Research On Secrecy Performance In Non-orthogonal Multiple Access Wiretap Channels

Information security is an unavoidable problem in the development of mobile Internet and Internet-of-things(IoT)technologies.The existing security technologies are based on cryptographic encryption algorithms,which assume the low computational ability of the eavesdroppers.However,with the development of new technologies,especially the emergence of quantum computing,the traditional encryption technology is becoming more and more unreliable.In addition,the disadvantages of traditional encryption technology are becoming more and more prominent in the face of new communication needs.It can not meet the secrecy,real-time and low power of the data transmission in the fifth generation mobile communication(5G)system.The physical layer security technology makes use of the inherent characteristics of the wireless transmission media to turn the uncertainty in the wireless channel into a "treasure",which makes the secrecy problem solved at the physical-layer level,and does not conflict with the traditional security means in the upper layer.It can not only be used as an independent security means,but also can be a supplement to the existing encryption technology.Furthermore,as one of the most promising 5G key technologies,non-orthogonal multiple access(NOMA)technology has the characteristics of high spectral effeciency,large device access,low delay and signaling overhead.The combination of physical-layer security and NOMA technology is a hot research topic.However,in the existing literature,the major of researchers only considered the secrecy problem from the downlink and neglected the secrecy of the uplink transmission.Some researchers studied the other form of secrecy NOMA model,multiple access wiretap channels,by considering the secrecy problem of uplink transmission.However,they only characterized the secrecy performance of the system,by studying the secrecy rate region,or the secure degrees of freedoms,or achievable coding schemes.Different from previous research,this paper focuses on the uplink NOMA wiretap channels.By establishing a variety of wiretap models,we fully study the individual secrecy performance in each model.The main contents include:1.Single-input single-output NOMA wiretap channels:In this mode,each node is assumed to be equipped with a single antenna.To avoid successive interference cancelation(SIC)in the eavesdropper,the transmitters are assumed to be homogeneous,that is,the average received power at the legitimate receiver or eavesdropper is equal.Such an assumption seems too ideal but does not harm our analysis.Actually,this hypothesis exists in satellite communication.The main contents of this chapter are summarized as:1)model the system,deduce the instantaneous secrecy capacity with SIC,and drive the statistics of the signal-to-noise ratios(SNRs)at both the legitimate receiver and the eavesdropper;2)derive the closed-form expression of secrecy performance in terms of positive secrecy capacity probability,secrecy outage probability,and effective secrecy throughput(EST).According to the expressions,the impacts of secrecy performance can be understood intuitively;and 3)propose three SIC order scheduling schemes,namely round-robin,channel-gain based,and secrecy-capacity based.With the help of simulation and mathematical analyses,three schemes are compared,and their advantages and disadvantages as well as applicable scenarios are pointed out.2.Single-input multiple-output NOMA wiretap channels:Extending the number of antennas at the legitimate receiver not only means that the secrecy performance has the spatial diversity gain,but also needs to consider what decoding method is used.To be specific,two common linear decoding methods are considered:zero forcing(ZF)and minimum mean square error(MMSE).In addition,we also study the case of the eavesdropper with multiple antennas.The main contents are:1)derive the closed-form expressions of the secrecy performance based on ZF-SIC and MMSE-SIC,respectively,making comparison of the two decoding methods;2)study the asymptotic behaviors of the secrecy performance,revealing the secrecy performance is only determined by the relative distance in the high SNR regime,and propose a novel SIC scheduling scheme,which is based on relative distance;and 3)study the problem of optimal power allocation under the constraint of the limited total power,and give an interesting solution to this problem.3.NOMA wiretap channels with multiple eavesdroppers:By expanding the single eavesdropper to the case of multiple ones,it greatly increases the complexity of the model.In order to simplify the model,the model is divided into two scenarios,non-colluding eavesdroppers and colluding eavesdroppers.The secrecy performance of these two scenarios is studied and analyzed with the same assumptions and decoding methods as the previous model.Specifically,1)derive the statistics of the SNR at the eavesdroppers for the non-colluding and colluding scenarios,and deduce the closed-form expressions of the secrecy performance and its asymptotic behaviors under the two scenarios;2)analyze the relationship of secrecy performance between non-colluding and colluding as well as single-eavesdropper scenarios;and 3)characterize the optimal power allocation problem under collusion,and propose another novel SIC scheduling scheme based on the maximum EST,which is conjectured to be optimal in achieving total maximum EST in a high SNR regime.