coln Laboratory on this SS com- munication system type was confined to exploring a viable filter realization. This communication approach apparently was dropped when full-scale work began on the F9C system. Fano later patented [208] the wide-band matched filter system concept, claiming improved performance in the presence of multipath. While Fano's invention, which originally suggested a reverse-driven magnetic-drum recording for signal generation, basically employed analog CRYSTAL OSCILLATOR fo+f' +fc, fo+fr BANDPASS MIXER AMPLIFIER f++++c NARROW NOISE BANDWIDTH BANDPASS forfr GENERATOR FILTER EXPANDER MIXER AMPLIFIER POWER to +f' AMPLIFIER INTELLIGENCE SIGNAL LOCAL OSCILLATOR AMPLITUDE MODULATOR MODULATOR fo+f' (a). Transmitter. Figure 2.32. Spilker's patent on a TR-SS system uses a novel bandwidth expansion mechanism to generate a wideband reference signal e', from a random narrowband key signal er Both transmitted signals (frequency offset by fc Hz), contain the intelligence signal as FM modula- tion, and the signal at fo + f' Hz additionally carries the key as AM modulation. NARROW INTELLIGENCE fotfc SIGNAL DETECTOR FILTER fo+fi++c MIXER to +f; tfc, fo+fi BANDPASS fo+fi+fc AMPLITUDE AMPLIFIER BANDWIDTH fo+fi+fc DETECTOR FILTER EXPANDER BANDPASS MIXER AMPLIFIER NARROW BANDPASS AMPLIFIER BANDPASS FILTER AND fo ffi fo+fr LIMITER SINUSOIDAL SIGNAL (b) Receiver. Figure 2.32 (continued). To demodulate at the receiver, the two received signals are cross-correlated to recover the AM modulated key, which is then bandwidth-expanded to produce the wideband reference signal needed to demodulate the intelligence. (Diagrams from [202].) The Historical Origins of Spread-Spectrum Communications Figure 2.33. Fano's elegant matched filter experiment consisted of transmitting an acoustic pulse into a chamber containing many reflectors. The upper signal shown here represents the sound sensed by a microphone in the room, and tape recorded. The tape was then reversed (not rewound) and replayed into the chamber, the micro- phone t