Design of a high capacity optical wireless line of sight communication link. Case study being Kampala, Uganda (A link between MTN Towers and the MTN Switching Centre in Mbuya)

Abstract

The increasing demand for wider bandwidths and higher transmission speeds has drawn attention to Optical Wireless Communication (OWC) which uses light as a carrier. This is because it guarantees higher capacity, cost effectiveness, high degree of security, flexibility and license-free operation compared to the traditional radio frequency (RF) communication systems. However, the major drawback of OWC in terrestrial applications is the threat of downtime caused by adverse weather conditions like rain and turbulence induced fluctuations called scintillations leading to fading. Therefore, a reliable high capacity link design after a proper analysis of the applicability of OWC in Uganda is required whereby the channel is represented by a mathematical model of the probability density functions (pdfs) of the fluctuating signal. Our major objective was to simulate a high capacity optical wireless line of sight communication link that takes into account the atmospheric weather effects in Uganda. The specific objectives were to compare two channel fading models and recommend an appropriate model for Uganda, design a link budget focusing on the case study. In our methodology, we identified the OWC channel parameters and the transmitter and receiver parameters obtaining their typical values from the literature and the case study. We computed the gains of the receiver and transmitter, free space loss, rain attenuation, beam divergence loss, power received and link margin as functions of these link parameters using their respective mathematical expressions. We then formulated a link budget of the OWC link. We used MATLAB software to simulate BER, outage probability and average capacity performance metrics of lognormal and gamma-gamma channel models as functions of the link parameters. For the case of weak turbulence conditions, the lognormal distribution model is used while for the case of moderate to strong turbulence strength, the gamma-gamma distribution is used. Based on the mathematical expressions and the case study, we present an accurate design for the estimation of the performance of the OWC link, taking into account realistic values for its parameters depending on the specific atmospheric conditions in the place where the OWC link will be installed. We therefore recommend the gamma-gamma model as the appropriate channel model for Uganda basing on the numerical results of the performance metrics.