FREQUENCY ECONOMY IN MOBILE RADIO BANDS 45 



rate at 150 mc may be as much as 15 cycles per second for a speed of 30 

 mph and increases as either the radio frequency or the speed of the 

 mobile unit is increased. The fast acting gain control needed to minimize 

 the flutter effects is obtained automatically with frequency modulation 

 but is more difficult to obtain with amphtude modulation. This factor is 

 one of the principal advantages of the use of FM instead of AM for 

 mobile radio systems. 



The co-channel interference to be expected between stations having 

 equal transmitter powers depends on the path loss statistics for both the 

 desired and undesired signals. At the edge of the desired coverage area 

 there must be a high probability that the desired signal will be strong 

 enough to be useful and only a small probability that the undesired 

 signal will be strong enough to be troublesome. The geographical separa- 

 tion needed between co-channel stations varies from about four to six 

 times the desired coverage radius when FM is used and from six to eight 

 times when AM is used.^ If the needs for mobile channels were uniformly 

 distributed geographically only a small part of the potential channel 

 assignments would ever be used in a given area. However, the needs for 

 mobile channels are usually concentrated in areas of high population 

 density so that a large percentage of the channel assignments may be 

 needed in the same area. 



The above estimates on co-channel spacing depend somewhat on the 

 antenna heights and the type of terrain, and assume that the same fre- 

 quency is used in both directions of transmission. When the two-fre- 

 quency method is used with adequate separation between the trans- 

 mitting and receiving frequencies, the co-channel spacings can be reduced 

 to about three to five times the coverage radius for FM and to about four 

 to six times for AM. This reduction of approximately 30 per cent is 

 possible because the most troublesome interfering path in the single 

 frequency method (from base transmitter to base receiver) can be elim- 

 inated in the two-frequency method by sufficient selectivity. 



The principal reason for using the single frequency method is to pro- 

 vide communication between tw^o mobile units when they are relatively 

 near each other but are beyond the range of the base station. When 

 transmission of all messages through the base station is desirable, or at 

 least not objectionable, the two-frequency method is preferable. It is 

 shown in a later section that close geographical and operational co- 

 ordination is needed to achieve maximum efficiency in the use of fre- 

 quency space and that this coordination can be obtained only with the 

 two-frequency method. 



* See reference in Footnote 3. / 



