This paper studies the downlink of a wireless system with an M-antenna base station, K single-antenna legitimate users and a single-antenna eavesdropper. A limited feedback-based scheduling and precoding scenario is considered that employs the orthogonal random beamforming (ORBF). The sum-rate of ORBF exhibits an identical growth rate as that of dirty paper coding for a large number of users. Unfortunately, for more realistic (low to moderate) K values, ORBF yields degraded performance. It is shown that having as many active beams as the number of transmit antennas is not always optimal for sparse network, thus scheduling strategies for ORBF with beam selection is investigated. Several user-beam selection algorithms featuring different complexity levels are proposed and their performance in terms of secrecy sum-rate is assessed. Numerical results are provided to illustrate the impact of the number of active beams on the secrecy performance.
This paper studies the downlink of a wireless system with an M-antenna base station, K single-antenna legitimate users and a single-antenna eavesdropper. A limited feedback-based scheduling and precoding scenario is considered that employs the orthogonal random beamforming (ORBF). The sum-rate of ORBF exhibits an identical growth rate as that of dirty paper coding for a large number of users. Unfortunately, for more realistic (low to moderate) K values, ORBF yields degraded performance. It is shown that having as many active beams as the number of transmit antennas is not always optimal for sparse network, thus scheduling strategies for ORBF with beam selection is investigated. Several user-beam selection algorithms featuring different complexity levels are proposed and their performance in terms of secrecy sum-rate is assessed. Numerical results are provided to illustrate the impact of the number of active beams on the secrecy performance.
This paper studies the beamforming design problem of a multiuser downlink network, assuming imperfect channel state information known to the base station. In this scenario, the base station is equipped with multiple antennas, and each user is wiretapped by a specific eavesdropper where each user or eavesdropper is equipped with one antenna. It is supposed that the base station employs transmit beamforming with a given requirement on sum transmitting power. The objective is to maximize the sum secrecy rate of the network. Due to the uncertainty of the channel, it is difficult to calculate the exact sum secrecy rate of the system. Thus, the maximum of lower bound of sum secrecy rate is considered. The optimization of the lower bound of sum secrecy rate still makes the considered beamforming design problem difficult to handle. To solve this problem, a beamforming design scheme is proposed to transform the original problem into a convex approximation problem, by employing semidefinite relaxation and first-order approximation technique based on Taylor expansion. In addition, with the advantage of low complexity, a zero-forcing-based beamforming method is presented in the case that base station is able to nullify the eavesdroppers' rate. When the base station does not have the ability, user selection algorithm would be in use. Numerical results show that the former strategy achieves better performance than the latter one, which is mainly due to the ability of optimizing beamforming direction, and both outperform the signal-to-leakage-and-noise ratio-based algorithm.
In this letter, we study on-off opportunistic beamforming for the multiuser downlink channel with a passive eavesdropper. Two opportunistic scheduling schemes exploiting multiuser diversity are investigated which require limited feedback of the effective signal-to-noise ratio (SNR) from the legitimate users. For the two scheduling schemes, we derive new closed-form expressions for the ergodic secrecy rate with on-off beamforming over Rayleigh fading channels. Numerical results are provided to verify our analytical results and illustrate the impact of multiuser diversity on the secrecy performance.
In this paper, the problem of secrecy sum-rate maximization is addressed for a multiuser multiple-input multiple-output wiretap channel. We consider the case of a single eavesdropper, when the information signals of the legitimate users are spatially multiplexed by using either zero forcing beamforming (ZFB) or Tomlinson-Harashima precoding (THP). By exploiting channel correlation of legitimate users with the eavesdropper, two low-complexity user selection schemes are investigated. In both schemes, the key idea is to extract multiuser diversity in order to protect information signals from eavesdropping rather than creating jamming interference. In the first scheme, user selection is performed in the null space of the eavesdropper's channel and an iterative power allocation method is proposed. In the second scheme, the eavesdropper is considered to be a selected authorized user but its allocated power is forced to be zero. Numerical results are provided to illustrate the secrecy sum-rate performance of the proposed schemes.
