A signal transfer time measurement apparatus measures the time from when a signal (radio wave) is transmitted from a base station to a terminal to when a signal (radio wave) transmitted from the terminal is received by the base station. Japanese Laid-Open Patent Publication Nos. 9-170364 and 2003-13644 and Japanese National Phase Laid-Open Patent Publication Nos. 2006-512515 and 2008-515315 each describe an electronic key system including such type of a signal transfer time measurement apparatus. When ID verification is performed between a vehicle and an electronic key through wireless communication, the signal transfer time measurement apparatus measures the time a signal is transferred between the vehicle and the electronic key. By measuring the signal transfer time, ID verification may be prohibited when the electronic key is connected to the vehicle using a relay in an unauthorized manner.
Referring to FIG. 11, a wideband signal may be used as a transmission signal (impulse) transmitted from a base station to a terminal. FIG. 12 illustrates changes in the signal to noise ratio (SNR) when changing the frequency band of the transmission signal from a narrowband to a wideband. As illustrated in FIG. 12, when widening the frequency band of the transmission signal, the power peak value of the transmission signal received by the base station increases, and the difference increases between the power level of the transmission signal and the power level of a noise wave. This improves the SNR, and increases the likeliness of communication establishment.