Borrowing Arrows With Thatched Boats: Exploiting the Reactive Primary Communications for Boosting Jamming-Assisted Proactive Eavesdropping

This paper investigates a cognitive information surveillance scenario, where a half-duplex legitimate monitor (E) aims to eavesdrop the frequency division multiple access based suspicious downlink broadcasting communication, which shares the same spectrum with a primary downlink broadcasting system. Specifically, the suspicious transmitter (ST) employs water-filling power allocation over all orthogonal frequency bands (FBs) to maximize the sum suspicious communication rate of all suspicious receivers (SRs), while the primary transmitter (PT) purposely maximizes the minimum communication rate among the primary receivers (PRs). Under this setup, we propose one novel jamming-assisted eavesdropping scheme for E to enhance its eavesdropping performance. In our scheme, instead of implementing passive eavesdropping over all FBs, E picks up certain FBs to jam and eavesdrops over the remaining FBs (due to the half-duplex constraint, E cannot eavesdrop over the FBs it decides to jam), and intends to control its transmit jamming beamforming over the jammed FBs, so as to enhance the interference at the corresponding SRs, even deliberately interfere with the corresponding PRs, such that the PT can reactively readjust its power to further strengthen and reduce its interference to the SRs over the jammed and eavesdroppable FBs, respectively. With this fashion, the ST will be deceived to reallocate more power over the eavesdroppable FBs and then E can acquire the better signal receiving quality accordingly. Our objective is to maximize the sum instantaneous eavesdropping rate at E, by jointly optimizing its jamming set and the corresponding jamming beamformers, subject to the constraints of its power budget and the maximum interference (interference temperature, IT) to the primary system. The optimization problem is non-convex and its approximate and sub-optimal solutions are provided by some convex optimization procedures, along with the optimal and greedy jamming set selections. Moreover, we also investigate the issue of sum ergodic eavesdropping rate maximization, and then interestingly reveal that, if E decides to jam, it should optimally jam over only one FB (such FB can be determined via the simple search) with tolerable maximum power and then eavesdrop over the remaining FBs. Our theoretical and simulation analysis reveals that, under the proposed jamming-assisted eavesdropping scheme, i) generally the obtained sum instantaneous (ergodic) eavesdropping rate of the monitor would monotonically increase and then tend to saturation as the monitor's power budget increases; ii) the larger the IT threshold is, the more strength the monitor can borrow from the primary system, thus the eavesdropping performance is better; iii) both the position and the number of transmit antennas of the monitor also play the important roles on the sum eavesdropping rate; iv) with the careful jamming set selection and jamming beamformers design, the proposed scheme achieves significant performance compared to competitive benchmarks.