Physical-layer Secret Communication In Multi-user Wireless Networks

Security is a basic requirement in wireless networks and also one of the key issuesfor wireless communications of next generation. Physical-layer security can projectconfdential message from being decoded by any eavesdropper and merge with tradi-tional security technologies. Physical-layer security can also be combined with otheradvanced wireless techniques to enhance secrecy capacity in various network models.Thus, providing physical-layer security for mobile users in future wireless networks isof both theoretical and practical importance. In this thesis, we introduce physical-layersecurity to multi-user networks as the security system. We discuss the feasibility ofproposed scheme by analyzing secrecy capacity or secrecy outage probability. More-over, we take into account security when assigning network radio resources in orderserve multiple users and enhance secrecy capacity. We provide theoretical basis andpractical solutions for the application of physical-layer security. In summary, the maincontributions and results of this thesis are as follows.1. We formulate an analytical framework for resource allocation in a downlinkOFDMA-based broadband network with coexistence of secure users (SU) and nor-mal users (NU). The problem is formulated as joint power and subcarrier allocationwith the objective of maximizing average aggregate information rate of all NU’s whilemaintaining an average secrecy rate for each individual SU under a total transmit pow-er constraint for the base station. Both long-term average power constraint and peakpower constraint are considered. We solve this problem in an asymptotically optimalmanner using dual decomposition. We also develop a suboptimal algorithm to reducethe computational cost. Numerical results show that the optimal policy achieves highersum rate for NUs while satisfying the basic secrecy rate requirements of SUs. More- over, the optimal policy consumes less power and subcarrier.2. In most existing works and previous part of this thesis, the eavesdropper in-volved is a passive model who just listens to the secrecy messages without other activebehaviororattack. However, anactiveeavesdropperisofmoresignifcanceinpracticalnetworks. Therefore, we consider secure downlink transmission of a cellular networkand introduce a novel false CSI attack model by a malicious user. The malicious userdeliberately reports a false CSI to the BS in order to obtain chance of eavesdrop. Weexamine the cases when secrecy leakage occurs to the system. To precisely evaluatethe detriment to the system security caused by the false CSI feedback, as well as thecost the malicious user has to pay, we propose three metrics, namely system secrecyleakage probability, system secrecy leakage and the malicious user’s own secrecy rateloss. To combat the malicious user’s false CSI feedback, a CSI compensation schemeat the BS is proposed and the three metrics are re-analyzed to evaluate the efcien-cy of the compensation. All the metrics are increasing with total transmit power andthe deducted value of the malicious user’s channel feedback and decreasing with usernumber and compensation value.3. Multicast is a signifcant type of service in multi-user network. In secrecymulticast service, users share the same channel and data. Both security and service re-quirements are needed in multicast networks. Traditional direct multicast sufers fromlow efciency and small coverage. Thus, we propose a cooperative secrecy multicastwhich allows part of users to work as relays and forward the multicast data to otherusers. The mathematical model for secrecy outage probability of cooperative multicastcommunication protected by physical-layer security technology is established. The an-alytical result of the secure outage probability of this cooperative scheme is derived.As a comparison, relay selection is investigated and the secure outage probability isanalyzed. Numerical results show that the cooperative multicast scheme outperformsdirect multicast in terms of outage. Especially when the number of users in the net-work is growing, the secure outage probability of the cooperative scheme evidentlydecreases.4. The assumption of perfect secrecy in multicast network is the correct channelfeedback from all the registered users. If multicast network sufers from false CSI at- tack, theconfdentialmessagesmightbeleaked. Hence, wefocusonthenegativeefectcaused by false CSI attack. Diferent from unicast, multicast serves a group of users si-multaneouslyandtheBSsetstransmissiontargetrateaccordingtotheequivalentgroupchannel. We study the successful scenarios of the attack as well as eavesdrop probabil-ity and secrecy loss. We further analyze the false CSI attack in cooperative multicastand the attacker’s strategies. Numerical results show that both eavesdrop probabilityand secrecy loss increase with the growing of channel deviation and decrease with therise of the user number.