Download 13 Introduction to Free-Space Laser Communication

13 Introduction to Free-Space Laser Communication from HAPs
M. Knapek, S. Arnon, F. Fidler, W. Leeb, D. Kedar
Abstract: This chapter introduces the basic concepts of free-space laser communications. The motivations for the deployment of laser communications are presented despite the relatively new technology compared to conventional microwave
communication. Although microwave communication is an established technology, the advantages of laser communication, as a data rate increase of more than a
factor 10, safety from eavesdropping, and a decrease of terminal size and energy
consumption, pose a strong argument for the use of laser communication. Basics
of link-budget calculation, state-of-the-art technology and fundamental concepts
of terminal design are given.
13.1 Motivation
Innovative technologies are required to satisfy the ever increasing bandwidth demand associated with new communication services. Free-space laser communications offers a promising solution. It allows information to be transmitted via a collimated laser beam at high data rates in the multi-Gigabit regime using compact,
low-mass terminals, while avoiding interference problems and without exhausting
the radio-frequency (RF) bandwidth. Compared to RF links, the small beam divergence (due to the shorter wavelength) allows interference-free and secure operation and offers a high antenna gain even with small telescope diameters. Typical
optical antenna diameters – i.e. of below 30 cm - will lead to a reduced flight terminal mass and small momentum disturbances onboard a satellite or a highaltitude platform (HAP). Furthermore, the size of a ground receiving station may
be quite compact, with a telescope diameter of only a few decimeters, enabling
transportable or even mobile stations.
While optical inter-satellite links are already operable, laser communication to
the ground suffers from atmospheric turbulence, cloud coverage, and harsh weather conditions such as rain or snow. To find a remedy, current research concentrates on optical communications from or to HAPs, which are positioned well
above the clouds and where the atmospheric impact on a laser beam is less severe
than directly above ground. Data gathered or stored at HAPs may be distributed to
several ground stations in a hybrid system, i.e. via optical and RF links. Alternatively, data may be routed to a ground station with – at that time – good atmospheric conditions via a laser link to a satellite or to another HAP.
Within this chapter, key concepts and major technological requirements of optical links operating from and to HAPs are investigated. Chapter 14 describes the
applications of free-space optical communications in HAP scenarios, where such a
link could serve as a broadband communication channel if data from several sensors or microwave (MW) communication terminals on board a HAP is to be
transmitted to a satellite or within an inter-HAP network situated above the
clouds. Also, the HAP could work as a data relay station, receiving information
from a satellite or a ground station. A relatively new application for free-space optical communications is quantum cryptography, which may use pairs of entangled
photons to distribute a secure key. There, HAPs could accommodate the source of
such pairs of photons and distribute them to users on the ground, to other HAPs,
or to a satellite.
Chapter 15 is dedicated to the atmosphere when acting as free-space optical
channel. Here we investigate analytical and numerical channel models, empirical
standard turbulence profiles, as well as link blockage caused by cloud coverage
and harsh weather conditions. Chapter 16 deals with techniques mitigating atmospheric effects, mainly focusing on adaptive optics and error correction methods to
reduce the outage probability of optical communications links from and to HAPs.
13.2 State-of-the-Art
In recent years, space laser communication systems have received increasing interest in Europe, mainly because of the success of the semi-conductor laser intersatellite link experiment (SILEX) initiated by the European Space Agency (ESA)
[1]. In 2001 ESA had started to operate an optical communication link using AlGaAs semiconductor lasers (at a wavelength of ~830 nm) between two satellites,
namely SPOT-4 (in low-Earth orbit) and ARTEMIS (a geostationary satellite),
transmitting at a data rate of 50 Mbit/s. A laser link between ARTEMIS and an
optical ground station (OGS) at Tenerife (Spain) was also established [2]. One of
the main goals of the SILEX project was to verify the pointing, acquisition, and
tracking (PAT) capabilities of the laser communication terminals (LCTs) onboard
the satellites. The PAT system has the task of setting up and maintaining the link,
which is difficult to achieve due to the small divergence of the laser beams [3]. Japan’s space agency JAXA launched its optical inter-orbit communications engineering test satellite (OICETS) in summer 2005 and laser communication links
with ARTEMIS as well as with different optical ground stations were successfully
established [4, 5, 6]. In the 1990’s the German company Tesat-Spacecom developed an inter-satellite optical communication terminal with a data rate of
5.6 Gbit/s [7]. The first two terminals of this type were launched on the American
military LEO satellite NFIRE in April 2007 and the German TerraSAR-X in June
2007 [8]. In May 2008 first inter-satellite links were successfully demonstrated at
a distance of up to 5000 km [9]. Fig. 13.1 shows an image of the LCT installed on
TerraSAR-X.
