Abstract: A modulation recognition algorithm for MQAM signals in the passband is presented. The proposed algorithm extracts the instantaneous envelope of the signals based on the Hilbert transform, and then the distribution vectors of the instantaneous energy are extracted. The algorithm extracting characters in the passband avoids the carrier frequency offset and the phase offset brought about by spectra shifting. The modulation recognition results of a few kinds of MQAM signals are given. The results show that the correct recognition probability of the algorithm is 100％ for 4QAM,8QAM,16QAM, it is above 95％ for 32QAM,128QAM, and above 90％ for 64QAM and 256QAM. The computer simulation verifies the validity of the algorithm.

Key words: quadrature amplitude modulation, modulation recognition, passband, instantaneous envelope