Research On Secure Wireless Transmission Method Using Hybrid Multi-antenna Structure

With the increasing development of massive multiple-input multiple-output(MIMO)technique,fully digital(FD)structure which requires one dedicated radio frequency(RF)chain per antenna is impractical for large-scale antenna arrays due to its high circuit cost and power consumption.To this end,the notion of hybrid digital and analog emerges.Compared to FD,HAD structure requires fewer number of RF chains to connect the low-dimensional digital beamformer and high-dimensional analog beamformer,which enjoys the advantages of simple circuit and low complexity,Therefore,it can strike a good balance among energy consumption,circuit cost and performance gains.Due to the broadcast nature of wireless communication,the confidential message is susceptible to the eavesdropper.How to ensure the secure transmission of confidential messages in HAD structure is of great importance.Therefore,the secure beamforming strategy based on HAD structure has attracted wide attentions from academia and industry.This thesis focuses on the physical layer security technology in the context of the HAD structure.Our main contributions and innovations are as follows:1)In order to obtain the channel state information of the intended user in the hybrid directional modulation(DM)system,an estimation method of direction of arrival(DOA)under HAD structure is proposed.The algorithm utilizes the estimation of signal parameters via rotation invariance techniques(ESPRIT)to obtain a set of feasible angles in digital part,and the optimal solution is selected with the aid of the criterion of maximizing received signal power,which is obtained by aligning the analog and digital phases.In addition,the machine learning histogram method is also invoked.By means of it,we note that the distribution of DOA estimation errors can be approximated as a Gaussian one.Finally,a robust hybrid DM beamforming method is proposed by using both the measured DOA and DOA measurement error distribution function.Simulation results show that the proposed hybrid ESPRIT angle estimation method can achieve a relatively accurate performance of the root of mean square error.Simultaneously,compared with the existing non-robust DM methods,the proposed robust hybrid DM beamforming method harvests a higher secrecy rate(SR)performance.2)To reduce the high power consumption induced by digital-to-analog converter(DAC)in hybrid structure,a secure hybrid beamforming algorithm based on low-resolution DAC is proposed in millimeter wave hybrid structure.With the aid of the additive quantization noise(AQN)model,the nonlinear output of the low-resolution DAC is approximately as a linear one,and an approximate analytical expression of secrecy rate lower bound is derived.By means of maximizing the lower bound on secrecy rate,a two-layer alternating iterative algorithm is proposed to optimize hybrid beamformers.Specifically,a gradient ascent method with low complexity is proposed for designing the analog beamforming matrix with the constraint of module-one.Simulation results show that the proposed alternating iterative algorithm can achieve a higher secrecy rate performance than existing methods.Moreover,the convergence of the proposed alogorithm can be ensured within several iterations,which verfied the validness of our devised method.