Research On Physical Layer Security And Optimization For Backscatter Communications

With the rapid development of the information and communication technology,the Internet of Things(IoT)technology has been widely applied to various application scenarios in our daily life,to realize the interconnection of information between people and things.But with the massive deployment of resource-limited wireless devices,the energy supply and low-power communication technology are key important issues in IoT systems.Backscatter communication(BC),which enables energy harvesting and ultra-low-power communication with the same radio-frequency(RF)signal,has received increasing attention in academia and industry as a cutting-edge technology to realize various IoT applications.However,it also faces severe security threats and vulnerabilities due to such limitations and shortcomings as follows.First,the instability,openness and broadcast nature of wireless channel make the BC more vulnerable to illegal users with passive or active attack methods,which in turn threatens the whole BC system.Second,due to limited energy and computation capability,BDs could not be able to exploit complex cryptographic algorithms to secure communication and authentication in BC systems.And with the large-scale deployment of backscatter devices(BDs),it is more difficult to apply existing cryptography-based security mechanisms to distribute and manage secret keys and identity information.Therefore,lightweight and efficient security solutions are urgently needed to achieve enough security for backscatter communication and its systems in various future plication scenarios.Physical layer security technology aims to provide lightweight and high security solutions for communication and authentication in wireless communication by exploiting uniqueness,randomness and reciprocity of wireless channels.It can reduce computation and energy consumption for resource-limited devices by avoiding the usage of complex cryptographic computation,and is one of the key technologies of future wireless network security.Although much progress has been made in research for physical layer security in traditional wireless communications and its application scenarios,it still lacks broad and comprehensive study in backscatter communications.Compared with traditional wireless communication technologies,BDs are unable to actively generate wireless RF signals and need to harvest energy from the received RF signals to support their operation.Due to these key features and limitations,it is difficult to utilize existing physical layer security mechanisms to solve the security issues of backscatter communications.Based on security issues and requirements in backscatter communications,we aim to develop some new physical layer security schemes in this dissertation from three aspects:BDs authentication,secure communication and key generation.The main contributions and innovations are summarized as follows:(1)To address the problems of identity spoofing,signal forgery and replay attacks in BC systems,we propose a multi-stage BD authentication and attack tracing scheme based on the physical spatial information of BDs to realize enhanced BD authentication security in both static and mobile BD cases.In the prior stage,we propose a preemptive two-factor authentication at an access point(AP)based on BD identity and its positing information,which is estimated from the received signal strength(RSS)and the angle of arrival(AoA)of BD backscattered signals.In the following stage,re-authentication is performed based on spatial correlation of signal source locations associated with BD in order to accurately detect potential attacks.Combined with these two authentication stages,it can achieve high security of BD authentication with high detection of identity-based attacks,signal forgery and replay attacks.Besides,we further propose a reciprocal channel-based method for BD re-authentication with better authentication performance in the mobile BD case when BDs can measure the RSS of signals.In case authentication failure due to attacks,the proposed scheme is capable of determining the number of attackers and localizing their positions by exploiting clustering-based analysis.Finally,we theoretically analyze the security of the proposed scheme and conduct numerical simulation with various settings to show its desirable security performance.(2)To secure the backscatter transmission between BDs and AP,we propose to inject artificial noise(AN)into multi-sine carrier signals to improve the security capacity of the backscatter channel.Since the signal waveform with injected AN will reduce the harvested energy of BDs,we characterize the achievable secrecy rate-energy region in the single-BD case and propose two waveform optimization algorithms to improve achievable secrecy rate and harvested energy with the limited transmit signal power.For a multi-BD system,we formulate an optimization problem to maximize the harvested energy and secrecy rate of all BDs by jointly considering the backscatter time,power splitting ratio and signal power allocation.Then,we propose an iterative algorithm to solve the non-convex problem of resource allocation and show its convergence and complexity.Finally,we conduct numerical simulations to show the performance in single-BD and multi-BD systems with our algorithms,and a proof-of-concept experiment by developing a BC platform to evaluate the secrecy rate energy.(3)For the security of backscatter communication between BDs,we propose a novel physical layer key generation scheme based on shared channel information extracted from the received superposed RF signals to support the key agreement between two BDs.By modeling and analyzing the BC system involving the RF source and two BDs,we construct a shared triangle channel model among an RF source and two BDs as shared randomness.With repeated patterns in RF signals,we propose a joint transceiver design of BD backscatter waveform and BD receiver to separate the downlink channel information and cascade channel information.By multiplying the downlink channel and cascade channel,we can obtain the triangle channel information as shared random secret resource for session key generation.Besides,we study the trade-off between the rate of secret key generation and harvested energy by modeling it as a joint optimization problem of power allocation,while BDs construct shared channel information and harvest energy from the same received signals.Finally,we conduct numerical simulations to evaluate the key generation and energy harvesting performance,and their trade-off under various system settings.