Research On The Modulation Technique Of The Null-space Projection Direction Based On Artificial Noise

Due to the broadcast nature of wireless transmission,the secrecy and privacy of confidential information is increasingly becoming an extremely important issue in wireless networks.Directional modulation(DM),as an emerging and promising technology in the field of physical layer secrecy in wireless networks,has attracted great research interest in academia and industry.Unlike the traditional upper-layer encryption technology,DM technology is not only independent of computational complexity,but also does not require key distribution and management.Additionally,the DM technology can recover the original constellation in the desired direction,but the constellation is distorted in the eavesdropping direction because of the interference of artificial noise,thereby which can further improve the secrecy performance of the system.The main contributions of this paper are follows:1)In order to make the designed DM synthesis algorithm suitable for the practical application scenarios,such as the relative motion between the desired receiver and the transmitter or one of them is in motion,an adaptive and robust beamforming algorithm is proposed to adapt to this dynamic situation.The proposed algorithm consists of the following three steps: first,the directions of arrival(DOA)of signals from the desired user and the eavesdropping user are estimated by the root multiple signal classification algorithm,and the relevant signal-to-noise ratios(SNR)are estimated based on the ratio of the corresponding eigenvalue to the minimum eigenvalue of the covariance matrix of the received signals.The intervals of DOA estimation errors are then predicted by the estimated values of DOA and SNR.Finally,a robust null-space projection beamforming scheme for DM system is proposed based on the afore-obtained estimates and predictions.The simulation results show that the proposed scheme can achieve lower bit error rate in the desired direction and higher secrecy rate compared with the traditional non-adaptive robust scheme and the non-adaptive non-robust scheme.In addition,the eye pattern achieved by the proposed scheme is clearer than those achieved by other schemes.2)In order to optimize the power allocation(PA)problem of useful signals and artificial noise,a PA strategy of maximizing the secrecy rate is proposed.Using the Lagrange multiplier method,we can derive the analytical expression of the proposed PA strategy based on maximizing secrecy rate.In order to verify the advantages of the proposed PA strategy,we take the null-space projection beamforming scheme as an example to derive the closed-form expression of its optimal PA strategy.The simulation results show that compared with the three typical PA parameters(such as ? =0.1,0.5,and 0.9),the optimal PA strategy shows an obvious advantage of secrecy rate performance gain over three fixed PA strategies,for example,the rate performance gain percentage is 62% when SNR is 30 dB and the number of antennas is 4.Furthermore,as the PA factor increases from 0 to 1,the achievable secrecy rate increases correspondingly in the low SNR region,while the secrecy rate in the medium and high SNR regions can be approximately considered as a concave function with respect to the PA factor.