The fluid antenna (FA) has emerged as a promising technology for enhancing wireless network performance due to its ability to dynamically adjust antenna positions and mitigate poor channel conditions. In this paper, we propose an FA-aided secure transmission scheme where the transmitter, equipped with an FA, intends to send confidential information to a legitimate receiver under the eavesdropping of multiple illegitimate receivers. To overcome the limited beamforming dimensionality, each symbol duration is divided into several sub-symbol periods, during which the FA activates different ports corresponding to varying legitimate and eavesdropping channel conditions. This dynamic switching enhances spatial degrees of freedom (DoF). During each sub-symbol period, the FA transmits a superposition of useful sig- nal and random noise with fine-tuned precoding factors, ensuring that the noise degrades eavesdropping channels while minimally affecting the legitimate channel. We consider a practical scenario where imperfect eavesdropping channel state information (ECSI) is available. A worst-case secrecy rate maximization problem is formulated via jointly optimizing the precoding factors, power allocation, and the receive beamforming vector. Numerical results demonstrate that the proposed scheme significantly improves secrecy performance compared to traditional secure schemes.
The fluid antenna (FA) has emerged as a promising technology for enhancing wireless network performance due to its ability to dynamically adjust antenna positions and mitigate poor channel conditions. In this paper, we propose an FA-aided secure transmission scheme where the transmitter, equipped with an FA, intends to send confidential information to a legitimate receiver under the eavesdropping of multiple illegitimate receivers. To overcome the limited beamforming dimensionality, each symbol duration is divided into several sub-symbol periods, during which the FA activates different ports corresponding to varying legitimate and eavesdropping channel conditions. This dynamic switching enhances spatial degrees of freedom (DoF). During each sub-symbol period, the FA transmits a superposition of useful sig- nal and random noise with fine-tuned precoding factors, ensuring that the noise degrades eavesdropping channels while minimally affecting the legitimate channel. We consider a practical scenario where imperfect eavesdropping channel state information (ECSI) is available. A worst-case secrecy rate maximization problem is formulated via jointly optimizing the precoding factors, power allocation, and the receive beamforming vector. Numerical results demonstrate that the proposed scheme significantly improves secrecy performance compared to traditional secure schemes.
For physical layer security with multiple antennas over wireless channels, we consider an artificial noise-aided secure beamforming system. A transmitter can send the confidential information to the legitimate user more securely without eavesdropping when an artificial jamming signal interfering eavesdroppers is transmitted with the confidential information signal. The transmitter splits its transmit power for both the information and jamming signals with a power splitting factor. Under such a system model, we investigate the impacts on secrecy performance of a power splitting factor, the numbers of antennas and eavesdroppers, and noise variance at the legitimate receivers and eavesdroppers by analyzing the expected secrecy rate. The optimal power splitting factor and limiting secrecy rate with large number of antennas are also derived. Moreover, we examine the expected secrecy rate loss caused by channel variation over time. It is shown that the secrecy rate loss is independent of system parameters like a power splitting factor, the numbers of antennas and eavesdroppers in a high signal to interference plus noise ratio (SINR) region. In a low SINR region, on the other hand, the secrecy rate loss changes with system parameters. Simulation results verify these observations on ergodic secrecy rate and its loss due to time-varying channels.
Recent researches have revealed that fluid antenna, a new position-switchable antenna technology, can make use of the spatial moments of deep fades of the interference signal occurred naturally due to multipath for multiple access. In this letter, this phenomenon is exploited in a physical layer security setup where a base station (BS) transmits an information-bearing signal to a legitimate user and an artificial noise (AN) signal to a potential eavesdropper while the user is equipped with a fluid antenna to overcome the AN signal. The proposed approach needs no effort from the base station (BS) to avoid the AN signal from harming the legitimate user. To enhance the secrecy rate, we study the power allocation of the information-bearing and AN signals if the channel state information (CSI) is available at the BS. Both perfect and imperfect CSI scenarios are investigated. Simulation results demonstrate that the proposed system, with a single fluid antenna with one radio-frequency (RF) chain at the user, achieves the secrecy rate that is comparable to the user utilizing maximal ratio combining (MRC) with many antennas instead, and is the only approach robust to CSI uncertainties of the eavesdropper, requiring no power allocation nor beamforming at the BS.
This paper considers a massive MIMO network that includes one multiple-antenna base station, one multiple-antenna eavesdropper, and K single-antenna users. The eavesdropper operates in passive mode and tries to overhear the confidential information from one of the users in the down-link transmission. In order to secure the confidential information, two artificial noise (AN)-aiding schemes are proposed. In the first scheme, AN is injected into the downlink training signals to prevent the eavesdropper from obtaining the correct channel state information of the eavesdropping link. In the second scheme, AN is deployed in both downlink training phase and payload data transmission phase to further degrade the eavesdropping channel. Analytical expressions and tight approximations of the achievable secrecy rate of the considered systems are derived with taking imperfect channel estimation and two types of precoding, i.e., maximum-ratio-transmission and zero-forcing, into consideration. Optimization algorithms for power allocation are proposed to enhance the secrecy performance of the proposed AN-aiding schemes. The results reveal that deploying AN in the downlink training phase of massive MIMO networks does not affect the downlink channel estimation process at users while enabling the system to suppress the downlink channel estimation process at eavesdropper. As a consequence, the proposed AN-aided schemes improve the system performance significantly. Furthermore, implementing AN in both phases allows the considered system having a flexible solution to maximize its secrecy performance at the price of higher complexity.
