In adaptive modulation systems, the receiver should identify the modulation employed at the transmitter. This paper presents a new modulation classification method based on amplitude moments and correlation between subcarriers in Orthogonal Frequency Division Multiplexing (OFDM) systems. In the OFDM systems, a frequency offset causes inter-carrier interference (ICI), and the amplitude moments of received signal include ICI components. The proposed method estimates and removes the ICI components using the correlation between subcarriers. Simulation results of classification error probability show that the proposed method is robust to frequency offset.
I. INTRODUCTIONAdaptive modulation is an important technique to improve communication efficiency in time-varying channel conditions. In the adaptive modulation systems, a transmitter selects the best modulation against the channel condition, and a receiver has to identify the modulation employed by the transmitter. Since modulation classification is an intermediate step between signal detection and demodulation, a misclassification results in burst errors on the demodulated data. Therefore, the receiver should identify the modulation accurately. The statistical information of the received signal is important and useful to identify the modulation without side information.The receiver has difficulties in regenerating a reference carrier frequency with sufficient accuracy from the received signal, because a channel condition changes over time. In orthogonal frequency division multiplexing (OFDM) systems, the carrier frequency offset causes inter-carrier interference (ICI).Many works were devoted to the statistical information in a modulation classification. Likelihood ratio methods were proposed in [1], [2], [3], where M-ary Phase Shift Keying (MPSK) was identified by an estimated likelihood function. The likelihood ratio method was extended to include unknown signal level in [4], and showed a good performance especially in low signal-to-noise power ratio (SNR). However, these methods have computational complexity. In order to classify modulations with reasonable complexity, modulation classification methods based on moments and cumulants were developed. Phase moments approach was proposed to classify MPSK in [5], and amplitude moments approach was presented to classify V.29 signals in [6], [7]. In [8], [9], a classification method based on joint moments of amplitude and phase were