In this paper, we consider a multiuser downlink wiretap network consisting of one base station (BS) equipped with AA antennas, NB single-antenna legitimate users, and NE single-antenna eavesdroppers over Nakagami-m fading channels. In particular, we introduce a joint secure transmission scheme that adopts transmit antenna selection at the BS and explores threshold-based selection diversity scheduling over legitimate users to achieve a good secrecy performance while maintaining low implementation complexity. More specifically, in an effort to quantify the secrecy performance of the considered system, two practical scenarios are investigated, i.e.: in Scenario I, the eavesdropper's channel state information (CSI) is unavailable at the BS, and in Scenario II, the eavesdropper's CSI is available at the BS. For Scenario I, novel exact closed-form expressions for the secrecy outage probability are derived, which are valid for general networks with an arbitrary number of legitimate users, antenna configurations, number of eavesdroppers, and the switched threshold. For Scenario II, we take into account the ergodic secrecy rate as the principle performance metric, and derive novel exact closed-form expressions for the ergodic secrecy rate. In addition, we also provide simple and asymptotic expressions for secrecy outage probability and ergodic secrecy rate under two distinct cases, i.e.: in Case I, the legitimate user is located close to the BS, and in Case II, both the legitimate user and eavesdropper are located close to the BS. Our important findings reveal that the secrecy diversity order is AAmA and the slope of secrecy rate is one under Case I, while the secrecy diversity order and the slope of secrecy rate collapse to zero under Case II, where the secrecy performance floor occurs. Finally, when the switched threshold is carefully selected, the considered scheduling scheme outperforms other well-known existing schemes in terms of the secrecy performance and complexity tradeoff.
Ergodic Secrecy Rate of K-User MISO Broadcast Channel With Improved Random Beamforming
In this paper, we study the secrecy performance of a K-user multi-input single-output (MISO) broadcast channel with random beamforming (RB), where the transmitter is equipped with N antennas, and the eavesdropper has Ne antennas. We first propose a signal-splitting random beamforming (SSRB) scheme for a single-user MISO wiretap channel, and then improve the SSRB scheme with power-minimizing (PM), which is called a PM-SSRB scheme. To improve the spectral efficiency of a multiuser network, we combine PM with non-orthogonal multiple access (NOMA), and propose a hybrid NOMA (H-NOMA) scheme and a signal-splitting NOMA (SS-NOMA) scheme for the K-user MISO wiretap channel, respectively. The sum ergodic secrecy rate of the K-user scenario is analyzed comprehensively, and a closed-form expression for the lower bound on the ergodic secrecy rate is also derived for the single-user case. Simulation results show that, compared with the traditional hybrid artificial fast-fading scheme (AFF) and artificial noise (AN) scheme, the proposed SSRB scheme and PM-SSRB scheme perform much better in terms of the ergodic secrecy rate in all power regimes. More importantly, when the eavesdropper has more antennas than the transmitter, our schemes always outperform the AFF and AN schemes. The PM-SSRB scheme is also shown to be superior to the secret-key AFF scheme. For the multiuser case, the SS-NOMA scheme can achieve higher ergodic secrecy rate than the H-NOMA scheme.
In this paper, we propose a novel pseudo-random beamforming technique with beam selection for improving physical-layer security (PLS) in a downlink cellular network where consists of a base station (BS) with Ntantennas, NMSlegitimate mobile stations (MSs), and NEeavesdroppers. In the proposed technique, the BS generates multiple candidates of beamforming matrix each of which consists of orthogonal beamforming vectors in a pseudo-random manner. Each legitimate MS opportunistically feeds back the received signal-to-interference-and-noise ratio (SINR) value for all beamforming vectors to the BS. The BS transmits data to the legitimate MSs with the optimal beamforming matrix among multiple beam forming matrices that maximizes the secrecy sum-rate. Simulation results show that the proposed technique outperforms the conventional random beamforming technique in terms of the achievable secrecy sum-rate.