The next-generation wireless local area networks (WLANs) supporting multimedia transactions will be required to operate at bit rates in excess of 20 Mbps. At such high bit rates, multipath is a major problem for radio-based data transmission within buildings. The technique proposed to overcome the limitations of the indoor radio channel for high bit rate data transmission is orthogonal frequency division multiplexing, or OFDM. A simulation model of a 25 Mbps OFDM system is used with measured channel data to predict the bit error rate for future WLANs. Measured samples of the indoor radio channel were obtained for fifty locations within a building at 5 GHz and 17 GHz using a vector network analyser. The frequency response of an FIR filter was then defined using the magnitude and phase data from each measurement to form a set of channel models. Computer simulations were performed to compare the BER performance for 16-carrier and 256-carrier OFDM against a single-carrier DQPSK system. It was found that for 256-carrier OFDM only 6% of measured channels at 5 GHz resulted in an error floor greater than 10^-5 compared with 46% for single-carrier DQPSK. At 17 GHz results were even more favourable with only 4% of channels giving error floors above 10^-5 for 256-carrier OFDM and 32% for single-carrier DQPSK. (5 pages)