WiMAX Performance at 4.9 GHz Jim Martin, Mike Westall

School of Computing Clemson University

jim.martin/westall@cs.clemson.edu

Presented at the 2010 IEEE Aerospace Conference Big Sky, Montana on 3/10/2010

1 This work was supported in part by the U.S. Department of Justice Grant 2006-IJ-CX-K035. The opinions and results described in this presentation do not necessarily reflect the opinions or recommendations of the National Institute of Justice or of the Department of Justice.

Outline

  Introduction – setting the stage   Background   Related work   Methodology   Analysis and results   Conclusions and future work

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Introduction

  WiMAX is a wireless network technology that can be deployed on a small scale, a campus-wide scale, a city-wide scale, a state-wide scale, or a national scale

  Our work explores the performance of WiMAX operating at 4.9 GHz in a University campus environment but the analysis and general results are applicable to space exploration

–  802.16d provides large pipes that can serve as backhaul links –  802.16e provides point-to-multipoint to support potentially many devices

operating latency/loss sensitive applications

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Background

  Worldwide Interoperability for Microwave Access (WiMAX) is a broadband wireless network technology with roots that go back to Multichannel Multipoint Distribution Service (MMDS).

  WiMAX specifies the MAC layer that manages the bandwidth and the physical layer that defines how bits are sent over the medium.

  The spectrum (and laws of physics and government spectrum policies) dictates the fundamental capabilities of a given WiMAX system.

–  Unlicensed : 2.4 GHz, 5 GHz –  Sort of unlicensed: 3.65 GHz, 4.9 GHz –  Licensed: 3.5 GHz, 2.5 GHz, –  Emerging: 700 MHz

  Long history….today the IEEE 802.16 group handles the standards evolution. The WiMAX Forum handles interoperability issues.

–  Profiles collect reasonable combinations of operating parameters, specific enough to ensure equipment from different vendors can interoperate.

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Background

  WiMAX is a centralized protocol that involves a base station and multiple subscriber stations.

  Current deployments are generally TDD. Future deployments will also support FDD (separate upstream and downstream channels)

  Downstream is simple- point-to-multipoint.   Upstream is more complex as there are multiple senders competing for

access. Upstream access is based on TDMA. Stations are told the starting time of a transmission and the allowed duration.

  Five service classes have been defined: unsolicited grant service (UGS), real-time polling service (rtPS), non-real-time polling service (nrtPS), best effort (BE) and Extended real-time variable rate (ERT-VR) service.

–  Difference primarily is in how requests for bandwidth are initiated and on the service qualities associated with a given service.

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Related Work

  Early studies of WiMAX involved simple analytic models.   More recent simulation-based studies focus on either IP packet

scheduling or scheduling over OFDM subchannels.   There are several studies of WiMAX over operational networks.

However to the best of our knowledge no other study had control of the network or knowledge from the vendor of implementation decisions.

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Motivations

  Interest by government agencies to use standards-based, widely deployed protocols. WiMAX is clearly an option for space applications.

  WiMAX equipment is much less accessible to the research community than other wireless technologies such as 802.11. Therefore, there are very few studies that provide insight in how an operational WiMAX network behaves.

  Given the complexity of a WiMAX system, models (analytic or simulation) can only approximate the behavior of operational networks.

  The WiMAX standards purposefully do not specify specific scheduling techniques. Therefore, implementations are likely to behave quite differently.

Our goal was to characterize the performance of a particular WiMAX implementation and to then correlate the results with expected results.

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Methodology

  Based on our equipment and configuration, we estimate the expected TCP/UDP throughput assuming best-case conditions.

  Perform experiments on Clemons’s WiMAX network to obtain achieved results.

–  Used iperf at fixed locations that provided stable operation at a given modulation/coding setting

  Embellish the results with an RF coverage analysis

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Clemson’s WiMAX Network

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WiMAX Equipment

Base station : (1) 4.9 GHz Hardened BS (MAVM-VMXDB, Harris Corporation) Client station: (4) 4.9 GHz Low Power Hardened Client (MAVM-VMCLH, Harris Corporation) (2) Low Power EasyST CPE (AirSpan Corporation)

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Harris WiMAX Operating Parameters: • 5 Mhz channel bandwidth • TDD operation • 256 OFDM subchannels • 1/8 guard interval • Symbol time: 50.0 usec • Frame time 0.01 seconds • 192 usable symbols per frame (96 symbols for each direction assuming a 50/50 split)

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Frame Format

Expected Results

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• In the best case, we find how much application data can be sent per frame time. • Assumptions:

• A single best effort TCP flow that always has data waiting for transmission over the link • IP packets are concatenated and sent as a single burst • Ideal channel

• Summary of results: • DS throughput ranges from 5.75 Mbps through 0.64 Mbps, US from 7.41 Mbps through 0.82 Mbps • DS has more overhead which explains the asymmetry

Achieved Results

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• On campus, we were not able to establish a link at 64-QAM ¾ • Achieved results were within 2% of expected results.

• Discrepancy is partially due to the overhead associated with concatenation. • Highlights of the RF coverage analysis

• Near line-of-sight was required, very sensitive to foliage • Coverage rarely exceeded a path distance of 0.5 miles (farthest distance that we observed an operational link was 1.2 miles).

Conclusions

  Our work provides insight in how an operational WiMAX network performs.

  Demonstrated that knowledge of equipment’s implementation choices is required to precisely correlate empirical results with expected results.

  Future work –  For a WiMAX profile that is appropriate for space missions, how might this

support anticipated application performance requirements and scaling requirements?

–  Space applications are likely to push standard protocols beyond their intended capacity, what are these limits and explore modified or extended variants of WiMAX?

–  It is likely that future space networks will be heterogeneous requiring disparate types of wireless networks to interoperate. For example, how can disparate systems such as 802.11, 802.16, and hybrid networks interoperate?

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