Energy Fundamentals of Radio

Chapter 6Digital Modulation Basics

Radio communication starts with a remarkable fact, which is that if we drive a transmitting antenna with a sinusoid, we can detect that sinusoid in a suitably built receiver (see Figure 6.1 for a cartoon rendering). Now, suppose we turn on both the transmitter and receiver, as well as the sinusoidal drive. If we just leave it on like this...no information is transmitted. Or maybe minimal information: detecting a sinusoid in the receiver indicates that everything is in working order, whereas detecting no sinusoid tells us that things are not in working order. So that is one bit of information at most.

Figure 6.1 Transmitting and receiving an unmodulated carrier.

This traveling electromagnetic (EM) wave is called a “carrier,” and the whole trick once we have the basic hardware in place is to modify this carrier in such a way that we encode our information on this far-traveling EM wave. We call this encoding process “modulation,” or “modulating the carrier.” In Section 1.5, we took a tentative stab at this by doing simple, on–off keying of the carrier. When it comes to using spectrum efficiently, we shall see that we can do much better than this.

6.1 Modulating a Carrier

We jump straight to the modulation technique that is most widely used in high-capacity networks today. This technique is called “quadrature amplitude modulation,” or QAM for ...