Lun Dong

Improving Wireless Physical Layer Security via Cooperating Relays

Physical (PHY) layer security approaches for wireless communications can prevent eavesdropping without upper layer data encryption. However, they are hampered by wireless channel conditions: absent feedback, they are typically feasible only when the source-destination channel is better than the source-eavesdropper channel. Node cooperation is a means to overcome this challenge and improve the performance of secure wireless communications. This paper addresses secure communications of one source-destination pair with the help of multiple cooperating relays in the presence of one or more eavesdroppers. Three cooperative schemes are considered: decode-and-forward (DF), amplify-and-forward (AF), and cooperative jamming (CJ). For these schemes, the relays transmit a weighted version of a reencoded noise-free message signal (for DF), a received noisy source signal (for AF), or a common jamming signal (for CJ). Novel system designs are proposed, consisting of the determination of relay weights and the allocation of transmit power, that maximize the achievable secrecy rate subject to a transmit power constraint, or, minimize the transmit power subject to a secrecy rate constraint. For DF in the presence of one eavesdropper, closed-form optimal solutions are derived for the relay weights. For other problems, since the optimal relay weights are difficult to obtain, several criteria are considered leading to suboptimal but simple solutions, i.e., the complete nulling of the message signals at all eavesdroppers (for DF and AF), or the complete nulling of jamming signal at the destination (for CJ). Based on the designed relay weights, for DF in the presence of multiple eavesdroppers, and for CJ in the presence of one eavesdropper, the optimal power allocation is obtained in closed-form; in all other cases the optimal power allocation is obtained via iterative algorithms. Numerical evaluation of the obtained secrecy rate and transmit power results show that the proposed design can significantly improve the performance of secure wireless communications.

Secure wireless communications via cooperation

The feasibility of physical-layer-based security approaches for wireless communications in the presence of one or more eavesdroppers is hampered by channel conditions. In this paper, cooperation is investigated as an approach to overcome this problem and improve the performance of secure communications. In particular, a decode-and-forward (DF) based cooperative protocol is considered, and the objective is to design the system for secrecy capacity maximization or transmit power minimization. System design for the DF-based cooperative protocol is first studied by assuming the availability of global channel state information (CSI). For the case of one eavesdropper, an iterative scheme is proposed to obtain the optimal solution for the problem of transmit power minimization. For the case of multiple eavesdroppers, the problem of secrecy capacity maximization or transmit power minimization is in general intractable. Suboptimal system design is proposed by adding an additional constraint, i.e., the complete nulling of signals at all eavesdroppers, which yields simple closed-form solutions for the aforementioned two problems. Then, the impact of imperfect CSI of eavesdroppers on system design is studied, in which the ergodic secrecy capacity is of interest.

Cooperative jamming for wireless physical layer security

Cooperative jamming is an approach that has been recently proposed for improving physical layer based security for wireless networks in the presence of an eavesdropper. While the source transmits its message to its destination, a relay node transmits a jamming signal to create interference at the eavesdropper. In this paper, a scenario in which the relay is equipped with multiple antennas is considered. A novel system design is proposed for determining the antenna weights and transmit power of source and relay, so that the system secrecy rate is maximized subject to a total transmit power constraint, or, the transmit power is minimized subject to a secrecy rate constraint. Since the optimal solutions to these problems are difficult to obtain, suboptimal closed-form solutions are proposed that introduce an additional constraint, i.e., the complete nulling of jamming signal at the destination.

Amplify-and-forward based cooperation for secure wireless communications

A physical layer approach to security for wireless networks is considered. In single-antenna wireless systems, such approaches are hampered by channel conditions in the presence of one or more eavesdroppers. Cooperation has the potential to overcome this problem and improve the security of of wireless communications. In this paper, an amplify-and-forward based cooperative protocol is proposed. Assuming availability of global channel state information, system design that maximizes the secrecy capacity is considered. Since the optimal solution to this problem is intractable, suboptimal closed-form solutions are proposed that optimize bounds on secrecy capacity for the case of a single eavesdropper, or that introduce additional constraints, such as nulling of signals at all eavesdroppers, for the case of multiple eavesdroppers.

A Cross-Layer Approach to Collaborative Beamforming for Wireless Ad Hoc Networks

Via collaborative beamforming, nodes in a wireless network are able to transmit a common message over long distances in an energy efficient fashion. However, the process of making available the same message to all collaborating nodes introduces delays. In this paper, a medium access control-physical (MAC-PHY) cross-layer scheme is proposed that enables collaborative beamforming at significantly reduced collaboration overhead. It consists of two phases. In the first phase, nodes transmit locally in a random access time-slotted fashion. Simultaneous transmissions from multiple source nodes are viewed as linear mixtures of all transmitted packets. In the second phase, a set of collaborating nodes, acting as a distributed antenna system, beamform the received analog waveform to one or more faraway destinations. This step requires multiplication of the received analog waveform by a complex weight, which is independently computed by each collaborating node, and which allows packets bound to the same destination to add coherently at the destination node. Assuming that each node has access to location information, the proposed scheme can achieve high throughput, which in certain cases exceeds one. Analyses of the average beampattern, networking performance, and symbol error probability corresponding to the proposed scheme are provided.

