FREQUENCY ECONOMY IN MOBILE RADIO BANDS 53 



separation between transmitters is greater than about 50 feet. For 

 example, with a 500-foot separation between the transmitting and re- 

 ceiving antennas, Table VII shows that the 150 mc requirement on r.f. 

 selectivity is 39 db. The bandwidth between the 39 db points on the 

 receiver selectivity characteristic determines the number of potential 

 channel assignments to be used in Table X. 



INCREASED EFFICIENCY OBTAINED BY COORDINATION 



The preceding selectivity and filtering requirements are severe and 

 in some cases virtually unattainable except at considerable sacrifice in 

 frequency space. The principal reason for these exacting requirements is 

 that the assumed unrestricted and independent operation results in large 

 differences in field intensities among closely spaced frequencies. In order 

 to pick out the weak signals from among the strong, sufficient selec- 

 tivity must be provided to suppress the potential interference to below 

 the minimum usable signal. 



An alternative is to reduce the level differences and hence the filter- 

 ing requirements by geographical and operational coordination. This 

 means that the level of the potential interference can be permitted to 

 be many db above set noise as long as it is always at least 10-20 db 

 below the desired signal at all possible locations. By proper coordination 

 the troublesome RF filtering problems can be eHminated within the co- 

 ordinated system and the remaining IF selectivity problems can be 

 minimized. 



The first step is to use the two frequency method of operation with 

 adequate separation between the frequencies used for the opposite 

 directions of transmission. In this way substantial RF filtering can be 

 obtained to eliminate the interference between one or more base trans- 

 mitters and a base receiver. This type of interference is particularly 

 troublesome between single frequency systems because of the relatively 

 high base transmitter power and because the high antennas at both 

 locations reduce the radio path loss to a minimum. The corresponding 

 possible interference between transmitters and receivers on different 

 mobile units is also reduced by the two frequency method but inter- 

 ference between mobile units is much less important because of the 

 lower power and much lower antenna heights. 



The potential interference between base transmitters and mobile re- 

 ceivers caused by insufficient total filtering can be reduced by locating 

 all base transmitters at or near a common point so the level differ- 

 ences between the desired and undesired signals will never be exces- 

 sive. When all transmitters radiate from a common antenna, a selec- 



