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Free Space Optical Communication: Challenges and
Mitigation Techniques
Hemani Kaushal1and Georges Kaddoum2
1Department of Electrical, Electronics and Communication Engineering, ITM University, Gurgaon, Haryana,
India-122017.
2Département de génie électrique, École de technologie supérieure, Montréal (Qc), Canada
Abstract—In recent years, free space optical (FSO)
communication has gained significant importance owing to
its unique features: large bandwidth, license free spectrum, high
data rate, easy and quick deployability, less power and low mass
requirement. FSO communication uses optical carrier in the
near infrared (IR) and visible band to establish either terrestrial
links within the Earth’s atmosphere or inter-satellite/deep space
links or ground-to-satellite/satellite-to-ground links. It also find
its applications in remote sensing, radio astronomy, military,
disaster recovery, last mile access, back-haul for wireless cellular
networks and many more. However, despite of great potential
of FSO communication, its performance is limited by the
adverse effects (viz., absorption, scattering and turbulence)
of the atmospheric channel. Out of these three effects, the
atmospheric turbulence is a major challenge that may lead to
serious degradation in the bit error rate (BER) performance of
the system and make the communication link infeasible. This
paper presents a comprehensive survey on various challenges
faced by FSO communication system for both terrestrial and
space links. It will provide details of various performance
mitigation techniques in order to have high link availability
and reliability of FSO system. The first part of the paper will
focus on various types of impairments that poses a serious
challenge to the performance of FSO system for both terrestrial
and space links. The latter part of the paper will provide the
reader with an exhaustive review of various techniques used
in FSO system both at physical layer as well as at the upper
layers (transport, network or link layer) to combat the adverse
effects of the atmosphere. Further, this survey uniquely offers
the current literature on FSO coding and modulation schemes
using various channel models and detection techniques. It also
presents a recently developed technique in FSO system using
orbital angular momentum to combat the effect of atmospheric
turbulence.
Index Terms—Free space optical communication, atmospheric
turbulence, aperture averaging, diversity, adaptive optics,
advanced modulation and coding techniques, hybrid RF/FSO,
ARQ, routing protocols, orbital angular momentum.
I. INTRODUCTION
A. FSO Communication - An Overview
In the recent few years, tremendous growth and
advancement has been observed in information and
communication technologies. With the increase in usage
of high speed internet, video-conferencing, live streaming
etc., the bandwidth and capacity requirements are increasing
drastically. This ever growing demand of increase in data and
multimedia services has led to congestion in conventionally
used radio frequency (RF) spectrum and arises a need to
shift from RF carrier to optical carrier. Unlike RF carrier
where spectrum usage is restricted, optical carrier does not
require any spectrum licensing and therefore, is an attractive
prospect for high bandwidth and capacity applications.
“Wireless optical communication” (WOC) is the technology
that uses optical carrier to transfer information from one
point to another through an unguided channel which may
be an atmosphere or free space. WOC is considered as a
next frontier for high speed broadband connection as it offers
extremely high bandwidth, ease of deployment, unlicensed
spectrum allocation, reduced power consumption (∼1/2 of
RF), reduced size (∼1/10 the diameter of RF antenna) and
improved channel security [1]. It can be classified into
two broad categories, namely indoor and outdoor wireless
optical communications. Indoor WOC uses IR or visible light
for communicating within a building where the possibility
of setting up a physical wired connection is cumbersome
[2]–[9]. Indoor WOC is classified into four generic system
configurations i.e., directed line-of-sight (LOS), non-directed
LOS, diffused and tracked. Outdoor WOC is also termed
as free space optical (FSO) communication. The FSO
communication systems are further classified into terrestrial
and space optical links that include building-to-building,
ground-to-satellite, satellite-to-ground, satellite-to-satellite,
satellite-to-airborne platforms (unmanned aerial vehicles
(UAVs) or balloons), [10]–[12] etc. Fig. 1 illustrates the
classification of WOC system.
Wireless Optical Communication System
Indoor System
Outdoor System
(FSO)
Terrestrial
Links
Space
Links
Inter-Orbital
Links (IOL)
Inter-Satellite
Links (ISL)
Deep Space
Links (DSL)
Directed
LOS
Non-
Directed
LOS
Diffused Tracked
Figure 1. Classification of wireless optical communication system
The FSO communication provides LOS communication
owing to its narrow transmit beamwidth and works in visible
and IR spectrum. The basic principle of FSO transmission is
similar to fiber optic communication except that unlike fiber
arXiv:1506.04836v1 [cs.IT] 16 Jun 2015
2
transmission, in this case the modulated data is transmitted
through unguided channel instead of guided optical fiber.
The initial work on FSO communication started almost
50 years back for defense and space applications where
US military used to send telegraph signal from one point
to another using sunlight powered devices. In year 1876,
Alexander Graham Bell demonstrated his first wireless
telephone system [13], [14] by converting sound waves to
electrical telephone signals and transmitted the voice signal
over few feets using sunlight as carrier. The device was called
“photo-phone” as it was the world’s first wireless telephone
system. Thereafter, with the discovery of first working
laser at Hughes Research Laboratories, Malibu, California
in 1960 [15], a great advancement was observed in FSO
technology. Large number of experiments were performed
in military and aerospace laboratories that demonstrated
ground-to-satellite, satellite-to-ground, satellite-to-satellite and
ground-to-ground links. It has resulted in various successful
experimentations like (i) airborne flight test system (AFTS)-
a link between aircraft and ground station at New
Mexico [16], (ii) laser cross link system (LCS)- full
duplex space-to-space link for geosynchronous system [17],
(iii) ground/orbiter lasercom demonstration (GOLD)- first
ground-to-space two way communication link [18], [19],
(iv) optical communication demonstrator (OCD)- laboratory
prototype for demonstrating high speed data transfer
from satellite-to-ground, (v) stratospheric optical payload
experiment STROPEX (CAPANINA Project)- high bit rate
optical downlink from airborne station to transportable
optical ground station [20], (vi) Mars laser communications
demonstration (MLCD)- provides up to 10 Mbps data transfer
between Earth and Mars [21], and (vii) airborne laser optical
link (LOLA)- first demonstration of a two-way optical link
between high altitude aircraft and GEO satellite (ARTEMIS)
[22]. Another mission by NASA is laser communication relay
demonstration (LCRD) to be launched in 2017 that will
demonstrate optical relay services for near earth and deep
space communication missions [23].
Over the last few years, massive expansion in FSO
technology has been observed due to huge advancement in
opto-electronic components and tremendous growth in the
market offering wireless optical devices. FSO communication
system seems to be one of the promising technology for
addressing the problem of huge bandwidth requirements
and “last mile bottleneck”. Commercially available FSO
equipments provide much higher data rates ranging from 10
Mbps to 10 Gbps [24], [25]. FSO link has been demonstrated
in laboratory up to 80 Gbps with average BER of 10−6without
forward error correction (FEC) coding [26]. Many optical
companies like Lightpointe in San Diego, fSONA in Canada,
CableFree Wireless Excellence in UK, AirFiber in California
etc., provide wide range of wireless optical routers, optical
wireless bridges, hybrid wireless bridges, switches etc., that
can support enterprise connectivity, last mile access, HDTV
broadcast link with almost 100% reliability in adverse weather
conditions.
B. Advantages of FSO Communication over RF
Communication
FSO communication system offers several advantages
over RF system. The major difference between FSO and
RF communication arises from the large difference in the
wavelength. For FSO system, under clear weather conditions
(visibility > 10 miles), the atmospheric transmission window
lies in the near infrared wavelength range between 700 nm
to 1600 nm. The transmission window for RF system lies
between 30 mm to 3 m. Therefore, RF wavelength is thousand
of times larger than optical wavelength. This high ratio of
wavelength leads to some interesting differences between the
two systems as given below:
(I) Huge modulation bandwidth: It is a well known
fact that increase in carrier frequency increases the
information carrying capacity of a communication
system. In RF and microwave communication systems,
the allowable bandwidth can be up to 20% of the
carrier frequency. In optical communication, even if the
bandwidth is taken to be 1% of carrier frequency (≈1016
Hz), the allowable bandwidth will be 100 THz. This
makes the usable bandwidth at an optical frequency in
the order of THz which is almost 105times that of a
typical RF carrier [27].
(II) Narrow beam divergence: The beam divergence is
proportional to λ/DR, where λis the carrier wavelength
and DRthe aperture diameter. Thus, the beam spread
offered by the optical carrier is narrower than that of RF
carrier. This leads to increase in the intensity of signal at
the receiver for a given transmitted power. Fig. 2 shows
the comparison of beam divergence for optical and RF
signals when sent back from Mars towards Earth [28].
~ 100 Earth Diameter
Beam Divergence (θdiv) = 2.44 (λ/DR)
~ 0.1 Earth Diameter
Earth
Earth
Mars
Mars
RF link
Optical Link
Figure 2. Comparison of optical and RF beam divergence from Mars towards
Earth [28]
(III) Less power and mass requirement: For a given
transmitter power level, the optical intensity is more at
the receiver due to its narrow beam divergence. Thus, a
smaller wavelength of optical carrier permits the FSO
designer to come up with a system that has smaller
antenna than RF system to achieve the same gain (as
antenna gain scales inversely proportional to the square
of operating wavelength). The typical size for the optical
system is 0.3 m vs 1.5 m for the spacecraft antenna [18].
3
Table I gives the power and mass comparison between
optical and RF communication systems using 10 W and
50 W for optical and Ka band systems, respectively at
2.5 Gbps.
Link
Optical
RF
GEO-LEO
Antenna Diameter
Mass
Power
10.2 cm (1.0)
65.3 kg (1.0)
93.8 W (1.0)
2.2 m (21.6)
152.8 kg (2.3)
213.9 W (2.3)
GEO-GEO
Antenna Diameter
Mass
Power
13.5 cm (1.0)
86.4 kg (1.0)
124.2 W (1.0)
2.1 m (15.6)
145.8 kg (1.7)
204.2 W (1.6)
LEO-LEO
Antenna Diameter
Mass
Power
3.6 cm (1.0)
23.0 kg (1.0)
33.1 W (1.0)
0.8 m (22.2)
55.6 kg (2.4)
77.8 W (2.3)
Table I
COMPARISON O F POW ER AN D MA SS FO R GE OSTATI ONA RY EARTH ORBIT
(GEO) AND LOW EART H OR BIT (LEO) LINKS USING OPTICAL AND R F
COMMUNICATION SYSTEMS (VALUES IN PARENTHESES ARE NORMALIZED
TO THE OPTICAL PARAMETERS)
(IV) High directivity: Since the optical wavelength is very
small, a very high directivity is obtained with small sized
antenna. The directivity of antenna is closely related to
its gain. The advantage of optical carrier over RF carrier
can be seen from the ratio of antenna directivity as given
below
Gain(optical)
Gain(RF)
=4π/θ2
div(optical)
4π/θ2
div(RF)
,(1)
where θdiv(optical) and θdiv(RF) are the optical and RF
beam divergence, respectively and are proportional to
λ/DR.
(V) Unlicensed spectrum: In RF system, interference
from adjacent carrier is the major problem due to
spectrum congestion. This requires the need of spectrum
licensing by regulatory authorities. But on the other hand,
optical system is free from spectrum licensing till now.
This reduces the initial set up cost and development time
[29].
(VI) High Security: FSO communication can not be
detected by spectrum analyzers or RF meters as FSO
laser beam is highly directional with very narrow beam
divergence. Any kind of interception is therefore very
difficult. Unlike RF signal, FSO signal cannot penetrate
walls which can therefore prevent eavesdropping [30].
In addition to the above advantages, FSO communication
offers secondary benefits as: (i) easily expandable and reduces
the size of network segments, (ii) light weight and compact,
(iii) easy and quick deployability, and (iv) can be used
where fiber optic cables cannot be used. However, despite
of many advantages, FSO communication system has its
own drawbacks over RF system. The main disadvantage is
the requirement of tight acquisition, tracking and pointing
(ATP) system due to narrow beam divergence. Also, FSO
communication is dependent upon unpredictable atmospheric
conditions that can degrade the performance of the system.
Another limiting factor, is the position of Sun relative to
the laser transmitter and receiver. In a particular alignment,
solar background radiations can increase and that will lead to
poor system performance [31]. This undoubtedly poses a great
challenge to FSO system designers.
C. Choice of wavelength in FSO communication
Wavelength selection in FSO communication is very
important design parameter as it affects link performance
and detector sensitivity of the system. Since antenna gain
is inversely proportion to operating wavelength, therefore,
it is more beneficial to operate at lower wavelengths.
