Abstract:
Optical wireless, also known as free-space optics (FSO), has become very popular in recent years as a cost-effective, license-free communication technique for high-rate ‘last-mile’ terrestrial communications [1]. However, the performance of FSO suffers heavily from atmospheric turbulence induced fading. Furthermore, FSO links require accurate pointing which may be disrupted due to the sway of high-rise buildings caused by thermal expansion, dynamic wind loads and weak earthquakes. In this thesis, we investigated the effects of atmospheric turbulence and pointing error on a multiple-input multiple-output (MIMO) intensity modulated with direct-detection FSO link (IM/DD). Initially, the effect of pointing jitter was studied for a FSO link with and without diversity in the absence of atmospheric turbulence. The power penalty of the single-input single-output (SISO) on the system at various values of jitter standard deviation was evaluated at a given bit-error rate (BER) of 10-9. The effect of beam width and detector size was also considered in these calculations. Next, the improvement in system performance was evaluated by the use of multiple lasers at the transmitter and multiple photodetectors at the receiver. For MIMO FSO both equal gain combining (EGC) and optimal combining (OC) was considered at the receiver. Next, to include the effect of atmospheric turbulence, the combined channel state distribution was obtained numerically. The Gamma-Gamma distribution was used to model atmospheric turbulence at it has demonstrated excellent fit with measurement data for a wide range of turbulence conditions (weak to strong). BER performance for the system at different values of pointing jitter standard deviation and for different beam widths was evaluated for different turbulence conditions. From these the system’s penalty due to turbulence and pointing error was obtained. Finally, the performance evaluations were extended for MIMO FSO system to find the improvement in receiver performance in comparison to the system without diversity.
