FREQUENCY ECONOMY IN MOBILE RADIO BANDS 57 



modulation bandwidth of ±8 kc and a frequency stability of ±.002 

 per cent; with these assumptions, the IF passband should be at least 22 

 kc in the 150 mc band and 34 kc in the 450 mc band. Narrower band- 

 widths could be used but this would result in a substantial sacrifice in 

 coverage under impulse noise conditions. 

 Five cases are considered: 



(1) Single Frequency Semi-Coordinated — In this case, substantially 

 no interference is expected from third order modulation problems, which 

 are avoided by careful selection of operating frequencies, but higher 

 order modulation products may be important. Base station locations 

 are unrelated geographically to other systems in same general area, 

 except that a minimum spacing of 500 feet between receiver and in- 

 terfering transmitter is assumed. 



(2) Single Frequency with Interference — In this case, the choice of 

 frequencies is unrestricted, but a 10 per cent chance of third order inter- 

 modulation interference is accepted within 500 feet of unwanted trans- 

 mitters, when transmitters are in operation 25 per cent of time. 



(3) Two Frequency Semi-coordinated — This is the same as (1), ex- 

 cept with two-frequency operation. 



(4) Two Frequency with Interference — Same as (2) except with two 

 frequency operation. 



(5) Fully Coordinated Broad-hand — This case assumes: (a) two fre- 

 quency operation with the land transmitters coordinated in location, 

 power, antenna height and emission of protective carriers ; (b) low power 

 mobile transmitters; (c) multiple land receivers; (d) no interference from 

 third or higher order intermodulation ; and (e) guard bands to protect 

 mobile and neighboring services from mutual interference. 



The number of usable channels that can be obtained in the same area 

 is estimated in Table XIII for frequencies near 150 mc. 



The minimum channel spacing shown in the first column of Table XIII 

 is calculated as follows: in cases (1), (2), (3) and (4), the extra band 

 radiation from the base transmitter is controlUng. As shown in Tables 

 III and V, to avoid interference for distances greater than 500 feet from 

 the interfering transmitter requires a guard band of about 50 kc. This 

 is added to the 22 kc required IF pass-band of which ±8 kc is allowed 

 for the FM signal, and ±3 kc for 0.002 per cent system instability. 

 In (5), the adjacent channel receiver selectivity is controlHng: Table IV 

 shows the required 60 db can be obtained in 15 kc, which added to the 

 required 22 kc IF band gives approximately 40 kc. 



It will be noted from Table VII that the assumption of a separation 

 of 500 feet between the receiver and the interfering transmitter requires 



