566 BELL SYSTEM TECHNICAL JOURNAL 



shifters in the receiving set. By taking several branch circuits in 

 parallel from the antennas to different sets of adjusting and receiving 

 apparatus the vertical signal components may be separated from each 

 other. 



Nature breaks the wave into several components and jumbles them 

 together. The first function of the MUSA system, as just described, 

 is to sort the components out again. Its second function is to correct 

 their differences so that they may be combined smoothly into a replica 

 of the original signal. To do this the received wave components are 

 separately detected and passed through individual delay circuits to 

 equalize their differences in transit time. They are then combined to 

 give a single output. As compared with a simple receiver the MUSA 

 receiving system gives (1) improvement in signal-to-noise ratio, as a 

 result of the sharp directive selectivity of the antenna ; (2) improvement 

 against selective fading distortion, by virtue of the equalization of the 

 time differences between the components before they are allowed to 

 mix; and (3) improvement against noise and distortion, because of the 

 diversity effect of combining the several components. 



Fortunately, it is found that the directive selection and the delay 

 compensation adjustments correct for one frequency are satisfactory 

 for a considerable band of frequencies adjacent thereto. Thus there is 

 offered the possibility of receiving a number of grouped channels 

 through one system and the prospect appears not only of improved 

 transmission but also of reduced cost per channel. 



The possibility of grouping channels at the transmitting station may 

 be conceived on the basis of either "multiple" or "multiplex" trans- 

 mission. In the multiple arrangement each channel has its own an- 

 tenna and its individual transmitter whose frequency is closely spaced 

 from and coordinated with the adjacent channels of the group. In 

 "multiplex" transmission, the channels are aggregated into a group at 

 low power and handled en bloc through a common high-power amplifier 

 and radiating system. Particularly in the multiplex case, there are 

 possibilities of important economies if the technical problems are 

 satisfactorily solved. Passing a multiplicity of channels simultane- 

 ously through a common-power amplifier involves interchannel inter- 

 ference due to modulation products which is not met with when only 

 one channel is present. Severe requirements are thereby placed on the 

 distortion characteristics of the power amplifier. 



It seems a fair conclusion that the tendency in the engineering solu- 

 tion of the problems of economy and growth in radio telephone de- 

 velopment (and perhaps also radio telegraph development) will be 

 toward channel grouping methods, especially for backbone routes 



I 



