Federico Clazzer

DVB-S2X-enabled precoding for high throughput satellite systems

Multi-user multiple-input multiple-output MU-MIMO has allowed recent releases of terrestrial long-term evolution LTE standards to achieve significant improvements in terms of offered system capacity. The publication of the DVB-S2X standard and particularly of its novel superframe structure is a key enabler for applying similar interference management techniques -such as precoding- to multibeam high throughput satellite HTS systems. This paper presents results from the European Space Agency-funded R&D activities concerning the practical issues that arise when precoding is applied over an aggressive frequency re-use HTS network. In addressing these issues, the paper also proposes pragmatic solutions that have been developed in order to overcome these limitations. Through the application of a comprehensive system simulator, it is demonstrated that important capacity gains beyond 40% are to be expected from applying precoding even after introducing a number of significant practical impairments. Copyright

Multi-receiver Aloha systems - a survey and new results

This paper focuses on the beneficial effects brought by the presence of multiple receivers to a slotted Aloha scheme. Starting from an analytical angle, we review and compare some recent results that characterize the throughput of such systems under different channel models, based on the assumption that incoming powers at receivers follow an i.i.d. distribution. While practical in some scenarios, this hypothesis does not hold when the path loss experienced by different users, or by a user seen by different receivers, starts to play a role. We shed light on this aspect by means of detailed simulations, and derive some relevant insights on the achievable diversity gain. The impact of successive interference cancellation in this context is also evaluated.

On the characterization of AIS traffic at the satellite

Satellite-aided Automatic Identification System (AIS) has drawn the attention from the maritime community for worldwide vessel tracking and monitoring. The satellite reception of AIS packets is viable, although the standard has been designed for inter-vessel communication and vessel collision avoidance. A model and an extensive analysis of the Medium Access (MAC) protocol performance of AIS can on the one hand shed light on possible protocol improvements and on the other hand help satellite operators to design satellite-aided AIS reception systems. The objective of the paper is to analytically model the AIS Self-Organized Time Division Multiple Access (SOTDMA) traffic pattern at the satellite and to investigate the realistic behavior of SOTDMA via simulations. In the paper we demonstrate that SOTDMA can be seen as a slotted random access protocol at the satellite which noteworthy simplifies the analysis of the protocol performance. Exploiting this connection, an optimization on the frequency of AIS packets generation is derived here, which maximizes the vessel tracking frequency.

On the impact of coverage range on AIS message reception at flying platforms

In the recent past, an increasing interest has been devoted to the possibility of receiving Automatic Identification System (AIS) messages via Low Earth Orbit (LEO) satellites. While the principle has been demonstrated to be a viable option for monitoring vessel traffic over oceans and vaste land areas, the achievable performance from a communications viewpoint is far from optimal. Recently, it was shown how AIS traffic seen at a satellite can be very accurately modeled resorting to simple random access schemes. Leveraging this result, in this work we propose a simple yet flexible analytical framework capable of predicting channel load and overall reception performance taking into account the spatial distribution of vessels as well as their traffic generation pattern. Feeding the model with ship speed and location data derived from experimental settings, we discuss the achievable efficiency for a typical LEO-satellite detecting AIS packets. Moreover, the impact of the receiver footprint on ground on the overall decoding performance is investigated, deriving some interesting insights on the benefits that could stem resorting to narrower-beam systems. In this direction, we discuss two cases: the usage of a LEO satellite with a directional antenna soon to be launched for AIS monitoring, and the possibility of using airliner for receiving vessel-generated traffic.

Optimum header positioning in successive interference cancellation (SIC) based Aloha

Random Access MAC protocols are simple and effective when the nature of the traffic is unpredictable and sporadic. In the following paper, investigations on the new Enhanced Contention Resolution ALOHA (ECRA) are presented, where some new aspects of the protocol are investigated. Mathematical derivation and numerical evaluation of the symbol interference probability after SIC are here provided. Results of the optimum header positioning which is found to be in the beginning and in the end of the packets, are exploited for the evaluation of ECRA throughput and Packet Error Rate (PER) under imperfect knowledge of packets positions. Remarkable gains in the maximum throughput are observed for ECRA w.r.t. Contention Resolution ALOHA (CRA) under this assumption.

