Download 1 - Capanina

1. Introduction to Free-Space Laser Communication from
HAPs 15p
1.1.
Motivation (Franz) 1.5p
Motivation, Scenarios, Applications…
Motivation for QKD
1.2.
State-of-the-Art (Franz) 3p
Laser Communication on Satellites, Communication experiments on the ground,
Technologies used
Trials, Experiments
Vibrations, some platform issues, environmental conditions,
PAT
Optical links have been demonstrated in various scenarios over the last decade. Inter-satellite links were
performed in Europe between the geostationary satellite Artemis and the LEO satellite Spot-4 on a semioperational basis [NIE02]. Downlinks were demonstrated from Artemis to the Optical Ground Station (OGS) on
Tenerife [ALO04], operated by ESA. An optical link from a stratospheric balloon platform at 22km altitude was
performed in Kiruna, Sweden, by the Institute of Communications and Navigation of the German Aerospace
Center (DLR) in August 2005 [HOR06][KNA06]. In 2006 DLR has demonstrated together with JAXA the
feasibility of direct optical LEO-downlinks in a link from the Japanese LEO satellite OICETS to the DLR OGS
near Munich, Germany [TAK07][PER07]. Several inter-satellite and downlink experiments are planned for end
of 2007 from the LEO satellites TerraSAR-X and NFIRE with the German Laser Communication Terminal at
5.6Gbps.
1.3.
Basic Design Considerations of Optical Communication
Links
1.3.1. Link Budgets (Walter) 2p
Some of the basic formulas with and without atmospheric effects
1.3.2. Atmospheric Effects on Quality-of-Service (Franz) 3p
Some of the basic formulas
influence on QoS (BER, packet-loss rate, avialability)
1.3.3. Services and Applications (Markus, Bernhard) 2p
Foreseen Services for optical wireless links (data transfer, voice, web-browsing,
in p2p link, in network)
1.3.4. Terminal Design Issues (Walter) 2p
Receiver Front-Ends, Lasers, Modulators, Terminal system issues and
environmental issues
1.4.
Radio Frequency vs. Optical Communication – some
considerations (Shlomi) 1.5p
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 multiwavelength schemes.
Optical frequencies are not subject to licences and tariffs, which are a major expense in radio
frequency 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-toground 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 frequency bands that 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 comprises 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 well-accepted. 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:
Capacity considerations
Payload considerations
Resource allocation
Technology maturity
Propagation issues
Privacy and security
Radio frequency
High capacity can be
achieved at the cost of high
bandwidth allocation and/or
modulation and multiplexing
schemes with high spectral
efficiency.
Frequency re-use also
increases capacity but adds
system complexity
Relatively bulky equipment
with relatively high power
consumption
Spectrum allocation is
restricted by regulation and
bandwidth is costly
Mature and well-accepted
technologies, constantly
upgraded
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
Very low inherent security
and high susceptibility to
interference and
eavesdropping.
- On the other hand,
broadcast and multi-cast are
possible
Optical communication
Optical communication
simply enables high data rate
communication. Capacity can
be increased using multiple
wavelength transmissions.
Very small equipment with
low power consumption
No licences or tariffs for use
of optic frequencies
Emerging technology, not
well penetrated in the global
market
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.
High security, further
augmented by quantum
cryptographic solutions.
- On the other hand,
alignment problems result
from the narrow and
directional beam.
References
1.
[NIE02] T. T. Nielsen and G. Oppenhaeuser, “In orbit test result of an operational intersatellite link
between ARTEMIS and SPOT4,” Proc. SPIE, 4635, pp. 1-15, 2002.
2.
3.
4.
5.
6.
[ALO04] A. Alonso, M. Reyes, and Z. Sodnik, "Performance of satellite-to-ground communications link
between ARTEMIS and the Optical Ground Station", Proc. SPIE, Optics in Atmospheric Propagation and
Adaptive Systems VII, 5572, pp. 372-383, 2004.
[HOR06] J. Horwath, M. Knapek, B. Epple, M. Brechtelsbauer, and B. Wilkerson, “Broadband backhaul
communication for stratospheric platforms: the Stratospheric Optical Payload Experiment (STROPEX),”
Proc. SPIE, Free-Space Laser Communications VI, 6304, San Diego, 2006.
[KNA06] M. Knapek, J. Horwath, N. Perlot and B. Wilkerson, “The DLR ground station in the Optical
Payload Experiment (STROPEX) - Results of the atmospheric measurement instruments”, Proc. SPIE,
Free-Space Laser Communications VI, 6304, San Diego, 2006.
[TAK07] Y. Takayama, T. Jono, M. Toyoshima, H. Kunimori, D. Giggenbach, N. Perlot, M. Knapek, K.
Shiratama, J. Abe, and K. Arai, “Tracking and pointing characteristics of OICETS optical terminal in
communication demonstrations with ground stations,” Proc. SPIE Photonics West, Free-Space Laser
Communication Technologies XIX, San Jose, Jan. 2007.
[PER07] 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.