Revealing transmit diversity mechanisms and their side-effects in commercial IEEE 802.11 cards

Service differentiation in WLAN has been traditionally faced at the MAC layer. However, some PHY layer parameters, such as the transmission power, the antenna, and the modulation/coding scheme, have a deep impact on network performance. Since the criterion for selecting these parameters is left to the vendor specific implementations, the performance spread of most experimental results about 802.11 WLAN could be affected by vendor proprietary schemes. The focus of this paper is an experimental analysis of the undisclosed antenna diversity mechanisms employed by some widely used cards (namely, the Atheros and Intel based cards), and a thorough understanding of the optimization goals which guided their design. Although these mechanisms have been introduced for improving the transmission robustness to fading, we proved that they may have dramatic side- effects on link performance, whenever the available antennas are not homogeneous.