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Terrestrial and satellite communication networks often rely on two-hop wireless architectures with an access channel followed by backhaul links. Examples include Cloud-Radio Access Networks (C-RAN) and Low-Earth Orbit (LEO) satellite systems. Furthermore, communication services characterized by the coexistence of heterogeneous requirements are emerging as key use cases. This paper studies the performance of critical service (CS) and non-critical service (NCS) for Internet of Things (IoT) systems sharing a grant-free channel consisting of radio access and backhaul segments. On the radio access segment, IoT devices send packets to a set of non-cooperative access points (APs) using slotted ALOHA (SA). The APs then forward correctly received messages to a base station over a shared wireless backhaul segment adopting SA. We study first a simplified erasure channel model, which is well suited for satellite applications. Then, in order to account for terrestrial scenarios, the impact of fading is considered. Among the main conclusions, we show that orthogonal inter-service resource allocation is generally preferred for NCS devices, while non-orthogonal protocols can improve the throughput and packet success rate of CS devices for both terrestrial and satellite scenarios., Comment: arXiv admin note: text overlap with arXiv:1909.10283

We consider a massive Internet of Things (IoT) scenario where indoor IoT devices access the network via optical wireless communication (OWC) IoT systems that relay data via a backhaul radio frequency (RF) low-power wide-area network (LP WAN). We propose a novel two-tier multi-rate Slotted ALOHA (SA) system model to design and analyse such hybrid OWC/RF networks. For a particular hybrid OWC/RF setup, we present an in-depth numerical investigation that provides insights into the proposed two-tier multi-rate hybrid OWC/RF SA system design., Comment: Accepted for publication in the IEEE Communications Letters

We consider a relay-aided Slotted ALOHA solution for uplink random access for an Optical Wireless Communications (OWC)-based Internet of Things (IoT). The first phase of uplink, the one between IoT devices and the relays, is realized using indoor OWC, while the second phase, between the relays and a base station, represents the long-range RF transmission based on low-power wide area network such as LoRaWAN and occurs outdoors. The throughput performance dependence on the OWC and RF channel conditions is observed. The behavior of the performance gain due to adding relays is highlighted and investigated under different channel and traffic conditions., Published in: 2020 12th International Symposium on Communication Systems, Networks and Digital Signal Processing (CSNDSP)

Wireless random access protocols are attracting a revived research interest as a simple yet effective solution for machine-type communications. In the quest to improve reliability and spectral efficiency of such schemes, the use of multiple receivers has recently emerged as a promising option. We study the potential of this approach considering a population of users that transmit data packets following a simple slotted ALOHA policy to a set of uncoordinated relays ( phase-1 ). These, in turn, independently forward – part of – what decoded towards a collecting sink ( phase-2 ). For an on-off fading channel model, we provide exact expressions for phase-1 throughput and packet loss rate for an arbitrary number of relays, characterising the benefits of multi-receiver schemes. Moreover, a lower bound on the minimum amount of phase-2 resources needed to deliver all information collected at the relays is provided. The bound is proven to be achievable via random linear coding when no constraints in terms of latency are set, with an overhead that approaches zero with the inverse of the packet length. We complement our study discussing a family of simple forwarding policies that require no packet-level coding, and optimising their performance based on the amount of available phase-2 resources. The behaviour of both random linear coding and simplified policies is also characterised when receivers are equipped with finite buffers, revealing non-trivial tradeoffs.

In this letter, we propose a short-frame (SF) approximation for the packet loss rate of Irregular Repetition Slotted ALOHA (IRSA) particularly suitable for very short frames, up to 50 slots. The reason for limiting the frame size is to allow low-latency communications, a typical requirement in Industrial Internet of Things (IIoT) applications. We extend the approximation of [1] (named EF) through a recursive evaluation of inter-related stopping sets (STs). The tighter approximation yields the user degree distribution that minimizes the packet loss rate (PLR). For a scenario with 10 slots and 5 users, the degree distribution optimized adopting the SF approximation results in a 20% lower PLR compared to the one optimized with the EF approximation.