However, higher wavelengths provide better link quality
and lower pointing induced signal fades [32]. Therefore, a
careful optimization of operating wavelength in the design
of FSO link helps in achieving better performance. The
choice of wavelength strongly depends on atmospheric
effects, attenuation and background noise power. Further,
the availability of transmitter and receiver components, eye
safety regulations and cost deeply impacts the selection of
wavelength in FSO design process.
The International Commission on Illumination [33] has
classified optical radiations into three categories: IR-A (700
nm to 1400 nm), IR-B (1400 nm to 3000 nm) and IR-C
(3000 nm to 1 mm). It can sub-classified into (i) near-infrared
(NIR) ranging from 750 nm to 1450 nm is a low attenuation
window and mainly used for fiber optics, (ii) short-infrared
(SIR) ranging from 1400 nm to 3000 nm out of which 1530
nm to 1560 nm is a dominant spectral range for long distance
communication, (iii) mid-infrared (MIR) ranging from 3000
nm to 8000 nm is used in military applications for guiding
missiles, (iv) long-infrared (LIR) ranging from 8000 nm to
15 µm is used in thermal imaging, and (v) far-infrared (FIR)
is ranging from 15 µm to 1 mm. Almost all commercially
available FSO system are using NIR and SIR wavelength
range since these wavelengths are also used in fiber optic
communication and their components are readily available in
market.
The wavelength selection for FSO communication has to
be eye and skin safe as certain wavelengths between 400 nm
to 1500 nm can cause potential eye hazards or damage to
retina [34]. Under International Electrotechanical Commission
(IEC), lasers are classified into four groups from Class 1 to
Class 4 depending upon their power and possible hazards [35].
Most of the FSO system uses Class 1 and 1M lasers. For same
safety class, FSO system operating at 1500 nm can transmit
more than 10 times optical power than system operating at
shorter operating wavelengths like 750 nm or 850 nm. It is
because cornea, the outer layer of the eye absorb the energy of
the light at 1550 nm and does not allow it to focus on retina.
Maximum possible exposure (MPE) [36] specifies a certain
laser power level up to which person can be exposed without
any hazardous effect on eye or skin. Table II summarizes
various wavelengths used in practical FSO communication for
space applications.
4
Mission Laser Wavelength Other parameters Application
Semi-conductor
Inter-satellite Link
Experiment (SILEX) [37]
AlGaAs laser
diode 830 nm
60 mW, 25 cm telescope size,
50Mbps, 6 µrad divergence, direct
detection
Inter-satellite
communication
Ground/Orbiter
Lasercomm
Demonstration (GOLD)
[19]
Argon-ion
laser/GaAs laser
Uplink: 514.5 nm
Downlink: 830 nm
13 W, 0.6 m and 1.2m tx. and rx.
telescopes size, respectively, 1.024
Mbps, 20 µrad divergence
Ground-to-satellite link
RF Optical System for
Aurora (ROSA) [38]
Diode pumped
Nd:YVO4 laser 1064 nm
6 W, 0.135 m and 10 m tx. and
rx. telescopes size, respectively,
320 kbps
Deep space missions
Deep Space Optical Link
Communications
Experiment (DOLCE)
[39]
Master oscillator
power amplifier
(MOPA)
1058 nm 1 W, 10-20 Mbps Inter-satellite/deep space
missions
Mars Orbiter Laser
Altimeter (MOLA) [40]
Diode pumped Q
switched
Cr:Nd:YAG
1064 nm
32.4 W, 420 µrad divergence, 10
Hz pulse rate, 618 bps, 850 µrad
receiver field of view (FOV)
Altimetry
General Atomics
Aeronautical Systems
(GA-ASI) & TESAT [41]
Nd:YAG 1064 nm 2.6 Gbps Remotely piloted aircraft
(RPA) to LEO
Altair UAV-to-ground
Lasercomm
Demonstration [42]
Laser diode 1550 nm
200 mW, 2.5 Gbps, 19.5 µrad
jitter error, 10 cm and 1 m uplink
and downlink telescopes size,
respectively
UAV-to-ground link
Mars Polar Lander [43] AlGaAs laser
diode 880 nm 400 nJ energy in 100 nsec pulses,
2.5 kHz rate, 128 kbps Spectroscopy
Cloud-Aerosol Lidar and
Infrared Pathfinder
Satellite Observation
(CALIPSO) [44]
Nd:YAG 532nm/1064nm 115 mJ energy, 20 Hz rate, 24 ns
pulse Altimetry
KIrari’s Optical Downlink
to Oberpfaffenhofen
(KIODO) [45]
AlGaAs laser
diode 847/810 nm
50 Mbps, 40 cm and 4 m tx. and
rx.telescopes size, respectively,
5µrad divergence
Satellite-to-ground
downlink
Airborne Laser Optical
Link (LOLA) [22]
Lumics fiber
laser diode 800 nm 300 mW, 50 Mbps Aircraft and GEO
satellite link
Tropospheric Emission
Spectrometer (TES) [46] Nd:YAG 1064 nm 360 W, 5 cm telescope size, 6.2
Mbps Interferometry
Galileo Optical
Experiment (GOPEX)
[47]
Nd:YAG 532 nm
250 mJ, 12 ns pulse width, 110
µrad divergence, 0.6 m primary
and 0.2 m secondary transmitter
telescope size, 12.19 x 12.19 mm
CCD array receiver
Deep space missions
Engineering Test Satellite
VI (ETS-VI) [48]
AlGaAs laser
diode (downlink)
Argon laser
(uplink)
Uplink: 510 nm
Downlink: 830 nm
13.8 mW, 1.024 Mbps
bidirectional link, direct detection,
7.5 cm spacecraft telescope size,
1.5 m Earth station telescope
Bi-directional
ground-to-satellite link
Optical Inter-orbit
Communications
Engineering Test Satellite
(OICETS) [49]
Laser Diode 819 nm 200 mW, 2.048 Mbps, direct
detection, 25 cm telescope size
Bi-directional Inter-orbit
link
Solid State Laser
Communications in Space
(SOLACOS) [50]
Diode pumped
Nd:YAG 1064 nm
1 W, 650 Mbps return channel and
10 Mbps forward channel, 15 cm
telescope size, coherent reception
GEO-GEO link
Short Range Optical
Inter-satellite Link
(SROIL) [51]
Diode pumped
Nd:YAG 1064 nm 40 W, 1.2 Gbps, 4 cm telescope
size, BPSK homodyne detection Inter-satellite link
Mars Laser
Communications
Demonstration (MLCD)
[52]
Fiber laser 1064 nm and
1076 nm
5 W, 1- 30 Mbps, 30 cm tx.
telescope size and 5 m and 1.6 m
rx. telescope size, 64 PPM
Deep space missions
Table II
WAVEL ENGTH S USED IN P RAC TIC AL FSO COMMUNICATION SYSTEMS
5
D. Related Surveys and Paper Contributions
Although, FSO communication has been studied in various
literatures before, however these surveys still lack to provide
the readers with comprehensive detail of every topic. For
example, a survey paper by Khalighi and Uysal [53]
has elaborated various issues in FSO link according to
communication theory prospective. They have presented
different types of losses encountered in terrestrial FSO
communication, details on FSO transceiver, channel coding,
modulation and ways to mitigate fading effects of atmospheric
turbulence. However, most of their work is centered around
terrestrial FSO communication. Similarly, Bloom et al. [54]
have quantitatively covered various aspects for the design
of FSO link. This paper has covered primary factors that
affect the performance of terrestrial FSO link - atmospheric
attenuation, scintillation, alignment or building motion, solar
interference and line-of-sight obstructions. It also provide
details on transmitter and receiver design, beam propagation
models and link budgeting of practical FSO link. Another
survey by Demers et al. in [55] solely focused on FSO
communication for next generation cellular networks. An
introductory paper on terrestrial FSO communication by
Ghassemlooy et al. [10] and Henniger et al. [29] provide
an overview of various challenges faced in the design of
FSO communication. Our survey is also related to various
challenges faced in FSO communication system with focus
on current status and latest research trends in this field. In our
work, we are intending to provide our readers with exhaustive
survey of FSO communication for both space and terrestrial
links. We have highlighted various performance mitigation
techniques at (i) physical level, and (ii) network level.
Our contributions in this paper are listed as follows:
(I) An exhaustive discussion on the basics of FSO
communication and its comparison with RF system have
been provided to make the readers better understand
the switching from conventional RF domain to optical
domain. This will make a quick and clear entry point
to the topic. It has also provided a comprehensive list
of various practical FSO systems deployed for space
application till date.
(II) A thorough discussion on various challenges faced
in FSO communication system both at terrestrial as well
as space links have been presented in this paper. We
have also listed out silent features of commonly used
atmospheric turbulence profile models.
(III) Effective mitigation techniques for reliable FSO
communication have been discussed in this paper. Most
of the surveys till date have covered mitigation techniques
only at physical layer. This paper uniquely presents
atmospheric mitigation techniques both at physical as
well as network/transport layer level. A detailed literature
survey has been presented for various FSO coding and
modulations schemes using various channel models and
detection techniques. It has covered the latest survey
materials where most of the papers are after the year
2007.
(IV) A recent approach of turbulence mitigation in FSO
system using orbital angular momentum (OAM) based on
channel coding or adaptive optics has been highlighted.
E. Paper Organization
The rest of the paper is organized as follows: Section II
describes major challenges faced by both terrestrial and space
FSO links. Section III presents various techniques both at
physical and upper layers (transport, network or link layer) to
mitigate the adverse effects of the atmosphere. This section
will also give the details of hybrid RF/FSO system that
provides a practical solution by backing up the FSO link with
low data rate RF link. Section IV gives a recent study on OAM
based FSO system using channel coding and adaptive optics
for turbulence mitigation. Section V discusses the future scope
of FSO technology and finally, the last section will conclude
the survey paper.
II. CHALLENGES IN FSO COMMUNICATION
FSO technology uses atmospheric channel as a propagating
medium whose properties are random function of space and
time. It makes FSO communication a random phenomena that
is dependent on weather and geographical location. Various
unpredictable environmental factors like clouds, snow, fog,
rain, haze, etc., cause strong attenuation in the optical signal
and limit the link distance at which FSO could be deployed.
This section will cover various challenges faced by system
designer in terrestrial as well as space FSO links.
A. Terrestrial Links
Terrestrial links include communication between
building-to-building, mountain-to-mountain or any other
kind of horizontal link between two ground stations. These
FSO network can be deployed with point-to-point or
point-to-multipoint or ring or mesh topology as shown in
Fig. 3. When a laser beam propagates through atmosphere,
it experiences power loss due to various factors and a
role of system design engineer is to carefully examine
the system design requirements in order to combat with
the random changes in the atmosphere. For reliable FSO
communication, the system design engineer need to have
thorough understanding of beam propagation through random
atmosphere and its associated losses. The various losses
encountered by the optical beam when propagating through
the atmospheric optical channel are:
(I) Absorption and scattering loss: The loss in the
atmospheric channel is mainly due to absorption and
scattering process and it is described by Beer’s law [56].
At visible and IR wavelengths, the principal atmospheric
absorbers are the molecules of water, carbon-dioxide
and ozone [57], [58]. The atmospheric absorption is
a wavelength dependent phenomenon. Some typical
values of molecular absorption coefficients are given in
Table III for clear weather conditions. The wavelength
range of FSO communication system is chosen to have
minimal absorption. This is referred to as atmospheric
transmission window. In this window, the attenuation
6
Point-to-multipoint
Point-to-point
Ring
Mesh
Figure 3. Terrestrial FSO links
due to molecular or aerosol absorption is less than
0.2 dB/km. There are several transmission windows
within the range of 700 - 1600 nm. Majority of FSO
systems are designed to operate in the windows of
780 - 850 nm and 1520 - 1600 nm. These wavelengths
have been chosen because of the readily availability of the
transmitter and detector components at these wavelengths.
The wavelength dependance of attenuation under different
weather conditions is commonly available in databases
like MORTRAN [59], LOWTRAN [60] and HITRAN.
