hat the noise-like carrier alone be transmitted on one channel, and that an information-bearing delay-modulated replica of the carrier also be NOISE DELAY LINE GENERATOR r(t) TAPPED FOR ADDITION STAGE EACH SYMBOL (WITH LIMITER) f=0 TO f2 r(1)+r(t-ro) r(t-to) MODULATION A.M. RECEIVER DELAY LINE VOLTAGE TO OUTPUT TRANSMIT TER r(t)+r(t-ro) r(t-ro+r(1-2rd) MULTIPLIER AND TO SIGN INDICATION INTEGRATOR (PER TAPPING) TYPE RESOLUTION Figure 2.19. Block diagram of a basic transmitted reference system using pure noise as a carrier and time-shift keying for data modulation. No effort has been made to separate the data and reference channels in this system proposed by the East German Professor F. H. Lange. His configuration is nearly identical to that secretly suggested in Project Hartwell a decade earlier. (Redrawn from [12].) The Historical Origins of Spread-Spectrum Communications transmitted at either the same frequency or at an offset frequency. A cross- correlation receiver still would be employed in this TR-SS system, but the carrier storage and synchronization problems of an SR-SS system would be traded for the headaches of a second channel. The Hartwell report noted that the SR system was cryptographically more secure than the TR system, which transmitted a copy of the wide-band carrier in the clear. Furthermore, it would be improper to transmit the intelligence- free wide-band carrier on the same channel as the intelligence-modulated carrier with a fixed delay T between them, since this delay-line addition would impose a characteristic cos(/) periodic ripple on the power spec- tral density of the transmitted signal. This ripple might be detectable on a panoramic receiver, compromising the covertness of the transmission. Although not mentioned in the report, it was realized at about the same time that multipath could produce a similar delay-line effect with similar results on any wide-band signal, including SR-SS transmissions. To close this revealing discussion of noise modulation, the Hartwell report suggested tha