552 BELL SYSTEM TECHNICAL JOURNAL 



antennas limits the attainable side-to-side ratio.^^ Reflection from aircraft 

 may also impose a practical limitation. Spacing the antennas laterally (on 

 two towers) would achieve freedom from these limitations. Another way 

 of coping with the antenna discrunination obstacle is to use short spans in 

 congested areas. This reduces the discrimination requirements particularly 

 because fading is reduced by shortening the spans. 



Conclusions as to Radio 



Of the systems included in Table VI we find that, for six routes, binary 

 PCM-AM, even without the potential frequency economy of dual polariza- 

 tion and/or single-frequency repeaters, has come close to being the most 

 economical of frequency space; quaternary PCM-AM shows a slight advan- 

 tage (which would be lost if the route spacing were less than fifteen degrees). 

 Even without dual polarization or single-frequency repeaters, the binary 

 PCM-AM occupancy is less, for more than 3 routes, than the occupancy 

 required by FDM whose hand width is 4 kc per channel. There is here an 

 excellent illustration of the possibility of a net saving in frequency space 

 through the use of tough wide-band systems. 



The power requirements also favor the low-base PCM systems. It should 

 be noted, in particular, that the linearity requirements with FDM demand 

 that the tabulated power of 80 watts be a very light load on the repeaters. 



Inspection of Table VII brings out the effectiveness of the coding prin- 

 ciple if very high-grade channels are required. Only with PCM (of low 

 base, as shown) are the occupancy and power requirements both within the 

 practical reabn. The non-PCM methods that achieve small occupancy, 

 comparable with that of low-base PCM, all require colossal amounts of 

 power. When the power requirement is reduced and the ruggedness in- 

 creased by use of band width, the occupancy becomes^ in turn, colossal. 

 This is illustrated by the two entries for PAM-FM. 



As route congestion increases without limit, any type of system that 

 permits exchange between bandwidth and ruggedness will always achieve 

 the minimum occupancy when bandwidth has been used to secure the 

 degree of ruggedness that avoids multiplying the frequency assignments. 

 Our studies have shown that, with the assumptions made, this result is valid 

 for channels of message grade when the congestion has reached a degree 

 that is by no means fantastic. We have accordingly prepared Table VIII 

 in which the dotted bars show the bandwidths (taken from Fig. 9-19) of the 

 various systems when their interference tolerances are alike and have values 

 of 18.5, 9, and 3 db.'^ While these systems, having the same tolerance, all 



^ Measurements made at the BTL radio laboratory at Holmdel, N. J. indicate that this 

 Umit to side-to-side ratio is of the order of 85 to 90 db. 



^ The AM pulse systems are here assumed to achieve the 6 db increase in tolerance by 

 suppressing the carrier. 



