channel is severely degraded by multipath delay time spread. In this paper. We propose a simple multipath delay time detection method, which has a merit of in serviceable, yet simple H/W realizability for $pi/4$ shift QPSK by detecting cross channel interference. A $pi/4$ shift QPSK signal originally has quadrature channel(Q-ch) component. Thus in order to measure CCI between in-phase channel(I-ch) and quadrature channel(Q-ch), which closely related to multipath delay time, Frequency doubling scheme(frequency doubler) and differential detector is proposed, which makes $pi/4$ shift QPSK signal look like BPSK and also makes it possible for CCI to be detected at I-ch detector output. To get an information from time varying I-ch output signal under the multipath lading environment, a method for obtaining the mean of the absolute value$(V_{MABS}(t))$ and another one for obtaining the root mean square value$(V_{RMS}(t))$ of CCI are proposed. Furthermore, a relationship between delay spread and CCI is also analyzed. In order to confirm theoretical results, computer simulation has been carried out under the quasi-static and Reyleigh distributed two ray multipath fading environments. A fairly good result was obtained. However it was also shown that this method is sensitive to bandwidth restriction to some extent. In addition, some idea for a simple hardware realization for the frequency doubler are given.