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Tech Report - University of Virginia

Tech Report - University of Virginia

Tech Report - University of

Opportunistic Random Access with TemporalFairness in Wireless NetworksChong Tang ∗ , Jagadeesh Balasubramani † , Lixing Song ∗ , Gordon Pettey ∗ , Shaoen Wu ∗ , Saâd Biaz †∗ School of Computing, The University of Southern Mississippi{chong.tang, lixing.song, gordon.pettey}@eagles.usm.edu, shaoen.wu@usm.edu† Computer Science and Software Engineering Department, Auburn Universityjzb0047@tigermail.auburn.edu, biazsaa@auburn.eduTechnical ReportAbstract—User diversity arises when wireless users experiencediverse channel conditions. This paper proposes three opportunisticrandom access algorithms that favor users with the bestchannel conditions in channel access. These algorithms operateon the contention mechanism of CSMA/CA and compute the contentionwindow based on channel conditions. In one algorithm,the contention windows of all users share the same lower boundzero. In another, the contention windows of users with differentchannel conditions are segmented without overlapping. The thirdalgorithm adopts a normal distribution to select the backoff valuein the contention window. These algorithms are also enhancedto provide temporal fairness and avoid starving the users withpoor channel conditions. We implemented a Linux-based testbedfor a real world performance evaluation and also developed thealgorithms into the NS3 simulator to conduct comprehensiveand controllable experiments. Extensive evaluations show thatthe proposed opportunistic access can significantly improve thenetwork performance in throughput, delay, and jitter over thecurrent default CSMA/CA method.I. INTRODUCTIONIn wireless networks, channel conditions are determinedby many factors such as fading, mobility, shadowing, andlocation. Because of spatial difference, wireless users oftenexperience different channel conditions, which is referred asuser or spatial diversity in wireless communication. Dueto spatial diversity, a wireless user with excellent channelcondition may be able to transmit data at the highest bit ratewhile another user with a poor link may not be able to transmitany data even at the lowest rate. Channel condition variationsalso lead to time diversity: a user may have a link of high bitrate now, but may have to use a low rate thereafter.The variations of channel conditions are often considereddetrimental in the traditional wireless communication because1) they are unpredictable and 2) each user is treated with nodifference. In recent years, opportunistic approaches have beenattempted to exploit the inherent randomness of wireless channelto improve wireless network performance and utilization,including opportunistic rate adaptation [11], transmission [1],scheduling [12]–[14] and routing [15]–[18]. Opportunisticprotocols exploit user diversity by granting higher priority tousers with good channel conditions and/or time diversity byextending the use of channel in good conditions.In this paper, we propose and evaluate three opportunisticmedia access schemes that exploit user diversity in randomaccess wireless networks. These schemes enable the userwith the best channel conditions in a random access wirelessnetwork to have the largest probability to access the sharedchannel at a certain moment, but they do not starve the userswith poor links. In the long run, each user probabilisticallyobtains a throughput proportional to its channel conditions interms of bit rate.The main contributions of this work consist of:• three distributed opportunistic random access algorithms• the development of a Linux-based testbed through prototypingthe algorithms into open source implementation• extensive performance evaluation on the testbed and theNS3 [25] simulator.The rest of this paper is organized as follows. The relatedcontention scheme of CSMA/CA is briefly reviewedin Section II. Then, Section III presents the problems thatmotivate this work. Next, Section IV discusses the threeproposed opportunistic access algorithms. Section V presentsthe implementation of a testbed, experiment settings, andperformance evaluations on the testbed and the NS3 simulator.Some observations are discussed in Section VI followed by therelated work in Section VII. Finally, Section VIII concludesthis paper.II. OVERVIEW OF CSMA/CANowadays, most random access networks such as IEEE802.11 [6] adopt CSMA/CA [4] as the core media accessmechanism. The opportunistic algorithms proposed in thiswork are therefore based on this mechanism. CSMA/CA isa contention-based MAC protocol. In a typical CSMA/CAnetwork, regardless of ad-hoc or infrastructure mode, all nodesthat have data to transmit on the shared wireless link mustundergo a contention procedure first. Only the node that winsthe contention can transmit while all others freeze the contentionprocedure until the winner completes its transmission.The contention is regulated by a Binary Exponential Backoffprocess. Each node maintains a contention window that has alower bound of “0” and an upper bound of CW that starts with

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