Fig. 13.1 The Laser Communication Terminal on TerraSAR-X with the periscope like coarse
pointing device (Artist’s Impression).
While these ventures all dealt with inter-satellite or ground-to-satellite communication, the aim of the European CAPANINA project was to develop optical
broadband technologies to be used on HAPs. Trials were carried out using a stratospheric balloon cruising at an altitude of 24 km for nine hours [10]. The DLR
(Deutsches Zentrum für Luft- und Raumfahrt) performed a 622 Mbit/s optical
downlink with a bit-error ratio (BER) better than 10-9 from the stratosphere to an
optical receiver on ground over a total link distance of 64 km. Also, a 1.25 Gbit/s
downlink was established within the CAPANINA trial, however without bit-error
measurements. Transmission was done at a wavelength of 1550 nm, employing intensity modulation (IM) and direct detection (DD). The data source with a pseudorandom bit-sequence (PRBS) of length 223-1 drove a laser diode module with an
output power of 1 mW which was amplified to 100 mW by an Erbium-doped fiber
amplifier (EDFA). Fig. 13.1 shows the stratospheric balloon as well as the optical
terminal developed for the CAPANINA experiments.
Fig. 13.2 Optical communication with a stratospheric balloon platform in the project Capanina.
On the left a schematic of the flight terminal (periscope type). On the right the balloon at 23 km
altitude and a distance of approximately 60 km to the ground station. The payload with the terminal was located below the balloon hanging on the flight train.
For the case of a horizontal laser link between two stratospheric HAPs, propagation simulations were performed [11, 12]. Together with link budget calculations
they showed that high data-rate laser communications between HAPs at an altitude of 20 km is feasible for link distances up to 600 km.
Not only HAPs, but also conventional airplanes flying at altitudes up to 10 km
are considered as platforms to accommodate laser communication terminals. As
part of the project LOLA (Liaison Optique Laser Aéeroportée), ESA established
links between an optical terminal onboard a Dassault Mystère 20 business jet and
the satellite ARTEMIS [13]. The goal of this trial was to verify the pointing ability
of the terminal mounted in the jet, which was flying at altitudes between 6 km and
10 km. Also, audio and video data were transmitted.
Two different receiver concepts are presently used for free-space optical communications. The coherent scheme uses a local laser oscillator (LO) to increase
the photodetector output signal. At the photodiode, the LO signal beats with the
communication signal, yielding a photocurrent proportional to the optical field
strength. This scheme features high sensitivity and low susceptibility to background radiation. In homodyning, the optical signal is directly transferred into the
baseband, while in heterodyning there is a frequency difference between the LO
and the signal, resulting in an intermediate frequency in the RF regime. Coherent
detection places strict requirements on the spectral purity of the used lasers and
demands that the received signal and the local oscillator have spatial phase fronts
nearly perfectly aligned over the active area of the detector [14, 15].
The Tesat Laser Communication Terminal has a sensitivity of about 35 photons/bit for a bit-error ratio of BER=10-9 based on a coherent binary phase-shift
keyed (BPSK) modulation at 1064 nm and 5.6 Gbps [7], which has been demonstrated in inter-satellite links. A laboratory BPSK receiver with only 16 photons/bit at 4 Mbit/s was demonstrated at DLR in 1994 [16].
In a direct detection receiver, the photodiode current is proportional to the
power of the received signal [15]. Hence any optical phase or polarization information is lost and the modulation format is restricted to intensity modulation (IM).
Direct detection offers advantages over coherent detection in terms of complexity
and cost, when the temporal coherence of the source or the LO cannot be sufficiently controlled, or when the spatial phase characteristics of the received wave is
disturbed (as in the case of atmospheric turbulence). Direct detection systems,
when using optically preamplified receivers, usually have a sensitivity around 70
to 80 photons/bit at a BER=10-9 [15, 17], while with APD photo detectors with
OOK modulation require more than 100 photons/bit. An extensive list with
demonstrated coherent and direct-detection communication receivers can be found
in [18].
