We now consider the effect of interference in communication systems. In real-world systems interference occurs from various sources, such as RF emissions from transmitters having carrier frequencies close to that of the carrier being demodulated. We also study interference because the analysis of systems in the presence of interference provides us with important insights into the behavior of systems operating in the presence of noise, which is the topic of Chapter 7. In this section we consider both linear modulation and angle modulation. It is important to understand the very different manner in which these two systems behave in the presence of interference. 3.3.1 Interference in Linear Modulation As a simple case of linear modulation in the presence of interference, we consider the received signal having the spectrum(single sided) shown in Figure 3.38. The received signal consists of three components: a carrier component, a pair of sidebands representing a sinusoidal message