58 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



Table XIII — Usable Channels in City at 150 mc 



* Includes three guard bands of 0.8 mc each to protect mobile and neighbor- 

 ing services from mutual interference. 



about 40 db RF selectivity, and from Table VIII that the 40 db selec- 

 tivity requires that all frequencies within d=3 mc need to be considered. 

 With 75 kc channel spacing, there are 80 potential assignments in 6 mc. 

 Table X indicates that 10 one-way channels can be found that are free 

 of mutual third order intermodulation interference. If the available 

 bandwidth were 12 mc the number of interference-free channels would 

 be doubled. 



By the same process from Table X, we derive the number of usable 

 channels shown for case (2). 



For cases (3) and (4), the methods are the same, but the number of 

 usable channels is reduced to one-half that shown for the single fre- 

 quency cases. 



In the fully coordinated broad-band system (case 5) a usable one-way 

 channel can be obtained every 40 kc. However, three guard bands total- 

 ing 2.4 mc are provided to protect both the mobile and neighboring 

 systems from mutual interference. If the available bandwidth were 12 

 mc the number of interference-free channels would be increased from 45 

 to 120 since no additional guard bands would be required. 



The comparison between various methods of operation given in Table 

 XIII applies to 150-mc channels operating in the same city. When the 

 channels are distributed more or less uniformly over a large area, the 

 number of usable channels is increased by several factors. The separation 

 between carrier frequencies in non-overlapping areas needs to be only 

 slightly greater than the IF pass-band of the receiver, say, 30 kc at 150 

 mc. The guard bands needed in one location can be used in other areas 

 at geographical separations less than co-channel spacings. Finally the 

 required geographical separation between co-channel stations is less 

 for the two frequencjy method than for the single frequency method and 

 is less for FM than it would be for AM. 



An estimate of the maximum number of usable channels within a large 