The blooming of internet of things (IoT) services calls for a paradigm shift in the design of communications systems. Short data packets sporadically transmitted by a multitude of low-cost low-power terminals require a radical change in relevant aspects of the protocol stack. For example, scheduling-based approaches may become inefficient at the medium access (MAC) layer, and alternatives such as uncoordinated access policies may be preferred. In this context random access (RA) in its simplest form, i.e., additive links on-line Hawaii area (ALOHA), may again become attractive as also proved by a number of technologies adopting it. The use of forward error correction (FEC) can improve its performance, yet a comprehensive analytical model including this aspect is still missing. In this paper, we provide a first attempt by deriving exact expressions for the packet loss rate and spectral efficiency of ALOHA with FEC, and extend the result also to time- and frequency-asynchronous ALOHA aided by FEC. We complement our study with extensive evaluations of the expressions for relevant cases of study, including an IoT system served by low-Earth orbit (LEO) satellites. Non-trivial outcomes show how time- and frequency-asynchronous ALOHA particularly benefit from the presence of FEC and become competitive with ALOHA.

The blooming of internet of things (IoT) services calls for a paradigm shift in the design of communications systems. Short data packets sporadically transmitted by a multitude of low-cost low-power terminals require a radical change in relevant aspects of the protocol stack. For example, scheduling-based approaches may become inefficient at the medium access (MAC) layer, and alternatives such as uncoordinated access policies may be preferred. In this context random access (RA) in its simplest form, i.e., additive links on-line Hawaii area (ALOHA), may again become attractive as also proved by a number of technologies adopting it. The use of forward error correction (FEC) can improve its performance, yet a comprehensive analytical model including this aspect is still missing. In this paper, we provide a first attempt by deriving exact expressions for the packet loss rate and spectral efficiency of ALOHA with FEC, and extend the result also to time- and frequency-asynchronous ALOHA aided by FEC. We complement our study with extensive evaluations of the expressions for relevant cases of study, including an IoT system served by low-Earth orbit (LEO) satellites. Non-trivial outcomes show how time- and frequency-asynchronous ALOHA particularly benefit from the presence of FEC and become competitive with ALOHA.

The flourishing of Internet of Things (IoT) applications, characterized by vast transmitter populations and the sporadic transmission of small data units, demands innovative solutions for the sharing of the wireless medium. In this context, satellite connectivity is an important enabler for all scenarios in which terminals are under-served by terrestrial communications and are thus fundamental for providing worldwide coverage. In turn, the design of medium access policies that attain efficient use of the scarce spectrum and can cope with flexible yet unpredictable IoT traffic is of the utmost importance. Starting from these remarks, we investigate in this work the coexistence of a quality of service (QoS)-constrained service with IoT traffic in a shared spectrum as alternative to a more traditional orthogonal allocation among the two services, with an eye on satellite applications. Leaning on analytical tools, we provide achievable rate regions, assuming a slotted ALOHA access method for IoT terminals and accounting for practical aspects, such as the transmission of short packets. Interesting trends emerge, showcasing the benefit of an overlay allocation with respect to segregating the resources for the two services.

Asynchronous random access protocols have become attractive for several satellite applications of interest, reducing the complexity at the transmitter side while granting good performance. These schemes typically follow two paradigms, using either (i) spread spectrum techniques or (ii) replication of the transmitted packets to achieve time diversity. In both cases, the receiver employs successive interference cancellation to resolve collisions. In this paper we offer a comparison of the two approaches in a narrowband scenario, discussing the relevant use case of the upcoming ITU VDES standard for maritime communications.

The impressive increase in market penetration of internet of things (IoT) services and the flourishing of targeted technology solutions forecast a prominent role for these applications in upcoming networks. For their success, the design of efficient grant-free based medium sharing policies is paramount, and represents a key challenge for beyond-5G systems. Motivated by this, the article reviews some recent results on the potential of receiver diversity, which is emerging as a very promising approach to boost performance of random access for machine-type communications.

In this paper, we explore the use of machine learning methods as an efficient alternative to correlation in performing packet detection. Targeting satellite-based massive machine type communications and internet of things scenarios, our focus is on a common channel shared among a large number of terminals via a fully asynchronous ALOHA protocol to attempt delivery of short data packets. In this setup, we test the performance of two algorithms, neural networks and random forest, which are shown to provide substantial improvements over {traditional} techniques. Excellent performance is demonstrated in terms of detection and false alarm probability also in the presence of collisions among user transmissions. The ability of machine learning to extract further information from incoming signals is also studied, discussing the possibility to classify detected preambles based on the level of interference they undergo.

