Research On Closed-loop Direction Finding Algorithm For Complex Signals

Along with the increasing development of the informatization,electronic warfare has been more widely used in the military field.Because some of the flaws in the satellite communication system are oftenused to cause the communication system to be destroyed,it is not possible for a communication satellite to work in a non-interference environment.The adaptive antenna can realize the interference detection and suppression in the complex electromagnetic environment,therefore,adaptive antennas have become a key technology for communication satellites to work in complex environments.The adaptive direction finding algorithm for communication satellites is studied in this thesis.Firstly,the planar and multi-beam shaping antenna array signal models are introduced,and the three optimal criteria for adaptive beamforming are given.The adaptive iterative algorithm based on the minimum mean square error(MSE)criterion is introduced.Then,the principle of closed-loop adaptive direction finding algorithm combined with the multi-beam shaping antenna pattern is analyzed and explained.On the basis of the closed-loop direction finding algorithm,the following contents are studied:1.Combined with the common ideas to achieve weak signal direction in the case that strong signal and weak signal exist at the same time in the space,the closed-loop direction finding algorithm of strong signal and weak signal based on orthogonal projection matrix is proposed.Firstly,the direction of arrival(DOA)of the strong signal is estimated by the closed loop direction finding algorithm.Then,the orthogonal projection matrix is constructed according to the direction of the strong signal to filter the strong signal in the array signal.Finally,the closed-loop adaptive direction finding algorithm is applied to the received signal after filtering out the strong signal,so that the weak signal direction is estimated in the strong signal environment.2.Inspired by the least mean square(LMS)algorithm in time domain,the frequency domain LMS algorithm is presented.And based on this,a closed-loop direction finding method for wideband signal in frequency domain is given.The algorithm first uses Discrete Fourier Transform to received data,which looks the wideband signal as superposition of narrowband signals.Then,LMS algorithm to the narrowband signals in different frequencies is applied to null signal respectively and geometric mean for the nulling results at different frequencies is used to get the final beam-forming pattern data.Finally,Inverse the Pattern data to get the spatial spectrum function,and then find the direction of signal according to the peak position of spatial spectrum.Compared with the incoherent signal subspace method(ISSM)algorithm which is the classical algorithm of wideband signal direction finding,it is verified that the closed-loop direction finding algorithm of wideband signal has a better performance.And the algorithm proposed in this thesis does not need to decompose the eigenvalues of the matrix in practice,what's more,the computational complexity of the proposed algorithm is lower than other open-loop high resolution direction finding algorithm.3.The programming framework of satellite multi-beam shaping antenna system and the engineering realization of closed-loop adaptive direction finding algorithm based on DSP are introduced.The realization scheme and process are given,and the implementation framework of the direction finding module and the main functions of each part of the frame are introduced in detail.Then,the software modules of closed-loop adaptive direction finding based on DSP are given in this thesis.At last,the DSP-based closed-loop direction finding algorithm is tested and the results are analyzed in detail.From the test results can be drawn,DSP-based closed-loop direction finding method can be applied in engineering practice,not only greatly reduces the hardware complexity,but also can achieve accurate signal direction,so it has a broad application prospects.