Effects of troposphere on Earth - space transmission links

New multimedia applications have appeared during the last decade and are becoming increasingly popular. In the past, the use of satellite for communications was mainly dedicated to TV broadcast and military applications; however interactive services based on both forward and return satellite links are available for some years and the marketing targets for these applications is mainly the people living in rural areas with no access to xDSL terrestrial infrastructures. Transmitting large amounts of data in (quasi) real-time requires high data rates and this increase in transmission velocity for both terrestrial and satellite services leads to a need for more and more frequency resources and to the development of new bandwidth-efficient coding and modulation schemes. Most of the proposed services are based on fixed transmission links from (or to) a geostationary satellite. The Ku-band (14 GHz/11 GHz) is allocated by the International Telecommunication Union for the Fixed Satellite Services but it is very populated and, therefore, it is difficult to use this band for high data rate transmissions in this context. On the other hand, the Ka-band (30 GHz/20 GHz) is also allocated for the Fixed Satellite Services and most of the providers are operating their satellites for interactive applications in these bands. The use of the Ka-band implies that new problems have to be understood and new challenges have to be overcome. One of the constraints introduced by the increase in frequency is the fact that the propagation phenomena will affect more strongly the transmitted signals than in the Ku-band. Indeed, the power fade induced by gases, clouds, scintillation and rain can reach tens of decibels (for example during rain events). Rain is the main contributor to the signal attenuation. For systems operating in the Ku-band, designers are considering a constant power margin dedicated to mitigate the signal fade in the transmission links when a propagation phenomenon occurs. As the occurrence probability of a rain event is about 10% in temperate regions, the power dedicated to this margin is only used during 10% of time. During the rest of the time, more power than needed is transmitted. This non-optimal use of the resource is acceptable in the Ku-band as the margin is very low (2 to 5 dB). However, as the margin has to be higher in the Ka-band (for example, 27 dB in Toulouse at 30 GHz for a tolerated interruption time of 1 hour per year), this approach is not applicable and has to be reviewed. The power margin can be used at Ka-band to mitigate propagation losses during rain events but also to increase the data rate (or the bandwidth efficiency) in clear sky conditions in order to ensure both acceptable capacities and availabilities for the system. Additionally, for a gateway to satellite (or satellite to gateway) link, site diversity techniques can also be used to increase link availability by operating two separated Earth stations. For the definition of the power margin, a simple cumulative distribution function of the attenuation is not enough. More complex propagation models are needed to design fade mitigation techniques and to estimate the availability and the capacity of the system: a good knowledge of the spatial and temporal behaviour of the propagation channel can be useful to define resource management techniques, to calibrate control loops for adaptive transmission links or to predict the percentage of time during which the link will be interrupted during one year. Since the relevant International Telecommunication Union recommendations only provides models for the