Legitimate monitor by proactive guarding for counter covert communications

Covert communication has been widely investigated to avoid the transmission behavior being overheard by the warder. However, covert communication may be illegitimately utilized by unauthorized parties to evade the supervision of authorized agencies, which leads to great challenges to information security. To meet the need for authorized parties to monitor and prevent illegitimate transmission between unauthorized nodes, a novel paradigm, called legitimate monitor, is proposed for counter covert communications. In the preceding covert communication system, the covert transmission rate is the focus. Differently, the core concern of the legitimate monitor system is the outage probability of the transmission between unauthorized nodes, which should be maximized to interrupt the potential but undetectable transmission. To achieve these goals effectively, a proactive guarding approach is proposed, where the authorized warder detects the transmission behavior and emits jamming signals to interfere with the potential transmission, simultaneously. In particular, the jamming power at the warder is optimized under cases where the instantaneous/statistical channel state information is available. Besides, the corresponding outage probability is derived to evaluate the system performance, which can also be simplified to scenarios with a passive warder. Numerical results demonstrate that proactive guarding outperforms the passive one, especially when the warder is not proximal to the unauthorized transmitter. In addition, the proposed jamming power allocation scheme also outperforms other benchmark schemes.