Performance analysis of a free space optical link with space time block code

Abstract

With the worldwide demand for larger bandwidth and greater mobility there is a rapid advancement in the area of broadband wireless communications. The high capacity and low loss of optical fiber has seen its exploding growth in the last few decades in the WANs and LANs. Free space optical (FSO) wireless communication has emerged as a viable technology for bridging the gap in existing high data rate fiber network and as a temporary backbone for rapidly deployable mobile wireless communication infrastructure. We believe that FSO will be one of the most unique and powerful tools to address connectivity bottlenecks that have been created in high-speed networks during the past decade due to the tremendous success and continued acceptance of the Internet. However, optical wave propagation through the air experiences fluctuation in amplitude and phase due to atmospheric turbulence. The intensity fluctuation, also known as scintillation is one of the most important factors that degrade the performance of an FSO communication link even under the clear sky condition. FSO systems offer capacities in the range of 100Mbps to 2.5 Gbps, and data rates as high as 160 Gbps, enabling optical transmission up to 2.5 Gbps of data, voice, and video communications through the air, thus allowing optical connectivity without deploying fiber optic cables or securing spectrum licenses. In this thesis, the performance of an FSO link using Alamouti-type Space Time Block Code (STBC) over log-normal atmospheric turbulence-induced fading channels has been investigated. Based on the modification of the Alamouti code presented by Simon and Vilnrotter, a generalized approach is adopted to consider space-time coded on-off keying (OOK) formats for improvement the Bit Error Rate (BER) performance. It is shown that the deployment of modified space-time coding is necessary for a FSO IM/DD link. The added diversity contributes a significant improvement in BER performance.