Tracking control of the mobile terminal in an active free-space optical communication system

This paper proposes a tracking control method for an active free space optics (FSO) telecommunication system that enables a mobile terminal to be tracked in user network areas with short-range coverage. The active FSO system consists of paired terminals comprising a transmitter with a laser diode (LD) and a receiver with a photo detector (PD). Each terminal incorporates a mechanism that controls the path of the laser beam emitted from the LD so as to align it with the optical axis of the PD regardless of positional changes between the terminals. This provides the system with a high-speed and ubiquitous optical communication capability over a conventional wireless network with a lower bit rate for control signal transmission. The axis alignment control requires the relative position and orientation between the terminals, which can be estimated on-line with the proposed method by using an extended Kalman filter. The performance of the tracking control method is confirmed experimentally using a prototype of the active FSO system.