Fair Coexistence of Scheduled and Random Access Wireless Networks: Unlicensed LTE/WiFi

We study the fair coexistence of scheduled and random access transmitters sharing the same frequency channel. Interest in coexistence is topical due to the need for emerging unlicensed LTE technologies to coexist fairly with WiFi. However, this interest is not confined to LTE/WiFi as coexistence is likely to become increasingly commonplace in IoT networks and beyond 5G. In this article we show that mixing scheduled and random access incurs and inherent throughput/delay cost, the cost of heterogeneity. We derive the joint proportional fair rate allocation, which casts useful light on current LTE/WiFi discussions. We present experimental results on inter-technology detection and consider the impact of imperfect carrier sensing.Comment: 14 pages, 8 figures, journa

Coding in 802.11 WLANs

Forward error correction (FEC) coding is widely used in communication systems to correct transmis- sion errors. In IEEE 802.11a/g transmitters, convolutional codes are used for FEC at the physical (PHY) layer. As is typical in wireless systems, only a limited choice of pre-speci¯ed coding rates is supported. These are implemented in hardware and thus di±cult to change, and the coding rates are selected with point to point operation in mind. This thesis is concerned with using FEC coding in 802.11 WLANs in more interesting ways that are better aligned with application requirements. For example, coding to support multicast tra±c rather than simple point to point tra±c; coding that is cognisant of the multiuser nature of the wireless channel; and coding which takes account of delay requirements as well as losses. We consider layering additional coding on top of the existing 802.11 PHY layer coding, and investigate the tradeo® between higher layer coding and PHY layer modulation and FEC coding as well as MAC layer scheduling. Firstly we consider the joint multicast performance of higher-layer fountain coding concatenated with 802.11a/g OFDM PHY modulation/coding. A study on the optimal choice of PHY rates with and without fountain coding is carried out for standard 802.11 WLANs. We ¯nd that, in contrast to studies in cellular networks, in 802.11a/g WLANs the PHY rate that optimizes uncoded multicast performance is also close to optimal for fountain-coded multicast tra±c. This indicates that in 802.11a/g WLANs cross-layer rate control for higher-layer fountain coding concatenated with physical layer modulation and FEC would bring few bene¯ts. Secondly, using experimental measurements taken in an outdoor environment, we model the chan- nel provided by outdoor 802.11 links as a hybrid binary symmetric/packet erasure channel. This hybrid channel o®ers capacity increases of more than 100% compared to a conventional packet erasure channel (PEC) over a wide range of RSSIs. Based upon the established channel model, we further consider the potential performance gains of adopting a binary symmetric channel (BSC) paradigm for multi-destination aggregations in 802.11 WLANs. We consider two BSC-based higher-layer coding approaches, i.e. superposition coding and a simpler time-sharing coding, for multi-destination aggre- gated packets. The performance results for both unicast and multicast tra±c, taking account of MAC layer overheads, demonstrate that increases in network throughput of more than 100% are possible over a wide range of channel conditions, and that the simpler time-sharing approach yields most of these gains and have minor loss of performance. Finally, we consider the proportional fair allocation of high-layer coding rates and airtimes in 802.11 WLANs, taking link losses and delay constraints into account. We ¯nd that a layered approach of separating MAC scheduling and higher-layer coding rate selection is optimal. The proportional fair coding rate and airtime allocation (i) assigns equal total airtime (i.e. airtime including both successful and failed transmissions) to every station in a WLAN, (ii) the station airtimes sum to unity (ensuring operation at the rate region boundary), and (iii) the optimal coding rate is selected to maximise goodput (treating packets decoded after the delay deadline as losses)

Survey of Spectrum Sharing for Inter-Technology Coexistence

Increasing capacity demands in emerging wireless technologies are expected to be met by network densification and spectrum bands open to multiple technologies. These will, in turn, increase the level of interference and also result in more complex inter-technology interactions, which will need to be managed through spectrum sharing mechanisms. Consequently, novel spectrum sharing mechanisms should be designed to allow spectrum access for multiple technologies, while efficiently utilizing the spectrum resources overall. Importantly, it is not trivial to design such efficient mechanisms, not only due to technical aspects, but also due to regulatory and business model constraints. In this survey we address spectrum sharing mechanisms for wireless inter-technology coexistence by means of a technology circle that incorporates in a unified, system-level view the technical and non-technical aspects. We thus systematically explore the spectrum sharing design space consisting of parameters at different layers. Using this framework, we present a literature review on inter-technology coexistence with a focus on wireless technologies with equal spectrum access rights, i.e. (i) primary/primary, (ii) secondary/secondary, and (iii) technologies operating in a spectrum commons. Moreover, we reflect on our literature review to identify possible spectrum sharing design solutions and performance evaluation approaches useful for future coexistence cases. Finally, we discuss spectrum sharing design challenges and suggest future research directions

