Design of infrared electronic-toll-collection systems with LEDs with irregular radiation pattern

Infrared light emitting diods (LEDs) with a half-intensity angle Φ_1/2 ≈ 10° are suitable for electronic-toll-collection (ETC) applications due to the requirement of a definite communication area between the on-board unit (OBU) and the road side unit (RSU). However, the radiation pattern of most low-cost commercial infrared LEDs are irregular to some extent leading to the difficulty to construct an infrared ETC system with a proper communication area. According to our measurements, most low-cost infrared LEDs with a half-intensity angle Φ_1/2 ≈ 10° have a common irregularity in their radiation pattern - a notch in the central direction. The unwanted influence of such irregularities can be reduced by a proper array of the constituent LEDs in the emitter. To analyze the system performance, the irregularity can be approximated by a linear combination of cosinⁿ functions. In this paper, we present some successful designs with the aid of an optimization algorithm to adjust the mounting angle of each component LED. In spite of the aforementioned irregularity, with our design the system can still have good performance in terms of the ability to withstand high signal attenuation and an extended communication area in the vehicle-traveling direction.