Jianhua Mo

Relay Placement for Physical Layer Security: A Secure Connection Perspective

This work studies the problem of secure connection in cooperative wireless communication with two relay strategies, decode-and-forward (DF) and randomize-and-forward (RF). The four-node scenario and cellular scenario are considered. For the typical four-node (source, destination, relay, and eavesdropper) scenario, we derive the optimal power allocation for the DF strategy and find that the RF strategy is always better than the DF to enhance secure connection. In cellular networks, we show that without relay, it is difficult to establish secure connections from the base station to the cell edge users. The effect of relay placement for the cell edge users is demonstrated by simulation. For both scenarios, we find that the benefit of relay transmission increases when path loss becomes severer.

Power and Subcarrier Allocation for Physical-Layer Security in OFDMA-Based Broadband Wireless Networks

Providing physical-layer security for mobile users in future broadband wireless networks is of both theoretical and practical importance. In this paper, we formulate an analytical framework for resource allocation in a downlink orthogonal frequency-division multiple access (OFDMA)-based broadband network with coexistence of secure users (SUs) and normal users (NUs). The SUs require secure data transmission at the physical layer while the NUs are served with conventional best-effort data traffic. The problem is formulated as joint power and subcarrier allocation with the objective of maximizing average aggregate information rate of all NUs while maintaining an average secrecy rate for each individual SU under a total transmit power constraint for the base station. We solve this problem in an asymptotically optimal manner using dual decomposition. Our analysis shows that an SU becomes a candidate competing for a subcarrier only if its channel gain on this subcarrier is the largest among all and exceeds the second largest by a certain threshold. Furthermore, while the power allocation for NUs follows the conventional water-filling principle, the power allocation for SUs depends on both its own channel gain and the largest channel gain among others. We also develop a suboptimal algorithm to reduce the computational cost. Numerical studies are conducted to evaluate the performance of the proposed algorithms in terms of the achievable pair of information rate for NU and secrecy rate for SU at different power consumptions.

Secure Beamforming for MIMO Two-Way Communications With an Untrusted Relay

This paper studies the secure beamforming design in a multiple-antenna three-node system where two source nodes exchange messages with the help of an untrusted relay node. The relay acts as both an essential signal forwarder and a potential eavesdropper. Both two-phase and three-phase two-way relay strategies are considered. Our goal is to jointly optimize the source and relay beamformers for maximizing the secrecy sum rate of the two-way communications. We first derive the optimal relay beamformer structures. Then, iterative algorithms are proposed to find source and relay beamformers jointly based on alternating optimization. Furthermore, we conduct asymptotic analysis on the maximum secrecy sum-rate. Our analysis shows that when all transmit powers approach infinity, the two-phase two-way relay scheme achieves the maximum secrecy sum rate if the source beamformers are designed such that the received signals at the relay align in the same direction. This reveals an important advantage of signal alignment technique in against eavesdropping. It is also shown that if the source powers approach zero, the three-phase scheme performs the best while the two-phase scheme is even worse than direct transmission. Simulation results have verified the efficiency of the proposed secure beamforming algorithms as well as the analytical findings.

Cooperative Secret Communication with Artificial Noise in Symmetric Interference Channel

We consider the symmetric Gaussian interference channel where two users try to enhance their secrecy rates in a cooperative manner. Artificial noise is introduced along with useful information. We derive the power control and artificial noise parameter for two kinds of optimal points, max-min point and single user point. It is shown that there exists a critical value Pc of the power constraint, below which the max-min point is an optimal point on the secrecy rate region, and above which time-sharing between single user points achieves larger secrecy rate pairs. It is also shown that artificial noise can help to enlarge the secrecy rate region, in particular on the single user point.

QoS-Aware Transmission Policies for OFDM Bidirectional Decode-and-Forward Relaying

Two-way relaying can considerably improve spectral efficiency in relay-assisted bidirectional communications. However, the benefits and flexible structure of orthogonal frequency division multiplexing (OFDM)-based two-way relay systems is much less exploited. Moreover, most of existing works have not considered quality-of-service (QoS) provisioning for two-way relaying. In this paper, we consider the OFDM-based bidirectional transmission where a pair of users exchange information via the assistance of a decode-and-forward (DF) relay. Each user can communicate with the other via three transmission modes: direct transmission, one-way relaying, and two-way relaying. We jointly optimize the transmission policies, including power allocation, transmission mode selection, and subcarrier assignment in order to maximize the weighted sum rates of the two users with diverse QoS guarantees. This is formulated as a mixed integer programming problem. By using the dual method, we efficiently solve the problem in an asymptotically optimal manner. Simulation results show that the proposed resource allocation scheme can substantially improve system performance compared with conventional schemes. A number of interesting insights are also obtained via comprehensive simulations.

