Research On Cooperative Jamming Techniques For Physical Layer Security

Confidential data traffic(e.g.,financial data,medical data,bank account,etc.)grows rapidly in recent years,fuelled by the popularity of wireless devices and the everincreasing demands on sensitive data-centric applications.However,wireless communication systems are particularly susceptible to security attacks(e.g.,malicious interception and eavesdropping)due to the fundamental characteristics of wireless medium,namely broadcast nature and superposition nature.It poses great challenges in ensuring reliable and secure transmission in the presence of eavesdroppers.Thus,guarantying the confidentiality is one of the top issues in future network designs.Currently,wireless communication secrecy is mostly addressed by utilizing cryptographic encryption and decryption technologies in the upper layers of the network protocol stack,such as advanced encryption standard(AES)and RSA(Rivest-Shamir-Adleman)encryption technique.However,these technologies have inherent difficulties and vulnerabilities in secret key distribution and high computational complexity.Hence,there is a strong need to develop low-complexity secure schemes in wireless systems with a large number of energy constrained wireless devices.Motivated by results in information theory,physical layer security techniques have been explored to enable exchange of confidential messages over wireless channels,in the presence of eavesdroppers,without relying on encryption at the higher layer.The fundamental of physical layer security is to exploit the physical properties of wireless channels(e.g.,fading,noise,and interference)to limit the amount of confidential information that can be extracted by eavesdroppers.Specifically,if the channel between the source and the destination(defined as the main channel)is more reliable than that between the source and the eavesdropper(defined as the eavesdropper channel),a secret message can be encoded in a way that allows it to be reliably decoded at the destination while causing substantial confusion at the eavesdropper.More importantly,the secrecy performance increases with the channel quality difference between these two channels.Multi-antenna technique is considered as an efficient and reliable way for enhancing the main channel quality and improving secrecy,due to the advantage of having spatial degrees of freedom(DoFs)and diversity gains.For example,in the wireless environment,channel gains vary over space dimensions.Hence,beamforming and precoding techniques can be utilized for secure transmission in dimensions where the main channel is reliable than the eavesdropper channel,and thus enhancing secrecy performance.For physical layer security,secrecy rate and secrecy outage probability(SOP)are often adopted as performance metrics.In addition to using beamforming and precoding techniques,it is often beneficial to emit artificial noise(AN)to deteriorate the information reception at eavesdroppers.Although transmission of AN effectively disrupts the eavesdropper channel,it decreases transmit power to be used for confidential signal,and indirectly reduces the signal-to-noise ratio(SNR)at the destination.Therefore,it is important to optimize the power allocation between the confidential signal and the AN signal for secrecy performance enhancement.Based on multi-antenna techniques,cooperative transmission can be utilized to further enhance secrecy.Cooperative security schemes mainly include cooperative forwarding and cooperative jamming(CJ).In cooperative forwarding scheme,the cooperative nodes can help forward the confidential signal to enhance the signal quality at the destination.However,it should be noted that two phases are needed for secure transmission from the source to the destination.In the first phase(i.e.,broadcasting phase),the cooperative nodes receive the confidential signal transmitted by the source.In the second phase,the cooperative nodes forward the confidential signal to the destination.It is important to maintain confidentiality against the eavesdropper during these two phases.Recent studies mainly concern the security protection in the second phase.However,there are few works on transmit designs for secure communication in the first phase.In particular,unreliable reception at the cooperative nodes during the first phase may pose a severe security risk.Therefore,we consider the problem of enhancing physical layer security by CJ scheme.In the CJ scheme,the cooperative nodes only emit jamming signals to interfere with eavesdroppers,without forwarding the confidential signal to the destination,and the source transmits confidential signal plus AN signal.In this case,the power allocation between the confidential signal and the AN signal is significant to enhance secrecy performance.Although the power allocation problem for multi-antenna secure transmission has been extensively studied,there are few works on optimization of power allocation for CJ scenario.Moreover,exisiting results cannot be easily generalized to CJ scheme.In this background,we systematically study CJ techniques for physical layer security,including CJ scheme for SOP minimization,and secrecy rate maximization.Specifically,we propose an adaptive secure transmission for the case that the cooperative node needs to provide service to its own user,taking both security performance and Quality of Service(QoS)guarantee into account.The main contributions can be summarized as follows:(1)In cooperative wireless networks with multiple independent eavesdroppers,we study a CJ scheme for SOP minimization.Firstly,we present a closed form expression for SOP.Then,the power allocation between the confidential signal and the AN signal is optimized for SOP minimization,subject to a secrecy rate constraint.Through strict mathematical proofs,we characterize the impacts of the number of eavesdroppers,transmit power,secrecy rate,and main channel quality on the transmit design and secrecy performance.When the number of eavesdroppers grows,the SOP increases.When the transmit power of the source increases,more power should be allocated to the AN signal,and the corresponding SOP decreases.When the secrecy rate increases,more power should be allocated to the confidential signal,and the corresponding SOP increases.When the main channel quality is improved,more power should be allocated to the AN signal,and the corresponding SOP decreases.(2)For cooperative wireless networks with multiple independent eavesdroppers,CJ scheme is studied.We provide an optimal power allocation between the confidential signal and the AN signal for maximizing the secrecy rate,subject to a SOP constraint.Specifically,we present interesting insights on the impact of the SOP constraint,and establish a condition under which positive secrecy rate can be achieved,and provide an accurate approximation for the optimal power allocation at high SNR regime.Unlike recent works on cooperative security that only concern about secrecy rate performance,we pay particular attention to secure energy efficiency(EE),defined as the ratio of secrecy rate to total power consumption.Through strict mathematical proofs,we characterize the impacts of the number of eavesdroppers and the main channel quality on the transmit design and secerecy performance.When the number of eavesdroppers grows,more power should be allocated to the AN signal,and the corresponding secrecy rate decreases.When the main channel quality is improved,more power should be allocated to the confidential signal,and the corresponding secrecy rate increases.(3)We study an adaptive secure transmission for physical layer security in cooperative wireless networks.Specifically,cooperative transmission scheme is explored to secure communication between the source and the destination,while ensuring that the cooperative node provides the desired QoS to its own user.In particular,we provide transmission scheme for secrecy rate maximization,subject to both security and QoS constraints.Unlike the conventional cooperative security with a fixed transmission scheme,we propose a mechanism for transmit strategy adaptation with security protection.Specifically,the cooperative transmission is replaced by the CJ scheme if either security or QoS constraint is not satisfied.Our design enables adaptive secure transmission,and thus is flexible and environment-adaptive.