S.No Wavelength (nm) Molecular Absorption (dB/km)
1. 550 0.13
2. 690 0.01
3. 850 0.41
4. 1550 0.01
Table III
MOLECU LAR A BS ORP TI ON AT TYPIC AL WAVELE NG THS [61]
Scattering of light is also responsible for degrading the
performance of FSO system. Like absorption, scattering
is also strongly wavelength dependent. If the atmospheric
particles are small in comparison with the optical
wavelength, then Rayleigh scattering is produced. This
scattering is quite prominent for FSO communication
around visible or ultraviolet range i.e., wavelengths below
1µm. However, it can be neglected at longer wavelengths
near IR range. Particles like air molecules and haze cause
Rayleigh scattering [62]. If the atmospheric particles size
are comparable with the optical wavelength, then Mie
scattering is produced. It is dominant near IR wavelength
range or longer. Aerosol particles, fog and haze are major
contributors of Mie scattering. If the atmospheric particles
are much larger than the optical wavelength like in case of
rain, snow and hail, then the scattering is better described
by geometrical optics model [63], [64].
Total atmospheric attenuation is represented by
atmospheric attenuation coefficient which is expressed
as combination of absorption and scattering of light. It is
therefore expressed as sum of four individual parameters
given as
γ=αm+αa+βm+βa,(2)
where αmand αaare molecular and aerosol absorption
coefficients, respectively and βmand βaare molecular
and aerosol scattering coefficients, respectively.
Various factors that cause absorption and scattering in
FSO system are as follows:
•Fog: The major contributor for atmospheric
attenuation is due to fog as it results in both
absorption and scattering. During dense fog
conditions when the visibility is even less than
50 m, attenuation can be more than 350 dB/km
[65]. This clearly shows that it could limit the
availability of FSO link. In such cases, very high
power lasers with special mitigation techniques
help to improve the chances of link availability.
Generally, 1550 nm lasers are preferred choice
during heavy attenuation because of their high
transmitted power. Although, some literature have
given evidence that attenuation during dense fog
are independent of choice of operating wavelength
[66]–[68]. Fig. 4 shows that during heavy fog i.e.,
low visibility weather condition, all wavelengths
(850, 950 and 1550 nm) are almost closely packed
following the same pattern implying that specific
attenuation is independent of choice of operating
wavelength. For light fog i.e., when the visibility
range is high (6 km), specific attenuation is quiet
less for 1550 nm as compared to 850 nm and
950 nm as shown in Fig. 5. Generally, carrier
class availability for all weather and geographical
conditions, FSO link distance should be limited to
140-500 meters [69].
0123456
0
5
10
15
20
25
30
35
Visibility in km
Specific attenuation in dB/km
850 nm
950 nm
1550 nm
Figure 4. Attenuation vs. visibility during heavy fog
•Rain: The impact of rain is not much pronounced
like that fog as rain droplets are significantly larger
7
6 8 10 12 14 16 18 20
0
0.5
1
1.5
2
2.5
Visibility in km
Specific attenuation in dB/km
850 nm
950 nm
1550 nm
Figure 5. Attenuation vs. visibility during light fog
(100 to 10,000 µm) in size than the wavelength
used in FSO communication. The attenuation loss
for light rain (2.5 mm/hr) to heavy rain (25 mm/hr)
ranges from 1 dB/km to 10 dB/km for wavelengths
around 850 nm and 1500 nm [70], [71]. For
this reason, the choice of hybrid RF/FSO system
improves the link availability especially for system
operating at 10 GHz frequency and above. This
topic is discussed in more details in Sec. III.
The modeling of rain attenuation prediction is done
using empirical methods proposed by International
Telecommunication Union- Radiocommunication
sector (ITU-R) for FSO communication [72]. The
specific attenuation, αrain (in dB/km) for a FSO
link is given by [73]
αrain =k1Rk2,(3)
where Ris rain rate in mm/hr and k1and k2are
the model parameters whose values depend upon
rain drop size and rain temperature. Table IV gives
the values of model parameters as recommended by
ITU-R. It is to be noted that rain accompanied by
Model Origin k1k2
Carbonneau France 1.076 0.67
Japan Japan 1.58 0.63
Table IV
RAIN ATTEN UATIO N MODEL P ROP OSE D BY ITU-R F OR FSO
COMMUNICATION [73].
low clouds result in very high attenuation. In order
to combat for huge power loss during heavy rain
accompanied by low clouds, high power lasers should
be used and sufficient link margin greater than 30 dB
should be achieved for maximum link availability of
FSO system [69].
•Snow: The size of snow particles are between fog and
rain particles. Therefore, attenuation due to snow is
more than rain but less than fog. During heavy snow,
the path of the laser beam is blocked due to increase
density of snow flakes in the propagation path or due
to the formation of ice on window pane. In this case,
its attenuation is comparable to fog ranging between
30-350 dB/km and this can significantly reduce the
link availability of FSO system. During heavy snow
(150 dB/km), FSO system can given 99.9 % link
availability if link margin of approx. 50 dB is chosen
[69]. For snow, attenuation is classified into dry
and wet snow attenuation. The specific attenuation
(dB/km), αsnow for snow rate Sin mm/hr is given
as [72]
αsnow =aSb,(4)
where the values of parameters aand bin dry and
wet snow are
Dry snow : a= 5.42 ×10−5+ 5.49, b = 1.38,
Wet snow : a= 1.02 ×10−4+ 3.78, b = 0.72.
(5)
(II) Atmospheric turbulence - horizontal link:
Atmospheric turbulence is a random phenomenon which
is caused by variation of temperature and pressure of
the atmosphere along the propagation path. It will result
in the formation of turbulent cells, also called eddies
of different sizes and of different refractive indices.
These eddies will act like a prism or lenses and will
eventually cause constructive or destructive interference
of the propagating beam. It will lead to redistribution
of signal energy resulting in random fluctuations in
the intensity and phase of the received signal. The
intensity fluctuations of the received signal is known
as scintillation and is measured in terms of scintillation
index (normalized variance of intensity fluctuations), σ2
I
given by [74]–[76]
σ2
I=I2− hIi2
hIi2=I2
hIi2−1,(6)
where Iis the irradiance (intensity) at some point in
the detector plane and the angle bracket hi denotes an
ensemble average. Scintillation index is expressed as
variance of log-amplitude, σ2
xas
σ2
I≈4σ2
x,for σ2
x<< 1.(7)
Scintillation index is function of refractive index structure
parameter, C2
n. This parameter determines the strength of
turbulence in the atmosphere. Clearly, C2
nwill vary with
time of day, geographical location and height. For near
ground horizontal link, value of C2
nis almost constant
and its typical value in case of weak turbulence is
10−17 m−3/2and for strong turbulence it can be up to
10−13 m−3/2or greater. Various empirical models of C2
n
have been proposed to estimate the turbulence profile
that are based on experimental measurements carried out
at variety of geographical locations, time of day, wind
speed, terrain type, etc [77]. Some of the commonly used
models are presented in Table V.
8
Models Range Comments
PAMELA
Model
[78]
Long (few
tens of kms)
- Robust model for different terrains
and weather type
- Sensitive to wind speed
- Does not perform well over
marine/overseas environment
NSLOT
Model [79]
Long (few
tens of kms)
- More accurate model for marine
propagation
- Surface roughness is ’hard-wired’ in
this model
-Temperature inversion i.e.,
(Tair −Tsur >0) is problematic
Fried Model
[80]
Short (in
meters)
- Support weak, strong and moderate
turbulence
Hufnagel and
Stanley Model
[81] Long (few
tens of kms)
-C2
nis proportional to h−1
- Not suitable for various site
conditions
Hufnagel
Valley Model
[82]–[84]
Long (few
tens of kms)
- Most popular model as it allows
easy variation of daytime and night
time profile by varying various site
parameters like wind speed,
iso-planatic angle and altitude
- Best suited for ground-to-satellite
uplink
- HV 5/7 is a generally used to
describe C2
nprofile during day time.
HV5/7 yields a coherence length of 5
cm and isoplanatic angle of 7µrad at
0.5 µm wavelength
Gurvich
Model [85]
Long (few
tens of kms)
- Covers all regimes of turbulence
from weak, moderate to strong
-C2
ndependance on altitude, h,
follows power law i.e., C2
n∝h−n
where ncould be 4/3, 2/3 or 0 for
unstable, neutral or stable
atmospheric conditions, respectively.
Von
Karman-Tatarski
Model [86],
[87]
Medium
(few kms)
- Make use of phase peturbations of
laser beam to estimate inner and
outer scale of turbulence
- Sensitive to change in temperature
difference
Greenwood
Model [88],
[89]
Long (few
tens of kms)
- Night time turbulence model for
astronomical imaging from
mountaintop sites
Submarine
Laser
Communication
(SLC) [90]
Model
Long (few
tens of kms)
- Well suited for day time turbulence
profile at inland sites
- Developed for AMOS observatory
in Maui, Hawaii
Clear 1 [91] Long (few
tens of kms)
- Well suited for night time turbulent
profile
- Averages and statistically interpolate
radiosonde observation measurements
obtained from large number of
meteorological conditions
Aeronomy
Laboratory
Model (ALM)
[92]
Long (few
tens of kms)
- Shows good agreement with radar
measurements
- Based on relationship proposed by
Tatarski [87] and works well with
radiosonde data
AFRL
Radiosonde
Model [93]
Long (few
tens of kms)
- Similar to ALM but with simpler
construction and more accurate
results as two seperate models are
used for troposphere and stratosphere
- Daytime measurements could give
erroneous results due to solar heating
of thermosonde probes
Table V
TURBUL ENC E PRO FILE MOD ELS FOR C2
n
For weak turbulence i.e., σ2
I<1, the intensity statistics is
given by log-normal distribution. For strong turbulence,
σ2
I≥1, the field amplitude is Rayleigh distributed which
means negative exponential statistics for the intensity
[80]. Besides these two models, a number of other
statistical models [94] are used in literature to describe
the scintillation statistics in either a regime of strong
turbulence (Kmodel) or all the regimes (I-Kand
Gamma-Gamma [95] models). For 3 <σ2
I< 4, the
intensity statistics is given by Kdistribution. Another
generalized form of Kdistribution that is applicable to all
conditions of atmospheric turbulence is I-K distribution.
However, I-K distribution is difficult to express in closed
form expressions. In that case, the Gamma-Gamma
distribution is used to successfully describe the
scintillation statistics for weak to strong turbulence [96].
Although Gamma-Gamma distribution is most widely
used to study the performance of FSO system, however,
in recent work proposed by Chatzidiamantis et al. [97],
DoubleWeibull distribution is suggested to be more
accurate model for atmospheric turbulence than the
Gamma-Gamma distribution, particularly for the cases
of moderate and strong turbulence. Another very latest
turbulence model proposed in [98] is Double Generalized
Gamma (Double GG) distribution which is suitable for
all regimes of turbulence and it covers almost all the
existing statistical models of irradiance fluctuations as
special cases.
Scintillation index for weak turbulence in case of plane
and spherical waves is expressed as
σ2
I=σ2
R= 1.23C2
nk7/6L11/6for plane wave,(8)
σ2
I= 0.4σ2
R= 0.5C2
nk7/6L11/6for spherical wave,(9)
where kis wave number (2π/λ), σ2
Rthe Rytov variance, L
the link distance. It is clear from Eqs.(8) and (9) that for
given link distance in case of weak turbulence conditions,
irradiance fluctuations will decrease at longer wavelength.
Scintillation index in case of strong turbulence is given
by
σ2
I= 1 + 0.86
σ4/5
R
, σ2
R>> 1 for plane wave,(10)
σ2
I= 1 + 2.73
σ4/5
R
, σ2
R>> 1 for spherical wave.(11)
Depending on the size of turbulent eddies and transmitter
beam size, three types of atmospheric turbulence effects
can be identified:
•Turbulence induced beam wander: Beam wander is a
phenomenon which is experienced when the size of
turbulence eddies are larger than the beam size. It
will result in random deflection of the beam from its
propagating path and leads to link failure. The rms
beam wander displacement is function of link length
(L), operating wavelength (λ), and initial beam size
(Wo) and is given by σ2
BW = 1.44C2
nL2W−1/3
o[99].
9
•Turbulence induced beam spreading: Beam spreading
takes into account when the size of the eddies are
smaller than the beam size. In this case, the incoming
beam will be diffracted and scattered independently
leading to distortion of the received wavefront.
•Turbulence induced beam scintillation: When the
eddy size is of the order of beam size, then the
eddies will act like lens that will focus and de-focus
the incoming beam. This will result in temporal and
spatial irradiance fluctuations of the laser beam and
is major cause of degradation in the performance of
FSO system.
(III) Beam divergence loss: As the optical beam
propagates through the atmosphere, beam divergence is
caused by diffraction near the receiver aperture. Some
fraction of the transmitted beam will not be collected
by the receiver and it will result in beam divergence
loss/geometrical loss. This loss increases with the link
length unless the size of the receiver collection aperture
is increased or receiver diversity is employed.
