Research On Modulation Classification Of Subcarriers Of OFDM Signals

Orthogonal frequency division multiplexing （OFDM） system is widely adopted bymany current and next-generation wireless communication specifications since therecent decade, due to its advantage of high data rates in multipath fading environments.In an OFDM non-cooperative communication system, before the OFDM signal beingdemodulated, not only its modulation type has to be recognized from single-carriersignals, but also the subcarrier modulation types should be identified. Meanwhile,because there are pilots and null subcarriers in standard OFDM signals, the subcarriermodulation classification is made a hard work.The subcarrier modulation classification for an OFDM signal under the standardprotocols is studied in this thesis. Firstly the mathematical non-cooperation model of anOFDM system is analyzed. Supposing each channel attenuation factor of the multipathis a Gauss random variable, the multipath channel is equivalent to a group of flatRayleigh fading channels in the OFDM system. The modulated subcarrier signals lessaffected by pilots are extracted using the combination characteristics of high ordercumulants, of which a subcarrier is randomly picked out to recognize. Secondly theqHLRT identification method for OFDM subcarriers is presented based on theparameter estimation and the Bayesian decision criterion. Thirdly a K-S testclassification method and the improved MKS₁and MKS₂methods for OFDMsubcarriers are put forward. Finally an identification method for OFDM subcarriers isproposed based on all available modulated subcarriers.The simulation results show that the methods proposed in this paper can effectivelyrecognize the subcarriers using fewer symbols under a variety of multipath channels inthe non-cooperation communication system, while the traditional high-order cumulantmethods are not able to identify the subcarriers of the standard OFDM signals. Whenfrequency offset existed in the OFDM system，K-S method and its modified methodsare less sensitive for frequency offset to some extent. Therefore, the methods presentedin this paper are valid and robust to a certain extent for the subcarriers of the standardOFDM system.