In this paper, we report on a mobile broadband wireless access (MBWA) system field trial with Fast Low-latency Access with Seamless Handoff Orthogonal Frequency Division Multiplexing (FLASH-OFDM), which was carried out at Sendai city in Japan. This paper describes coverage estimates of an downlink 64 quadrature amplitude modulation (QAM) signal under the condition of constant base-station output power when the signal bandwidth is up to 20.48 MHz. The estimates are based on the field trial results of 1.28 MHz signal bandwidth. The coverage estimates based on a propagation model are also discussed. The estimation results show that wider signal bandwidth yields narrower coverage area. 64 QAM can be used within 90 %, 80 %, 65 %, and 50 % of the field trial area when the signal bandwidths are 2.56 MHz, 5.12 MHz, 10.24 MHz, and 20.48 MHz, respectively. We consider that the downlink 64 QAM signal is useful to provide a broadband wireless data communication service to users in an urban area even if the signal bandwidth is 20 MHz.