Hui-Ming Wang

Distributed Beamforming for Physical-Layer Security of Two-Way Relay Networks

In this paper, we address the security of a two-way relay network in the presence of an eavesdropper, where each node is only equipped with single antenna. We propose two-phase distributed analog network coding, or distributed beamforming and power allocation to enhance the secrecy sum rate of the data exchange. In the first phase, the two terminals broadcast their information data simultaneously to all the relay nodes. In the second phase, three different security schemes are proposed: optimal beamforming, null-space beamforming, and artificial noise beamforming. In the first scheme, the objective is to achieve the maximum secrecy sum rate of the two terminals. Mathematically, the objective function is difficult to optimize. In the second scheme, we maximize the total information exchanged while we eliminate the information leakage completely, subject to the total transmission power constraint. We show that the problem has a unique and global optimum, which can be solved using bisection method. When the instantaneous channel state information of the eavesdropper is not available, we propose an artificial noise beamforming in the third scheme. We minimize the information transmission power so that the artificial noise power is maximized to eliminate information leakage, under the constraints of quality of service (QoS) required by terminals. It is a second-order convex cone programming (SOCP) problem, thus can be efficiently solved using interior point methods. Numerical results are provided and analyzed to show the properties and efficiency of the proposed designs.

Joint Cooperative Beamforming and Jamming to Secure AF Relay Systems With Individual Power Constraint and No Eavesdropper's CSI

Cooperative beamforming and jamming are two efficient schemes to improve the physical-layer security of a wireless relay system in the presence of passive eavesdroppers. However, in most works these two techniques are adopted separately. In this letter, we propose a joint cooperative beamforming and jamming scheme to enhance the security of a cooperative relay network, where a part of intermediate nodes adopt distributed beamforming while others jam the eavesdropper, simultaneously. Since the instantaneous channel state information (CSI) of the eavesdropper may not be known, we propose a cooperative artificial noise transmission based secrecy strategy, subjected to the individual power constraint of each node. The beamformer weights and power allocation can be obtained by solving a second-order convex cone programming (SOCP) together with a linear programming problem. Simulations show the joint scheme greatly improves the security.

Hybrid Cooperative Beamforming and Jamming for Physical-Layer Security of Two-Way Relay Networks

In this paper, we propose a hybrid cooperative beamforming and jamming scheme to enhance the physical-layer security of a single-antenna-equipped two-way relay network in the presence of an eavesdropper. The basic idea is that in both cooperative transmission phases, some intermediate nodes help to relay signals to the legitimate destination adopting distributed beamforming, while the remaining nodes jam the eavesdropper, simultaneously, which takes the data transmissions in both phases under protection. Two different schemes are proposed, with and without the instantaneous channel state information of the eavesdropper, respectively, and both are subjected to the more practical individual power constraint of each cooperative node. Under the general channel model, it is shown that both problems can be transformed into a semi-definite programming (SDP) problem with an additional rank-1 constraint. A current state of the art technique for handling such a problem is the semi-definite relaxation (SDR) and randomization techniques. In this paper, however, we propose a penalty function method incorporating the rank-1 constraint into the objective function. Although the so-obtained problem is not convex, we develop an efficient iterative algorithm to solve it. Each iteration is a convex SDP problem, thus it can be efficiently solved using the interior point method. When the channels are reciprocal such as in TDD mode, we show that the problems become second-order convex cone programming ones. Numerical evaluation results are provided and analyzed to show the properties and efficiency of the proposed hybrid security scheme, and also demonstrate that our optimization algorithms outperform the SDR technique.

Enhancing wireless secrecy via cooperation: signal design and optimization

Physical layer security, or information-theoretic security, has attracted considerable attention recently, due to its potential to enhance the transmission secrecy of wireless communications. Various secrecy signaling and coding schemes have been designed at the physical layer of wireless systems to guarantee confidentiality against information leakage to unauthorized receivers, among which the strategy based on the idea of node cooperation is promising. This article provides an overview of the recent research on enhancing wireless transmission secrecy via cooperation. We take a signal processing perspective and focus on the secrecy signal design and optimization techniques to increase secrecy performance. We also propose some future research directions on this topic.

Distributed space-frequency codes for cooperative communication systems with multiple carrier frequency offsets

In cooperative communications, due to the distributed nature, multiple different carrier frequency offsets (CFOs) may occur and make the channel time-varying. It is hard for the receiver to compensate multiple CFOs from multiple relay nodes simultaneously. Thus, the conventional space-time codes to collect cooperative diversity for co-located multi-input multi-output (MIMO) systems may not be applied directly. In this paper, we consider the cooperative transmission with multiple CFOs when the channels from relay nodes to destination node are flat (frequency-non-selective) fading. We approximate a flat fading channel with CFO as a block time-invariant intersymbol-interference (ISI) channel in the frequency domain. We then propose two distributed space-frequency codes (SFC) for such ISI channels in the frequency domain to achieve the cooperative full spatial diversity, where the space-frequency coding concept is different from the one in the literature and also has a different role. One is called frequency-reversal SFC and the other is called frequency-domain linear convolutive SFC. Furthermore, we show that, with only linear receivers, such as zero-forcing (ZF) and minimum mean square error (MMSE) receivers, our codes achieve the full cooperative diversity.

