1190 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER, 1953 

 GENERAL DISCUSSION OF FADING PHENOMENA 



The variations in the strength of a received radio signal known as 

 "fading^' are caused by variable or temporary conditions in the trans- 

 mission path. These conditions fall into two broad classes: those causing 

 partial obstruction of the path, and those causing multipath transmis- 

 sion. The latter class is believed to be the principal source of frequency- 

 selective fading. 



Multipath transmission involves the reception of more than one signal 

 ray, each of which travels over a different path between the transmitter 

 and receiver. Generally each such path has a different length between the 

 transmitter and receiver. Multipath transmission involves either or both 

 reflection or refraction of at least one (and in some cases all) of the re- 

 ceived rays. Since the conditions of the atmosphere are continuously 

 varying, the paths of the received rays are variable with time. The re- 

 ceived signal is the resultant of all the rays accepted by the receiving 

 antenna. The relative phase of the different received rays depends on (1) 

 the differences in the lengths of the paths over which they have travelled 

 and (2) the signal frequency. If the rays arrive at the receiving antenna 

 in nearly the same phase, they add and enhance the received signal; if 

 they arrive in phase opposition they partially cancel each other and 

 fading results. This fading is not only variable with time but also with 

 signal frequency, and is called "(frequency) selective fading". Since the 

 conditions which cause this kind of fading are substantially random, the 

 variation of fading with time on a statistical basis might be expected to 

 approach the Rayleigh distribution. There has been experimental con- 

 firmation of this. 



A number of other factors are involved which will not be treated here, 

 since the mechanism and effects of multipath transmission have been 

 discussed quite thoroughly in the previously mentioned reports by 

 Crawford and Jakes, DeLange and in an earlier paper.^ 



Results of Tests 



Fig. 1 shows three illustrations of the type of records obtained. On 

 each record the horizontal deflection of the cathode-ray- tube trace is 

 proportional to the radio signal frequency. The vertical deflection is 

 linearly proportional to the amplitude of the received signal, which 

 because of constant transmitter power is inversely proportional to the 



« H. T. Friis, Microwave Repeater Research, Bell System Tech. J., 27, Apr. 

 1948. 



