546 BELL SYSTEM TECHNICAL JOURNAL 



antennas alone determines the number of different frequencies. The same 

 receiving frequency may be used at the junction on two routes separated by 

 an angle sufficient to yield the required antenna discrimination. All inter- 

 vening routes must employ different frequencies. The frequencies so deter- 

 mined may be used for transmitting from the junction if they are staggered 

 with respect to the receiving frequencies. Take, for instance, the five-route 

 plan shown in Fig. 22. Suppose the directional discrimination needs to be 

 60 db for a particular system. The directional pattern shows that this 

 requirement is met at 85 degrees. Thus, routes 1 and 4 may use frequency 

 A, say. Routes 2 and 3 then must use different frequencies, B and C. 

 Thus, we have: 



Route 



Trans. Freq. 

 Rec. Freq. 

 or 



While the treatment of the route congestion problem outlined above is 

 oversimplified it enables us to make a broad survey having some significance. 



Table \T for 1000 4-kc message channels and Table \TI for 250 16-kc 

 "program" channels were derived on the above basis. The decibel figures 

 at the head of each column are the allowable interference ratios from Tables 

 IV and V increased by 30 db for differential fading.^" A single source of 

 mterference of the values given in the table is supposed to degrade the 

 circuit to the minimum requirements for a long circuit. In regenerative 

 PCM there is no accumulation of degradation due to interference occurring on 

 various spans. In non-quantized systems such degradations are cumulative. 

 However, when protection to the above values is provided, with no allowance 

 of 30 db for differential fading of the desired and interfering signals, the oc- 

 currence of simultaneous additional degradations is extremely unlikely. Pro- 

 tection against this severe fading at one point alternately protects against 

 the simultaneous occurrence of several less severe fades. 



The values of repeater power capacity shown in the table will satisfy the 

 noise requirements on a 133-span transcontinental nonregenerated circuit 

 with 15 db fades simultaneously on all spans. This is equivalent to pro- 

 viding for 13 simultaneous fades of 25 db or, statistically, for the fading 

 that is not likely to be exceeded except during a small fraction of the time. 

 The PCM systems employing regeneration are powered for 25 db fades on 

 any or all spans.'^ The free space span loss is 60 db. 



In computing the frequency occupancy we take cognizance of the fact 



^•^ The FDM-FM entrj' provides 1 db additional allowance for repeater crosstalk as 

 mentioned before. Repeater crosstalk is negligible in the other systems. 



^' No such distinction was made in Tables II and III. There, provision was made for 75 

 db span loss in all cases. 



