In this paper, we consider user scheduling and power allocation techniques for downlinks where transmitted message must be kept confidential from all other unintended users and the number of users is larger than that of access point (AP) antennas. In order to maximize network secrecy rate with a limited number of antennas, the AP schedules a user who can achieve the largest secrecy rate and exploits multiple antennas for transmitting artificial noise to the unintended users. We derive the closed-form expressions of the optimal power allocation for the artificial noise and the average network secrecy rate through asymptotic analysis. The analysis results provide the insightful information on the behaviors of the user scheduling and the effect of system parameters on the network secrecy rate performance. It is shown that opportunistic scheduling with artificial noise transmission technique is more effective than the conventional precoding techniques for multiple confidential message transmissions in terms of network secrecy rate. Furthermore, we have also discussed a transmission strategy for the worst case scenarios where all unintended users are treated as colluding eavesdroppers. The simulations confirm the validity of the analysis results and provide additional information on the network performance in general cases.
In this paper, we consider user scheduling and power allocation techniques for downlinks where transmitted message must be kept confidential from all other unintended users and the number of users is larger than that of access point (AP) antennas. In order to maximize network secrecy rate with a limited number of antennas, the AP schedules a user who can achieve the largest secrecy rate and exploits multiple antennas for transmitting artificial noise to the unintended users. We derive the closed-form expressions of the optimal power allocation for the artificial noise and the average network secrecy rate through asymptotic analysis. The analysis results provide the insightful information on the behaviors of the user scheduling and the effect of system parameters on the network secrecy rate performance. It is shown that opportunistic scheduling with artificial noise transmission technique is more effective than the conventional precoding techniques for multiple confidential message transmissions in terms of network secrecy rate. Furthermore, we have also discussed a transmission strategy for the worst case scenarios where all unintended users are treated as colluding eavesdroppers. The simulations confirm the validity of the analysis results and provide additional information on the network performance in general cases.
This paper studies opportunistic scheduling schemes to enhance the secrecy performance in multi-user multiple-input single-output (MU-MISO) non-orthogonal multiple access (NOMA) systems, in which a multiple antenna base station (BS) serves multiple single-antenna cell-center and cell-edge users in the presence of multiple single-antenna eavesdroppers. In order to improve the secrecy performance, we propose an opportunistic scheduling scheme, called the proposed antenna/users selection (PAUS) scheme. Additionally, we consider two practical eavesdropping scenarios, namely colluding and non-colluding eavesdroppers. We derive the exact closed-form expression for secrecy outage probability (SOP) and asymptotic SOP of the PAUS scheme under different eavesdropping scenarios. We provide the 2D golden section search-based algorithm to find the optimal values of the transmit power and power allocation coefficient that minimize the system secrecy outage performance. The developed analyses are verified by Monte Carlo simulations. The numerical results show that the PAUS scheme improves secrecy performances compared to the benchmark random antenna/users selection (RAUS) scheme. To provide further insights into the system secure performance, the effects of transmit signal-to-noise ratio (SNR), power allocation coefficient, the number of transmit antennas at the BS, number of cell-center and cell-edge users, and the eavesdropper placements are extensively evaluated and discussed.
In this paper, we consider secure communications in a multi-cell downlink multi-antenna system, where each multi-antenna base station (BS) sends confidential messages to one intended legitimate user (LU) with a passive multi-antenna eavesdropper trying to intercept the messages in this cell. To enhance the secure communications of the cell-edge users, the multiple BSs employ coordinated beamforming with the aid of artificial-noise (AN) beamforming. We conduct a mathematically rigorous secrecy performance analysis of the considered system and obtain new accurate closed-form expressions of a lower bound on the ergodic rate of each LU and an upper bound of the ergodic rate of each eavesdropper without assuming asymptotes for any system parameter (except for the “worst-case” scenario with zero noise power). Then, a lower bound on the ergodic secrecy rate (ESR) of each LU follows. Based on the derived analytical results, we propose a low-complexity algorithm to optimize the channel sate information (CSI) feedback bits allocation for the channels of target signal link and inter-cell interference links. Furthermore, we develop explicit sufficient conditions on the set of system parameters under which there are at least two BSs to coordinated secure transmission. Through both theoretical analysis and numerical results, we can verify that the obtained analytical lower bound on ESR of each LU as a function of the set of power allocation coefficients is generallyneither convex nor concave. Thus, we propose to employ some numerical method to find a sub-optimal power allocation solution. Finally, numerical results are also provided to validate our theoretical results and the proposed feedback bits and power allocation methods.
