FREQUENCY ECONOMY IN MOBILE RADIO BANDS 61 



lows : 



1. A fully coordinated system requires a band of several megacycles 

 that can be treated as a unit, but it offers substantial overall frequency 

 economy and freedom from interference that can be obtained in no other 

 way. This is particularly true in large metropolitan areas where the 

 demand is greatest. With the same equipment and the same standards 

 of quality and reliability, coordinated channels can always be spaced 

 much closer in frequency than uncoordinated systems. 



2. The advantages of coordination increase rapidly as the number of 

 channels per unit area is increased. However, in areas where only three 

 or four channels are required, the advantages of complete coordination 

 are sufficiently small that only the semi-coordination of careful frequency 

 allocation is required to preserve overall frequency economy. 



3. For maximum economy, where full coordination is not used, the 

 channels should be assigned as in FM and TV broadcasting first to areas 

 and then to users within areas. The allocation of a block of channels to 

 a particular service with a minimum of operational and geographical 

 restriction frequently results in an ever-increasing interference problem 

 as each additional station is placed in operation. 



4. Single-frequency operation is most suitable where the operational 

 need for single channel communication between mobile units (as con- 

 trasted with fixed-to mobile) is more important than frequency econ- 

 omy. 



5. A frequency separation between potential channel assignments of 

 25 kc in the 150 mc range, and 35 kc in the 450-mc range seems tech- 

 nically feasible; but adjacent channels with these minimum spacings 

 cannot be assigned in the same area. These values may be reduced to 

 about 20 and 30 kc, respectively, at the sacrifice of an appreciable reduc- 

 tion in coverage under impulse noise conditions. A further reduction in 

 channel spacing would not appreciably increase the total number of 

 usable channels, since the controlling factors are RF selectivity and 

 extra band radiation, rather than IF selectivity or the total number 

 of potential channel assignments. 



6. The average spacing needed between channels operating in the 

 same area varies from about 40 to 500 kc or more, depending on the 

 method of operation and the criterion of usability. 



7. The need is for a certain small number of channels in all areas, 

 plus a peaked demand in centers of population. In the semi-coordinated 

 cases, the maximum number of channels that can be allocated to the 

 peak area is a small fraction of the total number of channels available. 

 In the fully coordinated, broad-band case, there is much more fiexibil- 