(IV) Ambient light: Light from luminous bodies like
Sun, moon or fluorescent objects produces shot noise
and it interferes with the noise present at the detector.
Both the detector noise and ambient noise contribute
to total receiver noise and this result in some noise
flicker or disturbances. In order to minimize noise from
ambient light sources, FSO system should be operated at
higher wavelengths. The solar irradiance spectrum ranges
from 300 nm to more than 2000 nm showing peak at
around 500 nm. Thereafter, the pattern decreases with
the increase in the wavelength [100].
(V) Mis-alignment or building sway: The optical beam
used in FSO communication is highly directional with
very narrow beam divergence. Also, receivers used in
FSO links have limited FOV. Therefore, in order to
have 100% availability of FSO link, it is very essential
to maintain a constant LOS connection between the
transmitter and receiver. However, the building over
which FSO transceivers are mounted are in constant
motion due to variety of factors such as thermal
expansion, vibrations or high wind velocity. This will lead
to failure of FSO link due to mis-alignment or building
sway. It poses a great challenge for transceiver alignment
and one needs to have very accurate pointing and tracking
mechanisms to overcome FSO link failure.
Besides the above mentioned factors, there could be other
reasons for link failure. Since FSO system requires LOS
communication, any kind of physical obstruction can block
the beam path and cause a short and temporary interruptions
of the received signal. This adverse effect can be taken
care of by proper choice of system design parameters like
beam divergence, transmitter power, operating wavelength,
transmitter and receiver FOV [101].
B. Space Links
Space links include both ground-to-satellite/
satellite-to-ground links, inter-satellite links and deep
space links. Links between LEO to GEO are used for
transmitting gathered data from LEO to GEO which in turn
transmits data to other part of the Earth as shown in Fig. 6.
GEO Satellite
GEO Satellite
GEO Satellite
LEO Satellite
Ground Station
Satellite
UAV
Mobile User
Satellite
Mobile User
Satellite
Figure 6. Space FSO links
Many researchers in US, Europe and Japan are investigating
space-to-ground links using LEO (mobile FSO link).
Optical Inter-Orbit Communications Engineering Test Satellite
(OICETS) was the first successful bi-directional optical link
between KIRARI, the Japanese satellite and ESA’s Artemis in
2001 [102]. Also, successful operational inter-satellite optical
link was established between Artemis and Spot4 via SILEX
system [103]. An optical link between two LEO orbiting
satellites, Terra SAR-X and NFIRE, at 5.5 Gbps on a total
distance of 5500 km and at a speed of 25, 000 km/hr has been
established in 2008. A 2.5 Gbps experiment was performed
successfully between LEO satellite and ground station at 1
W laser power, 1064 nm wavelength using BPSK modulation
scheme [104]. An optical link at 2.5 Gbps was demonstrated
by NASA between ground station and UAV achieving a BER
of 10−9at 1550 nm wavelength [42]. These space links
have to face severe challenges due to adverse atmospheric
effects (in case of ground-to-satellite/satellite-to-ground links)
as discussed in previous section as well as require very tight
acquisition, tracking and pointing owing to its narrow beam
width.
(I) Pointing loss: Pointing error is one of the major
challenge in FSO communication that can result in link
failure. It is very essential to maintain pointing and
acquisition throughout the duration of communication. It
could arise due to many reasons such as satellite vibration
or platform jitter or any kind of stress in electronic or
mechanical devices. The effect of satellite vibration in
FSO system is described in [105]–[108]. Pointing error
can also be caused due to atmospheric turbulence induced
beam wander effect which can displace the beam from its
transmit path [109]. In any of the case, pointing error will
increase the chances of link failure or can significantly
10
reduce the amount of received power at the receiver
resulting in high probability of error. In order to achieve
sub micro-radian pointing accuracy, proper care has to be
taken to make the assembly vibration free and maintain
sufficient bandwidth control and dynamic range in order
to compensate for residual jitter [110].
Total pointing error, σpis sum of tracking error, σtrack
and point ahead error, σpa i.e., σ=σtrack+σpa . Tracking
error is primarily due to the noise associated with tracking
sensors or due to disturbances arising from mechanical
vibration of satellite. Point ahead error occurs if the
calculation of point ahead angle did not allow sufficient
transit time from satellite-to-ground and back again. It
could be due to error in Ephemeris data or point ahead
sensor error or calibration error or waveform deformation.
Pointing error loss is more when tracking LEO than GEO
satellite [111], [112]. Also, loss due pointing error is
more significant at visible wavelength and decreases at
higher wavelength due to inherent broadening of beam.
Pointing error has significant impact on BER performance
of FSO system. Fig. 7 shows the BER performance in the
presence of random jitter.
2 4 6 8 10 12
10−8
10−6
10−4
10−2
100
Average BER
Required SNR
θdiv /σjitter= 3
θdiv σjitter= 5
θdiv /σjitter=7
θdiv /σjitter= 10
No Jitter
Figure 7. BER vs SNR for different values of ratio of beam divergence angle
to random jitter
(II) Atmospheric turbulence- vertical link: For vertical
links, the value of C2
nchanges with altitude hunlike
horizontal link where its value is assumed to be constant.
With the increase in the altitude, the value of C2
n
decreases at the rate of h−4/3. Therefore, for vertical
links, the value of C2
nhas to integrated over the complete
propagation path extending from height of the receiver
above sea level to the top of the atmosphere (roughly up
to 40 kms). Due to this reason, the effect of atmospheric
turbulence from ground-to-satellite (uplink) is different
from satellite-to-ground (downlink).
Various empirical models of C2
nhave been proposed in
[113], [114] that describe the strength of the atmospheric
turbulence with respect to the altitude (as mentioned in
Table V). The most widely used model for vertical link
is Hufnagel Valley Boundary (HVB) model [115] given
by
C2
n(h)=0.00594hV
27 210−5h10 exp (−h/1000)
+2.7×10−16exp −h
1500
+Aexp −h
100 im−2/3,
(12)
where V2is the mean square value of the wind speed
in m/s, his the altitude in meters and Ais a parameter
whose value can be adjusted to fit various site conditions.
The parameter Ais given as
A= 1.29 ×10−12r−5/3
0λ2−1.61 ×10−13θ−5/3
0λ2
+3.89 ×10−15.
(13)
In the above equation, θ0is the isoplanatic angle [84]
(angular distance over which the atmospheric turbulence
is essentially unchanged) and r0is the atmospheric
coherence length [116]. The coherence length of the
atmosphere is an important parameter that is dependent
upon operating wavelength, C2
nand zenith angle θ. For
plane wave propagating from altitude hoto (ho+L)
(downlink),it is given as
r0=
0.423k2sec (θ)
ho+L
h0
C2
n(h)dh
−3/5
.(14)
For spherical wave (downlink), it is expressed as
r0=
0.423k2sec (θ)
ho+L
h0
C2
n(h)L+ho−h
L5/3
dh
−3/5
.
(15)
It is clear from above expressions that r0varies as λ6/5,
therefore, FSO link operating at higher wavelengths will
have less impact of turbulence than at lower wavelengths.
For the uplink, if transmitter beam size W0is of the order
of r0, significant beam wander takes place. For downlink,
angle of arrival fluctuation [74] increases as the value
of r0decreases. In case of weak turbulence, scintillation
index for plane wave (downlink) can be written in terms
of refractive index structure parameter, C2
nas
σ2
I=σ2
R≈2.24k7/6(sec (θ))11/6ho+L
h0
C2
n(h)h5/6dh.
(16)
It should be noted that weak fluctuation theory does
not hold true for larger zenith angles and smaller
wavelengths. In that case, scintillation index for moderate
to strong turbulence holds well and is given by [117]
σ2
I= exp
0.49σ2
R
1 + 1.11σ12/5
R7/6+0.51σ2
R
1 + 0.69σ12/5
R7/6
−1.
(17)
(III) Background noise: The main sources of
background noise are: (a) diffused extended background
noise from the atmosphere, (b) background noise from
the Sun and other stellar (point) objects and (c) scattered
11
light collected by the receiver [118]. The background
noise can be controlled by limiting the receiver optical
bandwidth. Single optical filter with very narrow
bandwidth in the order of approx. 0.05 nm can be used
to control the amount of background noise. In addition,
the other sources of noise in FSO system are detector
dark current, signal shot noise and thermal noise. Total
noise contribution is sum of background noise and noise
due to other sources.
(IV) Atmospheric seeing: The perturbations of the
optical beam associated with coherence length of the
atmosphere, r0is referred as atmospheric seeing effect.
When r0is significantly smaller than the receiver aperture
diameter DR,then it leads to the blurring of received
signal which is known as astronomical seeing which
is given as λ
/r0[119]. For a perfect optical collection
system, the spot size of the received signal in the focal
plane of the receiver is expressed as (2.44F λ/DR)where
Fis the focal length of receiver collecting optics. When
the optical beam propagates through atmosphere, then
DRis replaced by r0and therefore, the related signal
spot size at the focal plane is increased by the ratio DR
/r0
which effectively leads to increase in the background
noise. Also, larger FOV at the receiver can limit the
electrical bandwidth of the receiver thereby limiting the
data rate. This problem can be taken care of by use of
adaptive optics or array detectors.
(V) Angle of arrival fluctuations: Due to the presence
of turbulence in the atmosphere, the laser beam wavefront
arriving at the receiver will be distorted. This will lead
to spot motion or image dancing at the focal plane
of the receiver. This effect is called angle of arrival
fluctuations. However, this effect can be compensated by
use of adaptive optics or fast beam steering mirror. For
plane wave, the variance of angle of arrival fluctuations,
β2is expressed as [75]
β2=(1.64C2
nLl−1/3
o, DRlo
2.91C2
nLD−1/3
R, DRlo
,(18)
where DRis the diameter of collecting lens and lothe
inner scale of turbulent eddy.
III. MITIGATION TECHNIQUES
Atmospheric channel causes degradation in the quality of
received signal which deteriorate the BER performance of
the FSO system. In order to improve the reliability of FSO
system for all weather conditions, various types of mitigation
techniques are employed. Mitigation technique can be used
either at physical layer or at network layer. Multiple beam
transmission, increasing receiver FOV, adaptive optics, relay
transmission, hybrid RF/FSO etc., are some of the mitigation
techniques used at physical layer. Packet re-transmission (in
FSO link or network), network re-routing, quality of service
(QoS) control, data re-play are some of the methods used
in network layer in order to improve the performance and
availability of FSO system. Fig. 8 gives various mitigation
techniques used in FSO communication.
Turbulence Mitigation Techniques
Physical Layer Methods
TCP Upper Layer Methods
Aperture Averaging
Diversity
Adaptive Optics
Modulation &
Coding
Re-transmission
Reconfiguration and
Re-routing
Quality of Service
Control
Others: Re-play, DTN
Background Noise
Rejection
Hybrid RF/FSO link
Figure 8. Various techniques for mitigating atmospheric turbulence
A. Physical Layer Methods
•Aperture Averaging: This technique is used to mitigate
the effect of atmospheric turbulence by increasing the
size of the receiver aperture that average out relatively
fast fluctuations caused by the small-size eddies and helps
in reducing channel fading. The parameter that quantify
reduction in fading due to aperture averaging is called
aperture averaging factor, A. The parameter Ais defined
as ratio of variance of the signal fluctuations from a
receiver with aperture diameter DRto that from a receiver
with an infinite small aperture i.e.,
A=σ2
I(DR)
σ2
I(0) .(19)
Various approximations for the aperture averaging factor
have been given by Churnside [120], Andrew [121],
etc. The Churnside approximation of aperture averaging
factor for plane wave in weak turbulence region is given
by
A="1+1.07 kD2
R
4L7/6#−1
.(20)
Similarly, the approximated value for the aperture
averaging factor for spherical wave is given:
A="1+0.214 kD2
R
4L7/6#−1
.(21)
Therefore, increasing the aperture diameter reduces
atmospheric scintillation and improves the BER
performance of the system. Various literature on aperture
averaging can be found in [122]–[126]. NASA conducted
an experiment on aperture averaging using 1550 nm
operating wavelength through transmit aperture of
2.5 cm for propagation path of 1 km using Gaussian
beam. The receiver aperture diameter was varied up to
the size of 8 inches. It was seen from the results that
BER was around 10−3for small aperture diameter of
12
2 inches. However, with increase in aperture diameter to
8 inches, BER performance reached up to 10−12 or even
better [127]. In [128], irradiance statistics of a Gaussian
beam propagating through turbulent atmosphere along a
horizontal path was investigated. It was observed that for
moderate to strong turbulence regime, Gamma-Gamma
distribution provides the best fit to irradiance statistics.