We consider the physical layer security for the downlink system, where the confidential messages can be wiretapped by both the intended users (IUs) and the external eavesdroppers (EEs). Regularized channel inversion (RCI) precoding is adopted for multiuser communication because it can control the mutual information leakage among users. And, to enhance the secrecy performance, artificial noise (AN) is introduced to disturb external eavesdroppers. Considering the large-system regime, by using stochastic geometry, the secrecy outage probability (SOP) and mean secrecy rate for the nearest EE wiretap and the strongest EE wiretap scenarios are derived, respectively. Analytical and simulation results show that i) when the AN is adopted, the SOP exponentially decays with the number of transmission antennas, ii) in most of the power allocation case, per-user secrecy rate can be improved significantly, e.g., there is an almost 2.7 times of improvement for the particular transmission antennas number.
Wireless communication is particularly susceptible to eavesdropping due to its broadcast nature. Security and privacy issues have become increasingly critical for wireless networks. This paper considers the problem of secret communication in the multiuser downlink eavesdropped by a passive eavesdropper (EVE). The well-known zero-forcing preceding is adopted at the transmitter to produce concurrent data streams to the users, and at the same time an artificial noise (AN) is generated to prevent EVE from intercepting the information. We first derive an analytical closed-form expression for the secrecy sum-rate in the large system limit. We then use it as the objective function to optimize the power allocation (PA) between the information signals and the AN to maximize the secrecy sum-rate. A simple analytical expression of the optimal PA is derived in the high transmit power regime, which proves to be a near-optimal and generic strategy. The large system results are quite accurate for finite-size systems and thus can provide useful insights into system analysis and design. Our analytical and simulation results show that the AN assisted strategy achieves the same multiplexing gain as in the multiuser downlink without eavesdropping. Moreover, more power should be allocated to AN when the system serves fewer users and EVE has more antennas.
This paper investigates the secrecy performance of a multiuser system that utilizes transmit antenna selection scheme at the base station and adopts threshold-based selection diversity opportunistic scheduling over legitimate nodes. The legitimate transmission suffers from the presence of noncolluding (Non-Col) and colluding (Col) multiple passive eavesdroppers. Both the legitimate and eavesdropping nodes are assumed to suffer from co-channel interference (CCI) signals from independent channels that follow Rayleigh fading. Closed-form expressions for the probability density functions and cumulative density functions of the end-to-end signal-to-interference-plus-noise ratio for both eavesdropping scenarios in the presence of CCI signals are derived. In addition, closed-form expressions for the network secrecy outage probability (SOP) for Non-Col/Col scenarios are derived. At the high signal-to-noise ratio values, closed-form expressions for the asymptotic secrecy outage probabilities are obtained. Following this obtained asymptotic analysis, an optimization problem for power allocation is formulated and solved to improve the secrecy performance of the network by minimizing the asymptotic SOP for both Col and Non-Col cases. The derived analytical expressions are then validated using both simulations and numerical results.
In this paper, we investigate secure transmission in a cooperative non-orthogonal multiple access (NOMA) system in the presence of an active eavesdropper. To enhance secrecy performance, we propose two approaches. The first is a novel cooperative NOMA architecture, in which the cell-edge user generates a jamming signal to degrade the eavesdropper’s channel condition. The second is a novel opportunistic scheduling scheme, referred to as the minimum antenna selection (MAS) scheme, which selects the transmit antenna at the source node to minimize the maximum eavesdropper channel capacity between the cell-center and cell-edge users. We derive closed-form expressions for the secrecy outage probability (SOP) of both the cell-center and cell-edge users to characterize the impact of various network parameters on secrecy performance. Through numerical results, we analyze the impact of the source, cell-center, and cell-edge users’ transmit power on the SOP; the impact of the power allocation coefficient, target secrecy rate, and eavesdropper transmit power on the total SOP; and the impact of the distance between the source node and the eavesdropper, as well as the distance between the cell-center user and the eavesdropper, on the secrecy sum throughput (SST). These results demonstrate that the proposed cooperative NOMA architecture and the MAS scheme can effectively protect confidential messages against active eavesdropping attacks.