Despite the advantages and applications of FSO communication systems mentioned in the previous section, this technology presents some critical issues and limitations that must be commented. FSO is a line-of-sight (LOS) technology and, hence, transmitter and receiver must be free from physical obstructions. In other words, they must see each other. In addition, this technology must be designed to combat changes in the atmosphere that can deteriorate the performance.
One of the major challenges in FSO communication systems is the weather-dependent per-formance where heavy fog may result in a link outage due to the fact that fog can modify
1.2. CHALLENGES AND AIMS 5 the light characteristics. Rain and snow have little effect on FSO communication systems.
Another one is to mitigate the combined effect of atmospheric turbulence and dynamic misalignment to increase the distance, rate and reliability. The atmospheric turbulence pro-duces random fluctuations in the irradiance, also known as scintillation, beam spreading, beam wander, among others effects, as a result of variations in the refractive index along the communication link. The scintillation process produces random fluctuations in both the amplitude and the phase of the received optical signal; the effect of beam spreading is related to a loss of power at the receiver side caused by diffraction; and the effect of beam wander is related to an angular deviation of the beam from LOS [12,13]. Atmospheric turbulence along with absorption and scattering are considered as the three basic processes that affect optical wave propagation in the literature. Absorption and scattering refer to wavelength- and weather-dependent attenuation of optical wave. In addition to the effect of atmospheric turbulence, FSO communication links are strongly affected by pointing errors, resulting in serious misalignment of fixed-position laser communication systems.
Other challenges in FSO systems are related to safety and physical obstructions. Due to eye safety regulations, FSO systems are limited in the transmitted optical power since this technology uses lasers for transmission [10]. Physical obstructions such as birds, tree limbs, or other factors can temporarily or permanently block the laser LOS.
Finally, another challenge in FSO communication is to find the maximum reliable data rates, i.e., average channel capacity. As a fundamental step to achieve this, an accurate optical channel model is required for the atmospheric turbulence model and dynamic misalignment.
In general, the main objective of this thesis is to present advances in the field of FSO communication systems in order to cope with these challenges. This thesis is aimed at investigating the performance of advanced FSO communication systems with the goal of analyzing their benefits and limitations from a practical point of view. Therefore, the analysis and design of these communication systems are carried out, developing new results in two major areas:
(a) Ergodic capacity analysis.
(b) Generalized misalignment fading model for terrestrial FSO links.
On the one hand, the study of ergodic capacity represents the first research area of this thesis in which we analyze not only the ergodic capacity of FSO communication systems based on spatial diversity such as multiple-input/single-output (MISO), single-input/multiple-output (SIMO) and multiple-input/multiple-output (MIMO) FSO systems, but also the ergodic capacity of cooperative FSO systems based on detect-and-forward (DF) relaying. Hence, the objective of this research area is twofold: firstly, to develop new closed-form expressions that allow us to compute the ergodic capacity over the whole range of SNR values, as well as to study how this one is deteriorated by the effect of atmospheric turbulence and,
furthermore, how this one is also enhanced in relation to the ergodic capacity obtained by a single-input/single-output (SISO) FSO system; secondly, to include the effect of pointing errors in the study of the ergodic capacity (this effect has not been taken into account in the literature), as well as to include the effect of nonzero boresight pointing errors on SIMO and MIMO FSO systems, i.e., systems with more than one receiver aperture. The use of a more sophisticated pointing error model for the study of the ergodic capacity of SIMO and MIMO FSO systems has led to a second line of research that is focused on the modeling of generalized pointing errors.
On the other hand, the second research area of this thesis is the modeling of generalized pointing errors. Over the last decade, different statistical models have been proposed in the literature to model pointing errors. These models have been used in a large number of research articles with the goal of adding this effect to different studies, and giving them a higher degree of realism. Incorporating the effect of pointing errors results in an added difficulty in developing new closed-form expressions for the performance evaluation of FSO communication systems. Therefore, including such effect in the performance analysis is not only to give a greater degree of realism to the performance analysis, but also represents a great challenge from a mathematical point of view. In the general case, pointing errors are distributed according to the Beckmann distribution which is the focus of this research line.
In order to achieve all this, a number of research objectives have been set, which are outlined as follows:
1. Review the fundamental aspects of terrestrial FSO links and the statistical models that describe the atmospheric turbulence and dynamic misalignment. At the same time, understanding the limits and range of validity of each of the statistical models.
2. Investigate either approximate or exact closed-form expressions for the bit error-rate (BER), outage probability and ergodic capacity in order to evaluate the performance of FSO communication systems over atmospheric turbulence channels with pointing error effects. Moreover, all the theoretical studies derived from this thesis for the performance are verified by using Monte Carlo simulation.
3. Investigate the ergodic capacity of MISO, SIMO, MIMO and cooperative systems based on DF for FSO communication. The study of the ergodic capacity represents one of the great challenges of this thesis.
4. Investigate a new statistical model for generalized pointing errors that can be used in an easy way to study more complex FSO scenarios. In the general case, generalized pointing errors are modeled by the Beckmann distribution which is intractable from a mathematical point of view due to the fact that its density function appears in integral-form. This is another great challenge of this thesis, i.e, finding out an efficient way of including generalized pointing errors in FSO system design.