542 BELL SYSTEM TECHNICAL JOURNAL 



tightness with which the bands may be packed into the frequency range 

 without mutual interference. Because of the ruggedness of low-base PCM, 

 neighboring frequency bands can actually be allowed to overlap. Introduc- 

 tion of the proper spacing factors for satisfactory separation in frequency 

 between adjacent bands causes the PCM system to overtake the other 

 methods in effective utilization of frequency space, especially when inter- 

 secting routes are involved. 



The PCM-FM systems listed in Tables IV and V are of the second class 

 listed above, in which equivalent ruggedness rather than equivalent power 

 as compared to AM is the criterion. Thus, binary PCM-AM is compared 

 with a PCM-FM system having the same 9-db tolerance of interference. 

 The curves of Fig. 17 show that, with such a tolerance, the minimum PCM- 

 FM bandwidth is secured when the base is either three or four. We choose 

 the quaternary case here because the signal-to-noise ratios obtainable co- 

 incide with those of the binary. Likewise, either octonary or hexary 

 PCM-FM furnishes the optimum base for the 18.5 db tolerance possessed by 

 quaternary PCM-AM and, of the two, octonary is more suitable for our tabu- 

 lation. In determining the power required to override fluctuation noise in a 

 PCM-FM system designed for a specified tolerance of similar system inter- 

 ference, we must make sure that both the limiter and sheer are protected 

 against breaking. 



The values of repeater power capacity shown in Tables IV and V will 

 satisfy the noise requirements on a 133-span non-regenerated circuit with 

 75 db loss on all spans. For spans of 60 db free space loss the tabulated 

 power thus provides for 15 db fades simultaneously on all spans, or for 13 db 

 simultaneous fades of 25 db, or for a single fade of 36 db. PCM systems 

 employing regeneration on every span must be powered for the deepest 

 fade that is likely to be encountered. We have arbitrarily taken this to 

 be 25 db making the span loss 85 db. This is probably not a sufficient 

 allowance for some situations but will serve for illustrative purposes. If 

 regeneration were not practiced the power would be 25 — 15 = 10 db 

 lower from the fading allowance standpoint but would have to be increased 

 10 log 133 = 21 db for noise accumulation, so regeneration results in a 

 power saving of 1 1 db. If, with regeneration, we were to protect each span 

 against the deepest single fade (36 db) permitted by the power provided 

 for non-regenerative operation, the power advantage of regeneration would 

 disappear (36 — 15 — 21 = Odb). 



In general, when the power without regeneration protects against simul- 

 taneous fades upwards of just a few db there is little or no power advantage 

 in regeneration if we then protect each span against the deepest single fade 

 permitted when regeneration is not practiced. This is true even for large 

 numbers of spans. There remain, however, important advantages for 



