Research On Beampattern Design And Signal Processing For Transmit Diversity MIMO Radar

Comparing to the conventional phased-array radar,Multiple Input Multiple Output(MIMO)radar can change the working mode flexibly by exploring the transmit degrees of freedom(DOFs).The transmit diversity technique in MIMO radar can effectively extend the transmit DOFs.The flexible working modes can readily accommodate the complex environment.In recent years,new types of MIMO radar have attracted more and more attention,which introduce the frequency shift or time shift to realize the extention of the transmit DOFs with a better system performance in multiple dimensions.It can offer potential ways to solve the current MIMO radar problems such as relatively high beampattern design computational complexity and poor Doppler shift tolerance.This disertation mainly focuses on the transmit beampattern design and siganl processing alogorithms for the new transmit diversity based MIMO radar.The addressed main problems include transmit beampattern synthesis,subarray partrition for range resolution enhancement,space-time matched filter design for coherent Frequency Diverse Array(FDA)radar and Direction-of-arrival(DOA)estimation for space-time coding array(STCA)radar.The main contents of this thesis are summarized as follows:1.For the transmit beampattern design problem,we design a piecewise linear frequency modulation(PLFM)waveform to synthesize the desired transmit beampattern for coherent FDA radar.In transmit beampattern design,it is desirable to maximize the power around locations of interest.MIMO radar transmits orthogonal waveforms,which is hardly achievable in practical applications.Moreover,the constant modulus constraint and the computational burden limit the system performance.To deal with this problem,the PLFM waveform for coherent FDA radar was proposed to synthesize the desired transmit beampattern.By deriving the relationship between the beam direction and the transmit signal frequency,we find that different sub-band of transmitted signal corresponds to different angular sector.With the relationship between frequency band and mainlobe of beampattern,the beampattern synthesis is transformed into the design of waveform spectrum,which is realized by designing the PLFM waveform in this paper.Compared with the state-of-the-art technique,the proposed PLFM waveform in coherent FDA enjoys the advantage of the constant modulus,low sidelobe level and reduced computation complexity.In addition,the multi-dimensional ambiguity function and the transmit beampattern response are derived to examine the properties.2.For the matched filter design problem,we propose a space-time matched filter(STMF)in combination with the hybrid coding technique.By controlling the output signal-to-noise ratio,it can form deep nulls in specific directions under constraints of gains in different angles.Since the beamforming performance in traditional matched filter for coherent FDA is poor.Such filter structures merely control the main lobes of equivalent transmit beams towards target directions.It fails to form nulls at clutter directions.Besides,traditional adaptive beamforming algorithms by designing adaptive weight vectors are no longer applicable on account of the coupling between the spatial steering vector and the temporal transmitted waveform in the two-dimensional angle-time matched filter at the FDA receiver.The proposed STMF can be formulated as the quadratically constrained quadratic programing(QCQP)problem via the discretization manipulation.By relaxing quadratic constraints,the hard non-convex problem can be turned into the second-order cone programing to obtain optimal filter parameters via optimization toolbox.Furthermore,the hybrid coding technique is devised to jointly improve the range resolution by a factor equal to the element number.Both two-dimensional range-angle outputs and one-dimensional range profiles via filtering can demonstrate that the STMF with hybrid coding can effectively suppress the sidelobe clutter with a range resolution enhancement.3.For the range resolution reduction problem,we propose a subarray partition scheme for coherent FDA to enhance the range resolution.According to the established signal model of the coherent pulsed FDA,we quantitatively analyzed the linear relationship between the range resolution and the frequency shift.By this principle,we partition the transmit antenna array into multiple subarrays including regular non-overlapping,regular overlapping and irregular subarrays,wherein an identical carrier frequency is utilized in each subarray.Meanwhile,distinct carrier frequencies are adopted between subarrays.Moreover,the multidimensional ambiguity function is studied,wherein the angle-angle ambiguity function cut can show the angle resolution and the angular coverage ability.The range-angle ambiguity function cut can assess the performance of the range resolution and range sidelobe level.In comparison with the current methods,the proposed subarray partition scheme can improve the range resolution while maintaining the low range sidelobe level for coherent FDA radar.4.For the emerging STCA radar,a full angular coverage with a stable gain can be realized by merely transmitting a single waveform.We address the problem of direction-of-arrival(DOA)estimation in STCA.Firstly,we propose a beamspace multiple signal classification(MUSIC)algorithm applicable in case of small training samples,wherein the beamspace steering vector is devised to realize the DOA estimation in the equivalent transmit side.Next,since targets at the same range-doppler cell show characteristics of coherence,we design a transformation matrix to restore the rotational invariance property(RIP)of the receive array in the element space.Afterwards,the spatial smoothing is performed.In addition,the closedform expression of Cramer-Rao lower bound(CRLB)for angle estimation is derived.Theoretical performance analysis on the RMSEs,the probability of resolution as well as CRLB curve can demonstrate the effectiveness of proposed approaches.