Wireless Digital Modulation and Space-time Coding Forensics Detector for Spectrum Sensing

Modulation forensics detect the modulation type in wireless communications by the received signal only. It provides a powerful tool for spectrum sensing since by identifying the modulation type, the secondary users in cognitive radio systems can detect whether the primary user occupies the spectrum. In this paper, we investigate modulation forensics of linear digital modulations and space-time diagonal algebraic codes in slowly varying, frequency-selective fading channels. With unknown channel vector, and phase distortion at the receive side, we derive a composite test consisting of second-moment nonlinearity and maximum likelihood tests, and discuss the performance and the forensics system confidence measure. It is shown that the proposed algorithm achieves almost perfect identification of the space-time coding, and high accuracy rate of the modulation type.