As the wave expands outward from the transmitter, power density dissipates. This is modeled by the Friis Transmission Equation. In a free-space vacuum, power decays with the square of the distance ($d^2$). However, in real-world environments (urban canyons, indoor offices), the exponent is often much higher ($d^3$ to $d^5$). This physical constraint dictates the —the fundamental calculation of how much power is needed to bridge the distance between transmitter and receiver.

The content is organized to build knowledge from basic signals to complex system architectures: