To perform a complexity evaluation for an electromagnetic environment (EME), a new method based on the S-transform is proposed, which can simultaneously count the time occupancy, frequency occupancy, and energy occupancy in the time–frequency domain. The frequency coincidence, modulation similarity, and background noise intensity are selected as important evaluation indices, and their physical interpretations are analyzed and calculated. The Extreme Learning Machine (ELM) method is adopted to evaluate the environmental complexity. The proposed method (S-ELM) requires less training time and has a fast convergence rate. The simulation and experimental results confirm that the proposed method is accurate and efficient.

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