Frequency Economy in Mobile Radio Bands 



By KENNETH BULLINGTON 



(Manuscript received August 20, 1952) 



The various factors affecting the usability of mobile radio channels are 

 discussed, and estimates are obtained for the number of usable channels per 

 megacycle for several present and proposed methods of operation. The lack 

 of radio-frequency selectivity is the principal barrier to maximum frequency 

 economy, but this difficulty can be avoided by sufficient geographical and 

 operational coordination. 



The increasing demand for all types of radio services emphasizes the 

 need for efficient use of the radio frequency spectrum. In mobile radio 

 operation the number of usable channels that can be obtained in the 

 VHF and UHF mobile bands depends not only on the width of the in- 

 dividual channels, but also on how and where each channel is to be used. 

 Activity on the same frequency at neighboring locations, and on neigh- 

 boring frequencies at the same location both affect the usefulness of a 

 channel. Halving the channel spacing doubles the number of potential 

 assignments, but it does not double, and in some cases it does not ap- 

 preciably increase the number of usable channels. 



The usefulness of a single isolated channel is determined by the in- 

 tensity of its signal above the noise level. Because of the very wide varia- 

 tion in received signal strength caused by distance, terrain, building 

 shadowing, etc., the coverage area of a channel can be discussed only in 

 statistical terms. There are likely to be islands of poor signal-to-noise 

 ratio even close to the transmitter, and the coverage gradually fades out 

 into more spotty conditions at greater distance. 



If the same frequency is used at a neighboring location, the familiar 

 problem of co-channel interference arises. There will now be locations 

 where the desired signal is above noise, but the undesired signal is still 

 stronger. Thus, the coverage area of a channel is reduced by the existence 

 of the co-channel transmitter; again, it is possible to discuss this reduc- 

 tion only in statistical terms. 



When two channels are being operated on different frequencies in 

 the same general area, the coverage area of each is limited by signal-to- 



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