508 BELL SYSTEM TECHNICAL JOURNAL 



the full load capacity in accordance with Table I, at a point where the noise 

 power is — 189 dbw per cycle of bandwidth (15 db noise figure, NF). 



In most of these curves, plotted for 1000 message channels, the bandwidth 

 scale runs to hundreds of megacycles. We do not mean to imply that the 

 microwave transmission medium can be relied upon to transmit faithfully 

 such wide-band signals or that circuit techniques for producing them are 

 available. As suggested by Fig. 4, the 1000-channel system might be divided 

 into several groups of fewer channels to avoid frequency selective transmis- 

 sion difficulties or circuit limitations. The total frequency occupancy is not 

 altered by such a division, while the required power per group is reduced in 

 proportion to the number of channels.-^ 



Curves are shown of audio signal-to-noise ratio as a function of radio 

 signal bandwidth at constant power and at marginal power. Audio signal- 

 to-interference ratios are plotted against radio bandwidth for marginal ratio 

 of radio signal power to interfering signal power. By "marginal power", 

 we mean the radio signal power which just safely exceeds the threshold below 

 which noise or interference causes system failure. In the case of fluctuation 

 noise, any further increment of bandwidth from this point is untenable 

 without an increase in radio signal power. Points on the marginal power 

 curves show as abscissa the bandwidth at which minimum radio power is 

 required to obtain the audio signal-to-noise ratio given by the ordinate. In 

 calculating these curves, we have specified the marginal condition as 

 occurring when the peak disturbance is actually 3 db below the theoretical 

 value which just breaks the system. These relations are shown graphically 

 in Fig. 7. We have in this paper followed the accepted practice of ignoring 

 all fluctuation noise peaks exceeding the rms voltage by more than 12 db. 

 Radio signal power is taken as the power averaged over a cycle of the high 

 frequency in the FM wave, or, in the AM pulse case, over a cycle of the high 

 frequency when the pulse is maximum. A curve is included in Fig. 9 

 showing marginal AM radio pulse power values for various bandwidths of 

 fluctuation noise and a similar curve for FM is shown in Fig. 13. A noise fig- 

 ure of 15 db-- is assumed for the receiver. W^e have taken the noise band- 

 width as equal to the signal bandwidth throughout. This equality cannot 

 be quite attained in actual systems because of the departure of physical fil- 

 ters from ideal characteristics. In practice an allowance for frequency 

 instability would also have to be included. 



The relation of the PPM pulse to channel allotment time is shown in Fig. 8. 

 Pulses in channels adjacent in time can just touch when full load signals are 

 impressed on each. The slicer operates at half the pulse height which, for the 

 assumed pulse shape, is also the point of maximum slope. The width of the 



^' These statements are not exactly true for FDM and FDM-FM, where multiplex load 

 rating is used in the design. 



^ This means that tlie noise power is 189 db below a watt per cycle of bandwidth. 



