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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.