Channel Information Feedback Technology In Physical Layer Secure Transmission

Wireless physical layer secure transmission techniques,which provide new ways to solve the problem how to guarantee secure communication,utilize physical layer characteristics to design effective secure communication schemes.Such techniques are worth studying and have broad market prospects.Artificial noise assisted beam-forming,an important approach in physical layer secure transmission techniques,has received wide attention,because it doesn't need the prior knowledge of eavesdropper channel.Performance of artificial noise assisted beam-forming depends on the precision of the acquired channel state information(CSI)of the legitimate channel on the transmitter side.One of practical methods to acquire CSI is limited feedback.To take physical layer security into practical utilization,establishing a set of simple and efficient feedback schemes is a critical step.In this paper,we focus on feedback techniques in artificial noise assisted beam-forming scheme.Our work mainly includes three aspects of analyzing the effect of limited feedback on the security performance of artificial noise,designing a set of effective limited feedback protocols,and establishing an artificial noise assisted transmission system based on limited feedback.In this paper,we firstly summarize the research progress of feedback techniques,introduce basic principles of limited feedback,and analyze how imperfect feedback deteriorates the security performance of artificial noise assisted transmission strategy.Secondly,based on limited feedback with random vector quantization(RVQ)codebook,the problem how to select feedback bits number in artificial noise assisted beam-forming is studied.An upper bound of ergodic secrecy capacity loss of multiple-input-single-output(MISO)system adopting artificial noise assisted beam-forming is derived utilizing statistic features of quantization error of RVQ codebook.According to the derived upper bound,we calculate the minimum feedback bits number satisfying a given ergodic secrecy capacity loss.Also,we propose a method to adjust feedback bits number,whose accuracy is verified through simulation.To maintain a constant ergodic secrecy capacity loss,feedback bits number should be scaled linearly with the logarithmic transmitted signal-to-noise ratio(SNR)as well as(N_t-1)(N_t is the transmit antenna number).Unlike the current researches ignoring the low SNR case,our conclusion is suitable for all the SNR cases.Our work rectifies and completes the current researches.Thirdly,the effect of imperfect channel estimation and codebook feedback on the security of artificial noise assisted beam-forming are studied.A closed-form expression of lower bound of system ergodic secrecy capacity,whose accuracy is demonstrated through simulation,is derived considering the combined effect of estimation and codebook quantization errors.Furthermore,based on the derived lower bound,a suboptimal solution of power allocation ratio is calculated.Simulation results illustrate that system could achieve relatively good security performance with calculated sub-optimal solution.With SNR increasing,the optimal power allocation ratio decreases at first and then gradually increases.What's more,optimal power allocation ratio gradually approaches to that in perfect feedback case with the increasing of feedback bits number.Last but not least,an artificial noise assisted secure transmission system is designed according to the required performance indexes.With fully understanding of how limited feedback protocols affect the system security performance,we design codebook,feedback bits number,and pilot sequence length reasonably.Also,we design synchronization algorithms considering the features of burst communication mode and establish a mathematical simulation platform.Simulation results illustrate that our designed artificial noise assisted transmission scheme with limited feedback deteriorates eavesdropper's receiving performance severely,while it ensures relatively better receiving performance at the legitimate receiver.The designed system could provide communication security to some degree.