A Research On Physical Layer Security

In recent years,the applications of the Internet,especially the mobile Internet,have been booming and they are now parts of our daily lives.Under such circumstances,security of data transmission,as an eternal topic of communication,is ever more of paramount importance.However,ever since the Open Systems Interconnection Reference Model(OSI/RM)was defined in the 1980 s,there have been well-defined and well-applied security techniques for almost every layer except the physical layer.Eventually the physical layer would become a major security breach if no measures were taken.Being different from techniques of cryptography on the OSI/RM upper layers,the physical layer security techniques aim to protect the transmitted signal itself instead of the information it carries.To achieve this,most techniques use channel state information(CSI);others try to alter the ways the transmitted signals are generated in order to fix signals to a certain direction thus blocking the eavesdroppers,or to suppress certain features that are subject to blind detections.This thesis summerizes the background of and the current developments in the field of physical layer security,and briefly introduces the basics of Information Theory and Cyclostationarity Theory.Based upon these basics,this thesis proposed two novel algorithms for signal cyclostationary feature suppression.The first one is a constellationbased encryption and feature-suppression technique.It changes the phase and magnitude of the constellation randomly so as to alter the signal's higher-order cyclostationarity,thus suppressing its cyclostationary features.Improvement on communication performance of this algorithm is also provided.The second algorithm mainly deals with cyclic-prefixed Orthogonal Frequency Division Multiplexing(OFDM).It adds random number of redundant subcarriers to the system,causing the carrier seperation and the useful symbol length to change randomly.In such a manner,both the time domain feature,caused by employing cyclic prefix,and the inherent frequency domain feature are suppressed.Through analysis and MATLABR?simulation it's shown that while achieving the featuresuppression goals,such an algorithm scarcely affects the communication performance.