LEDs are the most commonly used light source for local-area communication systems operating from very short (< 1 m) to medium distances (5 km). Typical bit rates are tens of Mbit/s up to about 1 Gbit/s.

LEDs are non-linear devices and as such the series resistance, shunt resistance, and capacitance depend strongly on the applied voltage. A thorough analysis must take into account these non-linearities. However, much can be learned by considering the LED as a linear device, even though some important aspects of the LED modulation behavior cannot be inferred from the linear model. In this chapter, we first analyze LEDs as linear devices and subsequently discuss modulation characteristics, including some non-linear modulation characteristics.