Covert Communications in Wideband OFDMA Primary Networks

Protecting and hiding the transmission of information over wireless channels at the physical layer level is of great importance in several applications. An Ad-Hoc covert network (ACN) is a set of users that exchange information over a wireless channel with a low probability of being detected by an intercept receiver. The ACN achieves a low probability of detection by hiding its transmission within noise or by exploiting the RF spectrum of other man-made transmissions. OFDM-based wireless systems, like LTE, are very well known for their ability to adapt and overcome channel variations and interference, which make them vulnerable to exploitation. In this paper, we consider an ACN that hides its transmission by frequency hopping in the RF spectrum of an OFDM-based wireless primary network (PN). To minimize the interference on the PN and increase the performance of the ACN, we first employ a spectrum sensing method that takes advantage of the OFDM nature of the PN to identify unused subcarriers. We proceed by deriving the optimum ACN power allocation algorithm that maximizes its throughput, and satisfies interference constraints on the OFDM PN. The solution will demonstrate that this optimum algorithm does not rely on the LPD requirements of the ACN. Finally, the possibility of the ACN to operate at a low probability of being detected by an intercept receiver is discussed.