13.3 Basic Design Considerations of Optical Communication Links
13.3.1 Link Budgets
Only at a first glance are link budget calculations for optical free-space links the
same as those known for directional microwave links. At a closer look, the much
smaller wavelength, , the different technology available, and the pronounced influence of the atmospheric channel make quite a difference when estimating the
received optical power, PR, for a link with power PT available at the transmitter, a
link distance Z, a gain GT of the transmit antenna, and the effective area AR of the
telescope serving as receive antenna. As indicated in equation (13.1), implementation losses within the transmitter and the receiver, T and R, pointing losses LP,
and losses caused by the atmosphere, LA, may reduce the receive power considerably (T, R, LP, LA, all < 1). (Similar considerations would apply not only for the
data transmission beam but also for a beacon beam used for automatic tracking of
the two terminals).
PR  PT
GT AR
T  R LP L A
4 Z 2
(13.1)
Moreover, in an optical free-space link the quality of data transmission may further be deteriorated by background noise, as caused by celestial bodies or stray
light thereof. The second term in equation (13.2) can be considered as the basic
transmission loss. To a first approximation, the antenna gain GT is related to the
telescope diameters DT as
 D 
GT   T  .
  
2
(13.2)
The far-field on-axis gain expressed by equation (13.2) is that for a homogeneous
intensity distribution across the transmit aperture, for a perfect spatially coherent
laser radiation, and for diffraction-limited optics. In practice, the emitted beam often resembles a truncated Gaussian distribution, partially obscured by a secondary
mirror and antenna struts, and with some wavefront distortion caused by imperfect
optics. Hence the actual gain may be smaller by a factor of some 0.6 compared to
equation Error! Reference source not found.
The very high antenna gain GT achieved at optical frequencies (some 110 dB
for typical values of  and DT) is obtained at the expense of a very narrow beam
which requires ultra-precise pointing of the antenna axes. Even when implementing an active tracking control loop to align the antennas, some miss-pointing will
persist and the pointing loss LP and thus also the receive power will vary in a random manner.
As will be detailed in later chapters, the (time-varying) properties of the channel may become a limiting factor for an optical link, even without any clouds or
precipitation along the path. Clearly, losses caused by the atmosphere depend on
its density and on what we call the weather condition. They are most pronounced
close to the Earth’s surface and start to become negligible at a height of some
15 km. The resulting loss LA can be attributed to the physical effects of absorption
and scattering on one hand and turbulence, i.e. time-varying variation of the index
of refraction along the beam path on the other. Losses caused by turbulence may
be attributed to:

Increased beam divergence (as compared to propagation in vacuum);

Beam wander, caused by turbulent eddies large compared to the beam diameter;

Scintillation, i.e. fluctuations of the irradiance in the receiver plane, caused
by turbulent eddies small compared to the beam diameter.
We complete this subsection by presenting a dummy link budget for a link between a high-altitude platform (HAP) and a geostationary satellite in Table 13.1,
assuming a wavelength  = 1.55 µm, a link distance Z = 45 000 km, and diameters
of DT = DR = 0.18 m for the transmit and receive antenna. The resulting receive
power of PR = -47 dBm would allow the transmission of a data signal with data
rate of R = 1 Gbit/s at a bit-error ratio of BER = 10-9 when using on-off-keying,
return-to-zero format, and a receiver with an optical preamplifier. Using a state-ofthe-art receiver, the link margin would amount to 3.3 dB.
Table 13.1 Dummy link budget
parameter
symbol
value
unit
wavelength

1.55
µm
transmit power
PT
5.0
W
transmit power
PT
37.0
dBm
transmitter loss
T
-2.0
dB
receiver loss
R
-5.0
dB
transmit antenna gain
GT
111.2
dB
-71.0
dB
actual
transmission
loss,
0.6 GT AR
4Z 2
pointing loss
LP
-1.0
dB
atmospheric loss
LA
-2.0
dB
receive power
PR
-47.0
dBm
13.3.2 Atmospheric Effects and Background Radiation
Optical communication channels through the atmosphere can be modeled by attenuation of the transmitted signal, followed by the introduction of additive noise.
The attenuation term is a simplification of the underlying physical processes
caused by atmospheric effects and captures the change in signal power over the
course of the transmission. The noise in the model captures external interference,
e.g., due to background light.
