Research On Range-angle-dependent Characteristics Of Frequency Diverse Array Beampattern

Frequency diverse array(FDA)radar has received much attention in recent years.Compared with conventional phased-array radar,the elements of the FDA transmit the same waveform with distinct carrier frequencies,whereas all elements of the phased-array transmit the waveform with the same carrier frequency.FDA radar is also different from conventional multiple-input multiple-output(MIMO)radar.The former transmits frequency-overlapped and non-orthogonal waveforms to achieve coherent array gain,but the latter uses orthogonal waveforms to get spatial diversity gain without coherent array gain.The standard FDA adopts linearly increasing frequency offsets across the array elements to generate range-dependent transmit beampattern.However,the FDA beampattern is also coupled in both range dimension and angle dimension.As a consequence,the difficulty of array optimization and parameters estimation are significantly increased.To address these problems,this dissertation studies the rangedependent characteristics of FDA beampattern together with its optimization and potential new radar applications.The main contributions and originalities are summarized as follows:(1)Proposed a Costas sequence-based frequency offsets for low probability of interception(LPI)FDA radar: The nonlinear frequency offsets can produce pseudorandom energy distribution to decouple the range-angle beampattern,so that we can jointly estimate the range and angle of targets with matched filtering and subspace decomposition algorithm from the received signals.In addition,this pseudo-random energy distribution can be utilized for LPI radar.(2)Studied a multichannel mixing-based FDA receiver design approach: Different from conventional single-carrier mixing methods,here,the receiver uses the transmitted multi-carrier signals to perform multichannel frequency conversion,and then uses matched filtering for the baseband returns,so both the target range and angle parameters can be estimated from the output vector data model.The receiver can simultaneously deal with both frequency overlapped and non-overlapped FDA returns for the FDA radar using coherent and orthogonal waveforms,respectively.By utilizing the FDA range-angle dependence,the proposed receiver can adaptive suppress range-angle-dependent interferences,and can jointly estimate both range and angle of targets with subspace algorithms.(3)Proposed a modified nested-array design approach: Based on the nested-array design technique,the FDA degrees-of-freedom(DOFs)for a given number of array elements are further increased by optimally designing the array parameters and weights to detect and resolve more radar targets and suppress possible grating lobes.Moreover,an least squares(LS)-based arbitrary transmit-receive beampattern synthesis approach is proposed for large-scale arrays.This approach is superior to traditional polynomial factorization method.In particular,it is more advantageous to design a large-scale array beampattern with hundreds or even thousands of array elements.(4)Proposed a conceptual cognitive FDA radar technique: By jointly utilizing intelligent environment sensing and FDA radar range-dependent transmit beampatternm,the cognitive FDA radar adaptively optimizes its frequency offsets in the target tracking to focus the transmit beam towards the target position,so as to improve the receiver signal-to-interference-plus-noise ratio(SINR)and avoid possible interferences from other positions simultaneously.In addition,in order to overcome the disadvantages of that conventional high-gain radar transmit beams can be easily detected by unintended receivers,a series of low-gain transmit beams are designed with FDA to reduce the system visibility without degradation of radar performance.