We investigate the secrecy rate optimization problem in a wiretap channel with a single-antenna source, a single-antenna eavesdropper, and a multiple-antenna full-duplex (FD) destination. To fully utilize the spatial degrees-of-freedom of multiple antennas, the function of antennas at the destination is not predefined, i.e., each antenna can operate in a transmit or receive mode. We propose a low-complexity near-optimal joint optimization scheme by jointly applying the dynamic antenna mode switching (AMS) and optimal power allocation (OPA) techniques, to maximize the secrecy rate of the FD destination-based jamming (DBJ) system. The proposed joint optimization scheme is valid for two different eavesdropping channel state information (ECSI) availability cases, i.e., instantaneous ECSIs and statistical ECSIs. Specifically, closed-form expressions of OPA factors are first derived, and then the optimal transmit and receive antennas sets at the destination are determined by combining the OPA factor and applying a greedy-search-based AMS approach for both ECSIs availabilities, respectively. Moreover, through complexity analysis, the search complexity of the proposed scheme is proven to be significantly reduced compared with the exhaustive searching method. Simulation results verify the secrecy performance superiority of the proposed scheme over the conventional FD-DBJ method.
This paper studies secure wireless transmission from a multi-antenna transmitter to a single-antenna intended receiver overheard by multiple eavesdroppers with considering the imperfect channel state information (CSI) of wiretap channel.To enhance security of communication link, the artificial noise (AN) is generated at transmitter.We first design the robust joint optimal beamforming of secret signal and AN to minimize transmit power with constraints of security quality of service (QoS), i.e., minimum allowable signal-to-interference-and-noise ratio (SINR) at receiver and maximum tolerable SINR at eavesdroppers.The formulated design problem is shown to be nonconvex and we transfer it into linear matrix inequalities (LMIs).The semidefinite relaxation (SDR) technique is used and the approximated method is proved to solve the original problem exactly.To verify the robustness and tightness of proposed beamforming, we also provide a method to calculate the worst-case SINR at eavesdroppers for a designed transmit scheme using semidefinite programming (SDP).Additionally, the secrecy rate maximization is explored for fixed total transmit power.To tackle the nonconvexity of original formulation, we develop an iterative approach employing sequential parametric convex approximation (SPCA).The simulation results illustrate that the proposed robust transmit schemes can effectively improve the transmit performance.
Exploiting the randomness embedded in wireless channels, the physical-layer security techniques can provide additional security robustness against malicious eavesdropping at the physical layer of communication systems. To further this line of study, this thesis focuses on developing advanced multi-antenna transmission strategies for enhancing the secrecy performance of wireless communication systems. Assuming perfect channel feedback from the desired receiver and none from the eavesdropper, we consider beamforming with smartly injected artificial noise, aiming to confuse the eavesdropper whilst leaving the desired receiver unaffected. The system parameters are either chosen to be fixed for all transmissions, or adaptively adjusted based on the channel feedback from the desired receiver. For both cases, we provide novel and explicit design solutions for achieving the maximum throughput subject to a secrecy outage constraint. We then derive accurate approximations for the maximum throughput, and give new insights into the additional power cost for achieving a higher security level and the throughput gain of adaptive transmission over non-adaptive transmission. Having investigated the perfect feedback scenario, we then relax the feedback assumption and consider practical effects imposed by limited feedback constraints on the channel from the desired receiver to the transmitter. To deal with quantization error due to limited feedback whilst also accounting for unknown channel of the eavesdropper, we specify a connection outage constraint to guarantee the reliability of desired communication and a secrecy outage constraint to guard against malicious eavesdropping. Under dual outage constraints, we derive a novel optimized rate-adaptive transmission design to maximize the average secrecy throughput. New insights regarding the optimal power/feedback allocation and the number of feedback bits required for efficient quantization are obtained through our analysis. Our preceding study focused on isolated point-to-point transmission without any interference. However, the performance of contemporary wireless networks is often limited by interference from concurrent transmissions. To characterize the impact of interference on secure communication, we consider an interference-limited large-scale network. To confuse the malicious eavesdroppers, in addition to information-bearing signals, the transmitters are designed to generate artificial noise through either beamforming or antenna sectoring in a distributed manner. For both cases, using tools from stochastic geometry, we provide a statistical characterization of the reliability performance of desired communication and the secrecy performance against eavesdropping. We then investigate the networkwide secrecy throughput performance and optimize the transmit power allocation. Our analysis reveals interesting differences between the beamforming and sectoring approaches in terms of the optimal power allocation and the maximum networkwide secrecy throughput.