The Earth’s atmosphere extends approximately 700 km above the surface and
consists of several distinct layers [19]. Pronounced density is found within the
lowest 20 km, still influencing optical communication links from and to HAPs.
Atmospheric effects (cf. Chapter 15) on the beam propagation at optical wavelengths can be divided into absorption and scattering, which lead to a loss in
power, and turbulence, leading to intensity fluctuations (i.e. fading) at the receiver.
Absorption and scattering: When transmitting an optical signal from the
ground on a vertical path through the atmosphere, some 1 to 2 dB of atmospheric
loss have to be expected for clear skies, at zenith, and at a wavelength of
λ = 1550 nm due to absorption and scattering [20]. At 20 km height this value reduces to ~0.2 dB [21]. The variation of the atmospheric attenuation with zenith
angle ζ, which is the angle between zenith and the LOS between transmit and receive telescope, can be approximated as [22]
aatm    aatm 0  sec ,
(13.3)
which reduces the atmospheric loss to 0.2 to 0.8 dB for typical satellite-HAP links.
Unfortunately, even light clouds would interrupt the link in a HAP-to-ground or
inter-HAP communication scenario, causing attenuation of several tens of dB [23].
However, because the platforms are situated well above the clouds, free-space optical communication links are an ideal option for HAP-to-satellite transmission
[24].
Turbulence: Wind blowing over an aerodynamically rough region of the
Earth´s surface in the presence of a temperature gradient creates fluctuations in the
atmosphere´s refractive index known as optical turbulence. These changes in the
index of refraction cause turbulent eddies, acting as random optical lenses which
refract the propagating light. The effect of these lenses is an enlarged divergence
(i.e. beam spreading) and beam wander, scintillation, and phase-front distortions
resulting in intensity fluctuations at the receiver (i.e. fading) and deteriorating the
performance of coherent as well as single-mode coupled receivers (cf. later chapter).
Beam spread and beam wander: Because of random deflections during propagation through turbulent atmosphere, the beam profile moves off the LOS between
transmitter and receiver. The instantaneous center of the beam, i.e. the point of
maximum intensity, is randomly displaced in the receiver plane, which is commonly called beam wander [19]. It is caused mainly by large-scale turbulence near
the transmitter and therefore can typically be neglected for downlink scenarios.
Atmospheric turbulence also causes beam spread beyond the diffraction limited
divergence, θDL, leading to an effective divergence angle, θeff, which causes a degradation of the mean received optical power by a factor (θeff /θDL)2. For the calculation of this, additional, beam spread loss, the diffraction limited beam radius of a
Gaussian beam truncated by the transmit telescope [25] can be compared to the effective beam-radius of the same beam but in the presence of turbulence [19]. If the
turbulence is weak, relatively far away from the transmitting source, and turbulent
eddies are small compared to the beam diameter – e.g. in optical satellite-to-HAP
downlinks – it is found that the effective spot size is essentially the same as the
diffractive spot size. Hence, beam spread loss is negligible. In the uplink, where
the size of turbulent eddies situated just in front of the transmitter is large relative
to the beam diameter, the mean beam spread loss may be larger, ranging for example from 3 dB in a ground-to-satellite scenario to 0.03 dB in an HAP-tosatellite uplink [24].
Scintillation: A quantitative measure for the temporal effect of atmospheric
turbulence is the scintillation index, σ2I , i.e. the variance of intensity fluctuations
normalized to the square of the mean intensity [19]. The scintillation index is generally used to characterize the strength of turbulence for an optical link. Contrary
to some satellite-ground links the scintillation parameter σ2I is typically smaller
than 1 for inter-HAP scenarios, denoting weak turbulence conditions, ranging
down to values ≤ 0.025 for HAP-satellite links [24].
Phase-front distortions: When a laser beam propagates through the atmosphere
its phase-front gets perturbed, which deteriorates the performance of coherent receivers and reduces the coupling efficiency into a single-mode fiber [26]. If the
turbulent eddies are relatively small compared to the beam diameter, they lead to
noticeable phase-front distortions within the receiving aperture. As an example, in
a satellite-to-HAP downlink scenario the coupling loss into a standard singlemode fiber is smaller than 1.2 dB whereas for links to a ground station a coupling
loss of more than 7 dB has to be expected [26]. In uplink scenarios, where the turbulent eddies are right in front of the transmitter and comparatively large relative
to the optical beam, the phase-front disturbance are usually negligible within a
small receiving aperture [19].
