In the last few years, the Dynamic Spectrum Access (DSA) has been a flexible strategy for wireless spectrum management. In vehicular communications, OFDM is usually used. The detection of dynamically allocated signals is a problem for a receiver with DSA technique. The automatic modulation classifier can recognize and demodulate the dynamically allocated signals in the OFDM system. We design a layered modulation classifier based on high-order statistics to classify the unknown modulated signal. The aim of the classifier is to defeat the unknown channel effect. We show the classification algorithm of signal detection in multipath environments, and discuss the performance of simulation results.