This chapter lays the groundwork for understanding wireless communication systems. It begins by defining electrical signals used in communication systems. In the time domain, it explores measures like average power and energy, and defines several important signals. In the frequency domain, the chapter discusses power spectral density and signal bandwidths. It then establishes the relationship between electric and magnetic fields, explores power flux density, and provides a detailed treatment of various unit conversions. Next, the chapter discusses the important aspects of cellular antennas in line-of-sight conditions and defines key antenna parameters. Finally, it introduces basic digital modulation schemes, fundamental cellular network concepts and multiple access schemes.