Distributed gateways for terabit/s satellite : Study of smart gateway diversity

Abstract: Future Satcom systems will be required to provide Internet at high capacity to an increasing number of users. Because of demanding applications the throughput should approach the Tb/s. To achieve such a data rate, large bandwidths are required. So considering spectrum congestion below 15 GHz, communications should use Ka and Q/V bands. In those frequency bands, the signal is strongly affected by propagation impairments occurring in the troposphere. Attenuation (due to gas, clouds and rain), scintillation and depolarization are the predominant phenomena. To mitigate this fading, some techniques have been developed like ULPC (Uplink Control Power), site diversity or ACM (Adaptive Coding and Modulation). A novel propagation impairment mitigation technique to cope with impairments for the feeder links called gateway diversity is studied in this document. The principle of this diversity scheme is the following: some additional gateways are connected to others and are used to replace the ones undergoing impairments. Contrary to site diversity, there is no duplication of gateways. The redundant gateways can be assigned instead to any other gateway (or some of them, depending on the connectivity pattern). All these gateways are distant of hundreds of km. Thus the unavailability of many of them at the same time is unlikely because of the low spatial correlation of the propagation impairments. This technique requires a terrestrial connection between gateways to enable switchings and a logic control. A scenario taking into consideration the current technologies and the future requirements is studied. The influence of different ground or space segment parameters is investigated (transmitted power, location and number of redundant gateways). Results show that permanent availability can be maintained for any user regardless its position or the one of the feeding gateway. Moreover a high percentage of nominal capacity can be kept for a high percentage of time (90% of nominal capacity for 99.5% of the time) even when decreasing the transmitted power. Gateway diversity is therefore a promising technique in terms of performances and cost to mitigate the propagation impairments that constitutes today a major challenge for the use of Q/V frequency bands.