Applied Signal Processing Group

In Free-Space Optical (FSO) communication, information data is transmitted wirelessly using laser beams. Such links have the potential to provide virtually unlimited bandwidth without the expense of buried fibre optic cabling. Unfortunately, FSO link reliability is highly dependent on atmospheric conditions such as turbulence, rain, cloud, dust and fog. These effects cause fading of the received laser beam. In typical FSO channels, a deep fade can cause the loss of millions of data bits. Improvement in reliability can be gained by using multiple lasers and multiple apertures to create a multiple-input multiple output (MIMO) FSO channel. However, the atmospheric effects in multiple-laser, multiple-aperture FSO systems are largely unknown, and experimental analysis is lacking.

A comprehensive characterisation of this channel will be crucial in order to exploit the full capabilities of MIMO FSO links. This project aims to investigate these aspects by collecting channel measurements from both terrestrial and (if available) satellite-to-ground FSO links equipped with multiple-lasers and multiple detectors. Key parameters of interest are the temporal and spatial fading and background noise statistics under a range of climatic and environmental variations, including daily and seasonal variations for both clear-sky and impaired situations due to dust or clouds, for a variety of propagation lengths. Using these channel measurements, statistical models will be developed that describe the spatial-temporal behaviour of MIMO FSO channels.

Although free-space optical (FSO) links can usually support huge data rates, their performance is affected by fadingdue to atmospheric turbulence, plus meteorological conditions such as dust, clouds and fog. Whilstfading due to atmospheric turbulence can be mitigated using multiple-lasers and multiple-apertures,the large signal attenuation due to fog and cloud cover poses a formidable challenge. It appears theonly way around these problems is to use an additional radio frequency (RF) link, creating a hybrid FSO/RF channel. Inparticular, high frequency Ka-band (18-40 GHz) or V-band (40-70 GHz) links are most favourable forthis purpose to maintain data rates comparable to that of the FSO link.

At these frequencies, the FSOand RF channels operate in a complementary manner to each other. For example Ka-band RF linksare less affected by fog or cloud, but can be seriously degraded by rain. FSO links do not penetratefog/cloud well, but are reasonably resilient to rain. Thus, practical FSO systems are likely to use ahybrid approach that intelligently combines the advantages of FSO and RF links.Despite the obvious intuitive advantages of hybrid FSO/RF links, little is known regarding how tomodel the combined statistical behaviour of the component channels, and even less is known on howto efficiently combine them to improve system reliability. The aim of project is to characterise the jointstatistics of hybrid FSO/RF channels under a variety of atmospheric conditions. This will involve collecting sample measurements from simultaneous FSO and Ka-band RF transmissionsover a long propagation distance (approximately 20km) and mathematical modelling of the observed data.

The use of MIMO and hybrid RF/FSO techniques has great potential in significantly improving the reliability of free-space optical communication.However, very little is known on how to efficiently transmit information reliably using these channels.

This project will involve an information theoretic analysis of these channels to determinekey design parameters that underpin efficiency and reliability of the transmission of information. In particular the project will investigate the affects of non-ideal photodetection, scintillation, spatial and temporal correlation on the fundamental limits of FSO communication. Novel transmission strategies will then be investigated that achieve as close as possible to these fundamental limits, whilst remaining feasible to implement in practice.