1. Introduction

Telecommunication is one of the most important innovations in the history of mankind as it affords people the opportunity to communicate rapidly and reliably over long distances, often breaking physical and geographical barriers to make the world a global village as it is known today. One of the key ingredients in the heart of communication technologies over the last century is wireless communication. The advent of RF wireless communication techniques and protocols has been instrumental to the giant strides made in the communication domain as it eliminates the cumbrous requirement of lengthy wired connections, a requirement which has often been a great limitation for wired communication systems.

RF wireless communication systems enjoyed significant attention and penetration but soon became a victim of its success as more and more contents are demanded by users due to the proliferation of data, video, gaming and general broadband multimedia. These demands have prompted communication system engineers to explore

more efficient, faster and reliable wireless communication techniques. To this end, free space optical (FSO) communication has been a viable solution [1].

systems depending on the application for which they are deployed, and these applications include inter-chip communication, inter-vehicular communication, metropolitan area communication as well as satellite and space exploration.

Mitigating Turbulence-Induced Fading in Coherent FSO Links: An Adaptive Space-Time Code…

Concerted efforts being expended by researchers in the quest of effectively modeling the FSO channel are geared towards understanding the channel and serving as the template upon which FSO modulators, demodulators, receivers and other devices can work. Accurate mathematical models for FSO communication system are the basis upon which the development of high-performing hardware is established, despite huge technical challenge of turbulence-induced fading. This challenge is however being addressed using various techniques as summarized in

In the wavelength diversity schemes, the source information is encoded into different wavelengths obtainable from different constituents of the infrared

One of the most crucial steps in the attempts to mitigate the degradation in the

The lognormal distribution is one of the most widely employed for weak atmospheric turbulence distribution. Here, the irradiance value received at the receiver

> exp � ð Þ ln <sup>a</sup> <sup>þ</sup> <sup>2</sup>σ<sup>x</sup> <sup>2</sup> 8σ<sup>2</sup> x

!

<sup>x</sup> is the fading covariance.

(1)

performance of optical communication systems is accurate modeling of the atmospheric turbulence under different conditions. Below are some of the irradiance functions presented in terms of probability distribution functions.

> 1 2a 2πσ<sup>2</sup> x � �<sup>1</sup> 2

3. FSO system and channel models

DOI: http://dx.doi.org/10.5772/intechopen.84911

Figure 1.

spectrum.

Figure 1.

107

4. Turbulence models

4.1 Lognormal distribution

follows the distribution [9]

f <sup>A</sup>ð Þ¼ a

where μ<sup>x</sup> is the mean value of fading and σ<sup>2</sup>

Some turbulence mitigation techniques in FSO systems.

Free space optical communication is a communication technology that employs light as carrier by modulating baseband information with optical carriers often from laser beams through free space to the receiver [2]. The path of connection between FSO transmitters and the receivers are known as FSO links. Even though the very first optical system dates back to the eighteenth century, modern FSO communication systems were first widely deployed by the National Aeronautics and Space Administration (NASA) and have since become a promising broadband wireless access technology. FSO communication systems are also being combined with standard RF systems in order to form hybrid communication systems that harness the unique features of RF and FSO communication systems to enhance performance, capacity and reliability.

Characteristically, FSO communication systems are highly secure as they have high immunity to interference with the use of secure point-to-point line-of-sight links, they require no licensing or regulatory permission, and they are fast and can be easily deployed and operated compared to other systems like the fiber optic systems [3]. These features make FSO communication the favored option for the provision of high-speed links for a variety of next generation optical applications including broadcast, security, wireless backhaul at a data rate as high as 40 Gbps [4], fiber backup and last mile communication [5]. Finally, the ease of setup and cost effectiveness of FSO systems have made them the preferred option for restoring connection in case of disaster.

However, the performance of FSO communication systems are greatly affected by turbulence-induced fading [6–12], and different investigations are currently being explored to address this challenge. The inhomogeneity of the temperature and pressure of the atmosphere causes local variations in the refractive index as light propagates from the transmitter to the photoreceptor; these variations degrade the performance of FSO links significantly.

Geared towards the improvement of the performance of coherent FSO communication systems in the presence of atmospheric turbulence, this work examines the error reduction schemes currently being employed for FSO links and presents an adaptive space-time trellis code (STTC) scheme for coherent FSO links.

### 2. Free space optical communication: types and variants

In terms of reception technique, however, the most commonly reported variants of FSO communication systems are the direct detection/intensity modulated (IM/DD) FSO system and the coherent FSO system. The IM/DD FSO communication systems convey the information to be transmitted only on the intensity of the emitted light, and the receivers simply decode the information as the light changes in intensity. In coherent FSO communication systems, however, other signal properties such as phase and frequency may be employed in conveying the information. At the receiving end, as against simply observing changes in light intensity as in the case of IM/DD FSO systems, coherent FSO communication systems, first, mix the received field optically with a local oscillator before the actual photodetection. So far, more works on the IM/DD FSO communication system are being reported in literature owing to its simplicity of detection as less complex receivers and algorithms are required. Coherent FSO systems, though more complex, however, offer superior performance in terms of improved receiver sensitivity and background noise rejection [13].

Finally, in terms of communication range, FSO communication systems can be characterized into short-range, medium-range, long-range and inter-terrestrial FSO Mitigating Turbulence-Induced Fading in Coherent FSO Links: An Adaptive Space-Time Code… DOI: http://dx.doi.org/10.5772/intechopen.84911

systems depending on the application for which they are deployed, and these applications include inter-chip communication, inter-vehicular communication, metropolitan area communication as well as satellite and space exploration.
