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A Link Adaptation Mechanism For 802.11 Wireless LANs

IP.com Disclosure Number: IPCOM000015637D
Original Publication Date: 2002-Aug-18
Included in the Prior Art Database: 2003-Jun-20
Document File: 5 page(s) / 66K

Publishing Venue

IBM

Abstract

Abstract: A link adaptation mechanism is described that can be implemented in a compatible way with the current 802.11 specification. Using the mechanism, an 802.11 compliant transmitter is able to detect whether the quality of a link to a certain destination is improving or declining, and based on this information to switch to a higher or lower rate, respectively. Introduction Two wireless broadband LAN standards are being developed in the 5 GHz band, namely IEEE 802.11a and ETSI HIPERLAN/2. The physical layers of both standards are very similar: they both use a modulation technique called "Orthogonal Frequency Division Multiplexing (OFDM)" and can provide several transmission modes with data rates ranging from 6 Mbps up to 54 Mbps. This multi-rate capability enables a WLAN station to select an appropriate transmission mode based on the current radio channel quality to reach the best performance.

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A Link Adaptation Mechanism For 802.11 Wireless LANs

  Abstract: A link adaptation mechanism is described that can be implemented in a compatible way with the current 802.11 specification. Using the mechanism, an 802.11 compliant transmitter is able to detect whether the quality of a link to a certain destination is improving or declining, and based on this information to switch to a higher or lower rate, respectively.

Introduction

Two wireless broadband LAN standards are being developed in the 5 GHz band, namely IEEE 802.11a and ETSI HIPERLAN/2. The physical layers of both standards are very similar: they both use a modulation technique called

"Orthogonal Frequency Division Multiplexing (OFDM)" and can provide several transmission modes with data rates ranging from 6 Mbps up to 54 Mbps. This multi-rate capability enables a WLAN station to select an appropriate transmission mode based on the current radio channel quality to reach the best performance.

In general, adaptive adjustment of the transmission rate is achieved by having the receiver estimating the channel link quality, deriving from this estimation the rate to be used in future transmissions, and sending this information back to the transmitter. The main issues for an efficient link adaptation mechanism are the determination of the parameters to be used for the link quality estimation (e.g. packet error rate, signal to noise ratio, received signal strength, carrier to interference ratio, etc.), how to measure them, and how to select the appropriate rate out of the measurement results.

In HIPERLAN/2, it is the responsibility of an Access Point (AP) to dynamically select any of the available PHY modes for the down- and uplink transmissions. A Mobile Terminal (MT) continuously measures the quality of the downlink and suggests a suitable downlink transmission rate to the AP. For the uplink the AP itself performs the link quality estimation. The standard however does not specify how the link quality estimation and the corresponding transmission mode selection are performed.

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In 802.11 the link adaptation algorithm is intentionally left open. It only specifies which transmission rates are allowed for which types of MAC frames, but not how and when to switch between the permitted rates. Furthermore, there is no signaling mechanism specified which would allow a receiver to inform the transmitter about the quality of the communication channel or the rate to be used. The transmitter can change the rate any time between two consecutive packets, but not in the middle of a sequence of MAC frames belonging to the same packet. The rate at which a MAC frame is transmitted is coded in the header of the physical layer (the PLCP header) which is sent at a fixed rate
(e.g. 6 Mbps in case of 802.11a) supported by all stations. Thus, after having receiving the PLCP header, the receiver switches to the indicated rate to receive the MAC frame.

A link adaptation mechanism is described...