In an optical communication link the receiver not only detects the signal from
the transmitter but also unwanted light from celestial bodies [22, 27]. This may
have a degrading effect on system performance. The spectral radiance of electromagnetic radiation at all wavelengths λ from a self-emitting source at temperature
Tse is described by Planck’s law of black body radiation [28]:
R , TSE  
2hc 2

5
1
,
exp( hc / kTSE )  1
(13.4)
where h and k are Planck's and Boltzmann's constant, and c is the speed of light.
The major source of background radiation is the Sun. When the receiver is at the
HAP sunlight is reflected from Earth and a certain amount of background radiance
has to be expected also from the sky due to scattered sunlight [29, 30].
Fig. 13.3 Receiver sensitivity penalty of a single-mode coupled optically preamplified receiver at
a BER = 10−9 as a function of background noise power spectral density.
Fig. 13.3 gives an example for the sensitivity penalty, i.e. the required additional
optical power to arrive at a BER = 10-9, as a function of the background noise
power spectral density when using a single-mode coupled, optically preamplified,
direct-detection receiver [24]. In the case of blue sky (e.g. a satellite-to-HAP
downlink) or Earth as background source (e.g. a HAP-to-satellite uplink), no significant degradation of receiver performance is found when compared to the case
where there is no background light at all. Even when directly looking into the Sun,
only a deterioration of 1.4 dB compared to the optimum case has to be expected. It
is known that DD receivers suffer from background radiation but like in heterodyning, single-mode coupled receivers are less vulnerable to background light
than e.g. APD-based receivers because only one spatial mode is detected. The fiber provides an excellent spatial filter function [26].
13.3.3 Terminal Design Issues
Terminals for optical communication in space are mostly designed for bidirectional links. They comprise both a transmitter and a receiver that generally
share the optical antenna. Another peculiarity is the necessity of beam steering (or
pointing) capability with sub-microradian angular resolution.
laser,
modulator
optical
booster
amplifier
duplexer
fine
pointing
telescope
transmit
data
out
in
receive
data
electronic
amplification,
data recovery
coarse
pointing
point
ahead
photo
detector
optical
bandpass
filter
optical
pre-amplifier
acquisition &
tracking
detector
optical
electrical
Fig. 13.4 Block diagram of optical transceiver for space-to-space links
These requirements lead to a transceiver block diagram as shown in Fig. 13.4. To
achieve the highest possible antenna gain, the laser source preferably operates in a
single transverse mode. For impressing data, either direct (“internal”) modulation
or an external modulator is employed. The modulated beam passes an optical duplexer and a fine pointing assembly before it enters a telescope acting as transmit
antenna. A coarse pointing system provides for antenna steering.
The received radiation passes the antenna and the fine pointing assembly in reverse direction, and is then directed to the receive part of the terminal. One part
may first be optically amplified before being demodulated by the photodetector.
Another part is used for controlling the fine and coarse pointing mechanisms in
such a way that the acquisition and tracking detector is always hit centrally. For
scenarios with an appreciable transverse movement of the two terminals involved,
a point-ahead mechanism has to be inserted in either the receive or the transmit
path to take into account the slightly different direction of incoming and outgoing
beam.
The block diagram of Fig. 13.2 shows only a basic outline. It may be modified
in several respects, e.g. by providing

separate laser sources to generate extra beams for acquisition and tracking,

separate antennas for the outgoing and the incoming beam,

a single photodetector for data detection, acquisition, and tracking, etc.
It should be stressed, however, that engineering a laser terminal consists not
only of covering the data transmission and the pointing, acquisition and tracking
aspects, but also of designing space-qualifiable opto-mechanical structures and
proper interfacing with the spacecraft platform.
Data transmitter
The main parameters characterizing the optical source are wavelength, output
power, and modulation capability. Depending mainly on link distance, data rate,
and antenna diameters, an output power between 100 mW and several Watts will
be required. It should be emitted in a single transverse mode to achieve maximum
on-axis antenna gain. With diode laser sources (e.g. at  = 1.55 µm wavelength)
modulation may be achieved directly. In case of solid-state lasers (e.g. at
 = 1.06 µm), external modulators based on the electro-optic effect may be employed. Primarily, binary modulation formats such as on-off-keying or pulse position modulation are envisaged for space links.
