BAND W IDl II AM) TRAXSM ISS/ON PERFORMANCE 507 



only a few per cent of the pulse height. We deline signal bandwidth for the 

 pulse systems studied here as \/T, or 2/T in the r-f medium, i.e., double 

 sideband is assumed in all of the AM pulse cases. In assuming double 

 sideband, we bow to the obvious difficulty of dealing circuit-wise with single 

 sideband and its pulse demodulation problem. 



In the P^M systems we define the radio signal bandwidth as the peak-to- 

 peak frequency swing, ^3, plus two times the baseband width, 2Fh. 



We shall consider individually the following types of systems where the 

 meaning of the symbols is explained in Fig. 1. 



1. PPM-AM 3. PAM-AM 5. PCM-AM 7. FDM 



2. PPM-FM 4. PAM-FM 6. PCM-FM 8. FDM-FM 



The source of disturbance may be either fluctuation noise, a constant-fre- 

 quency interfering wave (CW), or a similar but independent system operat- 

 ing on the same frequency allocation. CW^ interference may fall anywhere 

 within the radio signal band. Interference from echoes, which is a special 

 case of similar system interference, is not treated. In certain cases echoes 

 such as might be produced by multiple reflections in waveguide connections 

 to the top of radio towers may be more detrimental than independent system 

 interference of the same amplitude. We assume that such echoes are sup- 

 pressed sufficiently by good design. 



Our first set of curves, Figs. 9-20, exhibits quantitatively the audio signal- 

 to-noise and audio signal-to-interference ratios which can be obtained with 

 increased radio bandwidth in the various systems. Audio signal is taken to 

 be the power of a test tone which fully loads one channel. Audio noise is 

 expressed as the total noise power in the channel. Audio interference is 

 expressed as the power of aU of the extraneous frequencies produced in a 

 channel by the assumed interfering signal. The term "radio bandwidth" 

 is intended to mean double-sideband width and does not imply that the 

 transmission is necessarily by radio. Two of the systems, PAM-AM and 

 FDM, are omitted from this study because, as has been pointed out earlier, 

 they do not provide a significant basis of exchange of bandwidth for suppres- 

 sion of noise and interference. The other systems possess this trading prop- 

 erty in varying degree as illustrated b}' the curves. The FDM and 

 PAM-AM systems are entered in Table I\' and discussed under Section III. 

 For comparison with the following curves it may be of interest to note here 

 that for 1000 4-kc message channels in FDM, the bandwidth (single side- 

 band) is 4 mc, and the received power for a 60-db audio signal-to-noise 

 ratio must be —77 dbw.-" This is the power in a sine wave which employs 



2" Throughout this paper wo shall use tlie alilneviation "(llnv" for power expressed in 

 decibels relative to one watt. 



