Study On Signal Processing For Future Sky-wave Over-the-horizon Radar System

Traditional sky-wave over-the-horizon radar(OTHR) is generally a bistatic phased array radar, with antennas linearly spaced in both transmit and receive bases. This work considers a future OTHR system by employing multiple input multiple output(MIMO) technique and 2D array structure. MIMO technique can improve the detection and estimation performances of OTHR by increasing transmit signals and propagation paths to the target. Non-collocated OTHR system is available by the mutually orthogonal transmit signals. A 2D array provides the potential of elevation resolution, so that OTHR is able to employ 2D beamforming and distinguish multipath. Besides, future OTHR system can design transmit waveform or receive filter adaptively for interference suppression, which makes OTHR cognitive to external environment.In the study on signal processing of future OTHR system, the main contributes of this work are:(1) Operating frequency selection for OTHR with 2D arrayTraditional OTHR cannot separate multipath signals from the amplitude backscattering ionogram and so is unable to select the optimal operating frequency for a mission. With 2D array, receive beamforming is employed in both the azimuth and elevation dimensions. By the ionosphere model and measurement in real-time, ray tracing technique provides multipath information. Hence, OTHR selects operating frequency for optimizing the signal-to-interference-plus-noise ratio(SINR) at the desired ground range.(2) Waveform design in cochannel interference and colored noise.High frequency(HF) environment is complex and time-varying. Instead of traditionally set waveform(such as linear frequency modulation, LFM), future OTHR can design transmit waveform adaptively based on environmental sensing. A cognitive scheme is proposed to support OTHR working in cochannel interference. Similarity constraint is employed for the consideration of mainlobe and sidelobes. The optimal solution for output SINR maximization under similarity constraint is derived, given in closed form.(3) Receive filter design for interference and noise suppression.Receive filter design is also considered for interference and noise suppression. The problem of SINR optimization under similarity constraint is solved. The analytical optimal filter is given. Compared with other signal and filter schemes, the proposed filter design is robust on output properties. Besides, receive filter design in OTHR can make use of the receive data directly, in no need of training data.(4) Discussion of target height estimation in OTHRTarget height estimation in OTHR is a difficult problem. Herein it is considered via 2D array and multipath signal. Multipath propagation can be produced by ionosphere layers and improved by MIMO technique. A 2D array provides elevation information which is related to ground range and height(altitude). The sufficient conditions for effective height estimation in OTHR are concluded, and the lower bound of estimation variance is derived.(5) Joint estimation of target state and ionospheric contamination in MIMO-OTHROTHR signals are contaminated when propagating through the ionosphere. This paper develops the receive signal model when MIMO-OTHR illuminates a vessel. Ionospheric contamination is modeled as frequency shift and sea-clutter is taken into consideration. The joint estimation of target state and ionospheric contamination is analyzed and the joint Cramer-Rao bound(CRB) is deduced。(6) Optimization of signal processing structure in non-collocated MIMO-OTHROTHR bases are widely separated and MIMO signals are processed jointly. Considering the non-collocated small-aperture arrays in OTHR, signal processing structure is optimized for improving the efficiency with little degradation of radar performance. Target detection and location methods are proposed according to the characteristics of signal processing output performance.