Feasibility analysis of a HAP-LEO optical link for data relay purposes

The aim of the paper is to prove the technical feasibility and the advantages of a joint RF and optical communication system. The reference scenario foresees the use of a HAP (high altitude platform) or multiple HAPs for data relay purposes. HAPs are based on airships or balloons placed at about 20 km height and they combine both the advantages of terrestrial networks (the highest mast in the town) and of satellites (a LEO in very low orbit). As earth observation (EO) satellites need the download of huge amount of data, HAPs could establish an optical link with a LEO EO satellite in order to exploit the high data rate. Optical space communications offer a wide bandwidth and low interference channel. By the use of this system, it is possible to increase the amount of information downloaded by a high altitude platform from a LEO satellite, reaching data rate up to 10 Gbit/s. There are some critical points to face in order to dimension this link. First of all, a fast pointing acquisition and tracking procedure must be implemented. Another point is related to the high relative speed between LEO satellite and HAP that leads to an important contribution of Doppler shift in the received signal spectrum, this phenomenon is not stochastic and it can be managed by a tunable filter. The third point to focus on, strictly related to the transmitter design, is the optical carrier choice. It is important to take into account the atmospheric layer from 20 to 100 km, in terms of absorption and scattering spectra, temperature and refractive index variations in altitude and time varying phenomena. The transmitter is designed choosing the best performing technology in different optical window frequencies in which the channel presents low absorption and scattering, this choice involves the modulation format, as well. Three optical windows are available: 850nm, 1064nm and 1550nm. The first one is the most affected by Doppler effect but offers high gain telescope and low cost components; devices that work in the second window can transmit high amount of power but the modulation constraints of laser source can make the design more complicated; the third window is the most used in terrestrial communication systems and many devices are available, the Doppler effect is lower than at other frequencies and for this carrier it is possible to carry out DPSK (differential phase shift keying) modulation schemes other than classical OOK (on off keying) techniques. Moreover, HAPs could use terrestrial electronics equipments in order to store the data received by the optical link (e.g. hard disks). The link between HAP and ground can be exploited by the use of high data rate RF links in Ka-band or X-band (mainly used in EO satellites), V-band (allocated for HAPs) or W-Band (the new frontier). Despite the impact by weather and atmospherics, it was demonstrated that the availability of W-band satellite links is at least 95% (Fragale, 2004). This scenario overcomes the limits imposed by the short time window for EO satellites data download, using an advanced "store and forward-data relay" concept