Orthogonal frequency-division multiplexing (OFDM) has received considerable interest as a means for delivering broadband data in highly dispersive wireless channels. Here we present the design, implementation, and performance of an OFDM system operating at 5.25 GHz, which provides 20 Mbits/s in an office environment. From propagation measurements, we describe a design methodology for determining the key parameters of the OFDM system, such as the number of subcarriers and the size of the guard period. The performance of the system is evaluated using experimental data obtained from a typical large open-plan office. Using a transmit power of +7 dBm, the OFDM system is capable of operating in a highly cluttered environment, providing usable error rates up to a range of 20 m. The importance of error control coding is shown, and we empirically derive the degree of correctability required to achieve packet failure rates of <1% from the cumulative distribution of errors per packet.