In this paper, we study the coexistence of critical and non-critical Internet of Things (IoT) services on a grant-free channel consisting of radio access and backhaul segments. On the radio access segment, IoT devices send packets to access points (APs) over an erasure collision channel using the slotted ALOHA protocol. Then, the APs forward correctly received messages to a base station (BS) over a shared wireless backhaul segment, modeled as an erasure collision channel. The APs hence play the role of uncoordinated relays that provide space diversity and may reduce performance losses caused by collisions. Both non-orthogonal and inter-service orthogonal resource sharing are considered and compared. Throughput and reliability metrics are analyzed, and numerical results are provided to assess the performance trade-offs between critical and non-critical IoT services., 6 pages, submitted as a conference paper

This work addresses the physical layer channel code design for an uncoordinated, frame- and slot-asynchronous random access protocol. Starting from the observation that collisions between two users yield very specific interference patterns, we define a surrogate channel model and propose different protograph low-density parity-check code designs. The proposed codes are both tested in a setup where the physical layer is abstracted, as well as on a more realistic channel model, where finite-length physical layer simulations of the entire asynchronous random access scheme, including decoding are carried out. We find that the abstracted physical layer model overestimates the performance when short blocks are considered. Additionally, the optimized codes show gains in supported channel traffic - a measure of the number of terminals that can be concurrently accommodated on the channel - of around 17% at a packet loss rate of 10^{-2} w.r.t. off-the-shelf codes., 29 pages, 9 Figures, Submitted for possible publication to MDPI Journal Algorithms

The possibility to detect Automatic Identification System (AIS) messages from low earth orbit (LEO) satellites paves the road for a plurality of new and unexplored services. Besides worldwide tracking of vessels, maritime traffic monitoring, analysis of vessel routes employing big data, and oceans monitoring are just few of the fields, where satellite-aided AIS is beneficial. Designed for ship-to-ship communication and collision avoidance, AIS satellite reception performs poorly in regions with a high density of vessels. This calls for the development of advanced satellite AIS receivers able to improve the decoding capabilities. In this context, our contribution focuses on the introduction of a new enhanced AIS receiver design and its performance evaluation. The enhanced receiver makes use of a coherent receiver for the low signal-to-noise ratio (SNR) region, while for medium to high SNRs, a differential Viterbi receiver is used. Additional novelty of our work is in the exploitation of previously decoded packets from one vessel that is still under the LEO reception range, to improve the vessel detection probability. The assessment of the performance against a common receiver is done making the use of a simple and tight model of the medium access (MAC) layer and the multi-packet reception (MPR) matrix for physical layer (PHY) representation. Performance results show the benefits of such enhanced receiver, especially when it is bundled with successive interference cancellation (SIC).

Grant-free access schemes are steadily gaining momentum for emerging machine type communications and internet of things applications. Relying on variations of the classical ALOHA protocol to allow a vast population of users share a common channel in an uncoordinated fashion, these solutions have recently been embedded in international standards as well as proprietary commercial products. In this context, the use of multiple low-complexity receivers that act as relays, collecting packets from users and forwarding them towards a central unit represents an interesting approach to increase the efficiency of the system, which finds natural application in both terrestrial and non-terrestrial settings. To gain insights on the potential of such a configuration, this paper investigates a two-tier slotted ALOHA system under an erasure channel model. Closed form expressions for both end-to-end throughput and packet loss rate are derived for an arbitrary number of receivers, and the role of correlation among wireless channels connecting users to relays is thoroughly discussed. Non-trivial tradeoffs and systems design hints are highlighted, pinpointing some fundamental benefits and limitations of receiver diversity for grant-free based machine type communications.

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.

