In this paper we investigate capacity optimization mechanisms for multi-beam satellite systems built on a realistic payload model. The first proposed mechanism deals with long term traffic variations, for which capacity optimization algorithms are proposed based on per-beam traffic requests. Due to the high asymmetry of the traffic, our algorithms provide time and spatial flexibility illuminating a specific set of beams within a window of several time-slots. Our algorithms maximize the amount of capacity actually offered while providing reduced power consumption. The second proposed mechanism deals with short-term traffic variations, for which we propose Network Coding (NC) based techniques at the link layer. The aim is to increase the offered capacity taking advantage of overlapping beam coverage, usually considered as a source of interference. This technique is meant to be applied not only in classical multibeam systems, but also on top of the per-beam capacity optimization as a method to deal with fast traffic unbalances not evaluated in the first mechanism. Analysis and simulations results show that system capacity can be increased up to 13% in the first case and up to 90% in the second case.

In this letter, we investigate the advantages of network coding (NC) combined with spatial diversity (SD) in scenarios with multiple sources, a single satellite and a single receiver. Each link source-satellite is modeled as an ON/OFF channel. We show that our system matches a wide number of realistic scenarios, from Wireless Sensor Networks (WSN) to Delay Tolerant Networks (DTN). We propose employing random linear network coding (RLNC) together with SD to reduce the system outage probability with respect to traditional SD scheme. The theoretical expressions we derive and the simulations performed show that the proposed scheme significantly reduces the system outage probability for a wide range of channel conditions. Moreover, we also propose a method for obtaining the optimal code rate constrained to a maximum system outage probability, which can serve as system design methodology.Index Terms-Network coding, spatial diversity, satellite, system outage.

We propose a multicast scheme for multibeam satellite systems exploiting both the multiuser and spatial diversity inherent in this type of systems while taking into account realistic physical distributions of User Terminals (UTs) over the coverage. Our proposed scheme makes use of the well-known Adaptive Coding and Modulation (ACM) feature in Digital Video Broadcasting over Satellite, 2nd Generation (DVB-S2) and Extension (DVB-S2X) standards but also incorporates a set of innovative features. First, multilink reception, that is, receivers that can syntonize different frequencies and/or polarizations, together with Network Coding (NC) is used to enable decoding of signals from adjacent beams (spatial diversity). Second, efficient and fair allocation of resources is achieved through Proportionally Fair Multicast (PFM) scheduling. Our results, obtained over realistic nonuniform UTs distributions, show average system multicast throughput gains up to 88% with regard to state-of-the-art multicast schemes. Furthermore, a complete cross-layer architecture is proposed, fully compliant with the standard providing Quality of Service (QoS) guarantees.

In this paper we investigate how to improve the availability (deliverability) of the Return Link Service (RLS) of the Search and Rescue service of Galileo (SAR/Galileo). More specifically, we focus on improving the probability that the emergency beacon receives the Return Link Message (RLM) sent in the I/NAV signal. It is wellknown that beacons in distress events may be in unpredictable and arbitrary position and particular surrounding conditions such as harsh weather (heavy rain) or suffer blocking (tilted ships, crashed aircraft, big waves, etc.). Hence, it is possible that some of the 4/8 SAR RLM frames which form the short/long RLM, sent by two satellites, are lost. In this paper we propose two solutions, which could be considered for the next generation of GALILEO, to improve the chances of obtaining the RLM even when the beacon loses some frames from both satellites. Our proposals take advantage of the spatial diversity inherent in the SAR/Galileo system. The first solution implies increasing and changing the bits of the I/NAV signal which signal the SAR RLM frames. The second solution also implies increasing and changing these bits and uses Network Coding (NC). Issues such as backward compatibility with the current system and complexity are also assessed in the paper.

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