Timing Alignment of Distributed Gateways: Theoretical Analysis and Experimental Demonstration

High data rates in satellite communications are achievable with the help of high throughput satellite (HTS) systems. To support the increased traffic, several ground-based downlink optimization techniques have been implemented. In HTS systems, multiple-gateway architectures are a requirement for a proper dimensioning of the feeder link. In general, to coordinate the operation of multiple gateways, high precision time synchronization is necessary for the ground-based optimization techniques. Using multiple-input multiple-output (MIMO) feeder links is an approach that aims to reduce the cost of implementing multiple-gateway applications and improve their performance. In this paper, the effect that timing misalignment has in the MIMO feeder links is studied. We introduce a mathematical model that describes the bandwidth limitation for a 2x2 MIMO scenario. Time distribution via optical fiber is considered for the synchronization in time of ground stations separated by a few tens of kilometers. The ability of such systems to distribute high precision time reference over fiber was tested through experiments. The White Rabbit standard was used as a commercial implementation of the optical fiber time distribution. Finally, we tested the proposed mathematical model of the bandwidth for the 2x2 MIMO feeder link with the residual timing misalignment due to imperfect time synchronization of the experimental setup. Results showed that the achieved synchronization allows to support channel bandwidths in the order of several GHz.