In case of aperture diameter larger than the coherence
length of the atmosphere, the irradiance statistics appear
to be log-normal.
It should be noted that increase in the receiver aperture
area will also increase the amount of background noise
collected by the receiver. Therefore, an optimum choice
of aperture diameter has to be made in order to increase
the power efficiency in FSO system.
•Diversity: Diversity technique for mitigating the effect
of turbulence in the atmosphere can operate on time,
frequency and space. In this case, instead of single
large aperture, an array of smaller receiver aperture
is used so that multiple copies of the signal that are
mutually uncorrelated can be transmitted either in time or
frequency or space. This will improve the link availability
and BER performance of the system. It also limits
the need of active tracking due to laser misalignment
[120], [129], [130]. GOLD demonstration in 1998 showed
that using four 514.5 nm multiple beams for uplink
transmission, scintillation index was drastically improved.
It was reported that the value of scintillation was 0.12
with two beams, however, its value reduces to 0.045
with four beams [131]. In order to achieve the full
advantage due to spatial diversity, the antennas separation
at transmitter or receiver should be at least or greater than
coherence length of the atmosphere to make the multiple
beams independent or at least uncorrelated. The effect of
correlation between multiple beams is presented in [129]
where it shows that a correlation of 0.25 among three
transmit apertures can decrease the diversity order by 1.
The situation worsens with the higher order correlation.
This is in contrast to RF communication where only
full spatial correlation results in loss of diversity. Also,
the gain due to diversity is more pronounced at high
turbulence level than at lower values [132], [133].
In case of receiver diversity (SIMO- single input multiple
output), diversity gain is achieved by averaging over
multiple independent signal paths. The signals can be
combined at the receiver using selection combining
(SC) or equal gain combining (EGC) or maximal
ratio combining (MRC). SC is simpler as compared
to other two, but gain in this case is low. The gain
achieved through MRC is slightly higher than EGC,
but at the expense of complexity and cost. Therefore,
implementation of EGC is preferred over MRC due
to its simplicity and comparable performance. In case
of receiver diversity using intensity modulation/direct
detection (IM/DD), it has been verified using wave-optics
and Monte Carlo simulations that effect of correlation
corresponding to small scale turbulence can be neglected,
irrespective of atmospheric turbulence condition [134].
For transmit diversity (MISO- multiple input single
output), special space time codes such as optical
Alamouti code is used [96], [135]. This code is designed
for only two transmit antenna but can be extended to
more number of antennas. The performance of optical
MIMO (multiple input multiple output) and RF MIMO
systems are almost equivalent. It increases the channel
capacity of the system almost linearly with the number
of transmitting antenna. Optical MIMO transmission with
advanced modulation schemes are studied in [136]–[143].
Fig. 9 shows that MIMO systems are more robust to
channel fading than SISO or point aperture. It has
been observed that when channel state information
(CSI) is available at the receiver, an improvement in
SNR is directly proportional to the number of transmit
and receiver antennae. In case of weak atmospheric
turbulence, the outage probability of Gaussian FSO
channel is proportional to [log (SNR)]2term whereas
for moderate to strong turbulence, it is proportional to
[log (SNR)] [138]. Optical MIMO FSO system using
M-ary Pulse Position Modulation (M-PPM) along with
multilevel coding (MLC) with low density parity check
(LDPC) codes provide excellent coding gain for turbulent
regimes. It has shown a drastic coding gain improvement
of 57.8 dB at BER = 10−6in strong turbulence conditions
[144].
FSO MIMO system performs well if the beams are
independent or uncorrelated. Otherwise, the performance
of FSO system is going to degrade. Another type of
diversity that is emerging these days is cooperative
diversity [145]–[147] which is a form of distributed
spatial diversity that enables multiple terminals to share
their resources by cooperative communication so that a
virtual antenna array can be built in a distributed fashion.
Here, instead of using multiple apertures at the transmitter
or receiver end, a single antenna is capable of achieving
huge diversity gain. It makes use of neighboring nodes to
form virtual antenna array and hence takes the advantage
of spatial diversity in distributed manner.
Time diversity with or without codes has also proven
to mitigate channel fading in FSO system. This
type of diversity is applicable to time selective
fading channels which allows repetitive symbols to be
transmitted over different coherence time period. If
data frame length exceed the channel coherence time,
then diversity can be employed by either coding or
interleaving. It was observed that in the presence of
time diversity, convolutional codes are good choice for
weak atmospheric turbulence and Turbo-codes provides
a significant coding gain for strong turbulence conditions
[148].
•Adaptive Optics: Adaptive optics (AO) is used to
mitigate the effect of atmospheric turbulence and helps
to deliver an undistorted beam through the atmosphere.
AO system is basically a closed loop control where the
beam is pre-corrected by putting the conjugate of the
atmospheric turbulence before transmitting it into the
atmosphere [149]–[151]. Increase in transmit power or
13
© 2009 ACADEMY PUBLISHER
Figure 9. BER performance comparison for SISO and MIMO links at
σ2
I= 0.8and using single aperture diameter = 20 cm [136]
using diversity can improve the performance of FSO
system. But in order have further improvement in SNR
with reduce transmit power requirement, AO have proved
to be very beneficial. The implementation of AO system
in Compensated Earth-Moon-Earth Retro-Reflector Laser
Link (CEMRLL) showed significant improvement in the
received SNR [152]. AO system makes use of wavefront
sensor, wavefront corrector and deformable mirrors either
at the transmitter or at the receiver optics to compensate
for the phase front fluctuations. Here, a part of the
received signal is sent to wavefront sensor that produces a
control signal for the actuators of wavefront corrector as
shown in Fig. 10. However, a real time wavefront control
Incoming
Distorted
Wavefront
Deformable
Mirror
Beam Splitter
Corrected
Wavefront
Atmospheric
Turbulence
Real Time Control
System
Wavefront
Sensor
Telescope
From Distant
Transmitter
Receiver
Figure 10. Conventional adaptive optics system
using conventional AO approach becomes quite difficult
for very strong turbulent conditions [151]–[154]. In such
situations, non-conventional AO approach is used which
is based on the optimization of received SNR or any other
system performance metric [155], [156]. Earlier, this
non-conventional approach was largely disregarded as it
imposed serious limitations for the control bandwidth.
But later with the development of high bandwidth
wavefront phase controllers e.g., deformable mirrors
based on micro-electromechanical systems (MEMS) and
with the development of new efficient algorithms, this
approach is gaining popularity these days. AO using
MEMS for long range FSO system is found in [157],
[158].
Designing of AO system requires that its closed loop
frequency should be at least four times Greenwood
frequency [159] (in Hz) given by
fG=
0.102k2sec (θ)
ho+L
ho
C2
n·vT(h)5/3dh
3/5
,
(22)
where vT(h)is the traversal component of wind speed.
This frequency tells the speed of AO system to respond
to fluctuations due to atmospheric turbulence.
•Modulation and Coding: In FSO communication, the
choice of modulation and coding schemes depends on
two main criteria: optical power efficiency and bandwidth
efficiency. Optical power efficiency can be measured
by computing optical power gain over On Off Keying
(OOK) provided both the modulation schemes have same
euclidean distance, dmin. Power efficient modulation
schemes are simpler to implement and are quite effective
in mitigating the effect of the turbulence for low data
rates. They have to abide with the eye safety regulations,
therefore, it limits the maximum propagation distance
during high turbulent conditions. Bandwidth efficiency on
the other hand, determines maximum data for a given link
length with a particular modulation scheme.
In general, FSO communication support variety of binary
and multilevel modulation formats. Out of these two
format, binary level format is most commonly used due
to its simplicity and high power efficiency. Most well
known binary modulation schemes are OOK and PPM.
OOK modulation scheme requires adaptive threshold
in turbulent atmospheric conditions for best results
[160], [161]. Due to its simplicity, OOK modulation
scheme is very popular in FSO communication
system and most commonly it is deployed with
IM/DD transmission and receive mechanism. Another
detection technique used in OOK modulation scheme is
maximum likelihood (ML) detection with perfect CSI
[162]. However, due to its implementation complexity,
this detection technique didn’t gain much popularity.
Maximum-likelihood sequence detection (MLSD) can be
employed when the receiver is having the knowledge of
joint temporal distribution of intensity fluctuations. Other
detection techniques [163]–[165] used at the receiver are
symbol by symbol maximum likelihood detection, blind
detection, VBLAST detection, etc.
In case of M-PPM, each symbol interval is divided into
Mtime slots and a non-zero optical pulse is placed
at these time slots while other slots are kept vacant.
For long distance communication, M-PPM scheme is
widely used because of its high peak-to-average power
ratio (PAPR) that improves its power efficiency [166].
Also, unlike OOK, M-PPM does not require adaptive
threshold. However, power efficient modulation scheme
14
Coding Modulation Channel Model Detection Techniques Reference
Convolutional OOK Gamma-Gamma Direct Detection [198]
- OOK Log-normal ML Detection with perfect
CSI
[199]
- OOK Log-normal and
Gamma-Gamma
ML sequence detection
(MLSD)
[200]
Turbo OOK and PPM - Direct Detection [201]
- OOK K Direct Detection [202]
- OOK K and IK Direct Detection [203]
- OOK Modified Rician Coherent Detection [204]
Reed Solomon PPM and DPSK Log-normal and Negative
exponential
Direct Detection [205]
- PPM Gamma-Gamma Direct Detection [206]
Space Time Trellis code OOK Negative exponential and K Direct Detection [207]
Bit-interleaved coded modulation
(BICM) and Multilevel coding (MLC)
PPM Poisson Direct Detection [208]
LDPC coded OFDM OOK, QAM, BPSK, QPSK Gamma-Gamma Direct Detection [194]
- PPM and dual pulse PPM Negative exponential Direct Detection [209]
- DPPM and OOK Log-normal Direct Detection [210]
Reed Solomon PPM Poisson Direct Detection [211]
- SIM DPSK Gamma-Gamma Direct Detection [212]
- SIM BPSK K Direct Detection [213]
Block, Convolutional, Turbo codes OOK Log-normal Direct Detection [214]
Turbo coded multi-carrier code
division multiple access (MC-CDMA)
SIM-PSK Gamma-Gamma Direct Detection [215]
Convolutional code PPM Gamma-Gamma Iterative Detection [216]
LDPC OOK Gamma-Gamma VBLAST-ZF Detection [217]
- PPM Gamma-Gamma VBLAST-ZF Detection [218]
- Differential pulse position width
modulation (DPPM+PWM)
Gaussian Direct Detection [219]
Hybrid channel (Non-uniform and
rate-compatible LDPC codes) &
Adaptive Codes
BPSK Kim model and
Gamma-Gamma (Hybrid
RF/FSO)
ML Detection [220]
Interleaved concatenated coding
(convolutional inner code and a
Reed-Solomon outer code)
Binary PPM Gaussian Iterative Detection [221]
Table VII
LITERATURE ON FSO CODING AND MODULATION SCHEMES USING VARIOUS CHANNEL MODELS AND DETECTION TECHNIQUES
may not be bandwidth efficient and therefore if the
system is bandwidth limited, then multi-level modulation
schemes are used. Here, the transmitted data can
take multiple amplitude levels and most commonly
used multi-level intensity modulation schemes are pulse
amplitude modulation (PAM) and quadrature amplitude
modulation (QAM) [167], [168]. However, the price paid
for bandwidth efficiency is the reduction in power level.
Therefore, these modulation schemes are not good choice
for high turbulent atmospheric conditions. It is reported
in many literatures that in case of high background
noise, M-PPM is considered to be optimum modulation
scheme on Poisson counting channel [169], [170]. With
the increase in order of Min M-PPM, the robustness
against background radiations increases even further due
to its low duty cycle and lesser integration interval of
photodiode. Owing to various advantages of PPM in
FSO communication, various variants of PPM have been
developed aiming to enhance the spectral efficiency of
the system. These modulation schemes are Differential
PPM (DPPM) [171], [172], Differential Amplitude PPM
(DAPPM) [173], Pulse Interval Modulation (PIM) [174].
A comparison of bandwidth requirement, PAPR and
capacity for all variants of PPM modulation schemes is
shown in Table VI.
Modulation
Schemes
•-PPM
DPPM
DAPPM
DPIM
Bandwidth
(Hz)
!"
log#
( + 1)!"
2log#
( + $)!"
2 $
( + 3)!"
