Andrea Munari

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.

Modern Random Access Protocols

Random access represents possibly the simplest and yet one of the best nknown approaches for sharing a channel among several users. Since ntheir introduction in the 1970s, random access schemes have been thoroughly nstudied and small variations of the pioneering Aloha protocol nhave since then become a key component of many communications standards, nranging from satellite networks to ad hoc and cellular scenarios. nA fundamental step forward for this old paradigm has been witnessed in nthe past few years, with the development of new solutions, mainly based non the principles of successive interference cancellation, which made it npossible to embrace constructively collisions among packets rather than enduring nthem as a waste of resources. These new lines of research have nrendered the performance of modern random access protocols competitive nwith that of their coordinated counterparts, paving the road for a nmultitude of new applications. nThis monograph explores the main ideas and design principles that nare behind some of such novel schemes, and aims at offering to the nreader an introduction to the analytical tools that can be used to model ntheir performance. After reviewing some relevant thoretical results for the random naccess channel, the volume focuses on slotted solutions that combine the napproach of diversity Aloha with successive interference cancellation, nand discusses their optimisation based on an analogy with the theory nof codes on graphs. The potential of modern random access is then nfurther explored considering two families of schemes: the former based non physical layer network coding to resolve collisions among users, and nthe latter leaning on the concept of receiver diversity. Finally, the opportunities nand the challenges encountered by random access solutions nrecently devised to operate in asynchronous, i.e., unslotted, scenarios nare reviewed and discussed.

mm-Wave Radar Based Gesture Recognition: Development and Evaluation of a Low-Power, Low-Complexity System

Gesture recognition is gaining attention as an attractive feature for the development of ubiquitous, context-aware, IoT applications. Use of radars as a primary or secondary system is tempting, as they can operate in darkness, high light intensity environments, and longer distances than many competitor systems. Starting from this observation, we present a generic, low-cost, mm-wave radar-based gesture recognition system. Among potential benefits of mm-wave radars are a high spatial resolution due to small wavelength, the availability of multiple antennas in a small area and the low interference due to the natural attenuation of mm-wave radiation. We experimentally evaluate our COTS solution considering eight different gestures and using two low-complexity classification algorithms: the unsupervised Self Organized Map (SOM) and the supervised Learning Vector Quantization (LVQ). To test robustness, we consider gestures performed by a human hand and a human body, at short and long distance. From our preliminary evaluations, we observe that LVQ and SOM correctly detect 75% and 60% of all gestures, respectively, from the raw, unprocessed data. The detection rate is significantly higher (>90%) for selected gesture groups. We argue that performance suffers due to inaccurate AoA estimation. Accordingly, we evaluate our system employing a two-radar setup that increases the estimation accuracy by 8-9%.

Asynchronous random access schemes for the VDES satellite uplink

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.

A Stochastic Geometry Approach to Asynchronous Aloha Full-Duplex Networks

In-band full-duplex is emerging as a promising solution to enhance throughput in wireless networks. Allowing nodes to simultaneously send and receive data over the same bandwidth can potentially double the system capacity, and a good degree of maturity has been reached for physical layer design, with practical demonstrations in simple topologies. However, the true potential of full-duplex at a system level is yet to be fully understood. In this paper, we introduce an analytical framework based on stochastic geometry that captures the behavior of large full-duplex networks implementing an asynchronous random access policy based on Aloha. Via exact expressions, we discuss the key tradeoffs that characterize these systems, exploring among the rest the role of transmission duration, imperfect self-interference cancellation, and fraction of full-duplex nodes in the network. We also provide protocol design principles, and our comparison with slotted systems sheds light on the performance loss induced by the lack of synchronism.

On the Capacity of Slotted Aloha With Ancillary Channels

In this letter, we focus on a legacy system operated with slotted-Aloha and complemented by a redundancy channel where nodes transmit replicas of their packets. The number of replicas follows a probability distribution, and successive interference cancelation is applied across channels. Leaning on the theory of codes on graphs and algebraic tools, we prove that the system can provide arbitrarily small error rate up to a certain load, beyond which packet losses have to be undergone with finite probability. Tight upper bounds on capacity are derived for both regions, characterizing the achievable performance as a function of the deployed ancillary resources. Simulation results for moderate MAC frame length are also provided.

A Stochastic Geometry Framework for Full-Duplex Machine Type Communications

This paper focuses on the role played by in-band full-duplex in asynchronous random access networks. With an eye on 5G, we tackle a scenario characterised by a large population of uncoordinated users exchanging sporadic traffic in the form of short data packets. In this context, we introduce an analytical framework based on stochastic geometry that captures the tradeoffs induced by the presence of some full-duplex links. Via closed-form expressions, we study the behaviour of the system as a function of the packet length, and derive the optimal fraction of full-duplex communications that shall be performed to maximise the network throughput. The role of imperfect self-interference cancellation is accurately accounted for, drawing interesting insights on the benefits, the design tradeoffs and the challenges to be solved when applying full duplex to machine type communications.