FREQUENCY ECONOMY IN MOBILE RADIO BANDS 59 



Table XIV — ^ Usable Channels in State or Large Area 



AT 150 MC 



* Assumes adjacent channels are not assigned in same area. 



area can be obtained by considering an area whose radius is about six 

 times the coverage radius of the individual transmitter. A larger area 

 is unnecessary because single frequency FM channel assignments can be 

 repeated at this distance, while a smaller area would tend to approach 

 the common area concept used above. The large area can be divided into 

 9 subareas, each of which can be treated in the manner used in Table 

 XIIL The results are shown in Table XIV, which again assumes an 

 FM modulation bandwidth of ±8 kc and d=0.002 per cent overall sys- 

 tem frequency stability. 



The entries in Table XIV are calculated as follows: Once again, the 

 smallest band to be considered is limited by the RF selectivity in mobile 

 receivers to 6 mc ; with 25 kc as the minimum channel spacing, there are 

 6000/25 = 240 potential assignments. From Table X, only 12 can be 

 found to be free of third order intermodulation . With 12 channels in 

 each of 9 subareas, there is a grand total of 108 channels usable in the 

 state or large area. With more frequency space, the usable number is 

 increased in proportion. 



By the same process, from Table X we derive the number shown for 

 case (2). 



In the two frequency cases, the co-channel separation can be made 

 smaller than in the single frequency cases, since the most troublesome 

 case of interference (that between base transmitters and base receivers) 

 is eased by RF selectivity. Thus, the co-channel separation needs to be 

 only about 0.7 that for single frequency operation, which means that 

 there are now effectively 18 instead of 9 subareas. It follows that the 

 grand total of usable channels is the same in cases (1) and (3) and cases 

 (2) and (4). 



In considering case (5), we note from Table XIII that 40 kc is the 

 minimum channel spacing usable in a single subarea. However, the larg- 

 est grand total of channels is found by using 50 kc spacing in the sub- 

 areas, and assigning the adjacent 25 kc channels to other subareas. 



