Before bits, there were waves. AM radio. FM radio. Broadcast television. Walkie-talkies. Vinyl records pressed from a needle wiggling at audio rates. Long-distance telephony built out of millions of voice channels stacked side by side in frequency. For most of the twentieth century, "communication" meant analog modulation, and the entire infrastructure of broadcasting, navigation, military signaling, and telephony was built around carriers, sidebands, and intermediate frequencies. Even today, when almost every payload at the application layer is digital, the physical layer of every radio, every satellite uplink, every Wi-Fi access point still speaks analog at the antenna. A digital bit is a digital bit, but it leaves the transmitter as a sinusoid that has been modulated, filtered, mixed, and amplified through circuits whose theory was nailed down in the 1930s. Understanding the analog story is the door to the rest of communications, and to a surprising amount of hardware security.
The chapter ahead has a single question driving it. Why must we modulate? Once you see why, every other architectural choice (sidebands, IFs, pre-emphasis, superheterodynes, capture effect, threshold) drops out as a consequence rather than a list of facts to memorize. We will keep reaching back to Chapter 3 (Fourier, Hilbert, sampling) because every modulation scheme is fundamentally a frequency-domain manipulation, and we will keep reaching forward to Chapter 9 (electromagnetic propagation) and Chapter 13 (antennas) because the whole point of modulation is to launch information into space and pull it back out somewhere else.