Untrusted bidirectional relay systems confront both internal and external eavesdropping problems, but existing studies generally focus on only one of them. This letter proposes a new scheme combining constellation superposition and artificial noise to resist both. First, we exploit constellation superposition to cause aliasing of the constellations at the untrusted relay to prevent its decoding. Then, we embed carefully designed artificial noise in the transmitted signal, making it difficult for an external eavesdropper to remove. Theoretical analysis and simulations verify that the proposed scheme maintains a high bit error rate for the untrusted relay while ensuring a lower bound of bit error rate for the external eavesdropper. In contrast, the bit error rate of the legitimate users decreases as the signal-to-noise ratio (SNR) at the transmitter increases.
Untrusted bidirectional relay systems confront both internal and external eavesdropping problems, but existing studies generally focus on only one of them. This letter proposes a new scheme combining constellation superposition and artificial noise to resist both. First, we exploit constellation superposition to cause aliasing of the constellations at the untrusted relay to prevent its decoding. Then, we embed carefully designed artificial noise in the transmitted signal, making it difficult for an external eavesdropper to remove. Theoretical analysis and simulations verify that the proposed scheme maintains a high bit error rate for the untrusted relay while ensuring a lower bound of bit error rate for the external eavesdropper. In contrast, the bit error rate of the legitimate users decreases as the signal-to-noise ratio (SNR) at the transmitter increases.
This letter studies the secure transmission protocol design for two-way cooperative systems, where two terminal users communicate via an untrusted relay. The relay acts as both a helper and a potential eavesdropper, from which the users' messages need to be kept secret. A constellation-rotation aided approach is proposed to prevent the relay from decoding these messages. By rotating the signal constellation with a proper angle, the complex-valued symbol can be fully represented by its real component. Then, either user utilizes only one dimension of the signal to deliver its information, with the other dimension carrying the artificial noise (AN). In this manner, the received signal-to-interference-plus-noise ratio (SINR) at the relay is degraded due to the injection of AN, thereby increasing the difficulty of eavesdropping. Moreover, the desired signal and the AN lie in different directions at the terminal users, which makes the signal detection free-of-interference. The approximate expression for the upper bound of the users' symbol error rate (SER) is derived based on which a criterion is developed to choose the rotation angle that can optimize the system SER performance. Simulation results demonstrate the superiority of our scheme compared to the competing candidates.
This paper proposes a constellation overlapping scheme to secure two-way untrusted relaying systems, where the relay acts as both a helper facilitating data transmission and an eavesdropper intercepting users' messages. A truncated-channel-inversion based approach is developed to make the signals transmitted from two users experience the same equivalent channel, thereby realizing full constellation overlapping at the relay. Consequently, it is extremely difficult for the relay to recover the users' individual signals, and data confidentiality is thus protected. The achieved error floor level at the untrusted relay is analyzed, and the truncation thresholds are optimized to maximize the sum rate for end-to-end information exchange. Simulation results demonstrate the superiority of our scheme in terms of security and transmission efficiency compared with the existing alternatives.
This paper studies the secure information exchange of two-way relaying systems, where two terminal users communicate bi-directionally with the assistance of an untrusted relay. Each user is assumed to have two messages with distinct priorities: high priority and low priority. The high-priority messages (HPM) are first transmitted, for which we propose a constellation overlapping scheme such that the received signals at the relay overlap with each other and a high error floor is created to prevent the untrusted relay from deciphering the information. Upon the completion of HPM exchange, each user sends its low-priority message (LPM) by superposing it onto the previously decoded HPM from the other user. By exploiting the difference between the error patterns for HPM at the terminal users and the relay, channel noises in various time slots can be aggregated at the relay to secure the LPM transmission. The symbol error probability (SEP) at the terminal users as well as the untrusted relay is analyzed, from which it is shown that HPM is guaranteed to have greater reliability and higher secrecy level than that of LPM. However, the transmission of LPM enjoys lower implementation complexity and reduced system overhead, which fully demonstrates that the message-prioritization-based unequal secrecy protection framework can realize a good performance-complexity tradeoff. Simulation results exhibit the superiority of the proposed design compared to the existing competing solutions.
In this article, a new relay-aided secure communication system is investigated, where a transmitter sends signals to a destination via an amplify-and-forward (AF) relay in the presence of an eavesdropper. We consider a general system configuration, where the source, relay, destination, and eavesdropper are all equipped with multiple antennas. In the practical scenarios of unknown eavesdropper's channel state information (CSI) and uncertainty of the eavesdropper's location, we aim to maximize the expected value of the system secrecy rate over the presumed distribution of the eavesdropper's channels, by exploiting the artificial noise (AN) transmitted by the source and relay nodes. The system design issue is formulated as a nonconvex stochastic optimization problem with a source transmission power constraint and a nonconvex relay transmission power constraint. A novel computational method is proposed to solve this challenging problem. The new method is developed based on an exact penalty function method together with a parallel stochastic decomposition algorithm. Numerical simulations are performed to study the effectiveness of the proposed scheme at various locations of the eavesdropper. Simulation results show that for most cases, secure communication can be achieved without the CSI knowledge of eavesdropper's channels, and the achievable secrecy rate follows the trend of a benchmark system where the eavesdropper's full CSI is available. In particular, the achievable system secrecy rate increases with the number of antennas at the legitimate users. Moreover, the optimal power allocated for the transmission of the AN increases with the system signal-to-noise ratio. The proposed computational method achieves a higher system secrecy rate than a conventional penalty function based approach.
Recent information theoretic results have shown that interference can be exploited in wireless relay-assisted networks to secure the information communicated between two legitimate network nodes from the potentially untrusted nodes. In this paper, we consider a two-hop multi-relay network with untrusted relays, where the destination node, aiming at securing the source node's information signal, designs and broadcasts artificial noise (AN) symbols to jam the reception by the multiple relays. By focusing on the case where long-term channel state information is available at the destination, the objective of the considered AN design is to increase the decoding error probability at all the relays. Given an average power constraint at the destination, we propose a unified AN scheme to optimally generate the phase and the power of the AN symbols to maximize the minimum average symbol error rate (ASER) at the relays. The proposed AN generation scheme accounts for practical modulations, such as square quadrature amplitude modulation and M-ary phase-shift-keying. In addition, it is not restricted to specific channel fading model. Numerical and simulation results clearly show that the proposed scheme achieves considerably higher ASER performance than various AN generations, including the frequently used Gaussian distribution.
This paper proposes a novel technique called constellation overlapping to secure two-way untrusted relaying systems. We first consider the symmetric case where two terminal users adopt the same modulation type. For this case, a truncated-channel-inversion-based approach is developed to make the signals from different users experience the same equivalent channel, thereby realizing full constellation overlapping at the relay. Consequently, an error floor is created at the untrusted relay, and data confidentiality is protected. Then, the asymmetric case is investigated, where different modulations are employed at the two users and the constellation overlapping cannot be realized. To address this issue, we develop a constellation virtualization method, which exploits the intrinsic user-cooperation mechanism to “virtualize” two identical constellations at the two users. In this manner, the constellation overlapping effect can still be achieved, without sacrificing much reliability performance. We optimize the truncation threshold to maximize the sum rate for end-to-end information exchange, and design the bit-to-symbol mapping criterion and the signal detection method as well. Unlike the existing physical layer security approaches that guarantee secrecy with additional resource consumptions, this method realizes security enhancement by letting different users mutually encrypt their signals over the air, which is more resource-efficient.