1,721,081 research outputs found
Performance Evaluation of 802.11ax OFDMA through Theoretical Analysis and Simulations
No full textWith the introduction of Orthogonal Frequency Division Multiple Access (OFDMA) in 802.11ax, the role of the Access Point (AP) in Wi-Fi networks changes significantly, thanks to the opportunity of implementing more complex scheduling logic to handle Downlink (DL) traffic flows and simultaneously act as coordinator of Multi User (MU) Uplink (UL) transmissions. In this context, it becomes necessary to develop reliable network analysis and simulation tools that allow for an in-depth investigation of the trade-offs involved in the usage of OFDMA, especially considering that the standard leaves much of the actual scheduling algorithmic details to vendor-specific implementation. In this work we present a series of results highlighting how several network settings have an impact on throughput and Head-of-Line Delay, in a network that employs multiple 802.11ax features such as OFDMA and the MU Enhanced Distributed Channel Access (EDCA) Parameter Set, while also containing legacy devices. The results are obtained via both the newly re-designed ns-3 wifi module and an original analytical framework, based on the well-established Bianchi 802.11 model
UAV Networks : Design Considerations
No full textThesis (Master's)--University of Washington, 2018Unmanned Aerial Vehicles have come a long way, from starting out as military reconnais- sance vehicles to a popular hobbyist’s tool. Significant research and development efforts from the commercial drone industry have significantly improved commercial UAVs and has got the wireless network and communications design community thinking about the possibility of realization and deployment of UAV networks. UAV networks, if realized, offer a significant edge over conventional wireless communication networks, they can be easily re-configured and re-arranged to handle varying traffic, can provide critical communication facilities in disaster hit regions etc. In this work, we explore the scenario where UAV air-to-ground communications between a low altitude platform such as a UAV flying happens over an open ground setting. An integral component of the above is to come up with a wireless channel model that depends more on physics rather than the empirical studies. Thus, this work develops a physics based wireless channel model for UAV air-ground link and validates the said model. This, to the best of the author’s knowledge has never been attempted before. The author then uses the validated model to look for rate-based optimization as a UAV flies right above a ground node in a straight line
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Layer 2.5 Network Coding to Improve 802.11 Network Efficiency
Full text linkThesis (Master's)--University of Washington, 2016-06The principles of Network Coding have been developed conceptually for over a decade (starting with key results c. 2003 on network coding for butterfly networks). Despite several subsequent extensions to wireless networks, there exists remarkably few implementations of Wireless Network Coding (WNC) techniques within existing wireless networking technology. In this thesis, we present a novel Layer 2.5 implementation for WNC system that can be retrofitted to existing 802.11 nodes (APs and client) with some minimal changes (i.e. is backward compatible with legacy nodes that do not implement WNC) to improve network throughput, when there exist significant intra-network symmetric traffic flows. We implement L2.5 WNC on an 802.11 software defined radio test-bed to experimentally collect data and demonstrate feasibility. We experiment using a simple network with 2 clients and 1 access point. The clients saturate the network by sending messages to each other through the access point whenever possible. Our experimental results show that on average, adding L2.5 network coding to an 802.11 network increases the network efficiency by 11.7% and throughput by 13.6%
Contributions to Smart Metering Protocol Design and Data Analytics
Full text linkThesis (Ph.D.)--University of Washington, 2015The next generation Advanced Metering Infrastructure (AMI), with the aid of two-way Smart Metering Network (SMN), is expected to support many advanced functions, such as remote reading and control, demand response, etc. In order to satisfy the communication requirements of these applications, the information transportation protocol design and data analytics are of fundamental importance to the design of SMN and associated research topics that need to be discussed. A suitable and well-designed Multiple Access Control (MAC) protocol is critical to the transportation of information in SMN. First, SMN traffic may be classified according to their communication requirements; thus each type may require a different MAC protocol which is specially designed for it. Furthermore, as discussed in Chapter 2, because the number of Smart Meters (communication nodes) involved in such a single SMN is much larger than those in today's local area networks, the traditional MAC protocols are unlikely to perform well in SMN. In order to solve this scalability issue for event-driven traffic, in Chapter 3, we propose two grouping based MAC protocols: TDMA-DCF and Group Leader DCF-TDMA. Both of them reduce the competitive random channel contentions in SMN effectively by dividing all the Smart Meters into several groups. The group division and management schemes are also clearly presented. We conduct comprehensive throughput and delay analysis on these two schemes for unsaturated traffic conditions and hidden node events. From the numerical results, we observe that the performance of these two grouping based MAC protocols are significantly better than those of traditional random access protocols. In order to solve the scalability issue for periodic data transportation, in Chapter 4, we propose a modified PCF scheme