2log#
PAPR
+ 1
2
$( + 1)
$ + 1
+ 1
2
Capacity
log#
2log#
+ 1
2 log#( ∙ $)
+ $
2log#
+ 3
Table VI
COMPARISON OF VARIANTS OF PPM MODULATION SCHEME
Optical sub-carrier intensity modulation (SIM) is another
modulation format where the base band signal modulates
the electrical RF sub-carrier (can be either analog or
digital) which is subsequently intensity modulated by
the optical carrier. Since sub-carrier signal is sinusoidal
signal, therefore a DC bias is added to omit negative
amplitude of the transmitted optical signal. SIM does
not require adaptive threshold unlike OOK scheme and
it is more bandwidth efficient than PPM scheme. Optical
SIM inherits the benefits from more mature RF system,
therefore, it makes the implementation process simpler.
Studies have reported that hybrid PPM-BPSK-SIM
15
gives better results than BPSK-SIM for all levels of
atmospheric turbulence [175]. SIM in conjunction with
diversity technique improves the BER performance of the
FSO system in the presence of atmospheric turbulence.
A 4x4 MIMO is considered to be the optimal choice
using BPSK-SIM for all turbulent conditions [176].
When this modulation scheme is used with different
RF sub-carriers which are frequency multiplexed, then
this scheme is know as multiple sub-carrier intensity
modulation (MSIM). In this case, each sub-carrier is a
narrow band signal and experience less distortion due
to inter symbol interference at high data rates. However,
the major disadvantage of SIM and MSIM is less power
efficiency than OOK or PPM.
Differential Phase Shift Keying (DPSK) has received
significant interest due to its power efficiency and 3 dB
improvement over OOK modulation [177]–[179]. Since it
has reduced power requirement than OOK, DPSK does
not have non-linear effects which in turn improves the
spectral efficiency of DPSK over OOK modulation. It is
reported that sensitivity of DPSK receiver can approach
quantum limit theory. However, the cost for implementing
DPSK based FSO system is high due to its increased
complexity in system design both at transmitter and
receiver level.
Error control coding also improves the performance of
FSO link by making use of different forward error
control (FEC) schemes including Reed-Solomon (RS)
codes, Turbo codes, convolutional codes, trellis-coded
modulation (TCM) and LDPC. The study of error
performance using error correction codes in fading
channel has been under research for many years
[180]–[182]. These codes add redundant information
to the transmitted message so that any kind of error
due to channel fading can be detected and corrected
at the receiver. The coherence time of FSO system
is in the order of milliseconds (about 0.1-10 ms),
therefore, the receiver design becomes too complex due
to large memories requirement for storing long data
frames [183]. On one side, it improves the coding gain
of the FSO system but on the other side it leads to
delay latencies and complexity of the system. Other
attractive option could be interleaving of the transmitted
symbols [184]. Since the duration of the fades are
random, no single maximum interleaving depth can be
used to render the channel completely memory less.
Furthermore, interleaving depths that correspond to time
separations of 1 ms between successive bits of a code
word require the encoder and decoder to contain very
large amounts of memory [185]. Coding gain provided
by RS and convolutional codes are sufficient in case
of weak atmospheric turbulence. The maximum coding
gain of convolutional code with constraint length = 3
and code rate = 1/2 for direct detection FSO system
using PPM with perfect interleaving is 7 dB for clear
weather conditions and 11 dB in moderate turbulent
conditions. The use of soft decision Viterbi decoding
in this case provided significant improvement in BER
of the system even when the interleaving depths were
insufficient to render the channel memory less [186].
RS codes provide good coding gain when implemented
with PPM. The performance of RS (255, 127) coded
PPM provides coding gain of 6 dB [187]. The
improvement in RS codes increases with the increase
in block length. RS coded PPM (63, 37, 64) at
code rate = 3/5 matches the performance of 64-PPM, but
RS (262143, 157285, 64) gives better performance at
BER = 10−6. In case of strong atmospheric turbulence,
Turbo, Trellis or LDPC codes are preferred. Turbo
codes can be arranged in any of the three different
configurations- parallel concatenated convolutional codes,
serial concatenated convolutional codes and hybrid
concatenated convolutional codes. Parallel concatenated
convolutional codes are most popularly used in which two
or more constituent systematic recursive convolutional
encoders are linked through an interleaver. For very high
data rate transmission, LDPC codes are preferred over
Turbo codes due to its reduced decoding complexity and
computational time. Variable rate LDPC codes can further
increase the channel capacity and provide good coding
gain [188], [189]. Various analysis has been carried out
for the use of LDPC codes in MIMO FSO system [190],
[191]. It was observed that LDPC coded MIMO FSO
system using M-PPM provides better performance over
uncoded system in case of strong atmospheric turbulence
and large background noise set to -170 dBJ. A coding
gain of 10-20 dB was observed over uncoded system
at BER = 10−12 [192], [193]. Also, bit interleaved
coded modulation (BICM) scheme proposed by I.B
Djordjevic [192] provide excellent coding gain when used
with LDPC coded FSO system. Orthogonal frequency
division multiplexing (OFDM) combined with suitable
error control coding is also considered a very good
modulation format for improving BER performance of
FSO IM/DD systems [194].
At the receiver, various efficient decoding algorithms
have been proposed to decode the generated codes.
Theoretically, ML decoding at the receiver can
provide better data recovery but its usage is limited
due to implementation complexity. Symbol-by-symbol
maximum a posterior (MAP) decoding algorithm is
computationally complex and is not a preferred choice
for implemention on VLSI chip. However, logarithmic
version of the MAP (log-MAP) algorithm [195] and the
Soft Output Viterbi Algorithm (SOVA) [196] are the
practical decoding algorithms for implementation using
Turbo codes. Out of these two, log-MAP algorithm
gives the best performance but it is computationally very
complex. Simplified-log-MAP algorithm performs very
close to the log-MAP and is less complex aswell [197].
Some of decoding algorithms used in LDPC codes are
belief propagation and message passing. VBLAST-ZF
(Vertical Bell Labs Layered Space Time Zero Forcing)
detection algorithm is suggested in LDPC coded MIMO
FSO system for reducing the decoding complexities at
the receiver. A summary of literature pertaining to
16
various coding and modulation schemes using various
channel models and detection techniques is presented in
Table VII.
•Background Noise Rejection: The major source of
background noise is due to day time solar radiations.
This can be mitigated with the help of spatial filters
along with suitable modulation technique that has high
peak-to-average power [53], [222]. The most suitable
modulation scheme is M-PPM to combat the effect
of solar background noise radiations (as the noise is
directly proportional to slot width). High order PPM
scheme is reported as potential modulation scheme for
inter-satellite links as it is more power efficient and
drastically reduce the solar background noise [223].
However, M-PPM is not suitable for bandwidth limited
system. In that case, DPIM is a preferred choice as it
does not require synchronization and is both capacity
and bandwidth efficient [89], [224]. Designing receiver
with narrow FOV and choosing filters with spectral
width less than 1 nm is another approach to reduce
the background noise [225]. It has been shown that
adaptive optics and deformable mirrors comprising of
array of actuators can result in significant improvement
due to background noise by reducing the receiver FOV.
Analysis presented in [226] shows that inter-planetary
FSO link between Earth and Mars has achieved 8.5
dB improvement in extreme background and turbulent
conditions using adaptive optics and array of actuators
(hundred 1 meter telescope) with PPM modulation
scheme. During moderate background conditions, the
improvement was decreased to 5.6 dB. Another analysis
presented in [227] gives the performance improvement of
6 dB by using array of 900 actuators and adaptive optic
technique with 16 PPM modulation scheme.
•Hybrid RF/FSO: The performance of FSO
communication is drastically affected by weather
conditions and atmospheric turbulence. This can lead
to link failures or poor BER performance of FSO
system. Therefore, in order to improve the reliability
and improve the availability of the link, it is wise to pair
up FSO system with a more reliable RF system. Such
systems are called hybrid RF/FSO and are capable of
providing high link availability even in adverse weather
conditions [69]. The major cause of signal degradation in
RF transmission is due to rain (as the carrier wavelength
is comparable to the size of the rain drop) and in FSO
communication is due to fog. So, the overall system
availability can be improved by using low data rate RF
link as a back up when FSO link is down. In [228],
the availability of an airborne hybrid RF/FSO link is
evaluated. It was observed that the FSO link provides
poor availability during low clouds conditions due to the
attenuation by cloud particles and temporal dispersion.
However, a significant improvement was observed
when a hybrid RF/FSO link was used as RF signals
are immune to cloud interference. The conventional
approach of hybrid RF/FSO causes inefficient use of
channel bandwidth [229]. Also, a continuous switching
between RF and FSO system could bring down the entire
system. Therefore, a new approach as suggested in [230]
gives symbol rate adaptive joint coding scheme wherein
both RF and FSO subsystem are active simultaneously
and saves channel bandwidth. Hybrid channel coding is
also capable of utilizing both the links by combining
non-uniform codes and rate adaptive codes where their
code rates are varied according to the channel conditions
[220].
Hybrid RF/FSO link provides great application in mobile
ad hoc networks (MANETs) [231]. A reconfigurable
networking environment can be formed in MANETs
by combination of wireless sensor network (WSN)
technology and mobile robotics. However, the
performance of this network is limited by the per
node throughput provided by RF based communication.
Therefore, the combination of RF and FSO provides
tremendous increase in per node throughput of MANETs.
The implementation of hybrid RF/FSO MANET with
real-time video data routing across 100 Mbps optical
link and 802 .11g RF transceiver has been studied in
[232].
The RF wireless network poses a strong limitation on its
capacity and throughput owing to growing development
in communication technology [233]. With the increasing
number of users, the chances of interference from
the neighboring nodes increases and that limits the
performance of the RF system. FSO system on the other
hand is highly directional and has very narrow beam
divergence. This makes FSO system immune from any
kind of interference. Therefore, the combination of FSO
and RF can help in solving the capacity scarcity problem
in RF networks. The throughput and capacity of hybrid
RF/FSO link is given in [234]–[236]
B. TCP Upper Layer Methods
There has been a lot of research on the performance
mitigation of atmospheric turbulence in physical layer. For
the last few years, researchers have gained attention to work
on modeling and performance evaluation of upper layers
including link layer, transport layer, application layer in order
to improve the performance of FSO communication [237],
[238]. In addition to the physical layer methods, various
techniques like re-transmission, re-routing, cross connection
between different layers, delay tolerant networking, etc., are
used to improve the performance of FSO in all weather
conditions [239]–[241].
•Re-transmission: A re-transmission protocol such as
automatic repeat request (ARQ) is widely used in
data communication for reliable data transfer. Here, the
transmission is carried out in the form of packets of
certain frame length. If due to some reason, the receiver
does not acknowledge the transmitted packet within
speculated time frame, then the packet is re-transmitted.
This process repeats till a positive acknowledgment
is received by the transmitter from the receiver or
the preset counter value is exceeded. So, this kind
17
of stop, wait and go-back-NARQ scheme results in
huge delay, large energy consumption and bandwidth
penalties due to re-transmission process [242], [243].
Therefore, another variant of ARQ is selective repeat
ARQ (SR-ARQ) in which data packets are continuously
transmitted from transmitter to the receiver without
the need to wait for individual acknowledgment from
the receiver. The receiver will continue to accept and
acknowledge the received frame. If any frame is not
acknowledged after certain period, it is assumed to be lost
and re-transmitted. ARQ protocol can be implemented
either at data link layer or at transport layer [244]. In
either case, the receiver terminal must have sufficient
data storage capability to buffer the received data at
least for the time period specified by window size.
In [241], [245], the performance of FSO system in
weak atmospheric turbulence has been studied when
the ARQ and FEC schemes are used in link layer.
Lee and Chan in [246] examined the performance of
transmission control protocol (TCP) and observed very
poor throughput even with 10 dB of link margin and
16 diversity transmitters/receivers over clear weather
conditions. The performance of TCP over FSO channel
as shown in Fig. 11 has been analyzed when SR-ARQ
scheme is used for the link layer in [247]. Here,
TCP-Reno and three-dimensional (3-D) Markov chain
model that includes the exponential back-off phase were
used for modeling the TCP operation. The throughput
of TCP was analyzed and it was observed that the
atmospheric turbulence has severe impact on TCP [246]
and use of SR-ARQ in link layer can significantly
maximize the throughput of TCP.