Receiver front end
Utmost receiver sensitivity is an extremely valuable asset in space applications,
not at least because no in-line amplification is possible. It is often characterized by
the minimum number of input photons per bit to achieve a bit-error ratio of
BER = 10-9. If there are no other sources of noise than that due to the quantum nature of radiation, a direct detection receiver 1 based on an ideal photodiode requires
n = 10 photons/bit while a receiver equipped with a perfect optical pre-amplifier
requires n = 38 photons/bit [31]. To what extent these quantum limits are reached
in practice depends on the engineer’s ability to make negligible the effect of other
noise contributions, as there are

1
multiplication noise and dark current in avalanche photodiodes,
Already when presenting Fig. 13.4, we tacitly assumed a so-called direct detection receiver.
Here we will not consider coherent receivers which make use not only of the intensity of the received wave but also of the information contained in its phase. Such receivers offer improved
sensitivity, however at the expense of a higher system complexity.

transistor noise and circuit noise in the receiver electronics,

out-of-band amplified spontaneous emission in case of optical amplification,

background radiation.
Examples of receiver sensitivities achievable today are listed in Table 13.2 [32].
(Note: For the wavelength and data rate chosen, receive powers of -38.4 dBm and
-50.3 dBm correspond to n = 1128 photons/bit and to n = 73 photons/bit, respectively).
Table 13.2 Real-world receiver sensitivities at  = 1.55 µm for a data rate of R = 1 Gbit/s and
on-off-keying under return-to-zero modulation.
Minimum input power for
BER = 10-9
avalanche photodiode receiver
-38.4 dBm
optically pre-amplified receiver
-50.3 dBm
13.4 Radio Frequency vs. Optical Communication – some considerations
It will be clear from the preceding sections that optical communications can provide the very large data-rates required for broadband communications, by virtue of
both the high frequency of the optical carrier and the vast spectrum available that
can be exploited for multi-wavelength schemes.
Optical frequencies are not subject to licences and tariffs, which can be a major
expense and challenge in RF systems. Furthermore, radio frequency system design
has to adapt to available spectrum allocations, while optical systems can be designed on the basis of preferred wavelengths and readily obtainable hardware
without consideration of spectrum availability, interference with other users etc.
Optical hardware is small and compact and economic in power consumption by
comparison with radio frequency equipment. While this may not be very important at the ground station gateway, these features are of paramount importance
on the HAP itself, where minimising payload size and weight is extremely important and energy expenditure is a major issue.
The optical carrier beam can be made very narrow in the interest of energy savings, which is particularly valuable for covert point-to-point links. The additional
features of optical quantum cryptography further render optical communications
safe in the face of attempts at interception or eavesdropping and guarantee a high
level of privacy, which is increasingly becoming recognised as an inherent hazard
of wireless communications. In contrast, radio frequency is suitable for broadcast
and point-to-multi-point links, which is important in many applications such as
live coverage of sports events and news reportage. However, the narrow beam in
FSO is also a drawback since it necessitates stringent transceiver alignment, and,
in the case of HAP-to-ground links, would require pointing and tracking systems
even with excellent station-keeping performance.
The robustness of optical and radio frequency wireless communication in various channel conditions is very different. The propagation of light through any medium is highly wavelength dependant and, in practice, only atmospheric “absorption windows” where attenuation due to absorption by atmospheric molecules and
particles is minimal, are possible for FSO. Scattering of light by atmospheric particles imposes a major limitation on HAP-to-ground links, particularly in the presence of fog or cloud, where the water droplets are of the same order of magnitude
as the radiation wavelength. This is termed Mie scattering, after the German scientist Gustav Mie (1869 - 1957) who first developed a mathematical theory describing this phenomenon of characteristic wavelength-dependant scattering with a
prominent forward-scattered lobe. The same issues constrain the efficacy of millimetre waves in the presence of rain, which is almost transparent to light (except
at wavelengths where the water vapour has absorption peaks). While atmospheric
attenuation due to absorption by water vapour, oxygen and other particulates must
be considered in calculating link budgets for all spectral allocations, it is evident
that the lower microwave frequency bands such as are used in WiMAX are robust
to atmospheric conditions. However, multipath phenomena must be considered for
radio frequency transmissions, and the terrain (mountainous/oceanic/etc.) may influence the received waveform. For HAP-to-HAP inter-platform links (IPLs) FSO
promises significant advantages, but field tests have yet to prove the feasibility
and true performance limitations of these links.
