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Smart antennas have the ability to cancel both cochannel interference (CCI) and intersymbol interference (ISI), and thus could be used in a cellular system to enhance coverage or increase capacity. A long standing problem is that of providing fast and efficient array adaptation in a random access packet-switching environment. This paper considers the media access control (MAC) design problem when smart antennas are used in a wireless data network and multipath fading is flat. The case of time dispersive fading will be considered in a companion paper. We propose a reservation-based MAC scheme in which the antenna weights will be computed from slot to slot to capture the actual interference and multipath fading. Performance of this scheme under the condition of flat Rayleigh fading is evaluated, and the training overhead is found, from which important design parameters for array training may be deduced. Bounds are also produced for the delay performance. Simulation results show that in an interference limited multicell environment, smart antennas can be used to significantly increase frequency reuse. If we require that the signal-to-interference-plus-noise (SINR) ratio must be at least 10 dB more than 90% of the time, then equipping each base station with eight antenna elements will allow two mobiles to simultaneously receive from the same base station. The number of training symbols required by the overhead in each transmission slot is between three and four times the number of antenna elements. We assume throughout that once adapted at the beginning of a packet, the antenna adapts throughout the packet to maintain the same SINR over the entire packet.