Free space optical communications are becoming a mature technology to cope with the needs of high data rate payloads for future low-earth orbiting observation satellites. However, they are strongly impacted by clouds. In this paper, we aim to find a network of optical ground stations maximizing the percentage of data acquired by a low-earth orbiting satellite that can be transferred to the Earth, taking into consideration cloud information. This problem can be separated in two parts and solved hierarchically: the selection of a network of optical ground stations and the assignment of downloads to visibility windows of the stations. We present theoretical and practical results regarding the complexity of the latter subproblem and propose a dynamic programming algorithm to solve it. We combine this algorithm with two methods for the enumeration of the stations, and compare them with a mixed integer linear program (MILP). Results show that even if the MILP can solve scenarios over small horizons, the hierarchical approaches outperform it in term of computation time while still achieving optimality for larger instances. KEYWORDS cloud uncertainties, combinatorial optimization, dynamic programming, free space optical communications, mixed-integer linear programming, network design INTRODUCTION AND STATE OF THE ARTFree space optical communications are seen as a key technology [11,13,16] to cope with the needs of high data rate payloads for future low-earth orbiting observation satellites in replacement or in addition to current radio-frequency technologies. While the latter are very mature and well proven technologies which have been used for decades, the former may be able to offer data rates beyond the reach of radio-frequency technologies.Current radio-frequency technologies mainly use X-band for download and thus can currently provide up to a few gigabits per second (Gbps) [3]. Their main advantage is that X-bands are not impacted by weather or atmospheric turbulences, thus allowing the establishment of communications at very low elevation angles (typically 5 • above the horizon). This leads to an increase in the contact duration between low-earth orbiting satellites and ground stations. One of their main drawbacks is their limited data rates and the need of frequency licensing in order to avoid interferences. This will be a major issue in the upcoming years due to the increase in the number of operational satellites and constellations orbiting around the Earth.Free space optical communications offer data rates orders of magnitude higher than current radio-frequency ones: targeted data rates go from some tens of gigabits per second to several terabits per second (Tbps). Moreover, thanks to their very narrow beam, they do not require frequency licensing and are hard to intercept by malicious observers. Finally, they Networks. 2019;73:234-253.wileyonlinelibrary.com/journal/net

Network virtualization is seen as a key networking paradigm for building diverse network services and architectures over a shared network infrastructure. Assigning network resources to virtual links and, more generally to virtual network topologies, efficiently and on-demand is one of the most challenging components of any network virtualization solution. This paper addresses the problem of on-line resource allocation of multiple virtual links on a Software Defined Network (SDN) infrastructure. The application context that is targeted is primarily the online provisioning of virtual overlay networks even if it can be broadened to address more general virtual networks. Considering an SDN physical infrastructure allows a complete freedom in choosing the optimal physical paths and the associated resources that support the virtual links with no interference from any other network function (such as routing). However, for the time being, forwarding in an SDN network is resource consuming with a noticeable impact on the size of the flow tables. Hence, forwarding (i.e. switching) resources should be carefully considered by the network resource allocation algorithm. This paper proposes a novel Integer-Linear formulation of the above cited problem by taking into account: (1) point-to-point as well as pointto-multipoint virtual links, each with an associated bandwidth requirement and a maximum transfer delay requirement, (2) two types of network resources, namely network links' bandwidth and nodes' switching resources, and (3) optionally, path splitting which allows a virtual link to be established on multiple physical paths. We report preliminary experimental results on a real network topology in an overloaded scenario (bandwidth requests largely exceed network capacity); they show that our algorithm outperforms shortest path heuristics with a gain on the admission rate (of virtual links requests) that ranges from 5 to 15% (compared to the most efficient heuristic) and with a computation time less than a few seconds.

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