FREQUENCY ECONOMY IN MOBILE RADIO BAND^ 43 



noise considerations. In addition, each channel may affect the other 

 because of spurious radiation from transmitters, insufficient receiver se- 

 lectivity, receiver oscillator radiation, etc. The recent trend toward re- 

 ceivers with greatly improved IF selectivity is worthwhile, but even 

 infinite IF selectivity cannot solve many of the present interference 

 problems. 



When three or more channels are operating in the same general area, 

 another type of interference occurs because of intermodulation in trans- 

 mitters or receivers. If it were technically feasible to build into the equip- 

 ment sufficient radio frequency selectivity to separate the working chan- 

 nels, this interference could be removed. In fact, this is not feasible, and 

 it is necessary to consider possible modulation products from channels 

 falling within a frequency band several percent wide. The number of 

 possible interference conditions that result from intermodulation (third 

 order) rises from 9 for 3 working channels to 50 for 5 channels, to 450 

 for 10 channels, and to 495,000 for 100 working channels. Some of 

 these interference combinations overlap and fall on the same channel; 

 but even considering all possible duplication, intermodulation inter- 

 ference rapidly becomes controlling as the number of closely spaced chan- 

 nels working in the same area is increased. 



It is not technically feasible to achieve enough radio frequency selec- 

 tivity to permit unrestricted and uncoordinated use of many channels 

 in a given area, unless the channels are, on the average, separated by 

 about 1 per cent of the operating frequency. For any kind of efficiency 

 of frequency utilization, it is necessary to have some coordination in the 

 location of fixed transmitters and in the use of channels. The maximum 

 efficiency of utilization requires the maximum coordination. 



The technical factors that determine channel width, channel spacing, 

 and the number of usable channels are described and tabulated below. 

 The first section discusses the principal factors that affect the useful- 

 ness of channels equipped with transmitters and receivers with perfect 

 filtering. This is followed by a consideration of the limitations imposed by 

 insufficient total filtering and by insufficient radio frequency filtering. 

 The next section shows the reduced requirements that are possible by 

 coordination between systems. Finally, the quantitative data are used 

 to illustrate the capabilities and efl^iciencies of various present and pro- 

 posed methods of mobile system operation. 



CHANNELS WITH PERFECT FILTERING 



It has been found by experiment that the radio path loss between an- 

 tennas in a mobile radio system can be ascribed to three principal fac- 