Offered load and stability controls in multi-hop wireless networks.

Ng Ping-chung.Thesis (M.Phil.)--Chinese University of Hong Kong, 2005.Includes bibliographical references (leaves 71-72).Abstracts in English and Chinese.Chapter Chapter 1 --- Introduction --- p.1Chapter 1.1 --- Overview and Motivation --- p.1Chapter 1.2 --- Background of Offered Load Control --- p.2Chapter 1.3 --- Background of Stability Control --- p.3Chapter 1.4 --- Organization of the Thesis --- p.4Chapter Chapter 2 --- Performance Problems and Solutions --- p.6Chapter 2.1 --- Simulation Set-up --- p.6Chapter 2.2 --- High Packet-Drop Rate --- p.7Chapter 2.3 --- Re-routing Instability --- p.8Chapter 2.3.1 --- Hidden-Node Problem --- p.8Chapter 2.3.2 --- Ineffectiveness of Solving Hidden-Node Problem with RTS/CTS …… --- p.9Chapter 2.4 --- Solutions to High-Packet Loss Rate and Re-routing Instability --- p.10Chapter 2.4.1 --- Link-Failure Re-routing --- p.11Chapter 2.4.2 --- Controlling Offered Load --- p.13Chapter 2.5 --- Verification of Simulation Results with Real-life Experimental Measurements --- p.14Chapter Chapter 3 --- Offered Load Control --- p.16Chapter 3.1 --- Capacity Limited by the Hidden-node and Exposed-node Problems --- p.16Chapter 3.1.1 --- Signal Capture --- p.18Chapter 3.1.2 --- Analysis of Vulnerable Period induced by Hidden Nodes --- p.20Chapter 3.1.3 --- Analysis of Vulnerable Period induced by Exposed Nodes --- p.21Chapter 3.1.4 --- Sustainable Throughput --- p.22Chapter 3.2 --- Capacity Limited by Carrier Sensing Property --- p.23Chapter 3.3 --- Numerical Results --- p.26Chapter 3.4 --- General Throughput Analysis of a Single Multi-hop Traffic Flow --- p.29Chapter 3.5 --- Throughput Analysis on Topologies with Variable Distances between Successive Nodes --- p.31Chapter Chapter 4 --- Discussions of Other Special Cases --- p.33Chapter 4.1 --- A Carrier-sensing Limited Example --- p.33Chapter 4.2 --- A Practical Solution to Improve Throughput --- p.34Chapter Chapter 5 --- Achieving Fairness in Other Network Topologies --- p.36Chapter 5.1 --- Lattice Topology --- p.36Chapter Chapter 6 --- Stability Control --- p.39Chapter 6.1 --- Ad-hoc routing protocols --- p.39Chapter 6.2 --- Proposed scheme --- p.40Chapter 6.2.1 --- Original AODV --- p.41Chapter 6.2.2 --- AODV with Proposed Scheme --- p.42Chapter 6.2.2.1 --- A Single Flow in a Single Chain of Nodes --- p.43Chapter 6.2.2.2 --- Real-break Case --- p.44Chapter 6.3 --- Improvements --- p.45Chapter Chapter 7 --- Impacts of Data Transmission Rate and Payload Size --- p.48Chapter 7.1 --- Signal Capture --- p.48Chapter 7.2 --- Vulnerable region --- p.50Chapter Chapter 8 --- Performance Enhancements in Multiple Flows --- p.53Chapter 8.1 --- Impacts of Re-routing Instability in Two Flow Topology --- p.53Chapter 8.2 --- Impacts of Vulnerable Periods in Multiple Flow Topologies --- p.55Chapter 8.2.1 --- The Vulnerable Period induced by Individual Hidden-terminal Flow --- p.57Chapter 8.2.2 --- The Number of Hidden-terminal Flows --- p.58Chapter 8.2.3 --- Correlation between Hidden-terminal Flows --- p.60Chapter Chapter 9 --- Conclusion --- p.63Chapter Appendix A: --- General Throughput Analysis of a Single Multi-hop Traffic Flow --- p.67Chapter A.l --- Capacity Limited by Hidden-node and Exposed-Node --- p.67Chapter A.1.1 --- Sustainable Throughput --- p.68Chapter A.2 --- Capacity Limited by Carrier Sensing Property --- p.68Bibliography --- p.7

