Signals and systems form the theoretical foundation of modern communication engineering, enabling the representation, analysis, and processing of information across diverse platforms. From analog circuits to digital transmission systems, the concepts of linearity, time-invariance, frequency response, and system stability remain central to engineering practice. The advancement of digital signal processing (DSP), wireless technologies, and IoT devices has significantly expanded the applications of signals and systems, making them indispensable for innovations in mobile networks, satellite communication, biomedical instrumentation, and smart infrastructure. This paper explores the applications of signals and systems in modern communication engineering, with emphasis on signal representation, system modeling, frequency domain analysis, and real-time processing. Case-based insights highlight applications in digital communication, biomedical signal analysis, and emerging 5G/6G networks. The study concludes that signals and systems provide not only analytical tools but also strategic enablers of technological progress in engineering domains.