BAND WIDTH AND TRANSMISSION PERFORMANCE 551 



These factors were multiplied by the product of bandwidth and number of 

 frequencies to obtain the dotted bars in Table \T. 



In regard to tlic "program grade" of circuit we must be more liberal in 

 our allowance for guard space. Our estimates for the band spacing factor 

 are: 



System of Table VII Factor U 



PPM-AM 4 



PPM-FM 4 



PAM-AM 4 



PAM-FM (narrow band) 4 



PAM-FM (wide band) 3 



PCM-AM (quaternary) 0.9 



PCM- AM (binary) 0.75 



PCM-FM (octonary) 1 • 5 



PCM-FM (quaternary) 1 . 5 



These factors were used to compute the dotted bars in Table VII. 



If transmission on two polarizations can be accomplished with mutual 

 cross-fire suppressed to a sufficient degree, half of the channels could be 

 transmitted by each polarization, on the same frequency, thus halving the 

 frequency occupancy. A probably unattainable cross-fire ratio seems neces- 

 sary to meet the requirements in the non-regenerative systems, if we remem- 

 ber that the interference produced by cross-fire accumulates from span to 

 span; but a suppression likely to be attainable, of the order of 15-20 db, 

 makes this frequency saving feasible in the rugged systems such as binary 

 PCM-AM or PCM-FM. The tables show entries for binary and quaternary 

 PCM-AM, assuming dual polarization transmission. 



If antennas could be improved to insure nearby discrimination ratios 

 adequate to allow use of the same frequency in and out and west and east, 

 the single-route occupancy would be halved again; with such a one-frequency 

 repeater plan the occupancy in a congested area is not, however, always 

 halved. Whenever the frequency requirements, as determined by the 

 terminal antenna directivity, result in two or more frequencies. A, B, C . . . 

 etc., there is no saving accruing from a one-frequency repeater plan, because 

 two-frequency routes can be accommodated with no additional frequencies 

 by suitably switching frequencies. It is only in the case of a system so 

 rugged that the terminal antenna directivity permits a single frequency, A, 

 to be used that the occupancy is reduced and it is then halved. Witli PCM 

 of low base this is a possibility and the tables include entries for this case. 

 As to achieving antenna characteristics suitable for one-frequency opera- 

 tion, it may be noted that reflection from a heavy rainfall in front of the 



