This paper proposes a message-prioritization based unequal secrecy protection framework for untrusted two- way relaying networks, where two terminal users communicate bidirectionally 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 devise a constellation overlapping method 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, users transmit their low priority messages (LPM) using a noise aggregation approach. To be specific, each user superposes its LPM 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. It is shown from simulation results 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 proposed unequal secrecy protection scheme can realize a good performance-complexity tradeoff.
This paper proposes a message-prioritization based unequal secrecy protection framework for untrusted two- way relaying networks, where two terminal users communicate bidirectionally 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 devise a constellation overlapping method 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, users transmit their low priority messages (LPM) using a noise aggregation approach. To be specific, each user superposes its LPM 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. It is shown from simulation results 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 proposed unequal secrecy protection scheme can realize a good performance-complexity tradeoff.
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.
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 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.
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 letter proposes a key exchange scheme for the untrusted relay system, where the relay not only assists in message transmission but also tries to recover messages. Terminal users initiate the process by transmitting a local key and utilize constellation overlapping techniques to create an overlapped constellation at the relay nodes, which increases the decoding error rate for relay eavesdropping but still poses a risk of key leakage. To address this, we propose a fine-grained entropy extraction algorithm that quantifies the overlapped constellation points into key bits, which are uniformly random bits from the relay’s perspective. In the constellation overlapping scenario, we can estimate the number of instances where the relay cannot distinguish. Leveraging this, our algorithm achieves remarkable levels of entropy utilization. QPSK enables perfect lossless entropy extraction, while other QAM modulation schemes can achieve a utilization rate of up to 96%.
A downlink single-input single-output non-orthogonal multiple access setting is considered, in which a base station (BS) is communicating with two legitimate users in two possible scenarios of unsecure environments: existence of an external eavesdropper and communicating through an untrusted relay. For the first scenario, a number of trusted cooperative half-duplex relays is employed to assist with the BS's transmission and secure its signals from the external eavesdropper. Various relaying schemes are proposed and analyzed for that matter: cooperative jamming, decode-and-forward, and amplify-and-forward. For each scheme, secure beamforming signals are devised at the relays to maximize the achievable secrecy rate regions. For the second scenario, with the untrusted relay, achievable secrecy rate regions are derived for two different relaying schemes, compress-and-forward and amplify-and-forward, under two different modes of operation. In the first mode, coined passive user mode, the users receive signals from both the BS and the untrusted relay and combine them to decode their messages. In the second mode, termed the active user mode, the users transmit a cooperative jamming signal simultaneously with the BS's transmission to further confuse the relay. Focusing on half-duplex nodes, the users cannot receive the BS's signal while jamming the relay, i.e., while being active, and rely only on the signals forwarded to them by the relay. It is shown that the best relaying scheme highly depends on the system parameters, in particular the distances between the nodes, and also on the part of the secrecy rate region at which the system is to operate.