There is an increasing demand on wireless communications in subway tunnels to provide video surveillance and sensory data for security,maintenance and train control,and to offer various communication or entertainment services(e.g.,Internet,etc.) to passengers as well.The wireless channel in tunnels is quite unique due to the confined space and the waveguide effects.Therefore,modeling the radio channel characteristics in tunnels is critically important for communication systems design or optimization.This paper investigates the key radio channel characteristics of a subway tunnel at 2.4 GHz and 5 GHz,such as the path loss,root mean square(RMS) delay spread,channel stationarity,Doppler shift,and channel capacity.The field measurements show that channel characteristics in tunnels are highly location-dependent and there exist abundant components in Doppler shift domain.In the straight section of the subway tunnel,the measured path loss exponents are close to1.6,lower than that in free space. There is an increasing demand on wireless communications in subway tunnels to provide video surveillance and sensory data for security, maintenance and train control, and to offer various communication or entertainment services (eg, Internet, etc.) To passengers as well. Wireless channel in tunnels is quite unique due to the confined space and the waveguide effects. ago, modeling the radio channel characteristics in tunnels is critically important for communication systems design or optimization. This paper investigates the key radio channel characteristics of a subway tunnel at 2.4 GHz and 5 GHz, such as the path loss, root mean square (RMS) delay spread, channel stationarity, Doppler shift, and channel capacity. The field measurements show that channel characteristics in tunnels are highly location-dependent and there exist abundant components in Doppler shift domain. the straight section of the subway tunnel, the measured path loss exponents are close to1.6, lower than that in free spac e.