Age of Information in Random Access Channels

In applications of remote sensing, estimation, and control, timely communication is not always ensured by high-rate communication. This work proposes distributed age-efficient transmission policies for random access channels with $M$ transmitters. In the first part of this work, we analyze the age performance of stationary randomized policies by relating the problem of finding age to the absorption time of a related Markov chain. In the second part of this work, we propose the notion of \emph{age-gain} of a packet to quantify how much the packet will reduce the instantaneous age of information at the receiver side upon successful delivery. We then utilize this notion to propose a transmission policy in which transmitters act in a distributed manner based on the age-gain of their available packets. In particular, each transmitter sends its latest packet only if its corresponding age-gain is beyond a certain threshold which could be computed adaptively using the collision feedback or found as a fixed value analytically in advance. Both methods improve age of information significantly compared to the state of the art. In the limit of large $M$, we prove that when the arrival rate is small (below $\frac{1}{eM}$), slotted ALOHA-type algorithms are asymptotically optimal. As the arrival rate increases beyond $\frac{1}{eM}$, while age increases under slotted ALOHA, it decreases significantly under the proposed age-based policies. For arrival rates $\theta$, $\theta=\frac{1}{o(M)}$, the proposed algorithms provide a multiplicative factor of at least two compared to the minimum age under slotted ALOHA (minimum over all arrival rates). We conclude that, as opposed to the common practice, it is beneficial to increase the sampling rate (and hence the arrival rate) and transmit packets selectively based on their age-gain

Wi-Fi QoS improvements for industrial automation

Digitalization caused a considerable increase in the use of industrial automation applications. Industrial automation applications use real-time traffic with strict requirements of connection of tens of devices, high-reliability, determinism, low-latency, and synchronization. The current solutions meeting these requirements are wired technologies. However, there is a need for wireless technologies for mobility,less complexity, and quick deployment. There are many studies on cellular technologies for industrial automation scenarios with strict reliability and latency requirements, but not many developments for wireless communications over unlicensed bands. Wireless Fidelity (Wi-Fi) is a commonly used and preferred technology in factory automation since it is supported by many applications and operates on a license free-band. However, there is still room for improving Wi-Fi systems performance for low-latency and high-reliable communication requirements in industrial automation use cases. There are various limitations in the current Wi-Fi system restraining the deployment for time-critical operations. For meeting the strict timing requirements of low delay and jitter in industrial automation applications, Quality of Service (QoS)in Wi-Fi needs to be improved. In this thesis, a new access category in Medium Access Control (MAC) layer for industrial automation applications is proposed.The performance improvement is analyzed with simulations, and a jitter definition for a Wi-Fi system is studied. Then, a fixed Modulation and Coding (MCS) link adaptation method and bounded delay is implemented for time-critical traffic in the simulation cases to observe performance changes. Finally, it is shown that the new access category with no backoff time can decrease the delay and jitter of time-critical applications. The improvements in Wi-Fi QoS are shown in comparison with the current standard, and additional enhancements about using a fixed modulation and coding scheme and implementation of a bounded delay are also analyzed in this thesi

Energy-efficient multi-criteria packet forwarding in multi-hop wireless networks

Reliable multi-hop packet forwarding is an important requirement for the implementation of realistic large-scale wireless ad-hoc networks. However, packet forwarding methods based on a single criterion, such as the traditional greedy geographic forwarding, are not sufficient in most realistic wireless settings because perfect-reception-within-rangecannot be assumed. Furthermore, methods where the selection of intermediate relaying nodes is performed at the transmitter-side do not adapt well to rapidly changing network environments. Although a few link-aware geographic forwarding schemes have been reported in the literature, the tradeoffs between multiple decision criteria and their impact on network metrics such as throughput, delay and energy consumption have not been studied. This dissertation presents a series of strategies aimed at addressing the challenges faced by the choice of relay nodes in error-prone dynamic wireless network environments. First, a single-criterion receiver-side relay election (RSRE) is introduced as a distributed alternative to the traditional transmitter-side relay selection. Contrary to the transmitter- side selection, at each hop, an optimal node is elected among receivers to relay packets toward the destination. Next, a multi-criteria RSRE, which factors multiple decision criteria in the election process at lower overhead cost, is proposed. A general cost metric in the form of a multi-parameter mapping function aggregates decision criteria into a single metric used to rank potential relay candidates. A two-criteria RSRE case study shows that a proper combination of greedy forwarding and link quality leads to higher energy efficiency and substantial improvement in the end-to-end delay. Last, mesh multi-path forwarding methods are examined. A generalized mesh construction algorithm in introduced to show impact of a mesh structure on network performance

