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The IEEE 802.11a standard using orthogonal frequency division multiplexing (OFDM) can provide data rates up to 54 Mbps, which makes it a good candidate for high-speed communications in wireless local area networks (WLANs). Although the standard is meant for indoor applications, we tested its performance in a city environment. Our measurements indicated that the users experience a significantly lower throughput than that promised by the standard, with the performance becoming worse as the speed of the users increased. To better understand the reasons for this behavior, we measured outdoor channels, corresponding to stationary and mobile users. Analysis of experimental data showed that the channel could vary even within the duration of one data frame. Thus, the training-based scheme used in 802.11a, which estimates the channel once during the initial part of the frame and then uses it to equalize the entire frame, results in imperfect channel equalization. This results in high bit-error rate (BER), causing packets to be discarded and increasing the probability of outage. The paper presents experimental results demonstrating the 802.11a weaknesses when operating in an outdoors mobile environment, and provides some recommendations for improving its performance.