Modulation recognition of PSK and QAM signals based on envelope spectrum analysis

Modulation recognition has been an essential part in non-cooperative communications. Without knowing any prior knowledge of the transmitted data, the received signals may be affected by intersymbol interference (ISI), carrier frequency offset, phase offset and timing delay. ISI and these unknown parameters would degrade the performance of many recognition methods. There have been many robust estimators of carrier frequency offset, phase offset and timing delay. However, the robust recognition methods combating ISI have not been well designed. Most blind channel equalization methods, which are designed to overcome the effects of ISI, have poor robustness to noise and carrier frequency offset. In this paper, a robust recognition scheme is proposed to recognize phase-shift keying (PSK) and quadrature amplitude modulation (QAM) signals in non-cooperative communication with ISI environment. The envelope spectrum of PSK and QAM signals are analyzed. Then the recognition scheme based on the ratio of spectral peak to spectral centroid (RSPSC) of envelope spectrum is presented. Such scheme achieves a good recognition performance in the presence of ISI. Spectral centroid is utilized to enhance the robustness to noises. By extracting the envelope spectrum, in addition, the method avoids being affected by carrier frequency offset, phase offset and timing delay. The feasibility of the proposed method has been proved in the respects of theory and simulations.