Coded Slotted ALOHA: A Graph-Based Method for Uncoordinated Multiple Access

In this paper, a random access scheme is introduced which relies on the combination of packet erasure correcting codes and successive interference cancellation (SIC). The scheme is named coded slotted ALOHA. A bipartite graph representation of the SIC process, resembling iterative decoding of generalized low-density parity-check codes over the erasure channel, is exploited to optimize the selection probabilities of the component erasure correcting codes via density evolution analysis. The capacity (in packets per slot) of the scheme is then analyzed in the context of the collision channel without feedback. Moreover, a capacity bound is developed and component code distributions tightly approaching the bound are derived.Comment: The final version to appear in IEEE Trans. Inf. Theory. 18 pages, 10 figure

Modern Random Access for Satellite Communications

The present PhD dissertation focuses on modern random access (RA) techniques. In the first part an slot- and frame-asynchronous RA scheme adopting replicas, successive interference cancellation and combining techniques is presented and its performance analysed. The comparison of both slot-synchronous and asynchronous RA at higher layer, follows. Next, the optimization procedure, for slot-synchronous RA with irregular repetitions, is extended to the Rayleigh block fading channel. Finally, random access with multiple receivers is considered.Comment: PhD Thesis, 196 page

On the Performance of Irregular Repetition Slotted ALOHA with an Age of Information Threshold

The present paper focuses on an IoT setting in which a large number of devices generate time-stamped updates addressed to a common gateway. Medium access is regulated following a grant-free approach, and the system aims at maintaining an up-to-date knowledge at the receiver, measured through the average network age of information (AoI). In this context, we propose a variation of the irregular repetition slotted ALOHA (IRSA) protocol. The scheme, referred to as age-threshold IRSA (AT-IRSA), leverages feedback provided by the receiver to admit to the channel only devices whose AoI exceeds a dynamically adapted target value. By means of detailed networks simulations, as well as of a simple yet tight analytical approximation, we demonstrate that the approach can more than halve the average network AoI compared to plain IRSA, and offers notable improvements over feedback-based state-of-the-art slotted ALOHA solutions recently proposed in the literature

Contribution to the development of a LoRa communications module for the AlainSat-1 CubeSat

The Remote sensing and Interference detector with radiomeTry and vegetation Analysis (RITA) is a 1U payload that will fly onboard Alainsat-1, a 3U CubeSat. Among other experiments, it will perform a proof of concept of a LoRa custom module for space-to-Earth communications between the satellite and a terrestrial network of Internet of Things sensors. This final degree thesis consists of the implementation of the protocols of the Media Access Control layer that will be used in the experiment mentioned above, precisely in the design, implementation, and testing of pure ALOHA, and the design of CSMA/CA with RTS/CTS.El sensor Remoto y detector de Interferencias por radiomeTría i Análisis de vegetación (RITA), es una carga útil de 1U que volará a bordo del CubeSat AlainSat-1 de 3 Unidades. De entre otros experimentos, ésta realizará una prueba de concepto de la utilización de un módulo LoRa desarrollado en la UPC para las comunicaciones espacio-tierra entre el satélite i una red terrestre de sensores de internet de las cosas. Este trabajo final de grado consiste en la implementación de los protocolos de la capa de control de acceso al medio que se utilizaran en el experimento mencionado anteriormente; concretamente en el diseño, implantación i testeo del ALOHA puro, i el diseño del CSMA/CA con RTS/CTS.El sensor Remot i detector d'Interferències per a radiomeTria i Anàlisi de vegetació (RITA), és una càrrega útil de 1U que volarà a bord del CubeSat de 3 Unitats Alainsat-1. Entre d'altres experiments, aquesta realitzarà un prova de concepte de la utilització d'un mòdul LoRa desenvolupat a la UPC per a comunicacions espai-terra entre el satèl·lit i una xarxa terrestre de sensors d'internet de les coses. Aquest treball final de grau consisteix en la implementació dels protocols de la capa de control d'accés al medi que s'utilitzaran en l'experiment mencionat anteriorment; concretament amb el disseny, implementació i test de l'ALOHA pur, i el disseny del CSMA/CA amb RTS/CTS

Impact of Segmentation and Capture on Slotted Aloha Systems Exploiting Interference Cancellation

The performance of the widely adopted slotted Aloha (SA) scheme has been recently improved thanks to the introduction of novel mechanisms, including interference cancellation (IC), packet segmentation, and slot slicing. The combined effect of these mechanisms in the presence of capture has however not yet deeply investigated, even if the resulting impact on the network behavior is determinant for properly quantifying the achievable throughput. To deal with this issue, this paper analyzes the influence of capture on a framed SA (FSA) system adopting IC, segmentation, and slicing, by considering a reliable decoding criterion that accounts for the actually experienced signal to interference-plus-noise ratio. A theoretical model is developed to evaluate the capture probability in fast and slow Rayleigh fading conditions, deriving closed-form expressions for the interference-limited case. The IC-based FSA throughput is then estimated adopting a Markov chain approach validated by Monte Carlo simulations. Finally, the performance of an actual system using a quadrature phase-shift keying modulation in conjunction with a turbo encoder is compared with that estimated by adopting the considered decoding criterion, in order to check its applicability to practical communication networks

Reliability-Latency Performance of Frameless ALOHA with and without Feedback

This paper presents a finite length analysis of multislot type frameless ALOHA based on a dynamic programming approach. The analysis is exact, but its evaluation is only feasible for moderate number of users due to the computational complexity. The analysis is then extended to derive continuous approximations of its key parameters, which, apart from providing an insight into the decoding process, make it possible to estimate the packet error rate with very low computational complexity. Finally, a feedback scheme is presented in which the slot access scheme is dynamically adapted according to the approximate analysis in order to minimize the packet error rate. The results indicate that the introduction of feedback can substantially improve the performance of frameless ALOH