Communication Systems Engineering with GNU Radio

5Digital Communications in Action: Design and Realization of a QPSK Modem

5.1 Digital Communication Concepts

5.1.1 What Is Digital Information?

Defining information for a digital transmission system is crucial for several reasons, but let us focus on some key ones (Figure 5.1). When designing a communication system, you are typically allocated a power and spectrum occupancy budget, so it is essential not to squander it by transmitting unnecessary data – data that carry no useful information. What constitutes unnecessary data depends on the nature of the information source, denoted as . For instance, when transmitting voice, it has been demonstrated that there is a significant amount of redundancy that can be eliminated. This redundancy is a characteristic of human language, facilitating comprehension by the majority of listeners. Similarly, in the case of video, it is obvious that when there is no motion, the images exhibit minimal changes. These examples underscore the possibility of extracting essential information from a data source, known as entropy, which can be quantified for any digital source. However, let us revisit the concept of information itself. According to Shannon [1948], the information produced by a digital source when the th message is transmitted is given ...