# Provision of Energy- and Wavelength-Efficient Traffic
## 1 Introduction  
Due to the extension of terrestrial 5G to satellite networks, the off-the-shelf microwave communication can no longer address the requirements of future extensive applications, such as remote sensing, Internet of Vehicles (IoV), and smart ocean [1; 2; 3]. Free-space optical communication has the superiorities of large-capacity, low-latency, excellent anti-interference, and terminal miniaturization, offering a promising means for satellite networking [4; 5]. Leveraging the WDM technique, the capacity of inter-satellite links (ISLs) can be greatly heightened to accommodate various services [6]. However, unlike the dense WDM-enabled terrestrial fiber links, wavelengths in each ISL are limited. On the one hand, the limited bandwidth of onboard optical amplifiers decides the sparse WDM ISLs. On the other hand, the crosstalk of filtering devices decreases the number of wavelengths in each ISL [7]. Driven by the limited single-satellite payload and massive data traffic requests, the distributed satellite cluster (DSC) has been designed to construct a large platform from the system perspective over the decade [8; 9; 10; 11; 12]. Satellites in the DSC are arranged on the sameor adjacent orbits to orderly and jointly accomplish spatial tasks. The typical architecture includes the F6 (Future, Fast, Flexible, Fractionated, Free-Flying) Program, space-based group, and GRACE [10; 11; 12]. The DSCNs are constructed by connecting multiple DSCNs. With the aid of high-speed laser ISLs, multigranular information, such as broadband remote sensing information and the narrowband Internet of Things (IoT) information, can be quickly transmitted and distributed in the DSCNs. Although the overall ability of the DSC system is enhanced via multi-satellite collaboration and WDM ISLs, energy consumption is a critical issue that cannot be neglected. Because the satellite is a kind of energy harvest system, the harvested energy is used for orbit keeping, signal processing, and ISLs establishment and maintenance [13]. A promising solution for wavelength and energy savings in the sparse WDM-enabled DSCNs is utilizing traffic grooming, where multiple traffic requests are multiplexed into a single lightpath that is then transmitted at the optical layer. Traffic grooming has been widely investigated in terrestrial optical networks over the past decades [14; 15; 16; 17; 18]. However, to our knowledge, no previous work has been focused on traffic grooming in optical satellite networks (OSNs) and considering energy and wavelength efficiency as well. In addition, most existing heuristics proposed for traffic grooming belong to DLG where traffic requests are directly groomed into wavelengths [16; 17; 18]. To actualize traffic grooming, the hybrid cross-connect architecture is maintained in every satellite node, including the optical cross-connect (OXC) unit which is functioned as wavelength routing operations, and the digital cross-connect (DXC) unit which is applied for electric switching and flow aggregation [16]. In this paper, we introduce the sub-wavelength, (e.g., OC-1) as the intermediate granularity and divide the traffic grooming into two phases, i.e., the TAASA and the SG. On the one hand, we can achieve a further segmentation of the fixed-capacity wavelength, targeting to enhance the utilization efficiency of wavelengths. On the other hand, the group transmission of traffic requests is realized, which helps to save transmission resources, especially for energy and wavelength channels. To obtain an optimal grooming scheme, the traffic requests, sub-wavelengths, and wavelengths can be treated as three sets of players to be matched with each other to minimize the energy consumption and the used wavelengths. We then resort to the matching algorithm to resolve the traffic grooming problem. The rest of the paper is structured as follows. The scenario of traffic grooming in the DSCNs is described, and the energy- and wavelength- minimized models are established in Section 2.