| With the development of technology and the proliferation of electronic devices in the military and civilian,wireless spectrum is becoming an increasingly scarce resources.In order to resolve the spectrum congestion and reduce the spatial competition and interference between radar and communication systems,the dual-function radar-communications(DFRC)system based on the complex platform could be used.DFRC system sends dual-function waveforms to achieve communication information transmission and at the same time to complete the detection and tracking of radar targets.The integration of radar and communication systems not only reduces the pressure caused by the lack of spectrum resources,but also enables hardware resources sharing,reduce the use of cost,and thus quickly become one of the hot research directions in recent years.Among them,dual-function waveform as the core of DFRC system,how to reasonably design it to meet the requirements of communication and radar system is undoubtedly a major difficulty.In conventional DFRC systems which transmit pulse waveforms,the communication transmission rate is limited by the pulse repetition frequency(PRF).While directly adjusting the PRF can increase the communication rate,it again introduces the problem of range ambiguity to radar detection.The use of frequency hopping(FH)waveforms in integrated systems can be a good solution to this problem.The information embedding of FH waveforms is achieved within fast time sub-pulses,which can increase the communication rate without modifying the system parameters.Therefore,this dissertation focuses on the DFRC waveform design problem of FH-based multiple-input multiple-output(MIMO).This dissertation first describes the application scenario,establishes the corresponding MIMObased DFRC system transceiver model,and describes in detail the DFRC system based on the shared implementation method,and discusses the processing method of common DFRC waveforms.To facilitate the verification and analysis of the effectiveness of the proposed scheme,the performance evaluation metrics commonly used for DFRC signals are given in this dissertation.Then,based on this,the DFRC system signal model based on FH-MIMO is further derived and its corresponding ambiguity function(AF)is analyzed.Using AF as the main index to measure radar performance,this dissertation proposes an information embedding strategy based on FH code alignment between sub-pulses.Specifically,the initial FH matrix is multiplied with the alignment matrix carrying communication information to obtain the FH matrix and then generate the corresponding FH waveform.By optimizing the initial FH matrix,we reduces the impact of the randomness of the communication information on the DFRC waveform detection function.Since the proposed initial FH matrix optimization problem is an integer programming,the corresponding genetic algorithm is designed in this dissertation to solve it offline.Simulation results show that the improved DFRC waveform has lower AF sidelobes,higher distance resolution,and better radar detection performance compared to the existing scheme.Finally,for the communication security problem faced by the DFRC system with FH-MIMO,two information embedding strategies are proposed in this dissertation.The first hybrid information embedding strategy based on phase compensation of the transmitting array is suitable for the application scenario where target detection is performed for the entire airspace.At this time,the DFRC platform still sends omnidirectional orthogonal waveforms.Through the phase compensation of the transmitting array,this dissertation achieves three dimensions of communication information embedding,which further improves the communication rate while increasing the difficulty of eavesdropping.The second multi-beamforming-based information embedding strategy is applicable to the application scenario of detecting or tracking targets in a certain direction in the airspace.In this case,the DFRC platform sends the relevant waveforms after beamforming.The optimization problem proposed for the beamforming matrix takes the reduction of the beam sidelobe to mainlobe integrated power ratio as the objective function.The optimized beamforming matrix generates DFRC waveforms with the mainlobe aimed at the radar target and the sidelobes aimed at the communication users,and it can realize different communication messages to different users.The simulation results show that both strategies can improve the physical layer security of communication. |
