imizing densities (Pi(f)) over the available bandwidth W, as a function of frequency f, for three dif- ferent power levels. The optimal distribution may be viewed as the gravity-driven result of pouring a fluid, having a total volume proportional to the available trans- mitter power, into a rectangular vessel of width W, whose bottom profile in the f direction corresponds uniformly, and by the same proportion, to the power density N(f) of the additively interfering Gaussian noise. [32], [33], Nelson Blachman [34], [35], and Lloyd Welch [36], all then asso- ciated with SS system developments. Driven by the intense interest in the theories of Wiener and Shannon, the Institute of Radio Engineers (IRE) formed the Professional Group on Information Theory, which commenced publishing in 1953 [37]. The first three chairmen of this Group were, in order, Nathan Marchand, William G. Tuller, and Louis deRosa. Marchand and deRosa, close friends, were at that time playing key roles in the development of SS systems; Tuller had inde- pendently but rather heuristically arrived at one of Shannon's capacity for- mulas. 2.1.3 Correlator Mechanization One of the difficult problems which Guanella faced (by this account with- out any knowledge of Wiener's work) was to fabricate a device which will perform a weighted correlation computation on two inputs. Specifically, a means was needed for taking two inputs x1(t) and x2(t) and computing y(t) = The Historical Origins of Spread-Spectrum Communications where y(t) is the device output and w(t) is the weighting function. The dif- ficulty here is not with the weighting (i.e., filtering) operation, but with the prior multiplication of x1(t) by x2(t), and in particular with the range of inputs over which accurate multiplication can be accomplished. As shall be seen later, the ability to mechanize the correlation operation precisely is essen- tial in building high-performance SS systems. In 1942, Nathan Marchand, then a 26-year-old engineer working for ITT's Federal Telephone and Radio Corpora