A well-researched phenomenon impacting optical wireless links through the
atmosphere is turbulence, which can challenge the performance of the link due to
fades and scintillation. While numerous solutions exist to mitigate the effects of
turbulence (aperture averaging, multiple beams, etc.), this phenomenon represents
a clear drawback of FSO by comparison with radio frequencies.
Another consideration influencing the choice of communication modality is the
maturity of the technologies. Radio communication is a very mature technology
that has penetrated almost all spheres of wireless communication and is wellaccepted. Equipment is readily available and considerable practical expertise has
been amassed throughout the world. In contrast, FSO is still an emerging technology and development costs of new niche systems would probably be quite high.
Innumerable obstacles could delay the successful implementation of an optical
wireless solution, despite the favourable performance that was predicted by theoretical analysis.
An overall comparison of radio frequency and optical communication can best
be presented in tabular form, as shown in the following:
Table 13.3
Capacity considerations
Radio frequency
Optical communication
High capacity can be
achieved at the cost of
high bandwidth allocation and/or modulation
and multiplexing
schemes with high spectral efficiency.
Optical communication
simply enables high data
rate communication. Capacity can be increased
using multiple wavelength transmissions.
Frequency re-use also
increases capacity but
adds system complexity
Payload considerations
Relatively bulky equipment with relatively high
power consumption
Very small equipment
with low power consumption
Resource allocation
Spectrum allocation is
restricted by regulation
and bandwidth is costly
No licenses or tariffs for
use of optic frequencies
Technology maturity
Mature and wellaccepted technologies,
constantly upgraded
Emerging technology, not
well penetrated in the
global market
Propagation issues
Most radio frequencies
are robust in typical atmospheric conditions,
although multipath can
be a drawback in certain
conditions and rain can
hamper transmission at
millimetre wavelengths
Absorption, scattering
and turbulence challenge
the performance of optical wireless links through
the atmosphere. The
presence of clouds in the
propagation path can be
prohibitive to FSO links.
Privacy and security
Very low inherent security and high susceptibility to interference and
eavesdropping.
High security, further
augmented by quantum
cryptographic solutions.
– On the other hand,
broadcast and multi-cast
are possible
– On the other hand,
alignment problems result
from the narrow and directional beam.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
Planche G, Chorvalli V (2004) SILEX in-orbit performances.," Proceedings of the 5th International Conference on Space Optics (ICSO 2004),
Toulouse, France 2004:403-410
Toyoshima M, Yamakawa S, Yamawaki T, Arai K, Reyes-Garcia-Talavera
M, Alonso A, Sodnik Z, Demelenne B (2005) Long-term statistics of laser
beam propagation in an optical ground-to-geostationary satellite communication link. IEEE Transactions on Antennas and Propagation 53:842-850
Baister G, Gatenby PV (1994) Pointing, acquisition, and tracking for optical space communications. IEEE Electronics and Communication Engineering Journal 6:271-280
Jono T, Takayama Y, Ohinata K, Kura N, Koyama Y, Arai K, Shiratama
K, Sodnik Z, Bird A, Demelenne B (2006) Demonstrations of ARTEMISOICETS inter-satellite laser communications. Proceedings of the 24th
AIAA International Communications Satellite Systems Conference, San
Diego, USA 2006:5461
Jono T, Takayama Y, Shiratama K, Mase I, Demelenne B, Sodnik Z, Bird
A, Toyoshima M, Kunimori H, Giggenbach D, Perlot N, Knapek M, Arai
K (2007) Overview of the inter-orbit and the orbit-to-ground laser communication demonstration by OICETS. Proceedings of the SPIE 6457: 645702
N. Perlot, M. Knapek, D. Giggenbach, J. Horwath, M. Brechtelsbauer, Y.
Takayama, and T. Jono, "Results of the optical downlink experiment
KIODO from OICETS satellite to Optical Ground Station Oberpfaffenhofen (OGS-OP)," Proc. SPIE Photonics West, Free-Space Laser Communication Technologies XIX, San Jose, Jan. 2007.
