54 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



tivity or filtering requirement of about 40 db (instead of the values 

 shown in Table III) is sufficient for a reasonable signal to interference 

 ratio plus an allowance for differential path losses resulting from stand- 

 ing wave effects. 



The RF selectivity or intermodulation problem in the mobile receiver 

 can be eliminated by reducing the power level at the first converter to 

 about 75 db below one watt. This can be done by providing a simple 

 automatic gain control in the RF stage of the mobile receiver. In regions 

 where the desired and undesired signals are weak the receiver has full 

 sensitivity, while at locations near the transmitters both the desired 

 and undesired signals are reduced in level before reaching the first con- 

 verter. The result is that the intermodulation products generated in the 

 receiver are reduced about 3 db for every db that the desired signal is 

 lowered and the distortion becomes negligible before the output signal- 

 to-noise ratio is reduced appreciably. In order that the a.g.c. circuit can 

 be fully effective it is necessary that the transmitters be grouped to- 

 gether and that the desired carrier be transmitted to control the gain of 

 the receiver. 



Grouping the base transmitters at or near a common point together 

 with the associated measures of transmitting the carriers and using 

 a.g.c. greatly reduces the requirements on the mobile receiver, but these 

 measures complicate the design of the base transmitter. The intermodu- 

 lation products generated in the closely associated transmitters result 

 in potential interference both within and outside of the desired trans- 

 mitting band. The intermodulation that falls on the mobile receiver fre- 

 quencies needs to be suppressed by at least 25 db below^ the carrier on 

 any channel to prevent mutual interference within the coordinated 

 system. The intermodulation that appears as extra band radiation out- 

 side the frequency range of the coordinated system must be suppressed 

 by RF filters. The guard band needed to prevent mutual interference 

 between the coordinated system and its neighbors is small compared 

 with the frequency space that is saved by the close spacing of the chan- 

 nels within the coordinated system. 



In the direction of transmission from the mobile transmitters to the 

 base receivers, the above coordinating methods cannot be used but 

 equally effective ones are available. The RF selectivity requirements 

 shown in Table VII can be reduced 20 db by using 20 db less power in 

 the mobile transmitter than in the base transmitter. This measure is 

 somewhat analogous to the use of a.g.c. in the opposite direction of 

 transmission; a further step would be automatic control of the radiated 

 power but this complication does not appear to be necessary. 



In order to regain the full coverage area, multiple base receivers at 