Secure beamforming for MIMO two-way transmission with an untrusted relay

From security perspective, a friendly relay may help to keep the confidential messages from being eavesdropped, while an untrusted relay may intentionally eavesdrop the messages when relaying. This paper studies the secure beamforming for multiple-input multiple-output (MIMO) two-way communications, where two source nodes exchange information with the help of an untrusted relay node. The relay adopts amplify-and-forward (AF) strategy and acts as both an essential helper and a potential eavesdropper. Our goal is to maximize the secrecy sum rate of the bidirectional links by jointly optimizing the source and relay beamformers. For the two-phase two-way relay scheme, we first derive the optimal structure of the relay beamformer and then propose an iterative algorithm to jointly optimize the source and relay beamformers. Then, a comprehensive study on the asymptotical performance is conducted by letting the source and relay powers approach zero or infinity. In particular, we show that when all powers approach infinity, the two-way relay scheme achieves the maximum secrecy rate if the transceiver beamformers are designed such that the received signals at the relay can be aligned to be parallel.

QoS-aware policies for OFDM bidirectional transmission with decode-and-forward relaying

In this paper, we consider the orthogonal frequency division multiplexing (OFDM)-based bidirectional transmission where a pair of users exchange information with the assistance of a decode-and-forward (DF) relay. Each user can communicate with the other via three transmission modes: direct transmission, one-way relaying, and two-way relaying. We jointly optimize the transmission policies, including power allocation, transmission mode selection, and subcarrier-node assignment for maximizing the weighted sum rates of the two users with quality-of-service (QoS) guarantees. We formulate the joint optimization problem as a mixed integer programming problem. By using the dual method, we solve the problem efficiently in an asymptotically optimal manner. Particularly, we derive the capacity region of two-way DF relaying in parallel relay channels. Simulation results show that the proposed resource-allocation scheme can substantially improve system performance compared with the conventional schemes.

Power and Subcarrier Allocation for Physical-Layer Security in OFDMA Networks

Providing physical-layer security for mobile users in future broadband wireless networks is of both theoretical and practical importance. In this paper, we formulate an analytical framework for resource allocation in a downlink OFDMA-based broadband network with coexistence of secure users (SU) and normal users (NU). The problem is formulated as joint power and subcarrier allocation with the objective of maximizing average aggregate information rate of all NUs while maintaining an average secrecy rate for each individual SU under a total transmit power constraint for the base station. We solve this problem in an asymptotically optimal manner using dual decomposition. Our analysis shows that an SU becomes a candidate competing for a subcarrier only if its channel gain on this subcarrier is the largest among all and exceeds the second largest by a certain threshold. Furthermore, while the power allocation for NUs follows the conventional water-filling principle, the power allocation for SUs depends on both its own channel gain and the largest channel gain among others. We also design a suboptimal algorithm to reduce the computational cost. Numerical studies are conducted to evaluate the performance of the proposed algorithms in terms of the achievable pair of information rate for NUs and secrecy rate for SU at different power consumptions.

Degrees of freedom of MIMO two-way X relay channel

In this paper, we study the degrees of freedom of a multiple-input multiple-output (MIMO) two-way X relay channel, i.e., a system with two groups of source nodes and one relay node, where each of the two source nodes in one group wants to exchange independent messages with both the two source nodes in the other group via the relay node. We only consider the symmetric case where each source node is equipped with M antennas while the relay is equipped with N antennas. We first show that the upper bound of the degrees of freedom is 2N when N ≤ 2M. Then by applying physical layer network coding and joint interference cancellation, we propose a novel transmission scheme for the considered network. We show that this scheme can always achieve this upper bound when N ≤ ⌊4M over 3⌋.

Hash function mapping design utilizing probability distribution for pre-image resistance

Hash functions are often used to protect the integrity of information. In general, the design of hash functions should satisfy three standards: pre-image resistance, second pre-image resistance and collision resistance. The design of hash functions in the literature assumes that the messages to be transmitted are equally probable. In this paper, we focus on the pre-image resistance and investigate the problem of mapping design for hash function utilizing the unequal occurrence probabilities of the messages. We first present a necessary condition for the optimal mapping and then introduce a heuristic algorithm. Simulation experiments are carried out to evaluate the performance of the proposed new design. It is shown that the probability of successful attack can be significantly reduced compared with the conventional design. Our algorithm can be useful in scenarios where the attacker has limited ability or time to estimate the probability distribution of the messages. To our best knowledge, this work is the first attempt of making use of the message distribution in designing hash functions for information security.