabilities are developed. These data then give the maximum capability of the tropospheric scatter system when all relevant design parameters are optimized. It is suggested that long-haul toll performance criteria should be based on six tandem links. This leads to a per-link requirement for channel signal-to-weighted thermal noise ratio with quadruple diversity operation of 45.7 db, and 53 db for the channel signal-toweighted nonlinear noise ratio. Difficulties in the measurement of thermal noise are discussed. It is believed that a suitable approach is to specify single-link performance in terms of a distribution obtained with one-to-two minute averaged samples. Data are also developed for the minimum permissible system bandwidth and the transmitted spectrum as a function of channel capacity. Finally, it is considered that the performance in multilink multichannel circuits may be limited by nonlinear noise introduced by multipath effects related to selective fading. In this respect, further research is necessary in order to obtain reliable computational techniques for the intermodulation noise associated with the multichannel system.