Title page for ETD etd-02272004-133201

Type of Document Master's Thesis
Author Ramachandran, Shyamal
Author's Email Address shyamal@vt.edu
URN etd-02272004-133201
Title Link Adaptation Algorithm and Metric for IEEE Standard 802.16
Degree Master of Science
Department Electrical and Computer Engineering
Advisory Committee
Advisor Name Title
Bostian, Charles W. Committee Chair
Midkiff, Scott F. Committee Member
Sweeney, Dennis G. Committee Member
  • TCP/IP
  • Congestion Control
  • Link Adaptation
  • IEEE 802.16
  • WirelessMAN
Date of Defense 2004-02-12
Availability unrestricted
Broadband wireless access (BWA) is a promising emerging technology. In the

past, most BWA systems were based on proprietary implementations. The Institute of

Electrical and Electronics Engineers (IEEE) 802.16 task group recently standardized the

physical (PHY) and medium-access control (MAC) layers for BWA systems. To operate

in a wide range of physical channel conditions, the standard defines a robust and flexible

PHY. A wide range of modulation and coding schemes are defined. While the standard

provides a framework for implementing link adaptation, it does not define how exactly

adaptation algorithms should be developed.

This thesis develops a link adaptation algorithm for the IEEE 802.16 standard’s

WirelessMAN air interface. This algorithm attempts to minimize the end-to-end delay in

the system by selecting the optimal PHY burst profile on the air interface. The IEEE

802.16 standard recommends measuring C/(N+I) at the receiver to initiate a change in the

burst profile, based on the comparison of the instantaneous the C/(N+I) with preset

C/(N+I) thresholds. This research determines the C/(N+I) thresholds for the standard

specified channel Type 1. To determine the precise C/(N+I) thresholds, the end-to-end(ETE) delay performance of IEEE 802.16 is studied for different PHY burst profiles at

varying signal-to-noise ratio values. Based on these performance results, we demonstrate

that link layer ETE delay does not reflect the physical channel condition and is therefore

not suitable for use as the criterion for the determination of the C/(N+I) thresholds. The

IEEE 802.16 standard specifies that ARQ should not be implemented at the MAC layer.

Our results demonstrate that this design decision renders the link layer metrics incapable

of use in the link adaptation algorithm.

Transmission Control Protocol (TCP) delay is identified as a suitable metric to

serve as the link quality indicator. Our results show that buffering and retransmissions at

the transport layer cause ETE TCP delay to rise exponentially below certain SNR values.

We use TCP delay as the criterion to determine the SNR entry and exit thresholds for

each of the PHY burst profiles. We present a simple link adaptation algorithm that

attempts to minimize the end-to-end TCP delay based on the measured signal-to-noise

ratio (SNR).

The effects of Internet latency, TCP’s performance enhancement features and

network traffic on the adaptation algorithm are also studied. Our results show that delay

in the Internet can considerably affect the C/(N+I) thresholds used in the LA algorithm.

We also show that the load on the network also impacts the C/(N+I) thresholds

significantly. We demonstrate that it is essential to characterize Internet delays and

network load correctly, while developing the LA algorithm. We also demonstrate that

TCP’s performance enhancement features do not have a significant impact on TCP delays

over lossy wireless links.

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