536 BELL SYSTEM TECHNICAL JOURNAL 



upon signal-to-noise ratio. Both tables apply to long systems comprising 

 133 repeaters spaced 30 miles apart. The span loss is 75 db. Three re- 

 shaping or regeneration plans are shown: reshaping at every repeater, reshap- 

 ing five times in the complete system, and no reshaping within the system. 

 In the case of PCM, reshaping becomes a true regeneration which com- 

 pletely removes noise accumulated in the transmission link; while, in the 

 non-quantized pulse systems, reshaping restores the original pulse shape 

 but retains timing or amplitude errors. In the case of FM systems, reshap- 

 ing is accomplished by the amplitude limiter which removes envelope 

 variations arising from noise, but does not suppress the accompanying fre- 

 quency shifts. Reshaping, in contrast to regeneration, is only a partial 

 prevention of cumulative effects but, as shown by the tables, it has definite 

 value in enabUng the use of wider transmission bands wdth corresponding 

 smaller amounts of power than permissible without reshaping. Regenera- 

 tion completely removes errors in both the amplitude and in the time. The 

 maxunum bandwidth which can be used is independent of the number of 

 regenerations, but the signal power must be increased in proportion to the 

 number of spans covered before regeneration. 



In the case of non-regenerative radio transmission systems, we may regard 

 the 75 db span loss as 60 db free space loss plus a fading allowance of 15 db. 

 This allowance is intended to cover the increment of noise caused by fading 

 in the entire system. Available data on distribution of fading are too meager 

 to permit generalization, but indicate that on some routes, at least, the total 

 degradation suffered would rarely be worse than that produced by 15 db 

 simultaneous fades on all spans; and hence a design based on 75 db span loss 

 should satisfy the noise requhements except for an extremely small fraction 

 of the time. In other words a few spans of the non-regenerative system 

 may fade deeply but, in regard to total accumulated noise, the system is 

 credited with the higher signal-to-noise ratio occurring on spans which are 

 not simultaneously in fading minima. In regenerative PCM systems no 

 credit accrues from a higher signal-to-noise ratio occurring between pomts 

 of regeneration, and protection must be provided against the worst condition 

 that is likely to occur in any section included between regeneration points. 

 However, the values of minimum power obtained by lavish use of band- 

 width exhibited in Tables II and III may be particularly significant for 

 wave guide transmission systems; and hence the assumption that all spans 

 have the same loss is appropriate here. 



The outstanding features of Tables II and III are the extremely small 

 amounts of power needed in the PCM systems with only moderate expendi- 

 tures of bandwidth as compared with the non-quantized system. These 



