MB-OFDM ultra wideband radio will have to coexist with narrowband radio services that operate over the 3.1-10.6 GHz band. In order to coexist with these other radio systems without becoming a source of harmful interference, MB-OFDM system designers will have to develop methods to dynamically detect their presence so that the appropriate avoidance techniques can be implemented whenever it is determined that MB-OFDM transmissions will pose a source of harmful interference. This paper investigates the feasibility of detecting a downlink transmission from an indoor fixed service (FS) system when the MB-OFDM radio collects measurements over a number of consecutive OFDM symbol periods. The analysis shows that when the fixed service signal received by the MB-OFDM device is at a power level of -98 dBm, that it is possible to achieve a false alarm probability of 10-4 and a missed detection probability of ~10-6 when using the measurements that are obtained over 70 consecutive OFDM symbol periods (~22 us). Since the fixed service sub-channel time duration is 200 us, our results imply that the required measurement time is reasonable with respect to the duration of time that the fixed service signal is on the air