Free space optical communication has become an attractive solution for data transmission due to its cost-effective, license-free and high bandwidth characteristics. The performance of such systems, however, is highly affected by the effects of optical turbulence. Multiple laser transmitters and/or multiple aperture receivers can be used to mitigate the turbulence fading by exploiting the advantages of spatial diversity. In this work, the performance of a link employing spatial diversity at the receiver was investigated by analyzing two figures of merit: Bit Error Rate and Link Availability. The statistical properties of the photocurrent generated at the receiver – required for evaluating Bit Error Rate and Link Availability – were obtained by means of computer simulation using the Monte Carlo method. The results presented here can be very helpful in designing a free space optical link with spatial diversity, since they are given in function of system configurations and impairment conditions.
Recent studies have been carried out, in which the statistical properties of the received signal of free space
communication systems are assessed by means of simulations. The obtained results, however, have not brought
information about the temporal behavior of the generated photocurrent. This work describes how this can be
accomplished through stochastic processes simulations. Although most of the theory on stochastic field simulation has
been developed for application in mechanics and fluids engineering, it is well suited for signal analysis in wireless
optical systems. Basically, if it is possible to artificially create a signal with realistic properties that is transmitted over
the atmospheric channel, it can be used for system analysis and design without the need of mounting a real transceiver.
This dramatically reduces development costs and time. A technique that has been widely applied in engineering is the
spectral representation method. The concept of spectral representation of Gaussian random processes was introduced in
1944. Its use, however, in generating simulations of random processes was only proposed in 1972. This method deals
with the summation of a large number of weighted trigonometric functions. This work shows how to apply this technique
in order to assess the generation of turbulence-corrupted signals using the above mentioned method and its application in simulating data transmission over the atmospheric channel. As figure of merit, system Bit Error Rate and the eye-figure
of the received signal are obtained.
In this work a theoretical investigation regarding the availability of a Free Space Optical communication system was
carried out. The statistical distribution of the photocurrent generated at the receiver by a Gaussian beam after propagating
through the random medium was investigated. The photocurrent's relative frequencies were obtained by means of
computer simulation and the cases where beam wandering was present were taken into account. An experimental set up
was mounted for comparisons purposes. Finally, with the knowledge of the photocurrent statistical distribution, the
system availability and the Bit Error Rate were estimated under several system configurations and impairments
In this work the statistical distribution of the photocurrent generated at the receiver by detecting a Gaussian beam wave
after propagating through the turbulent medium was investigated. The results are presented as photocurrent's relative
frequencies obtained by means of computer simulation. It was taken into account the cases where random beam
misalignment was present. With knowledge of the photocurrent statistical distribution the system bit error rate (BER),
which is an important parameter of a digital communication system, was obtained. An experimental set up was mounted
in order to collect data for comparisons with the theoretical results. The normalized relative frequencies of both
simulated and measured current were plotted in the same graphic. The distributions have shown to be very similar to
each other, what leads to the conclusion that the theory provides reliable results.