Enhancing Contention Resolution ALOHA Using Combining Techniques

Recently, random access protocols have acquired a new wave of interest, not only from the satellite communication community, but also from researchers active in fields, such as Internet of Things and machine-to-machine. Asynchronous (slot- and frame-wise) ALOHA-like random access protocols, are very attractive for such applications, enabling low complexity transmitters and avoiding time synchronization requirements. Evolutions of ALOHA employ time diversity through proactive replication of packets, but the time diversity is not fully exploited at the receiver. Combining techniques, as selection combining and maximal-ratio combining, are beneficial and are adopted in the enhanced contention resolution ALOHA (ECRA) scheme, presented here. A tight approximation of the packet loss rate for asynchronous random access, including ECRA, well suited for the low channel load region is derived. Finally, ECRA is evaluated in terms of spectral efficiency, throughput and packet loss rate in comparison with recent protocols, showing that it is able to largely outperform both slotted synchronous and asynchronous schemes.

Performance, cost analysis, and ground segment design of ultra high throughput multi‐spot beam satellite networks applying different capacity enhancing techniques

There is an ever present demand for increasingly higher data rates in multi-spot beam satellite networks. This can be enabled, by shifting carrier frequencies to higher bands, such as the Q/V-band, where more bandwidth is available. Furthermore, the available bandwidth has to be used as efficiently as possible, which requires efficient capacity enhancing techniques. The present paper identifies and analyzes ground segment capacity enhancing techniques for ultra high throughput multi-spot beam satellite networks operating in Q/V-band in the feeder link and in Ka-band in the user link. The impact of several capacity enhancing techniques on system performance is analyzed using a realistic time step-based system simulator. Their impact on the ground segment costs is also analyzed. The examined capacity enhancing techniques are as follows: (i) reduced carrier spacing with adjacent channel interference cancellation; (ii) full frequency reuse with co-channel interference cancellation and scheduling; (iii) four different smart gateway diversity techniques and; (iv) employing optical feeder links as an alternative to radio frequency feeder links. The paper shows that these techniques are capable of improving system performance at the expense of an increase in cost related to the complexity and maturity of the applied capacity-increasing technique.

LDPC code performance and optimum code rate for Contention Resolution Diversity ALOHA

In this paper we investigate several central aspects of using the unslotted Contention Resolution Diversity ALOHA (CRA) scheme with Successive Interference Cancellation (SIC). A model for the co-user-interference under consideration of time-, phase- and frequency-offsets is presented. With this model, the throughput and Packet Loss Ratio (PLR) performance of CRA is investigated when using Low Density Parity Check (LDPC) codes, showing that CRA can achieve remarkable performance also in these conditions. Moreover, the throughput and PLR gain of CRA in presence of power unbalance is studied, showing that significant gains can be expected. Following this, the central question is answered which channel rates optimize the goodput of CRA, showing that for CRA with few replicas a low, robust channel rate achieves optimum while for high numbers of replicas a high channel rate performs optimum. Finally an investigation of the interference distribution of an asynchronous TDMA scheme with replicas and the SIC operations in it are shown.

Irregular repetition slotted ALOHA over the Rayleigh block fading channel with capture

Random access protocols relying on the transmission of packet replicas in multiple slots and exploiting interference cancellation at the receiver have been shown to achieve performance competitive with that of orthogonal schemes. So far the optimization of the repetition degree profile, defining the probability for a user to transmit a given number of replicas, has mainly been performed targeting the collision channel model. In this paper the analysis is extended to a block fading channel model, also assuming capture effect at the receiver. Density evolution equations are developed for the new setting and, based on them, some repetition degree profiles are optimized and analyzed via Monte Carlo simulation in a finite frame length setting. The derived distributions are shown to achieve throughputs largely exceeding 1 [packet/slot].

Neighbor discovery in wireless networks: A graph-based analysis and optimization

The neighbor discovery (ND) procedure is typically performed during the network setup and allows nodes to learn about the existence of neighbors, exploiting the transmission of broadcast messages. Battery powered nodes impose many constraints on the ND protocol, such as low latency and energy efficiency. This calls for the deployment of dedicated and efficient ND protocols allowing nodes to be idle and save energy. We propose and analyze a new ND protocol, named collision resolution birthday protocol (CRBP), that drastically improves the performance of the birthday protocol (BP) making the use of multiple transmitted packets per node and successive interference cancellation (SIC). Via a bipartite graph analysis, we optimize the number of transmitted packets per node in order to maximize the discovery rate, under the asymptotically large frame assumption. Numerical simulations are performed for evaluating the goodness of the optimization also for finite frame durations.