Performance Analysis of TCP over Free-Space
Optical Links with ARQ-SR
Vuong V. Mai, Truong C. Thang, and Anh T. Pham
Computer Communications Lab., The University of Aizu
Aizuwakamatsu, Fukushima, Japan 965-8580
Emaill: {m5162101,thang,pham}@u-aizu.ac.jp
Abstract—This paper presents an analytical study on the
performance of transmission control protocol (TCP) over free-
space optical (FSO) links when the automatic-repeat request,
selective repeat (ARQ-SR) scheme is used for the link layer. Using
a three-dimensional (3-D) Markov model, we analytically derive
the TCP throughput and energy-throughput efficiency, which is
the the ratio between the TCP throughput and the average energy
for transmitting an unit data of TCP. In the numerical analysis,
we quantitatively discuss the impact of various FSO physical
and link layer parameters on the trade-off between the energy
consumption and the TCP throughput.
I. INTRODUCTION
Free-space optical (FSO) communication is a transmission
technology in which the optical signal is transmitted through
the atmosphere. FSO has recently emerged as an alternative
solution for broadband wireless communications thanks to its
advantages such as cost-effective, license-free, high data rate,
and quick deployment [1]. In FSO systems, one of major
performance-degrading factors, particularly for extended links,
is the atmospheric turbulence [2]. Over past decade or so, there
have been many studies focusing on the impact of atmospheric
turbulence on the performance of FSO systems in the physical
layer [3]– [7].
Recently, several studies has been devoted to the modeling
and performance evaluation of upper layers, including the ap-
plication, transport and link layers, over the FSO atmospheric
turbulence links [8]– [14]. In [8], Lee et al. focused solely on
the transport layer and analyzed the TCP throughput over the
clear turbulence atmospheric optical channels. In [9]– [11],
the performance of FSO systems when the automatic-repeat
request (ARQ) and FEC schemes are used in the link layer
has been studied taking into account the FSO log-normal
turbulence channels. In [12], the performance analysis of
voice transfer application over the FSO systems have been
presented. Most recently, studies investigating the effect of
turbulence channels on the MAC layer performance in FSO
systems have been reported in [13] and [14]. In these studies,
the impact of FSO channel conditions on the performance
of upper layers has been separately studied. In this paper,
we focus on the cross-layer analysis of the transport layer,
the link layer and FSO turbulence channel. The aim of our
study is to investigate the impact of both the link layer and
FSO turbulence channels on the performance of transport
layer employing the popular Transmission Control Protocol
(TCP). In our analysis, the TCP-Reno is considered, and a
Figure 1: Network scenario with a TCP connection over FSO
link.
three-dimensional (3-D) Markov chain model that includes
the exponential back-off phase is used for modeling the TCP
operation. The exponential back-off phase is necessary for
operation of TCP over FSO link as the probability of this
event is considerably high due to the bust loss caused by
the atmospheric turbulence. In the link layer, we use the
automatic-repeat request, selective repeat (ARQ-SR) scheme
thanks to its ability to offer both loss-recovery efficiency and
simplicity [9]– [11]. In the physical layer, we employ the sub-
carrier binary phase-shift keying (SC-BPSK) system, and the
log-normal turbulence channel model is also assumed for the
case of weak-to-moderate turbulence conditions.
It is worth mentioning that the understanding of the com-
bined effect of both ARQ-SR and FSO link on the performance
of TCP connections would enable us to investigate the cross-
layer optimization, which is critical to the design of the
FSO networks due to the harsh condition of the atmospheric
channels. In addition, we newly define a joint throughput-
energy efficiency parameter, which is the the ratio between
the TCP throughput and the average energy for transmitting
an unit data of TCP. By using this parameter, we can optimize
FSO system parameters for the trade-off between the energy
consumption and the TCP throughput in various contexts of
ARQ-SR and FSO turbulence channels.
The remainder of the paper is organized as follows. Section
II describes the network model, ARQ-SR scheme, FSO chan-
nel model and their impact on the TCP segment-loss prob-
ability and end-to-end transmission delay. The 3-D Markov
chain model for TCP-Reno is presented in Section III. Finally,
Sections IV and V show the numerical results and conclusions,
18th European Conference on Network and Optical Communications & 8th Conference on Optical Cabling and Infrastructure - NOC/OC&I 2013
ISBN: 978-1-4673-5822-4, July 10-12, 2013, Graz, Austria
18th European Conference on Network and Optical Communications & 8th Conference on Optical Cabling and Infrastructure - NOC/OC&I 2013
ISBN: 978-1-4673-5822-4, July 10-12, 2013, Graz, Austria
18th European Conference on Network and Optical Communications & 8th Conference on Optical Cabling and Infrastructure - NOC/OC&I 2013
ISBN: 978-1-4673-5822-4, July 10-12, 2013, Graz, Austria
18th European Conference on Network and Optical Communications & 8th Conference on Optical Cabling and Infrastructure - NOC/OC&I 2013
ISBN: 978-1-4673-5822-4, July 10-12, 2013, Graz, Austria
18th European Conference on Network and Optical Communications & 8th Conference on Optical Cabling and Infrastructure - NOC/OC&I 2013
ISBN: 978-1-4673-5822-4, July 10-12, 2013, Graz, Austria
18th European Conference on Network and Optical Communications & 8th Conference on Optical Cabling and Infrastructure - NOC/OC&I 2013
ISBN: 978-1-4673-5822-4, July 10-12, 2013, Graz, Austria
105
Figure 11. TCP connection over FSO link when used with SR-ARQ [247]
Another variant of ARQ that has been studied by
various researcher is hybrid ARQ (HARQ) which uses
a combination of FEC coding and ARQ error control
[248]–[250]. The outage probability of different HARQ
scheme in the strong turbulence regimes has been
investigated and it was found that good performance gain
can be achieved using this scheme [251]. However, this
scheme has large bandwidth penalties and delay latencies.
Recently, combination of cooperative diversity with ARQ
(CARQ) has gained interest that has shown remarkable
results for combating turbulence induced fading in FSO
channel. A modified version of CARQ with lesser
transmission delays and improved energy consumption
is M-CARQ. This modified scheme allows relay nodes
to store a copy of frames for more efficient response to
transmission failure due to atmospheric turbulence [252],
[253].
Another protocol i.e., Rateless Round Robin protocol is
used in FSO networks which is based on the combination
of channel coding (FEC) and packet level coding (a
form of ARQ) [254], [255]. FEC is applied to the
transmitted data and reverse link acknowledgment. Cyclic
redundancy check (CRC) is used to verify the integrity
of the received packets after FEC decoding [256]. It is
observed that Rateless Round Robin is an effective error
control design for practical FSO applications even during
very strong turbulence when the channel availability is
less than 45% [257].
•Reconfiguration and re-routing: Path reconfiguration
and data re-routing is carried out in order to increase
the availability and reliability of the FSO link during
loss of LOS or adverse atmospheric conditions or device
failure. Through dynamic reconfiguration of the nodes
in FSO network using physical and logical control
mechanism, link availability is improved drastically. In
physical layer, reconfiguration is achieved using pointing,
acquisition and tracking (PAT) and in logical layer, it
makes use of autonomous reconfiguration algorithms
and heuristics. Here, the data packets are re-routed
through other existing links that could be either an
optical link or low data rate RF link. Various topology
control mechanism and re-routing algorithms have been
investigated for FSO network [258]–[262]. Autonomous
topology control and beam reconfiguration is achieved
through: (a) the topology discovery and monitoring
process, (b) the decision making process by which a
topology change has to be made, (c) the dynamic and
autonomous re-direction of beams (based on algorithms)
to new receiver nodes in the network, and (d) the
dynamic control of these beams for link re-direction
[259]. Therefore, reconfiguration and re-routing improves
the reliability of FSO link but at the cost of huge
processing delays. A good design engineer has to ensure
the restoration of link through reconfigurability without
significant impact in delay and at reduce cost. For this,
routing protocol should be designed in such a way
so that during re-routing process, the path which has
minimum delay or least number of hops should be given
priority. Sometimes, all the routing routes are computed
prior to their actual need and are stored in routing
tables. Such type of routing is classified as ‘proactive
routing’ protocol. However, this routing protocol is not
suitable for large networks as it imposes high overhead
to the network and that makes it bandwidth inefficient.
Examples of proactive routing protocol are: Destination
sequenced distance-vector (DSDV) routing protocol
[263], optimized link state (OLS) routing protocol [264],
wireless routing protocol (WRP) [265]. Another routing
protocol that generates very less overhead as compare
18
to proactive routing is called ‘reactive routing’ and
computes new routes only ‘on demand’. A new route is
established only during the failure of the existing route.
However, this leads to prolonged latency in data delivery.
Examples of reactive routing protocols are: Ad hoc
on-demand (AODV) routing [266], location aided routing
(LAR) [267], temporary ordered routing (TOR) protocol
and dynamic source routing (DSR) [268]. Combination
of both proactive and reactive routing protocols is called
‘hybrid routing’ protocol. It divides the network into
clusters and apply proactive route updates within each
cluster and reactive routing across different clusters.
Examples of hybrid routing protocol are zone routing
protocol (ZRP) [269] and Ad hoc on-demand distance
vector hybrid (AODVH) [270]. A comparison of some
of popular routing protocols that can be implemented in
wireless network are presented in Table VIII. Further, the
routing protocols can be classified as negotiation-based
[271], multipath-based [272], [273], query-based [274],
[275], QoS-based [276], [277], or coherent-based routing
[278] as shown in Fig. 12.
Based on
Negotiation
Routing Protocols:
Based on routing criteria
Based on
Multipath
Based on Query
Based on QoS
Based on coherent and non-
coherent processing
Figure 12. Classification of routing protocols
•Quality of Service Control: The QoS in FSO
communication is measured in terms of data rate,
latency, delay jitter, data loss, energy consumption,
reliability and throughput efficiency. The data transfer
from one node to another in FSO communication
system should meet the given requirements of special
QoS class otherwise the offered services can not be
used by end-users in a satisfying way. For this, the
main challenge in FSO network is to optimize the
performance of communication system measured in
(a) end-to-end connection delay, (b) delay variation,
(c) packet rejection rate, and (d) overhead [90]. It
is suggested in some literatures that modification in
different layers improve the QoS of FSO system. Various
methods that address application level QoS control are:
application admission control algorithm [292], multipath
and multi-speed (MMSPEED) routing protocols proposed
in [293], tunneling [294], DSR protocol [269], etc. A
routing algorithm that improves the QoS of network
and medium access control (MAC) layer is proposed
in [295]. The MAC layer QoS can be classified into
(a) channel access policies, (b) scheduling and buffer
management, and (c) error control. This routing protocol
provides energy efficient real time FSO communication.
Some of the QoS routing protocols implemented in
wireless networks are: multipath routing protocol (MRP)
[296], sequential assignment routing (SAR) protocol
Protocol Classification Energy
Consumption
Route
Overhead
Scalability
Low Energy
Adaptive
Clustering
Hierarchy
(LEACH)
[279]
Hierarchical High High Good
Geographic
Adaptive
Fidelity
(GAF) [280]
Location Low Average Good
Geographical
and Energy
Aware
Routing
(GEAR)
[281]
Location Low Average Limited
Directed
Diffusion
(DD) [282]
Flat Low Low Limited
Rumor
Routing
(RR) [283]
Flat Low Low Good
Gradient
Based
Routing
(GBR) [284]
Flat Low Low Limited
Adaptive
Threshold
Sensitive
Energy
Efficient
Network
(ATEEN)
[285]
Hierarchical High High Good
Dynamic
Source
Routing
(DSR) [286]
Reactive Average Average Limited
Location
Aided
Routing
(LAR) [287]
Reactive Low Low Limited
Link Quality
Source
Routing
(LQSR)
[288]
Reactive Low Low Limited
Temporally
Ordered
Routing
Algorithm
(TORA)
[289]
Reactive Low Average High
Zone
Routing
Protocol
(ZRP) [290]
Hybrid Low Low High
Ad Hoc On
Demand
Distance
Vector
(AODV)
[291]
Reactive High High Limited
Table VIII
COMPARISON OF REAL TIME ROUTING PROTOCOLS
19
[297], energy-aware QoS routing protocol [298], SPEED
[299], multi constrained QoS multi-path routing (MCMP)
protocol [300], QoS-based energy-efficient sensor routing
(QuESt) protocol [301], etc.
•Others: Re-playing is another technique to promote
end-to-end connectivity of FSO link. If re-routing or
re-transmission is not possible, then FSO network will
replay up to 5 sec of data from the edge node
[302]. Delay (or disruption) tolerant networking (DTN)
technique is applied for the networks with intermittent
connectivity and therefore, it is a good candidate for FSO
communication having extreme atmospheric conditions
[303], [304].