Flexible Spectrum Assignment for Local Wireless Networks

In this dissertation, we consider the problem of assigning spectrum to wireless local-area networks (WLANs). In line with recent IEEE 802.11 amendments and newer hardware capabilities, we consider situations where wireless nodes have the ability to adapt not only their channel center-frequency but also their channel width. This capability brings an important additional degree of freedom, which adds more granularity and potentially enables more efficient spectrum assignments. However, it also comes with new challenges; when consuming a varying amount of spectrum, the nodes should not only seek to reduce interference, but they should also seek to efficiently fill the available spectrum. Furthermore, the performances obtained in practice are especially difficult to predict when nodes employ variable bandwidths. We first propose an algorithm that acts in a decentralized way, with no communication among the neighboring access points (APs). Despite being decentralized, this algorithm is self-organizing and solves an explicit tradeoff between interference mitigation and efficient spectrum usage. In order for the APs to continuously adapt their spectrum to neighboring conditions while using only one network interface, this algorithm relies on a new kind of measurement, during which the APs monitor their surrounding networks for short durations. We implement this algorithm on a testbed and observe drastic performance gains compared to default spectrum assignments, or compared to efficient assignments using a fixed channel width. Next, we propose a procedure to explicitly predict the performance achievable in practice, when nodes operate with arbitrary spectrum configurations, traffic intensities, transmit powers, etc. This problem is notoriously difficult, as it requires capturing several complex interactions that take place at the MAC and PHY layers. Rather than trying to find an explicit model acting at this level of generality, we explore a different point in the design space. Using a limited number of real-world measurements, we use supervised machine-learning techniques to learn implicit performance models. We observe that these models largely outperform other measurement-based models based on SINR, and that they perform well, even when they are used to predict performance in contexts very different from the context prevailing during the initial set of measurements used for learning. We then build a second algorithm that uses the above-mentioned learned models to assign the spectrum. This algorithm is distributed and collaborative, meaning that neighboring APs have to exchange a limited amount of control traffic. It is also utility-optimal -- a feature enabled both by the presence of a model for predicting performance and the ability of APs to collaboratively take decisions. We implement this algorithm on a testbed, and we design a simple scheme that enables neighboring APs to discover themselves and to implement collaboration using their wired backbone network. We observe that it is possible to effectively gear the performance obtained in practice towards different objectives (in terms of efficiency and/or fairness), depending on the utility functions optimized by the nodes. Finally, we study the problem of scheduling packets both in time and frequency domains. Such ways of scheduling packets have been made possible by recent progress in system design, which make it possible to dynamically tune and negotiate the spectrum band [...

Fair coexistence of scheduled and random access wireless networks: unlicensed LTE/WiFi

We study the fair coexistence of scheduled and random access transmitters sharing the same frequency channel. Interest in coexistence is topical due to the need for emerging unlicensed LTE technologies to coexist fairly withWiFi. However, this interest is not confined to LTE/WiFi as coexistence is likely to become increasingly commonplace in IoT networks and beyond 5G. In this paper, we show that mixing scheduled and random access incurs an inherent throughput/delay cost and the cost of heterogeneity. We derive the joint proportional fair rate allocation, which casts useful light on current LTE/WiFi discussions. We present experimental results on inter-technology detection and consider the impact of imperfect carrier sensing.This work was supported in part by the Science Foundation Ireland under Grant 11/PI/1177 and Grant 13/RC/207, in part by the European Commission in the framework of the H2020-ICT-2014-2 Project Flex5Gware under Grant 671563, and in part by the Spanish Ministry of Economy and the FEDER regional development fund through SINERGIA Project under Grant TEC2015-71303-R