Thermal noise in multi-section radio links

Thermal noise in multi-section radio links

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Microwave radio links are used for the long-distance transmission of large groups of telephone channels and for television signals. Thermal noise effects arise in each section of the link, and the magnitude of the resulting noise in the overall circuit is one of the major parameters of the overall transmission circuit.The paper outlines methods of calculating the thermal noise in a long radio link when the individual radio-transmission sections are subject to fading. Various methods of expressing fading statistics are discussed, and it is proposed that the statistics of the individual path shall be expressed in terms of the three first hourly moments of the fading ratio and the statistics of these moments.A technique is described for combining the effects of fluctuating fading in successive (tandem-connected) paths to obtain the hourly fading moments of the overall circuit. A method is given of converting the overall moments into an overall circuit fading distribution; this is expressed in terms of an ‘augmented log-normal distribution’, which can readily be translated into the distribution of thermal noise in the overall connection.The noise requirements for long-distance telephone circuits are specified for a 2500 km circuit, although any particular project will generally be much shorter. To avoid the difficulties involved in subdividing the noise permitted for 2500 km, it is proposed instead to treat such path performance data as can be made available as if it were representative of all the sections which would enter into a 2500 km circuit (synthetic overall circuit).Thermal noise can be calculated for the synthetic circuit and compared with the requirements; this comparison will show whether the actual paths for which data are available are suitable to form part of an overall connection.

Inspec keywords: noise; radio links

Subjects: Radio links and equipment


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