Multi-Antenna Transmission With Artificial Noise Against Randomly Distributed Eavesdroppers

In this paper, we study the secure multi-antenna transmission with artificial noise (AN) under slow fading channels coexisting with randomly located eavesdroppers. We provide a comprehensive secrecy performance analysis and system design/optimization under a stochastic geometry framework. Specifically, we first evaluate the secrecy outage performance, and derive a closed-form expression for the optimal power allocation ratio of the information signal power to the total transmit power that minimizes the secrecy outage probability (SOP). Subject to a SOP constraint, we then propose a dynamic parameter transmission scheme (DPTS) and a static parameter transmission scheme (SPTS) to maximize secrecy throughput, and provide explicit solutions on the optimal transmission parameters, including the wiretap code rates, the on-off transmission threshold and the power allocation ratio. Our results give new insight into secure transmission designs. For example, secrecy rate is a concave function of the power allocation ratio in DPTS, and AN plays a significant role under SOP constraints and in dense eavesdropper scenarios. In SPTS, transmission probability is a concave function of the power allocation ratio, and secrecy throughput is a quasi-concave function of the secrecy rate. Numerical results are demonstrated to validate our theoretical analysis.

Joint Cooperative Beamforming, Jamming, and Power Allocation to Secure AF Relay Systems

The idea of multiuser (nodes) cooperation is an efficient way to improve the physical-layer security of a wireless transmission in the presence of passive eavesdroppers. However, due to the half-duplex constraint of the practical transceivers, two phases are required for one round of data transmission, which grants the eavesdroppers two opportunities to wiretap the information. Therefore, protecting the data transmissions in both phases is critical. Toward this end, we propose a joint cooperative beamforming, jamming, and power-allocation scheme to enhance the security of an amplify-and-forward (AF) cooperative relay network in this paper. Different from the existing works assuming that the source node always uses its total power, we show that the secrecy rate is a quasi-concave function of the power of the source node so that allocating its total power may not be optimal. The beamformer design and power optimization problem can be solved by a bisection method together with a generalized eigenvalue decomposition, which has a semiclosed form and is computationally very convenient. Simulations show that the joint scheme greatly improves the security.

On the Design of Relay Selection Strategies in Regenerative Cooperative Networks with Outdated CSI

Opportunistic relay selection is considered as an efficient approach to implement cooperative communication in multi-relay cooperative networks owing to its low implementation complexity. However, due to channel fluctuations, the channel state information (CSI) employed in relay selection process may differ from the exact CSI in data forwarding, in other words, the CSI is outdated. Addressing this issue, this paper focuses on the design of relay selection strategies in decode-and-forward (DF) cooperative networks with outdated CSIs being available in relay judgement, as well as their performance analysis. We resort to two main cases, 1) only outdated CSIs are available in relay selection process; 2) both outdated CSIs and statistical channel information are available in selection procedure, and propose three different relay selection schemes. The expressions of outage probability are derived, and asymptotic analysis in high SNR region is also carried out. Simulation results are finally provided, which not only validate our theoretical analysis, but also shed light on the way of designing relay selection strategies in various scenarios.

On the Secrecy Throughput Maximization for MISO Cognitive Radio Network in Slow Fading Channels

This paper studies the secure multiple-antenna transmission in slow fading channels for the cognitive radio network, where a multiple-input, single-output, multieavesdropper (MISOME) primary network coexisting with a multiple-input single-output secondary user (SU) pair. The SU can get the transmission opportunity to achieve its own data traffic by providing the secrecy guarantee for the PU with artificial noise. Different from the existing works, which adopt the instantaneous secrecy rate as the performance metric, with only the statistical channel state information (CSI) of the eavesdroppers, we maximize the secrecy throughput of the PU by designing and optimizing the beamforming, rate parameters of the wiretap code adopted by the PU, and power allocation between the information signal and the artificial noise of the SU, subjected to the secrecy outage constraint at the PU and a throughput constraint at the SU. We propose two design strategies: 1) nonadaptive secure transmission strategy (NASTS) and 2) adaptive secure transmission strategy, which are based on the statistical and instantaneous CSIs of the primary and secondary links, respectively. For both strategies, the exact rate parameters can be optimized through numerical methods. Moreover, we derive an explicit approximation for the optimal rate parameters of the NASTS at high SNR regime. Numerical results are illustrated to show the efficiency of the proposed schemes.

Physical Layer Security in Heterogeneous Cellular Networks

The heterogeneous cellular network (HCN) is a promising approach to the deployment of 5G cellular networks. This paper comprehensively studies physical layer security in a multitier HCN where base stations (BSs), authorized users, and eavesdroppers are all randomly located. We first propose an access threshold-based secrecy mobile association policy that associates each user with the BS providing the maximum truncated average received signal power beyond a threshold. Under the proposed policy, we investigate the connection probability and secrecy probability of a randomly located user and provide tractable expressions for the two metrics. Asymptotic analysis reveals that setting a larger access threshold increases the connection probability while decreases the secrecy probability. We further evaluate the network-wide secrecy throughput and the minimum secrecy throughput per user with both connection and secrecy probability constraints. We show that introducing a properly chosen access threshold significantly enhances the secrecy throughput performance of a HCN.