Free Space Optical Communication

The last few decades have seen rapid advances in information and communication technology. We commonly use broadband technology with high-speed Internet connectivity at our homes, offices, and in our mobile devices. The bandwidth and high-capacity requirements due to the increased use of Internet and broadband services have exceeded our expectations in twenty-first century. Wireless optical communication (WOC) uses optical carrier in the near-infrared (IR) and visible bands and is considered a viable solution for realizing very high-speed and large-capacity communication links. It is a line-of-sight communication using a laser to transmit the information signal between two transceivers over an unguided channel which may be either the atmosphere or free space.

This chapter focuses on statistical description, physical characteristics, and modeling of free-space optical channel. The primary factors characterizing an atmospheric communication channel include atmospheric attenuation (both due to scattering and absorption) and turbulence. This chapter will provide good understanding of various types of atmospheric losses due to absorption, scattering, and turbulence. Section 2.1 presents various types of atmospheric losses due to molecular constituents and particulates present in the atmosphere. Although absorption and scattering significantly decrease the power level of the transmitted beam, the random fluctuations in the intensity of received signal due to turbulence in the atmosphere can severely degrade the wavefront quality of the transmitted beam. Statistical description of atmospheric turbulence and its effect on Gaussian beam will be discussed in this section. Section 2.2 presents various turbulence channel models. Finally, Sect. 2.3 describes various techniques to mitigate the effect of atmospheric turbulence.

The link design plays an important role in the implementation of any FSO system. Before designing a link, the requirements have to be specified very carefully and clearly understood to build up system modules in a cost-effective way to meet the given requirements. Once the requirements, e.g., data rate, range, bit error rate, acquisition time, etc., are fixed, the system-level design implementation goes through various levels of iterations until the “best” solution that fits the requirement set is identified. In this chapter, the details of FSO system modules, technology options, and link design issues are discussed.

For ground-to-satellite laser communication link, it is essential to establish line-of-sight (LOS) path between ground-based station and the onboard satellite to ensure reliability of the system. Since the optical beam is very narrow, it requires a very accurate acquisition, tracking, and pointing (ATP) subsystems especially for long-distance communication. The process of aligning the transmitter in the field of view (FOV) of receiver is called “pointing,” and the process of aligning the receiver in the arrival direction of the beam is called “spatial acquisition.” It is a process defined by the ability of the receiver to search within initial uncertainty region for detecting the optical beam from the transmitter. Maintaining both “pointing” and “acquisition” throughout the communication time period is referred as “spatial tracking.” For FSO communication system, it is desirable to have an efficient performance of acquisition system within shortest acquisition time. Very fast and precise pointing and tracking system with good accuracy has to be incorporated into the system to maintain the LOS between transmitter and receiver. One of the major sources of pointing errors also called “boresight errors” arises due to stress, noise, structure fabrication, etc., of various electronic and mechanical components in the satellite. The pointing error is further increased due to turbulence-induced beam wander (discussed in Chap. 2) which may deviate the optical beam from its LOS. Further, the vibration of satellite platform and ATP assembly may complicate the pointing process and results in random pointing jitter. This will lead to significant displacement of transmitted beam at the receiver which will increase pointing error and cause significant degradation of the system performance. This chapter begins with the description of acquisition link in Sect. 4.1. It covers mathematical modeling, beam divergence, and power criteria required to establish the acquisition link. Details of tracking and pointing errors are given in Sect. 4.2. In Sect. 4.3, the concept and importance of ATP system is highlighted. Finally, ATP link budget is discussed in Sect. 4.4.

The turbulence in an FSO link causes fluctuations in the intensity and the phase of the received signal. These fluctuations ultimately lead to the degradation in the link error probability and limit the system performance. In this chapter, bit error rate (BER) performance of coherent subcarrier (SC-BPSK and SC-QPSK) and noncoherent (OOK and \(\mathbb{M}\)-PPM) schemes in turbulent atmospheric environment have been analyzed. Multilevel modulation schemes which are more bandwidth efficient are also discussed in this chapter. Section 5.1 gives the system model which includes a brief description on transmitter, atmospheric channel, and receiver. In Sect. 5.2, the BER performance of FSO system in atmospheric turbulent channel for both coherent (SC-BPSK and SC-QPSK) and noncoherent (OOK and \(\mathbb{M}\)-PPM) modulation schemes have been analyzed. Other bandwidth-efficient modulation techniques like pulse- amplitude modulation (PAM), differential pulse-position modulation (DPPM), and differential amplitude pulse position modulation (DAPPM) are discussed in this section. Finally, in the last section, BER performance of different modulation schemes are summarized.

As seen in the previous chapter, turbulence in the atmosphere causes random fluctuations of the phase and amplitude of the received signal leading to deep signal fades or surges. These fluctuations of the received signal severely degrade the link performance, particularly over a link distance of 1 km or longer or if communication is taking place with moving platform. Moreover, the propagating pulse may experience pulse broadening due to atmospheric scattering that further deteriorates the performance of the received signal and limits the application of FSO to short-range links. Various techniques have been proposed in literature to mitigate the effect of atmospheric turbulence like aperture averaging, diversity, channel coding, adaptive optics, etc.

Aperture averaging technique is the simplest form of spatial diversity where the size of receiver aperture is larger than the fading correlation length. This technique averages the scintillation and thus significantly reduce the atmospheric turbulence especially in strong turbulence. Diversity technique makes use of multiple transmitter or receiver or both, and it can significantly combat the atmospheric fading by creating additional spatial degree of freedom. It allows multiple beams to propagate through different atmospheric spatially coherent cells, and, therefore, the likelihood that all the beams are simultaneously being affected by turbulent atmosphere is reduced than that of single beam. Channel coding like Reed-Solomon codes, LDPC codes, etc., significantly improves the bit error rate of the system ranging from 5 to 15 dB depending upon the strength of turbulence in the atmosphere. In the subsequent sections, all these performance improvement techniques will be discussed in details.

When an optical signal propagates from ground to satellite, various effects of atmosphere, viz., absorption, scattering, and turbulence, will degrade the link performance. The loss arising due to beam propagation in space and effects of atmospheric turbulence are considered in link feasibility study. In this chapter, the received optical signal power in the presence of various losses and background noise are evaluated for a given BER in weak atmospheric turbulence. Subsequently, link power budget is made and the link power margin is determined. Improvement in the link margin has been worked out with transmit diversity and coding scheme when used separately.