A Joint Modulation Identification and Frequency Offset Correction Algorithm for Qam Systems

Modulation identification is one of the most challenging steps for blind receivers. If frequency offset is present in the system, this process becomes even more difficult. The proposed method is based on the estimation of the modulation types for quadrature amplitude modulation (QAM) even before correcting the frequency offset. The modulation order is estimated using the probability density information (pdf) of the amplitude of the received signal. As carrier frequency offset causes a cumulative rotation for every successive sample, amplitude variation of the signal gives very important clues about the order. Using the density information and the number of clusters, the modulation order is estimated. After estimating the modulation type, frequency offset is corrected. If the constellation mapping of the incoming signal is circular, an algorithm defined for phase shift keying (PSK) mappings is used; on the other hand, if it is rectangular, algorithms defined for square constellation mappings are used.