IV. ORB ITAL ANGULAR MOMENTUM FOR FSO SYSTEM
Angular momentum is one of the most fundamental physical
quantity in both classical and quantum mechanics. It is
classified as spin angular momentum (SAM) and orbital
angular momentum (OAM). SAM is associated with the spin
of the photon and thus, it is related with polarization. On
the other hand, OAM is associated with the helicity photon
wavefront and therefore, it is related to spatial distribution.
It was reported in [305] that a beam having helical shaped
phase front described by azimuthal phase term exp(ilθ) carries
OAM of ¯
lh per photon where lis topological charge with
any integer value, θthe azimuth angle and ¯
hthe Plank’s
constant hdivided by 2π. Therefore, unlike SAM, which
can take only two possible states of ±¯
h, the OAM can have
infinite number of states corresponding to different values of
l. In principle, infinite number of bits is carried by OAM of
single photon. This makes OAM a potential candidate for high
capacity communication systems. Also, orthogonality among
beams with different OAM states allow additional degree of
freedom by multiplexing of information carrying OAM beams.
The possibility to generate and analyze states with different
OAM by using interferometric or holographic methods
[306]–[308] permit the development of energy-efficient FSO
communication systems. Further, OAM based FSO system has
provided good performance in atmospheric turbulence when
used with suitable encoding or modulation format or adaptive
optics system.
An OAM beam is formed by attaching azimuthal phase
term to the Gaussian beam as U(r, θ) = A(r)·exp (ilθ).
Here, A(r)is the amplitude at the waist of the Gaussian beam
and rthe radial distance from the center axis of the Gaussian
beam. When data is encoded to OAM beam, it is expressed
as U(r, θ, t) = S(t)·A(r)·exp (ilθ)where S(t)is the
data to be transmitted. With multiplexing of Ninformation
carrying OAM beams, the resultant field is expressed as
UMux (r, θ, t) = PN
m=1 Sm(t)·Am(r)·exp (ilmθ). It is
to be noted that OAM of individual beam is not modified
when propagated through free space or spherical lenses. For
de-multiplexing OAM beams, an inverse of azimuthal phase
term exp [i(−ln)θ]is used and received de-multiplexed beam
is then given as
URx (r, θ, t) = exp [i(−ln)θ]·
N
X
m=1
Sm(t)·Am(r)·exp (ilmθ).
(23)
The capacity and spectral efficiency of FSO links using
OAM is increased by employing several techniques like: (i)
combining multiple beams with different OAM values, (ii)
using positive or negative OAM values, (iii) using wavelength
division multiplexing (WDM), (iv) polarization multiplexing,
or (v) using two groups of concentric rings. Recent reports
in [309] have demonstrated 2.56 Tbps data rate transmission
with spectral efficiency of 95.7 bps/Hz using four light beams
with 32 OAM modes employing 16 quadrature amplitude
modulation (QAM). In [310], 100.8 Tbps data transmission
has been reported using 42 wavelengths with 24 modes. This
shows that OAM has tremendous potential for increasing
the capacity of FSO system. However, these high capacity
transmissions were limited to short transmission distances only
where the effect of turbulence was not considered. Therefore,
OAM based FSO system is capable of delivering huge data
returns in inter-satellite links or deep space mission where
atmospheric turbulence does not pose any problem.
It has been reported that OAM beams are highly
sensitive even in case of weak atmospheric turbulence due
to redistribution of energy among various OAM states
leading to time varying crosstalk [311]. An OAM beam
has a doughnut shape with less power and large phase
fluctuations in the center. Since the orthogonality of the beam
is dependent upon helical phase front, therefore practical
implementation of OAM based FSO system in the presence
of atmospheric turbulence seems to be challenging. Single
information carrying OAM state result in random and bursty
error in the presence of atmospheric turbulence. In case of
multiplexed data channels having different OAM values, the
crosstalk among adjacent channels degrade the performance
of the system [312]. In [313], experimental investigation was
carried out for OAM based multiplexed FSO communication
link through emulated atmospheric turbulence. The results
indicated that turbulence induced signal fading and crosstalk
could significantly deteriorate the link performance. However,
very recently researchers have reported that the effect of
atmospheric turbulence is mitigated in OAM based FSO
system by using suitable channel coding and wavefront
correction techniques. Channel codes are used to correct
the random errors caused by atmospheric turbulence for
single OAM state and wavefront correctors take care of
cross talk among adjacent OAM states. Various channel
coding techniques like RS codes in [314], LDPC codes
in [315] have proved quite beneficial in improving the
performance of OAM based FSO system. Other techniques
like holographic ghost imaging system [316], adaptive optics
[317], [318] help in controlling OAM crosstalk and thus
is capable of providing high data rates even in adverse
atmospheric conditions. Combination of error correcting codes
and wavefront correction methods have also shown good
results using OAM based FSO system in the presence of
atmospheric turbulence. In [319], RS codes in combination
20
Data input
Data input
Data input
Data input
Data output
Data output
Data output
Data output
Modulator 1
Modulator 2
Modulator 3
Laser 3
Laser N
SLM+
Hologram 1
SLM+
Hologram 2
SLM+
Hologram 3
SLM+
Hologram N
Encoder 1
Encoder 2
Encoder 3
Encoder N
Modulator N
Laser 1
Laser 2
MUX
for
OAM
Beams
FSO
Channel
De-
MUX
for
OAM
Beams
SLM+
Hologram 1
SLM+
Hologram 2
SLM+
Hologram 3
SLM+
Hologram N
Demodulator 1
Demodulator 2
Demodulator 3
Detector 3
Detector 4
Decoder 1
Decoder 2
Decoder 3
Decoder N
Demodulator N
Detector 1
Detector 2
Electrical
encoded
data bits
Electrical
modulated
data bits
Optical
beam
OAM
beam
OAM
multiplexed
beam
OAM
de
-
multiplexed
beam
Optical
beam
Electrical
modulated
data bits
Electrical
coded data
bits
Figure 13. FSO communication system using multiplexed OAM beams and channel coding
with Shark-Hartmann wavefront correction method has been
adopted to combat the detrimental effect of atmospheric
turbulence. It was shown that the effect of atmospheric
turbulence was reduced and satisfactory BER performance
was achieved even during strong atmospheric turbulence.
Fig. 13 shows the block diagram representation of OAM based
FSO multiplexed system using channel coding technique. The
transmitter consists encoder, modulator, laser source, computer
controlled hologram along with spatial light modulator (SLM)
and multiplexer. The data from different independent sources
are encoded, modulated and transformed into OAM beams by
adding a spiral phase mask with different charges (l). These
OAM beams are then multiplexed together and sent onto FSO
channel. The multiplexing of OAM beams is considered as a
form of spatial multiplexing which is capable of enhancing the
capacity and spectral efficiency of the FSO link. At the receiver
side, OAM beams are de-multiplexed and an inverse spiral
phase mask with charge (-l) is used to remove the azimuthal
phase term exp (ilθ) of OAM beam to recover the plane phase
front of the beam. This optical beam is then passed through
detector, demodulator and decoder to recover information data.
V. FUTURE SCOPE OF FSO COMMUNICATION
FSO communication seems to have promising future as it
provides a cost effective connectivity alternatives for several
applications like: last mile access, cell cite back haul for
mobile networks, fiber backup and much more. The FSO
communication has experienced a rapid growth in the last
few years despite of various crisis in the global market. This
technology has demonstrated less capital expenditure with
huge returns in very little time due to (i) easy availability
of components, (ii) quick deployment (as it does not seek
permission from municipal corporation for digging up of
street), and (iii) no licensing fee required. FSO communication
provides a back up protection for fiber based system in
case of accidental fiber damage. Also, FSO technology is
used to provide high speed data connectivity for distance
ranging from few cms ( like in optical interconnect networks)
up to few meters/kms (like in wireless local area networks
(WLAN), metro area extensions, wireless body area network
(WBAN), etc.). This technology has proved to outreach the
capacity of RF wireless link by providing 10 Gbps wireless
optical link. With the availability of 10G Ethernet switches
in the market these days, FSO technology is capable of
providing promising gigabit Ethernet access for high rise
network enterprise or bandwidth intensive applications (e.g.,
medical imaging, HDTV, hospitals for transferring large
digital imaging files or telecommunication) or intra campus
connections. FSO technology provides good solution for
cellular carriers using 4G technology to cater their large
bandwidth and multimedia requirement by providing a back
haul connection between cell towers. Using ultra short pulse
(USP) laser, FSO communication provide up to 10 Gbps back
haul connection without deploying fiber cables. It is believed
21
that FSO technology is the ultimate solution for providing
high capacity last mile connectivity up to residential access.
Instead of hybrid fiber-coax systems, hybrid fiber-FSO system
may cater the high bandwidth and high data rate requirements
of end users.
FSO technology allows connectivity to remote places where
physical access to 3G or 4G signals is difficult. It involve
integration of terrestrial and space networks with the help of
High Altitude Platforms (HAPs)/UAVs by providing last mile
connectivity to sensitive areas (e.g., disaster relief, battlefields,
etc.) where high bandwidth and accessibility are necessary. An
upcoming project by Facebook is an example that will allow
internet access to sub-urban or remote areas by providing
aerial connectivity to users using FSO link. It is proposed
that for sub-urban areas in limited geographical regions,
solar-powered high altitude drones will be used to deliver
reliable internet connections via FSO links. For places where
deployment of drones is uneconomical or impractical (like in
low population density areas), LEO and GEO satellites can
be used to provide internet access to the ground using FSO
link. Hybrid RF/FSO communication has been proposed for
wireless sensor network due to their low energy consumption
requirement.
FSO interconnects (FSOI) over very short distances like
chip-to-chip or board-to-board have gained popularity these
days as it potentially addresses complex communication
requirement in optoelectronic devices. FSOI technology offers
the potential to build interconnection networks with higher
speed, lower power dissipation and more compact packages
than possible with electronic VLSI technology. However, the
cost of optoelectronic devices, their integration and overall
packaging makes FSOI a costly affair. A throughput upto
1 Tbps per printed circuit board (PCB) board has been
experimentally demonstrated in [320] using 1000 channels per
PCB with 1 mm optical beam array at 1 Gbps per channel.
FSO technology when used over a mobile platform can be
deployed in armed forces as it demands secure transmission of
information on the battlefield. Intelligence, Surveillance, and
Reconnaissance (ISR) platforms can deploy this technology
as they require to disseminate large amount of images and
videos to the fighting forces, mostly in real time. Besides, this
technology can be a good alternative to acoustic and tethered
underwater communications for short distances. It can be used
for various applications like undersea explorations, monitoring
ocean currents and winds for improving weather forecast,
providing tsunami warning by measuring seismic activities,
etc. It can be used by navy surface ships for communicating
with underwater submarines. The acoustic technology that is
currently being used is not suitable for high data rates in real
time environment. However, FSO underwater technology is
capable of providing high data rates in real time applications.
This technology can also be used with underwater sensor
nodes that collects long term geo-physical data. At present,
various devices are recovered to off load their data before they
are deployed again, which is a resource intensive task. In the
future, these underwater sensor nodes can use FSO technology
to wirelessly off load their data to an interrogating underwater
vehicle equipped with an optical modem.
VI. CONCLUSIONS
The tremendous growth in the number of multimedia users
and internet traffic in the recent years has incurred a substantial
strain on RF system operating at low data rates. Due to this
huge explosion in information technology that is driving the
information business to higher and higher data rates, there
is a need to switch from RF domain to optical domain.
FSO communication is capable of providing LOS wireless
connection between remotes sites with very high bandwidths.
This technology is considered to be the promising technology
in near future which can meet very high speed and huge
capacity requirements of current day communication market.
However, in order to fully utilize the terabit capacity of FSO
system, it has to overcome various challenges offered by
heterogeneous nature of atmospheric channel. FSO system
is vulnerable towards various atmospheric phenomenon like
absorption, scattering, atmospheric turbulence and adverse
weather conditions. Various techniques implemented either
at physical layer or at network layer help to combat the
detrimental effect of atmosphere on the quality of the laser
beam. Several fading mitigation techniques that were initially
proposed for RF works well for FSO communication aswell
e.g., diversity, adaptive optics, error control codes, modulation,
etc. Besides this, the complementary nature of RF and FSO has
motivated the design of hybrid RF/FSO system which ensure
carrier class availability for almost all weather conditions.
Also, modifications in the upper layers of TCP model like
application, transport and link layer with suitable protocols and
algorithms help in improving the reliability of FSO system.
Hence, it is clear that after so much advancement in FSO
communication, this technology seems to have very high
growth prospects in the near future. Many commercial product
for FSO terrestrial and space links are already available in
market and hopefully very soon this technology will bring
worldwide telecommunication revolution.
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