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Performance Characteristics

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1Performance Characteristics of an OperationalWiMAX NetworkJames Martin, Member, IEEE, and James Westall, Affiliate, IEEE CSAbstract—The term WiMAX is used to refer to a collection of standards, products, and service offerings derived from the IEEE 802.16family of standards for metropolitan area wireless networks. A significant body of published research in the WiMAX domain exists, butthe focus of much of it is on the use of analytic or simulation models to evaluate aspects of physical layer protocols, medium accesscontrol protocols, or proposed scheduling algorithms. It this paper we describe performance characteristics of an operational WiMAXtestbed upon which we were able to conduct controlled experiments in the absence of competing traffic. We characterize latency,throughput, protocol overhead, packet loss, and the impact of WiMAX on TCP dynamics.Index Terms—WiMAX, IEEE 802.16, wireless, performance.✦1 INTRODUCTIONThe term WiMAX, an acronym for Worldwide Interop-erability for Microwave Access, is commonly used torefer to a collection of standards, products, and serviceofferings derived from the IEEE 802.16 family of stan-dards [1], [2]. The IEEE standards include many imple-mentation options that are left to equipment vendors.Conflicting design choices can make interoperation ofthe equipment of multiple vendors problematic. TheWiMAX forum was organized by equipment vendors in2001 to define operational profiles, certify interoperabil-ity, and promote the use of the technology. A discussionof the roles of the IEEE and the WiMAX forum in thedevelopment of the standards and profiles can be foundin [3].The equipment described in this paper is compli-ant with the IEEE 802.16-2004 standard [1] which issometimes called IEEE 802.16d or “fixed WiMAX” be-cause it does not support seamless handoff for mobileclients. The subsequent amendment, IEEE 802.16e-2005[2], sometimes called 802.16e, added support for mobileclients. Perspectives on the evolution of WiMAX may befound in [4], [5], [6]. A very thorough discussion of theWiMAX physical layer is provided in [4]. A discussion ofWiMAX as it relates to alternative wireless technologiesis found in [7].At present WiMAX usage is not widespread whencompared to competing access network technologies. Inurban areas it competes with WiFi mesh technology, andin both urban and suburban areas it competes with cableand DSL service. Although originally designed to pro-vide metropolitan area network (MAN) service, its maincompetitive advantage against competing technologieslies in sparsely populated rural areas where the cost ofproviding high bandwidth fiber coverage is prohibitive.• The authors are with the School of Computing, Clemson University,Clemson, SC 29634.E-mail: see http://www.cs.clemson.eduWhere commercial offerings do exist, the serviceproviders are generally unwilling to release usage andperformance data. Furthermore, it is very difficult toconduct controlled experiments that measure best casethroughput and latency on an operational public net-work. Therefore, virtually all published studies of theperformance characteristics of WiMAX systems havebeen derived from simulation or analytic models.The objective of this paper is to augment the resultsobtained in simulation studies of hypothetical equip-ment with measured results obtained from an opera-tional WiMAX testbed. Our focus is upon characteriz-ing the latency, throughput, and overhead properties ofthe network. The important issue of coverage will beaddressed in a subsequent paper.The testbed is deployed on the campus of ClemsonUniversity and uses M/A-COM’s Vida WiMAX equip-ment. The network operates in the 4.9 gigahertz (GHz)public safety band which is comprised of ten channels offive MHz each spanning 50 MHz of spectrum between4940 and 4990 MHz. This spectrum was allocated by theFederal Communications Commission (FCC) in 2002 forfixed and mobile wireless services in support of publicsafety [8]. Both base and subscriber stations operatingin this spectrum are limited to no more than 27 dBm oftransmitter power and no more than 40 dBm of effectiveisotropic radiated power. Although a WiMAX Forumprofile for 4.9 GHz has not yet been defined, WiMAXequipment vendors have agreed on a set of operatingparameters allowing interoperability. These parametersare consistent with the 802.16-2004 standard and areused in equipment currently offered by Airspan, M/A-COM, and Nortel.The authors of this paper were funded by the NationalInstitute of Justice to evaluate the suitability WiMAXequipment operating in this spectrum for use in publicsafety applications. The remainder of the paper reportson aspects of that study. The results reported in thispaper were obtained during an evaluation period in2which the network was under the operational controlof the authors and operating under a license held by theClemson University Police Department and the City ofClemson Police Department.2 THE WIMAX TESTBEDThe equipment used in the study includes a M/A-COMVIDA Broadband MAVM-VMXBD hardened base sta-tion, M/A-COM VIDA Broadband MAVM-VMCLL sub-scriber stations, and Airspan EasyST subscriber stations.In the remainder of this section we review aspects of the802.16-2004 standard that pertain to this equipment andthis study.2.1 The Physical LayerThe 802.16-2004 standard defines single carrier (SC), or-thogonal frequency division multiplexing (OFDM), andorthogonal frequency division multiple access (OFDMA)modes of operation at the physical layer. The M/A-COMequipment implements only OFDM operation.Operational parameters that bound the capacity of anOFDM WiMAX physical layer include:• channel bandwidth,• number of data-carrying subchannels,• modulation and forward error correction (FEC) tech-nique and,• duplexing mode (time or frequency division duplex-ing).The M/A-COM equipment can operate in any oneof the ten channels of the 4.9 GHz spectrum. The 5Mhz bandwidth is partitioned into 256 subchannels asspecified in the standard: eight pilot channels are usedin physical layer synchronization; 55 channels are usedas guard bands; and 192 channels carry data. A nullcarrier is transmitted on the remaining center frequencychannel. The OFDM timing data used by the M/A-COMequipment is summarized in the terminology of p. 428of the standard in Table 1.TABLE 1OFDM Timing DataParameter ValueBandwidth 5.000 × 10+6Nf f t2.560 × 10+2Sampling frequency 5.760 × 10+6Subcarrier spacing 2.250 × 10+4Useful symbol time


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