Analysis of convergent beam to improve sensor node positioning performance in optics-based WSN

It is proposed that a method to improve sensor node positioning performance in optics-based Wireless Sensor Network (WSN) system. This system consists of base-station with laser source and sensor nodes with MEMS CCR as a reflector. As a method to position the sensor node, Time of Flight and direction angle between base-station and sensor node are used. The problem in this system is that the intensity of the laser decreases as it propagates forward because of divergence angle and that the reflect area of MEMS CCR is too small to reflect enough power. This makes it hard to measure the TOF and direction angle and as a result the measured distance is very short. To resolve this problem, convergent beam is introduced. The convergent beam has a characteristic that there exists focal point on the optical axis so the beam can be focused on that point. If the focal length is adjusted far away from the base-station, the cross sectional area of the beam decreases as the beam propagates and the intensity of the beam increases enough to use TOF. After this point, the laser beam propagates as like divergent beam. This has the effect that using more powerful laser at nearer to the sensor node. Also by the theoretical analysis of the convergent beam, the offset from the optical axis which can detect MEMS CCR is obtained. To analyze the effect of convergent beam, the beam propagation function is introduced and by this, simulation is conducted.