with the aid of cognitive radio technology, in which the Smart Meters are allowed to use the white space to report the periodic data to the data aggregator as secondary users. The comprehensive throughput analysis is also presented. The numerical and simulation results through NS-3 show that the modified PCF with cognitive radio significantly outperforms the traditional one in SMN. In Chapter 5, we focus on a specific type of periodic reporting data, power factor measurement, which is an important information to the power grid security and infrequently changing. Therefore, we exploit the time-invariant nature of this measurement and propose a new MAC protocol by using compressive sensing, a famous signal processing and reconstruction technique. The simulation results show that this proposed scheme can solve the scalability issue effectively and outperform the traditional MAC protocols. An effective analysis of Smart Metering data is critical to demand response, one of the most important applications in SMN, whereby customers use aggregated power consumption data (available locally via Smart Meters) and real-time pricing information (send pre-emptively from the utility) to schedule their future energy use to enhance conservation goals. This requires enabling customers to determine disaggregated consumption at individual appliance level from aggregated power consumption data. Therefore, in Chapter 6, we propose a real-time disaggregation algorithm based on a new Markov Chain model for the power consumption of individual electrical appliances. The model incorporates both temporal and inter-device correlations which are used to estimate the appliance-level disaggregated power consumption through the application of the Viterbi Algorithm. The performance is vetted through significant testing with real minute-resolution power consumption data and proves the accurate estimation
Distributed Multiple Access for OFDMA Femtocells
No full textThesis (Ph.D.)--University of Washington, 2014Demand for data services in cellular networks is growing exponentially, due to the proliferation of high-end, multimedia-enabled mobile devices. To meet this challenge, cellular operators are moving toward a heterogeneous network architecture consisting of macrocells for wide-area coverage and smaller cells such as femtocells for capacity boost in local hotspots. The primary problem in such heterogeneous networks is mitigating inter-cell interference especially in dense deployments of residential femtocells. Traditionally, interference in wide-area cellular networks has been studied from a multi-cell resource allocation perspective, where radio resources, e.g. power, bandwidth, are allocated to different cells to reduce interference. This approach generally assumes a fully loaded network (i.e. many simultaneous active users in a cell) where the system performance is insensitive to the activity of a single user. This assumption is not suitable for femtocells which are designed to serve very few users and thus lacks the presumed traffic aggregation. In this thesis, we first discuss the infeasibility of applying the classical multi-cell resource allocation framework to the femtocell case. Then, we motivate the case of using distributed, random access protocols as apposed to centralized interference mitigation techniques in the context of LTE femtocells. For this, we employ queuing analysis to compare the performance of centralized resource allocation schemes represented by an ideal power control to the performance of a simple random access protocols. The results show that, with a high probability, a simple un-optimized random access protocol such as Aloha would perform much better than an optimized power controlled network in most of the unsaturated traffic scenarios. This result serves as a strong motivation for the design of OFDMA-aware random access protocols that utilize the inherent frequency diversity of OFDMA to improve the MAC performance. Next, we analyze OFDMA-Aloha which is the simplest protocol that utilize the frequency-dimension in OFDMA to improve random access performance. The protocol attempts to reduce the packet retransmission time using collision resolution over the frequency domain by switching subchannels randomly after each collision. However, this comes at the expense of expanded time scale, or larger slot size due to lower channel rates. We showed that when the network is lightly loaded, the reduction in the collision rate outweighs the effect of expanded slot size thus in these situations OFDMA-Aloha enjoys smaller packet delays than the single channel Aloha. Then, we propose the Exponential Backoff in Frequency (EBF) algorithm to address the case when multiple packets need to be transmitted over multiple subchannels simultaneously. Instead of spreading the packets uniformly over K subchannels as would be done in a multichannel variant of Aloha (K-Aloha), the EBF algorithm keeps packet transmissions clustered at few frequency branches using a synchronized binary tree branching process over the K subchannels. Collisions are handled by reducing the accessed bandwidth exponentially and using an access probability that is inversely proportional to the accessed bandwidth. Analysis of a lightly loaded network, shows that EBF enjoys considerably less packet delay compared to the basic K-Aloha protocol. Finally, we propose the OFDM-based Reservation Random access (OFDM-RR) protocol that utilizes the frequency-dimension in a different way. In this protocol, reservation requests are transmitted on randomly selected subchannels and the indices of these subchannels are then used to create an implicit ordering among competing nodes for conflict resolution. Our analysis shows that beyond some critical value of the network load, OFDM-RR significantly improves the system throughput compared to the canonical Reservation-Aloha (R-Aloha) protocol
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