Lange R, Smutny B (2007) Homodyne BPSK-based optical inter-satellite
communication links. Proceedings of the SPIE 6457: 645703/1-645703/9
Kudielka K (2007) Laser communications in space – applications and systems. 20th LEOS summit, doi: 10.1109/LEOS.2007.4382392
Fields RA (2008) The near-field infrared experiment (NFIRE) satellite
program laser communication terminal (LCT): International cooperation
for joint LCT experiments. Proceedings of Laser Science XXIV:LWA1
Horwath J, Knapek M, Epple B, Brechtelsbauer M, Wilkerson B (2006)
Broadband backhaul communication for stratospheric platforms: the
stratospheric optical payload experiment (STROPEX). Proceedings of the
SPIE. doi:10.1117/12.680824
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
Giggenbach D, Purvinskis R, Werner M, Holzbock M (2002) Stratospheric
optical interplatform links for high altitude platforms. Proceedings 20th
AIAA International Communications Satellite Systems Conference and
Exhibit, Montreal, Canada. 2002:1910
Katimertzoglou E, Vouyioukas D, Veltsistas P, Constantinou P (2007)
Optical interplatform links scenarios for 20 km altitude. Proceedings 16th
IST Mobile and Wireless Communications Summit, Budapest, Hungary.
2007:1-5
Sodnik Z, Furch B, Lutz H (2006) Free-Space Laser Communication Activities in Europe: SILEX and beyond. 19th LEOS summit,
doi:10.1109/LEOS.2006.278845
Leeb WR (1989) Heterodyne and homodyne detection in optical space
communications. Proceedings of the SPIE 1131:216-227
Winzer PJ (2002) Optical transmitters, receivers, and noise. In: .G.Proakis
(ed.), Wiley Encyclopedia of Telecommunications, Wiley, New York
B. Wandernoth, “5 photon/bit low complexity 2 Mbit/s PSK transmission
breadboard experiment with homodyne receiver applying synchronization
bits and convolutional coding,” Proc. Eur. Conf. Opt. Commun. (ECOC),
Vol. 1, pp. 59-62, 1994.
Fidler F (2008) Optical backhaul links between HAPs and satellites in the
multi-Gigabit regime. IEEE Globecom, New Orleans, USA. doi:
10.1109/GLOCOMW.2008.ECP.100
A. Majumdar, J. Ricklin, Free-Space Laser Communications – Principles
and Advances, in: Optical and Fiber Communications Reports, Springer
Science+Business Media, New York, 2008.
Andrews LC, Phillips RL (2005) Laser beam propagation through random
media. 2nd edn. SPIE Press
Pratt WK (1969) Laser communication systems. 1st edn. John Wiley &
Sons Inc, New York
Giggenbach D, Purvinskis R, Werner M, Holzbock M (2002) Stratospheric
optical interplatform links for high altitude platforms. 20th AIAA International Communications Satellite Systems Conference and Exhibit, Montreal, Canada 20:1910.
Hemmati H (2006) Deep space optical communications. 1st edn. John
Wiley & Sons Inc, New York
Horwath J, Knapek M, Epple B, Brechtelsbauer M, Wilkerson B (2006)
Broadband backhaul communication for stratospheric platforms: the
stratospheric optical payload experiment (STROPEX). Proceedings of the
SPIE. doi:10.1117/12.680824
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
Fidler F (2008) Optical backhaul links between HAPs and satellites in the
multi-Gigabit regime. IEEE Globecom, New Orleans, USA. doi:
10.1109/GLOCOMW.2008.ECP.100
Kogelnik H, Li T (1966) Laser beams and resonators. Proceedings of the
IEEE 54:1312-1329
Fidler F, Wallner O (2008) Application of single-mode coupled receivers
in optical satellite to high-altitude platform communications. EURASIP
Journal on Wireless Communications and Networking 2008:864031/1864031/7
Leeb WR (1989) Degradation of signal to noise ratio in optical free space
data links due to background illumination. Applied Optics 28:3443-3449
Goody RM, Yung YL (1989) Atmospheric radiation: theoretical basis. 2nd
edn. Oxford University Press
Fidler F (2007) Optical Communications from high-altitude platforms. Dissertation, Vienna University of Technology, Austria
Pfennigbauer M (2004) Design of optical space-to-ground links for the International Space Station. Dissertation, Vienna University of Technology,
Austria
G. Einarsson, Principles of Lightwave Communications, John Wiley &
Sons, New York, 1996.
F. Fidler, Optical Communications from High-Altitude Platforms, Dissertation, Vienna University of Technology, Austria, Sept. 2007.