BAM) Winill AM) TRAXSMISSIOX I'l'.KIOKM A \CK 4'>3 



ference l)et\vct'n systems in a congested area is taken into account, certain 

 wide-band methods, less \ulneral)le to interference, ma}' he as or more 

 etlicient in the use of frecjuency space than other narrower hand muUipiex 

 methods. 



The principal pur|)()se of (liis pai)er is to examine, for various systems, the 

 rehitions governing the exchange l)etween freciuency space and transmission 

 advantages. 



It will be shown that the preferred multiplex method depends in jjart 

 upon : 



1. 'J1ie grade of facility required; low-grade and high-grade channels lead 

 to difTerent preferences. These preferences also are influenced Ijy the length 

 of circuit. 



2. The nature of the transmission obstacle over which advantage is 

 sought. These obstacles may be: (a) intrasystem distortion (phase distor- 

 tion, overload distortion, etc.) and noise; (b) intersystem interference as 

 between similar radio systems or between different tyj)es of radio systems, 

 operating on the same frequency. 



Other factors beside the transmission considerations discussed here are 

 likely to be involved in a practical multiplex application; hence the system 

 preferences arrived at in this study may not be the controlling factors in 

 practice. 



Before a detailed analysis is undertaken, it may be helpful to e.xamine and 

 comment upon the chart shown in Fig. 1. All of the multiplex methods 

 shown here have been studied sufficiently to i)ermit their approximate 

 evaluation with the aid of some theoretical considerations and subject to 

 certain qualifications as pointed out from time to time. Variations and 

 combinations of these are possible,* some of which will be discussed later. 



In addition to the two general classifications of frequency and time 

 division there is a third type based on carrier phase discrimination. A 

 familiar example is the quadrature carrier system,^ which is capable of 

 yielding two channels for each double sideband width. In another form^ 

 each of X channels is modulated simultaneously on A 2 carriers with a 

 different set of carrier phases provided for each channel. Time division 

 multiplex may be regarded as a kind of phase discrimination in which the 

 signal is modulated on harmonic carriers so i)hase(l as to balance out excej)! 

 during the channel sami)ling time intervals. In true phase discrimination. 



' .\ loiuprelR-nsisc lislinii ;uul (lisiussiim ol' \ari()us romhinaliuns will ho louiul in a 

 recciU paper by V. D. Laiulon, "Theoretical .Viialysis of Various Systems of .Miihipk-.\ 

 Transmission" K.C.A. Reviav, vol. IX, numbers 2 and 3, June-Sept. 1948, pp. 287-351, 

 438-482. 



^ H. Nyquist, "Certain Topics in 'lY-legrajjli Transmission 'I'lieoiA," A.I.E.F.. Iraiis., 

 .\pril, 1928, pp. 617-644. 



« \V. R. Bennett, "Time Division .Multiplex Svstems," Bell Svs. Tech. Jl. Vol. 20, pp. 

 199-221, April. 1941. 



