This invention relates system for classifying a modulation format of a communication signal. The system includes a receiving antenna system, to receive the communication signal, a preprocessing module, to evaluate a Fourier transform of a plurality of samples of the received communication signal, normalize each discrete sample of the Fourier transform, quantize each normalized sample based on a number of quantization levels (Q), a graphical analyzer to construct a undirected graph by tracing amplitude of the each quantized sample; and a classification module to extract one or more modulation classification (MC) features from the undirected graph; and to determine the modulation format of the received communication signal based on the extracted MC features.