Link Adaptation Algorithm and Metric for IEEE Standard 802.16
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 standardâs
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
The effects of Internet latency, TCPâs 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
TCPâs performance enhancement features do not have a significant impact on TCP delays
over lossy wireless links.
Advisor:Dennis G. Sweeney; Charles W. Bostian; Scott F. Midkiff
School:Virginia Polytechnic Institute and State University
School Location:USA - Virginia
Source Type:Master's Thesis
Keywords:electrical and computer engineering
Date of Publication:03/26/2004