Misunderstandings about link adaptation for frequency selective fading channels

Modern wireless communication standards like HiperLAN/2 or IEEE 802.11a provide a high degree of flexibility on the physical layer (PHY) allowing the data link control (DLC) layer to choose transmission parameters with respect to the currently observed link quality. This possibility of so-called link adaptation (LA) is also a key element for meeting quality of service (QoS) requirements of higher layers. Most LA algorithms take an estimate of the signal-to-noise ratio (SNR) or carrier-to-interference ratio (CIR), respectively, as the only input from the PHY layer. For frequency selective channels, however, the SNR alone does not adequately describe the channel quality. It is shown in this paper that system performance can be significantly improved if the LA scheme is based on a more detailed description of the channel situation. Methods for selecting the PHY mode based on the estimated channel transfer function are presented and compared to conventional algorithms.