2.1  What You Will Learn

• The main characteristic of a digital communication system: the system adopts a finite set of $M$ distinct symbols to convey information. Along the process, these symbols can be transformed into analog waveforms, but the essential characteristic is that the receiver will look after information that allows it to estimate one among the $M$ possible symbols. This differs from the strategy of an analog communication system that tries to have the received signal as similar (analogous) to the input signal as possible.
• Basic concepts such as symbol rate, bit error rate, symbol error rate and oversampling.

The main goal of this chapter is to simulate and interpret the results of a digital communication system using PAM signals.

Digital communications concerns a vast number of techniques. The approach adopted in this chapter is to make concrete some of the fundamental concepts. With this goal a hands-on description of an almost complete simulation is presented in the context of simple $M$-ary PAM systems using baseband signals with the spectrum centered around DC. Later, ASK, PSK and FSK illustrate simple strategies that use frequency upconversion to center the spectrum at a desired high frequency determined by the carrier signal.

When using the multiplication by a sinusoidal carrier to perform frequency upconversion of a baseband signal with bandwidth $\mathrm{\text{BW}}$, the new passband signal has bandwidth $2\mathrm{\text{BW}}$. However, this disadvantage can be compensated. For example, two carriers in phase quadrature such as a cosine and sine of the same frequency can be used to compose a quadrature amplitude modulation (QAM) system from two distinct PAM signals.¹ QAM is widely adopted in many modern communication systems and PAM can be seen as a simpler special case, which motivates concentrating in PAM in this chapter and deferring the discussion about QAM to Chapter 3.