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The study of the fundamental limits of covert communications, where a transmitter Alice wants to send information to a desired recipient Bob without detection of that transmission by an attentive and capable warden Willie, has emerged recently as a topic of great research interest. Previous work has assumed that the distribution of the multipath fading on the channel, which we term the channel distribution information (CDI), is known to Alice, hence facilitating her characterization of Willie's capabilities to detect the signal. However, in practice, Willie tends to be passive and the environment heterogeneous, implying a lack of signaling interchange between the transmitter and Willie makes it difficult if not impossible for Alice to estimate the CDI exactly and provide covertness guarantees. In this paper, we address this issue by developing covert communication schemes for various assumptions on Alice's imperfect CDI: 1) when the transmitter knows the channel distribution is within some distance of a nominal distribution; 2) when only the mean and variance of the channel distribution are available; 3) when Alice knows the channel distribution is complex Gaussian but the variance is unknown. In each case, we formulate new optimization problems to find the power allocations that maximize covert rate subject to a covertness requirement under uncertain CDI. Moreover, Willie may be able to estimate CDI exactly or be faced with an error in estimation, and we investigate two these issues. Numerical results are presented to compare the proposed schemes from various aspects, for attaining desirable covert system performance. [ABSTRACT FROM AUTHOR]

Covert communication is often limited in rate because it is difficult to hide the signal in the background noise. Recent work has shown that jamming can significantly improve the rate at which covert communication can be conducted, but this jamming signal has a devastating effect on the receiver. To tackle this issue, we equip the jammer with multiple antennas so that it can employ beamforming to place the intended receiver in the null space of the jamming. Moreover, in order to increase rate in this network, we equip the transmitter with an antenna array to be able to perform 3-dimensional (3D) beamforming toward its intended recipient. To evaluate this design, we formulate an optimization problem and present an iterative algorithm to solve it. Moreover, since Willie is passive, it is possible that Alice is not able to estimate the location of Willie perfectly. Hence, we investigate a new practical scenario in which the information about Willie's location is imperfect at Alice. Numerical results consider both the rate of covert communication with the proposed architecture and the gap between the result from our optimization and that obtained from exhaustive search. Numerical results highlight that the optimality gap is on average approximately 10%, and 3D beamforming compared to 2D beamforming increases covert rate by approximately 70%. [ABSTRACT FROM AUTHOR]

In this paper, we address the problem of joint covert communication and secure transmission in untrusted relaying networks when multiple wardens exist in the network. We first consider a system model consisting of one source, one untrusted amplify-and-forward relay, one destination, and one warden. For covert communications, our aim is to prevent the warden from detecting the presence of communications via the source-relay-destination link. For secure transmission, our aim is to prevent the untrusted relay from decoding the source signal. To satisfy these requirements, we propose that the destination and the source inject jamming signals during the source-to-relay and relay-to-destination transmission phases, respectively. For the considered system model, we propose a power allocation strategy to maximize the secrecy rate and satisfy the covert requirements in both of the phases. Given that the proposed optimization problem is non-convex, we employ the successive convex approximation (SCA) approach to derive a tractable solution. To obtain further insights, we generalize our analytical results to consider multiple untrusted relays and multiple wardens. We focus on two scenarios of non-colluding and colluding wardens. Our results highlight that as the number of relays increases, the achievable secrecy rate increases while the average detection error probability decreases. This is a novel observation of a fundamental trade-off between covert requirement and secrecy performance. [ABSTRACT FROM AUTHOR]

To prevent detection of communication between legitimate users by warden and guarantee a strong security in wireless networks, covert communication technique is recommended. The covert communication is based on hiding the main message in the background of noise which poses challenges such as low transmission rate. On the other hand, the transmit power in the conventional networks is supplied by separate built-in batteries, which are sometimes hard to be recharged or replaced especially in the military applications. In this work, to tackle the low transmission rate, we employ three dimensions beamforming in which we can focus the main beam on the legitimate user and minimize the leakage information at the warden. Moreover, in the proposed network, we intend to employ cooperative jamming (CJ) to increase covert rat in which the jammer is equipped to multiple antennas. Furthermore, to tackle the recharging or replacing batteries in the proposed network, we employ the energy harvesting (EH) technique. The simulation results reveal that the energy harvesting from the jammer's signal reduce the total power consumption. Moreover, the simulation results show that the optimality gap is about 50% according to energy harvesting technique, and the 3D beamforming increases covert rate about 70% compared to two-dimensional beamforming method. [ABSTRACT FROM AUTHOR]

This paper proposes an adaptive transmission scheme for two-way amplify-and-forward (AF) relaying over optical wireless communications. The two terminals utilize subcarrier phase shift keying (S-PSK) intensity modulation for data transmission, and the atmospheric turbulence impacts the transmitted signals. The proposed transmission scheme exploits the efficient utilization of the optical wireless channel capacity by adjusting the modulation order of S-PSK. While meeting predetermined bit error rate (BER) requirements for each terminal, the selected modulation order of each terminal corresponds to the instantaneous condition of turbulence-induced fading. A new expression is derived for the spectral efficiency of the proposed two-way relaying scheme, and then performance investigations are carried out under various turbulence conditions. Moreover, we derive a closed form expression for the BER performance and analyze the proposed system model from the outage probability point of view. Numerical examples illustrate that the proposed two-way relaying scheme enjoys better spectral efficiency compared with the traditional direct transmission scheme in the high signal-to-noise-ratio regime. The simulation results show that for a fixed turbulence, the TWR saves the electrical signal-to-noise-ratio (SNR) about 2.5 dB compared to the direct transmission scheme when the target spectral efficiency is set to 1.2 bits/s/Hz. [ABSTRACT FROM AUTHOR]