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Cooperative relaying is utilized as an efficient method for data communication in wireless sensor networks and the Internet of Things. However, sometimes due to the necessity of multi-hop relaying in such communication networks, it is challenging to guarantee the secrecy of cooperative transmissions when the relays may themselves be eavesdroppers, i.e., we may face with the untrusted relaying scenario where the relays are both necessary helpers and potential adversary. To obviate this issue, a new cooperative jamming scheme is proposed in this paper, in which the data can be confidentially communicated from the source to the destination through multiple untrusted relays. In our proposed secure transmission scheme, all the legitimate nodes contribute to providing secure communication by intelligently injecting artificial noises to the network in different communication phases. For the sake of analysis, we consider a multi-hop untrusted relaying network with two successive intermediate nodes, i.e, a three-hop communications network. Given this system model, a new closed-form expression is presented in the high signal-to-noise ratio (SNR) regime for the Ergodic secrecy rate (ESR). Furthermore, we evaluate the high-SNR slope and power offset of the ESR to gain an insightful comparison of the proposed secure transmission scheme and the state-of-arts. Our numerical results highlight that the proposed secure transmission scheme provides better secrecy rate performance compared with the two-hop untrusted relaying as well as the direct transmission schemes. [ABSTRACT FROM AUTHOR]

By considering an adversary jammer in a communication network, we investigate the secrecy performance of a cooperative wireless network comprised of a source, a destination and an untrusted amplify-and-forward relay. We assume that either the source or the destination, as well as the jammer are equipped with large-scale multiple antennas systems, while the rest are equipped with a single-antenna. To prevent the untrusted relay from intercepting the source message, the destination sends an intended jamming noise to the relay, which is referred to as destination-assisted cooperative jamming. On the other hand, the role of the jammer is to disturb the communication by transmitting jamming signals toward the untrusted relay. Given this system model, novel closed-form expressions are extracted for the ergodic secrecy rate (ESR) with Rayleigh fading channels. We further evaluate the ESR at high signal-to-noise ratio (SNR) and then determine the high SNR slope and power offset of the ESR to get some insights into the network. Next, with the aim of maximizing the instantaneous secrecy rate, we derive new closed-form solutions for the optimal power allocation (OPA). Numerical examples depict that the presented OPA considerably improves the system secrecy rate compared to the equal power allocation (EPA) which reveals the priority of our optimized network. We also illustrate that by increasing the number of jammer antennas, the ESR performance of both the OPA and EPA is reduced. The results state that unlike the EPA technique, increasing the number of source antennas enhances the ESR of the proposed OPA technique which reveals the priority of our OPA. [ABSTRACT FROM AUTHOR]

In this paper, we examine the secrecy performance of two-way relaying between a large-scale multiple antenna base station (BS) and a single antenna mobile user (MU) in the presence of a multiple antenna external jammer. We consider the untrusted relaying scenario, where an amplify-and-forward relay is both a necessary helper and a potential eavesdropper to the BS and MU transmissions. With the aim of maximizing the instantaneous secrecy sum rate, we derive new closed-form solutions for the optimal power allocation (OPA) under the following three scenarios: 1) without jamming (WoJ) where the jammer is not activated; 2) friendly jamming (FJ) where the jamming signal is known a priori at the BS and MU; and 3) Gaussian noise jamming (GNJ) where the jamming signal is unknown at the BS and MU. Based on our OPA solutions, new closed-form expressions are derived for the ergodic secrecy sum rate (ESSR) with Rayleigh fading channels. Furthermore, we characterize the high signal-to-noise ratio slope and power offset of the ESSR to provide a fundamental comparison of the WoJ, FJ, and GNJ scenarios. Based on our analysis, we propose a simple relay selection criterion for the scenario when the BS and MU are assisted by multiple untrusted relays. Numerical examples are presented to demonstrate the impact of the jammer’s location, number of antennas and the advantage of the proposed relay selection criterion on the secrecy performance. Our findings highlight that similar to the trusted relaying scenario, the ESSR of all three secure transmission scenarios improve as the number of untrusted relays increases. Moreover, we highlight an interesting insight that the OPA for the GNJ scenario results in the same ESSR performance as the OPA for the WoJ scenario. [ABSTRACT FROM AUTHOR]

The authors study the transmit power allocation (PA) problem for a network of two multi-antenna terminals (one of which is a massivemultiple-input and multiple-output (MIMO) terminal) and a two-way, amplify-and-forward relay. The relay is limited to a single antenna. Using perfect channel state information, the terminals employ beamforming with maximumratio- transmission and maximum-ratio-combining for transmission and reception, respectively. The authors investigate two practical problems, namely; (i) maximising the sum rate subject to a total power constraint (ii) maximising the sumrate when one of the terminals must exceed a target signal-to-noise ratio (SNR). For the first case, the authors derive the closed-form optimal PA and for the second, the authors derive a sub-optimal PA. In both cases, the resulting sum rates are a function of instantaneous channel gains. Thus by averaging over the Nakagami-m distribution and exploiting the weak law of large numbers, the authors derive the closed-form ergodic sum rates. Finally, the simulation results validate the theoretical analysis and show the sum-rate improvements over uniform PA. For example, to achieve 4 bit/s/Hz, a uniform allocation needs 1 dB more than the authors' optimal allocation. When one of the SNRs must exceed a target value, the gap between the authors' sub-optimal PA and random PA increases to 2 dB. [ABSTRACT FROM AUTHOR]