【作者】 蒋开伟；

【导师】 荆涛；

【作者基本信息】 北京交通大学 ， 通信与信息系统， 2018， 博士

【摘要】 信息安全是当前移动互联网和物联网技术发展过程中不可回避的问题,现有的安全技术基于密码学的加密算法,寄托于窃听者的计算能力有限。然而随着新技术的发展,尤其是量子计算的出现,让传统加密技术越来越不可靠;此外,面对新的通信业务需求,传统加密技术的弊端也越发凸显,不能满足第五代移动通信(5G)系统对于数据传输的安全性、实时性以及高效性的要求。而物理层安全技术利用无线传输媒介内在特性,将无线信道中的不确定性“变废为宝”,使得安全性问题在最底层得以解决,并且与传统的基于上层的安全手段并不冲突,既可作为独立的安全手段,也可以是现有加密技术的补充。另一方面,非正交多址技术作为比较有前途的5G关键技术之一,具有频谱利用率高、设备接入量大、时延以及信令开销低等特点。将物理层安全和非正交多址技术结合起来是当前研究者趋之若鹜的热点。然而,在现有的非正交多址安全研究中,大多数研究者仅从下行考虑安全问题,忽略了上行传输的安全性是同样重要的;尽管部分研究者在另一种形态的非正交多址安全模型——多址窃听信道——的研究中考虑了上行传输的安全问题,但通常仅从整体上考虑系统安全性,研究的内容包括:安全速率域,安全自由度以及编码方案用于取得安全速率域。有别于前人研究,本篇论文将针对上行非正交多址窃听信道,通过建立多种模型,由简单到复杂,深入浅出,充分研究多用户中个体的安全性能问题。主要内容包括:1.研究分析单输入单输出非正交多址窃听信道的安全性能。以窃听信道中各个节点均是单天线的模型为切入点,并考虑排除窃听者具有连续干扰消除解调的能力,将窃听模型中的发送者假设成齐次的,即接收端收到的信号平均功率是相等的,此种假设看似过于理想,但不影响安全性能的研究分析结果,且存在于卫星通信等场景中,具体而言:1)在发送者齐次性的窃听信道模型下,给出瞬时安全容量表达式,推导得出合法接收端和窃听端的信干噪比的统计特性,确立安全性能分析目标;2)推导出以正安全容量概率、安全中断概率和有效安全吞吐量为指标的安全性能闭式表达式,据此能直观了解影响安全性能的因素;3)提出三种连续干扰消除调度策略,即轮询、基于信道增益、基于安全容量,通过数学证明加仿真分析比较三种策略,指出其中的优劣和适用场景。2.研究分析单输入多输出非正交多址窃听信道的安全性能。扩展合法接收端的天线数,不仅仅意味着安全性能关于空间分集有增益,还需要考虑使用何种解调方式,在此考虑两种常见的线性解调方式:迫零和最小均方误差,此外,也考虑了窃听者是多天线的情况,具体而言:1)推导出基于迫零和基于最小均方误差并结合连续干扰消除的解调方式的安全性能闭式表达式,且比较两种解调方式的优劣;2)研究分析安全性能的渐进特性,给出了高信噪比下,安全性能和相对距离之间的关系。受此启发,提出基于相对距离的最优连续干扰消除调度策略;3)研究在总功率受限的条件下,如何分配功率使得总安全性能最大化,并给出此类最优解的图解法。3.研究分析多窃听者非正交多址窃听信道的安全性能。将单窃听者扩展到多窃听者的情形,大大增加模型的复杂度,为了简化,将模型分成窃听者不串通和窃听者串通两种场景。沿用前述模型中的一些假设和解调方式,分别对两种场景的安全性能进行研究分析,具体而言:1)根据窃听者不串通和串通两种场景,首先分别给出窃听端的信干噪比统计特性,并据此推导出这两种场景下的安全性能闭式表达式及其渐进特性;2)根据所得安全性能表达式,研究分析窃听者串通与不串通以及单窃听者条件下的安全性能的关系;3)研究串通条件下的最优功率分配问题,提出基于最大有效安全吞吐量的连续干扰消除调度策略,并推测该策略在此模型中是最佳的。更多还原

【Abstract】 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.更多还原