Ill-posed Theory Framework And Methods For Guaranteeing The Security Of Wireless Physical-layer

Wireless physical-layer security technique is a method, which guarantees wireless securetransmission through using the diversity and time-variation of wireless channel and theuniqueness and interoperability of the channel characteristics between transceivers. Wirelessphysical-layer security technique can be divided into two types by the mechanism forguaranteeing security: the information theory based physical-layer security technique and thesignal processing based physical-layer security technique. The signal processing basedphysical-layer security technique can be available without the channel state information ofeavesdropper and the assumption that the legitimate users have more excellent channel state.Thus, more and more researchers engage in the wireless physical-layer security researching areas.The technique can increase the difficulties of intercepting signal and restoring information byeavesdropper while ensure the communication quality of legitimate users through randomizingthe transmitted signal by means of signal processing to protect transmission in physical layer.The unified theoretical framework can be used to not only analyze existing methods but alsoguide the formation of new methods. However, there is no unified theoretical framework andefficient physical-layer secure transmission method in existing research. Based on this, thedissertation focuses on the theoretical framework and efficient physical-layer secure transmissionmethod of the techniques.In the dissertation, research content is divided into two parts: the theoretical framework ofwireless physical-layer security is proposed, which reveals that the existing signal processingbased physical-layer security methods are all the special cases of the framework. Furthermore,multiform efficient physical-layer security communication methods in the framework areresearched. The concrete contents are as follows:The ill-posed theoretical framework for guaranteeing wireless physical-layer security isproposed, which includes the first kind ill-posed secure theory based on non-unique solutionsand the second kind ill-posed secure theory based on instable solutions of an ill-posed problem.The first one utilizes the non-uniqueness of ill-posed problem to introduce redundancy for thetransmitted precoding parameters, which is used to randomize transmitted signals forguaranteeing physical-layer security. The second one utilizes the instability of ill-posed problemto introduce redundancy for the transmitted precoding parameters, which can be used toguarantee physical-layer security at a little cost of legitimate users’performance. The frameworkis useful for not only analyzing the existing physical-layer secure transmitting methods but alsoproviding theoretical basis for new methods. The concept of unsafe factor is also proposed,which is used to research the security of the theoretical framework.A wireless physical-layer secure transmission mechanism, joint array redundancymechanism, is proposed for adapting the MIMO systems. As limited number of the transmitterantennas, which is introduced by applying the first kind ill-posed secure theory to the MIMOsystems, we expand the condition of non-unique solutions to ill-posed problem. This expansiontranslates the traditional unique solution or inexistent solution into joint ill-posed problem of non-unique solution. Based on it, two joint array redundancy physical-layer security methods areproposed. Both methods utilize the idea of joint array to introduce redundancy of the transmittedpreprocess parameters. The method of joint random weighting vector utilizes the redundancy torandomize the transmitted signals. And the method of joint random artificial noise uses theredundancy to produce artificial noise, which does not affect legitimate users while guaranteeingphysical-layer security.The secure precoding method for wireless MIMO multigroup multicast system isproposed, which is based on established wireless physical-layer security joint optimizationmodel for MIMO multigroup multicast system. This method maximizes minimum SINR oflegitimate users subjecting to transmitted power and the maximum SINR of eavesdropper, byutilizing the concept of artificial noise that based on the first kind ill-posed secure theory. Weconvert the problem of physical-layer security for wireless MIMO multigroup multicast systeminto convex optimization problem. And two low complexity suboptimal methods forsingle-stream and multi-stream multicast system respectively are proposed, which are all basedon the joint optimization model of wireless physical-layer security and utilize the decouplednature of signal to leakage plus noise ratio to reduce complexity. For multi-stream multicastsystem, the concept of generalized Rayleigh entropy is used for simplifying the solving oforiginal problem. The robust physical-layer secure precoding methods for multicast system underthe condition of imperfect-CSI are also researched.Three joint optimization methods, considering wireless physical-layer security andresource allocation for multiuser MISO-OFDMA system, are proposed. Firstly, a low complexitysuboptimal physical-layer secure transmission method that based on spatial correlation andrandom weighting vector is proposed, which involves user selection and resource allocation.Then, based on zero forcing (ZF) beamforming and artificial noise (AN), the original problem isformulated as an AN aided secrecy rate maximization problem, which maximizes the sum ofsecrecy rate by jointly designing user selection and the AN covariance. We show that theproblem can be handled by performing an exhaustive search in which a sequence of semidefiniterelaxation programs (SDPs) are involved, leading to a tractable user selection and resourceallocation method. At last, a subcarrier-reference (SR) transmission scheme is proposed withdeliberate signal randomization to achieve LPI in multiuser MISO-OFDMA systems. For eachuser, one of the allocated subcarriers is chosen by the transmitter to send reference signals, andothers are chosen to send the user’s information symbols. By some deliberate signalrandomization, the eavesdropper cannot detect the transmitted symbols, while the authorizedusers can operate the system successfully without knowledge of the channels bysubcarrier-reference demodulation.The random multiuser secure transmission mechanism for multiuser communicationsystem is proposed, which is based on the first kind ill-posed secure theory. The mechanismutilizes independent random fading channel characteristics to introduce redundancy forrandomizing transmitted signal and guaranteeing physical-layer security. Considering amulti-user SISO system, a random time slot assignment scheme is proposed for enhancingwireless security. For each time slot, single antenna transmitter selects a user randomly. Signals of the intended user are pre-processed before transmitting using known wireless channel stateinformation, which not only offsets the impact of multipath channels but also producescharacteristic mark for intended user. Eavesdropper cannot demodulate the signal and couldn’tknow the intended receiver. By exploiting the dimensions provided by multicarrier systems forrandom user selection and beamforming, a physical-layer security method is proposed.Meanwhile, hypothesis testing is used to determine which subcarriers belong to him by anyintended user without user selection information.A wireless physical-layer security transmission mechanism for SISO system,perturbation redundancy mechanism is proposed, which is based on the second kind ill-posedsecure theory framework. The mechanism utilizes adding tiny perturbation to equivalent channelof legitimate user for introducing redundancy in order to randomize transmitted signal andguaranteeing wireless physical-layer security. The concept of relaxed mean square error isintroduced, which is used to introduce redundancy for pre-coding weight. The relaxationconverts the physical-layer security problem to the problem of randomizing transmitted signaland a novel physical-layer encryption method is proposed. At last, a method making use ofmulti-path redundancy and time diversity of wireless channel is proposed by using thecharacteristics of wideband wireless channel itself adequately. The method randomizestransmitted signals and makes the signal to be focused in time and space at legitimate user at thesame time, which achieves the aim of secure transmission.