56 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



Table XII — Channel Spacing versus System Stability- 

 Coordinated Systems in Same Areas 



mitted on an individual carrier. Single-channel operation seems to be 

 the only practical arrangement for transmission from the mobile trans- 

 mitter to the base receiver. In the other direction of transmission, from 

 base transmitter to mobile receiver, the question naturally arises whether 

 additional frequency economy could be achieved by multichannel meth- 

 ods. In this case individual carrier operation is also indicated for trans- 

 mission and economic reasons. The multiple echoes that exist at street 

 level in urban areas Umit the number of usable channels that can be 

 transmitted on a single carrier.^ While the exact number is somewhat 

 indefinite, it appears to be less than about 20 and perhaps less than 10 

 channels. In addition the selectivity and linearity requirements on 

 multi-channel receivers (even for two channels) are much more severe 

 than for single channel equipment. From these considerations it appears 

 that the use of more expensive receivers and channel separation equip- 

 ment in each mobile unit is not economically feasible. 



frequency economy in present and proposed mobile SYSTEMS 



The technical factors given above provide a basis for estimating the 

 number of usable mobile channels that can be obtained in a given band- 

 width. This bandwidth must be sufficiently large to be isolated by RF 

 filtering if the results are to be well defined. 



The following examples assume two different geographical distribu- 

 tions: (1) the number of usable channels with overlapping coverage 

 areas that can be obtained within a city or metropolitan area, and (2) 

 the number of usable channels that can be obtained when the channels 

 are distributed more or less uniformly over a state or other large area. 

 The examples are based on the use of frequency modulation with a 



• Young, W. R., Jr., and L. Y. Lacy, Echoes in Transmission at 450 Megacycles 

 from Land-to-Car Radio Units. I.R.E., Proc, pp. 255-258, March, 1950. 