Weighted Cross-Layer Cooperative Beamforming for Wireless Networks

In wireless networks, implementing cooperative beamforming (CB) can enable long-range communications in an energy efficient manner. By appropriately weighting and forwarding message signals, the cooperating nodes form one or more beams to cooperatively transmit one or more message signals to the desired destinations. In this paper, a cross-layer CB framework recently proposed by the authors is revisited and optimal weight design is considered. For single-beam beamforming, closed-form optimal weights that maximize the received signal-to-noise ratio (SNR) or signal-to-interference-plus-noise ratio (SINR) under a transmit power constraint are derived. It is shown that these weights also achieve maximal spectral efficiency. For multibeam beamforming, determining the weights that maximize spectral efficiency is in general a difficult problem, and two suboptimal weight designs (co-phasing weights and nulling weights) are proposed. Co-phasing weights allow desired signals to combine coherently at destinations and require local channel state information (CSI); nulling weights completely cancel interference at destinations and require global CSI.

Cooperative Beamforming for Wireless Ad Hoc Networks

Via collaborative beamforming, nodes in a wireless network are able to transmit a common message over long distances in an energy efficient fashion. However, the process of making available the same message to all collaborating nodes introduces delays. In this paper, a MAC-PHY cross-layer scheme is proposed that enables collaborative beamforming at significantly reduced collaboration overhead. It consists of two phases. In the first phase, nodes transmit locally in a random access time-slotted fashion. Simultaneous transmissions from multiple source nodes are viewed as linear mixtures of all transmitted packets. In the second phase, a set of collaborating nodes, acting as a distributed antenna system, beamform the received analog waveform to one or more faraway destinations. This step requires multiplication of the received analog waveform by a complex weight, which is independently computed by each cooperating node, and which allows packets bound to the same destination to add coherently at the destination node. Assuming that each node has access to location information, the proposed scheme can achieve high throughput, which in certain cases exceeds one. An analysis of the symbol error probability corresponding to the proposed scheme is provided.

Multichannel ALLIANCES: A Cooperative Cross-Layer Scheme for Wireless Networks

A random access protocol named ALLow improved access in the network via cooperation and energy savings (ALLIANCES) was recently proposed, that achieves high throughput by resolving collisions in wireless networks. ALLIANCES exploits diversity provided by user cooperation. In this paper we propose a multichannel extension of ALLIANCES that in addition to cooperation diversity can exploit multipath diversity. The total bandwidth is divided into noninterfering subchannels and each packet occupies one subchannel for its transmission. First, two schemes are proposed for rate-limited traffic. Users transmit packets on all subchannels. Collisions on a subchannel are resolved via cooperative transmissions, involving either the subchannel on which they occurred only, or all subchannels in a shared fashion. Second, for the case of bursty traffic, a random subchannel selection scheme is proposed to adaptively control the number of transmitted packets for each active user and, thus, keep collision orders small. Third, to accommodate heterogeneous traffic with diverse quality of service requirements, a fixed subchannel selection scheme is presented, where packets with the same traffic type are allocated to the same cluster of subchannels and predefined traffic priorities are taken into account. Analytical performance characterization of the proposed schemes at the physical layer is provided. The analysis provides insight on the relationship between achievable diversity and parameters such as collision order, number of relays, channel length, and number of carriers per subchannel.

A High-Throughput Cross-Layer Scheme for Distributed Wireless Ad Hoc Networks

In wireless ad hoc networks, distributed nodes can collaboratively form an antenna array for long-distance communications to achieve high energy efficiency. In recent work, Ochiai, et al., have shown that such collaborative beamforming can achieve a statistically nice beampattern with a narrow main lobe and low side lobes. However, the process of collaboration introduces significant delay, since all collaborating nodes need access to the same information. In this paper, a technique that significantly reduces the collaboration overhead is proposed. It consists of two phases. In the first phase, nodes transmit locally in a random access fashion. Collisions, when they occur, are viewed as linear mixtures of the collided packets. In the second phase, a set of cooperating nodes acts as a distributed antenna system and beamform the received analog waveform to one or more faraway destinations. This step requires multiplication of the received analog waveform by a complex number, which is independently computed by each cooperating node, and which enables separation of the collided packets based on their final destination. The scheme requires that each node has global knowledge of the network coordinates. The proposed scheme can achieve high throughput, which in certain cases exceeds one.

A comparison of cooperative beamforming to direct transmission based on spectral efficiency

We recently proposed a cross-layer cooperative beamforming approach for transmitting signals of competing users to a far away destination in an energy efficient fashion. This was a two-stage scheme that relied on cooperating nodes to appropriately weight and forward the received signal, so that the signal of a particular node adds coherently at its destination only. In this paper we propose a variant of our previous approach that achieves maximum spectral efficiency. We compare the performance of the proposed approach to existing ones, in order to determine the best transmission scheme given certain network parameters, such as number of sources, number of cooperating nodes, power budgets and path loss.