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 like 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., 13 pages, 7 figures, accepted for publication in IEEE Transactions on Communications. Old title: Exploiting Combination Techniques in Random Access MAC Protocols: Enhanced Contention Resolution ALOHA

Graph-based random access protocols have emerged as an efficient solution to tackle the medium sharing problem for machine type communications. Their remarkable performance is well-understood for scenarios in which devices communicate with a one-hop receiver devoted to collecting packets. Many practical applications, however, foresee the receiver to also act as sender towards a second-hop terminal. Under a half-duplex operation constraint, the two tasks would typically be fulfilled by means of time sharing, triggering a tradeoff between the amount of collected and transmitted traffic at the intermediate node. In this perspective, the recent rise of effective self-interference cancellation techniques poses the intriguing question of whether and how much a full-duplex receiver could improve performance in such a setting. To cast light on the issue, we refine the asymptotic analysis of successive interference cancellation procedures at the receiver of a graph-based random access scheme. In doing so, we account for the degradation in decoding probability due to residual self-interference when operating in full-duplex mode, and derive a tight lower bound to the achievable packet loss rate. We then analyse the throughput as a function of some key parameters that drive the system, identifying relevant tradeoffs and design hints.

The paper focuses on the satellite uplink for the new VHF data exchange system (VDES), which aims at providing a worldwide messaging service for vessels. In this context, we investigate the use of some recently proposed asynchronous schemes for the narrowband random access mode of the standard. Remarkable improvements over the basic VDES self-organised TDMA return link access are shown in terms of both spectral efficiency and system coverage when considering realistic ships distributions. The study offers relevant insights for the ongoing discussions on the MAC layer design for upcoming versions of the VDES standard.

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 per- formance 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]., Presented at ICC 2017

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.

This article surveys the unsatisfying communication capabilities in the harsh Arctic regions. By melting of the ice caps, the undergoing change of the north polar area opens new opportunities and, therefore, increases the need for human and machine-to-machine communications. The emerging demands and challenges are reviewed. New satellite systems, architectures, and technologies being deemed as key enablers are highlighted. As also addressed in this article, satellite links could additionally backhaul terrestrial networks for local communications.

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 cancelation in this context is also evaluated.

In this paper, we discuss the impact of the capture effect and multi-user detection on the reception of automatic identification system messages at the satellite. Resorting to simple yet beneficial tools such as the multi-packet reception matrix and the random access approximation of the medium access layer, an insightful model is presented. A comparison with the destructive collision model is also carried out and performance evaluation with vessel distribution and vessel speed distribution gathered from real data are derived. The more realistic physical layer improves the results accuracy and provides tradeoffs and future research directions.

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 are 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.

Random Access (RA) Medium Access (MAC) protocols are simple and effective when the nature of the traffic is impulsive and unpredictable. Recently, evolutions of RA have shown impressive improvements in the throughput, but no investigation on the impact on the higher layers has been carried out, at the best of our knowledge. The objective of the paper is to investigate recently presented RA solutions in terms of layer 3 throughput when generic packet length distribution is considered. The definition of layer 3 throughput is presented for both slotted and non slotted protocols. In the latter case, a comprehensive analytical framework is developed, where different levels of knowledge of the packet distribution are considered. The results show that non slotted protocols that use Forward Error Correction (FEC) schemes are able to outperform slotted protocols under the same conditions. On the other hand, the layer 3 throughput of non slotted protocols is more sensitive to changes in the form of the packet length distribution than in slotted protocols.

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.

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.

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., Accepted for publication in the IEEE International Conference on Communications (ICC) 2013

This paper addresses the design of protograph low-density parity-check (LDPC) codes for an asynchronous random access (RA) scheme. At the receiver successive interference cancellation (SIC) is assumed to recover packets subject to Gaussian interference and to remove their replicas from the received signal. Since the interference typically affects the beginning or the end of a packet we provide a dedicated code design resulting in gains w.r.t. standard channel codes. The benefit of the proposed codes is illustrated in terms of packet error rate versus channel load results. A gain of 0.1 in supported channel load at a packet loss rate of 10−2 is shown.

Ensuring a correct signal integrity within the entire FlexRay network and for all the possible environmental situations is mandatory for reliable operation of the distributed application. However, this is a goal difficult to reach due to the large number of parameters that influence the signal integrity. The use of simulation is a natural answer to efficiently support space exploration. We discuss in this work how the TEODACS test approach supports the validation process of the simulation models for FlexRay topologies and provides trustfulness for the simulation results even if hardware reference is not available. Further, we introduce a new method for the advanced analysis and evaluation of signal integrity in FlexRay networks.