294 BELL SYSTEM TECHNICAL JOURNAL 



considerable phase difference so that a given frequency is not Hkely to fade 

 into the noise level at all antennas simultaneously. By employing separate 

 receivers for each antenna and suitably combining or selecting the demodu- 

 lated outputs, a system is obtained which is much less susceptible to fading. 

 Such a method is called space diversity reception. Inasmuch as fading over 

 a given combination of paths is highly selective with respect to frequency 

 much the same effect is obtained by frequency diversity reception. When 

 this method is employed the intelligence is transmitted on two or more fre- 

 quencies simultaneously, and then received by separate receivers from a 

 single antenna and the resulting demodulated signals combined or selected 

 as for space diversity. 



In te'egraph transmission large differences in delay over two separate 

 propagation paths cause the telegraph signal transitions to arrive at different 

 instants over the two paths. Thus, there are intervals of overlap when a 

 marking condition is received over one path and a spacing condition over a 

 second path. When two components of nearly equal amplitude arrive at 

 nearly 180° phase difference a signal transition may involve large and sudden 

 amplitude and phase changes. The resulting transients in the bandpass 

 networks of the receiving equipment may cause fortuitous distortions con- 

 siderably greater than the difference in delay times over the two paths. 

 The wider the pass band of the receiving system the shorter the duration of 

 these fortuitous transients and hence the less the distortion. This phe- 

 nomenon is one of the determining factors in the selection of bandwidth and 

 frequency shift to be used in a given application of FS telegraphy. It 

 becomes of increasing importance when the circuits are long and at higher 

 signaling speeds such as are used in time-division multiplex methods. 



In an AM system the effect of large differences in path lengths is usually a 

 filling in of the spacing intervals with resulting marking bias. In an FS 

 system the overlap time and associated transients may add to either mark- 

 ing or spacing intervals in a random fashion depending on the amplitude and 

 phase conditions at each transition. The overlapping of the mark and space 

 frequencies in FS transmission can sometimes be heard in an AM receiver as 

 short pips of audio tone at each transition, the audio tone being the beat 

 between the two frequencies. 



Use of Superimposed Phase Modulation 



Superimposed phase modulation has sometimes been employed as a 

 simple means for achieving a certain amount of frequency diversity both in 

 AM and FS telegraph systems. This consists in causing the radiated signal 

 to oscillate continuously through a small phase angle at a rate relatively 

 high compared to the dotting speed. Phase modulation spreads the energy 

 of the signal over a wider frequency band so that the complete loss of the 