Supporting Real-Time Communication in CSMA-Based Networks : the VTP-CSMA Virtual Token Passing Approach

Tese de doutoramento. Engenharia Electrotécnica e de Computadores. Faculdade de Engenharia. Universidade do Porto. 200

Priority Communications for Critical Situations on Mobile Networks

[ES] En la actualidad, las redes públicas de comunicación están ampliamente desplegadas en todo el territorio. Como las redes públicas no contemplan un uso priorizado de los recurso, los cuerpos de seguridad tienden a utilizar redes privadas de uso específico. Estas redes privadas satisfacen los requisitos marcados pero, a cambio, los costes de despliegue y mantenimiento son muy elevados, lo cual limita su despliegue y disponibilidad. Además, la interconexión entre distintas redes privadas no siempre es posible, lo que supone un gran problema cuando la emergencia se produce en zonas fronterizas. Estos grandes inconvenientes justifican un estudio minucioso sobre nuevos mecanismos de priorización en la gestión de recursos radio que permitan hacer uso de las redes públicas por parte de los cuerpos de seguridad y emergencias. Para ello se ha analizado el marco tecnológico actual, se ha contactado con distintos cuerpos de seguridad para averiguar los requisitos de comunicación actuales y los deseables. Caracterizado el sistema, se han definido distintos escenarios realistas utilizados en simulación masivas para finalmente demostrar cómo una red pública es capaz de cursar todo el tráfico que actualmente cursa una red privada en una situación de emergencia.[EN] Technical evaluation for enhancement and priorization of calls during a emergency situation over 2G and 3G networksDíaz Sendra, S. (2012). Priority Communications for Critical Situations on Mobile Networks. http://hdl.handle.net/10251/27446.Archivo delegad

SCHEDULING IN PACKET SWITCHED CELLULAR WIRELESS SYSTEMS

In cellular wireless networks where users have independent fading channels, throughput for delay tolerant applications has been greatly increased on the downlink by using opportunistic schedulers at the base station. These schedulers exploit the multiuser diversity inherent in cellular systems. An interesting question is how opportunistic schedulers will provide Quality of Service(QoS) guarantees for a mix of data traffic and traffic from delay-sensitive multimedia applications. In the first part of this dissertation, we completely characterize the scheduled rate, delay and packet service times experienced by mobile users in a packet switched cellular wireless system in terms of a configurable base station scheduler metric. The metric used has a general form, combining an estimate of a mobile user's channel quality with the scheduling delay experienced by the user. In addition to quantifying the scheduler performance, our analysis highlights the inherent trade-off between system throughput and the delay experienced by mobile users with opportunistic scheduling. We also use this analysis to study the effect of prioritized voice users on data users in a cellular wireless system with delay constrained opportunistic scheduling. Our statistical analysis of the forward link is validated by extensive simulations of a system architecture based on the CDMA 1xEV-DO system. The increase in data traffic from mobiles to the base station has led to a growing interest in a scheduled reverse link in the 1xEV-DO system. We address the reverse link scheduling problem in a multi-cell scenario with interference constraints both within and outside the cell. This approach leads to a co-operative scheduling algorithm where each base station in a cellular network maximizes the sum of mobile data transmission rates subject to linear constraints on (1) the maximum received power for individual mobiles(2) the total interference caused by scheduled mobiles to (a) traffic and control channels of other mobiles within the cell and (b) mobiles in neighboring cells. Simulations of the reverse link structure based on the 1xEV-D0 system highlight the distinct advantages of this algorithm in ensuring predictable inter-cell interference and higher aggregate cell throughputs