FREQUENCY ECONOMY IN MOBILE RADIO BANDS 55 



different locations are needed and this use of space diversity techniques 

 provides an opportunity to pick the receiver having the best signal-to- 

 noise ratio. Moreover, the low power in the mobile transmitter together 

 with the better RF filters that are possible in fixed locations reduces the 

 critical bandwidth within which intermodulation interference can arise 

 to about =b0.4 mc at 150 mc and to about d=0.6 mc in the 450 mc range. 

 In these bandwidths approximately 20-25 channels can be obtained 

 which with random location of the mobile units would be divided more 

 or less uniformly among five or more base receiving stations. Since no 

 more than 4 or 5 channels would be operating within the critical RF 

 bandwidth at any one receiving location, the possibility of intermodula- 

 tion interference is almost negligible. Finally, an off-channel squelch 

 circuit is provided which disables the base receiver at a location where 

 serious adjacent channel interference is most hkely to occur and forces 

 the choice of another base receiver in a different location. Another ef- 

 fect of the off- channel squelch circuit is that it keeps the base receiver 

 quiet during idle times, and in this respect it is analagous to the advan- 

 tage gained in the mobile receiver by continuous transmission of the 

 desired carrier at the base transmitter. 



Most of the above coordinating methods tend to emphasize and to 

 increase the characteristic differences between the two directions of 

 transmission. The net effects are that greater frequency economy is ob- 

 tained and that the electrical requirements are reduced on the mobile 

 equipment where size, weight and power are critical and where cost 

 savings are important because of the large number involved. An increase 

 in complexity occurs at the multi-channel base station but this seems 

 economically justified because the cost can be divided among many 

 working channels. 



When the above methods of coordination are fully utilized, the RF 

 requirements are eliminated in the mobile equipment and can be met 

 in the base station equipment. In addition, the IF selectivity require- 

 ment on nearby channels is reduced to about 40 db in the mobile re- 

 ceiver and to about 60 db in the base receiver. The extra band radiation 

 requirement on nearby channels is reduced to about 25-40 db in the base 

 transmitter, depending on whether one or more than one antenna is 

 used ; and to about 60 db in the mobile transmitter. 



These requirements coupled with the data given in Tables II, IV 

 and V lead to the frequency separation between coordinated channels 

 operating in the same area as given in Table XII. The channel spacings 

 are shown for AM and for FM with a frequency swing of ±5 kc (which 

 requires a bandwidth of ±8 kc for good quaUty). 



The spacings shown in Table XII assume that each channel is trans- 



