Wen-Qin Wang

Space–Time Coding MIMO-OFDM SAR for High-Resolution Imaging

Multiple-input and multiple-output (MIMO) radar has received renewed attention in recent years, but little work on MIMO synthetic aperture radar (SAR) remote sensing has been reported. This paper presents a scheme of space-time coding MIMO orthogonal frequency division multiplexing (OFDM) SAR for high-resolution imaging. This system employs MIMO configuration in the elevation direction and the Alamouti space-time coding scheme in the azimuth direction, along with the use of OFDM waveform diversity and displaced phase center antenna (DPCA) techniques. As an orthogonal transmission waveform is required for this novel space-time coding MIMO-OFDM SAR system, one kind of OFDM linearly frequency modulated waveforms is investigated. The matched filtering and multibeam forming in the elevation direction are detailed. Additionally, the corresponding mathematical relations and signal models for high-resolution imaging are formed. In this way, efficient spatial diversity gain and improved range resolution are obtained by the MIMO configuration, and the requirement of pulse repeated frequency for wide-swath remote sensing is reduced by the DPCA technique. The feasibility is validated by numerical simulation results.

Transmit Subaperturing for Range and Angle Estimation in Frequency Diverse Array Radar

Different from conventional phased-array, which provides only angle-dependent beampattern, frequency diverse array (FDA) employs a small frequency increment across the antenna elements and results in a range-angle-dependent beampattern. This beampattern offers a potential to localize the targets in two dimensions in terms of slant ranges and azimuth angles. However, it is difficult to obtain the target location information from a standard FDA radar due to the couplings in range and angle responses. In this paper, we propose a transmit subaperturing scheme on the FDA radar for range and angle estimation of targets. The essence is to divide the FDA elements into multiple subarrays and optimize the transmit beamspace matrix with the use of convex optimization. We also discuss several practical issues for designing the FDA radar system parameters. Since the subarrays offer decoupled range and angle responses, the targets can be located using the beamspace-based multiple signal classification algorithm. The range and angle estimation performance is evaluated by comparing with the Cramér-Rao lower bound.

Range-Angle Dependent Transmit Beampattern Synthesis for Linear Frequency Diverse Arrays

Phased-array is widely used in communication and radar systems, but the beam steering is fixed in an angle for all the ranges. In this paper, we propose a range-angle dependent beampattern synthesis scheme for linear frequency diverse array (FDA) using the discrete spheroidal sequence, with an aim to focus the transmit energy in a desired two-dimensional spatial section. Different from conventional phased-arrays, FDA employs a small frequency increment, compared to the carrier frequency across the array elements. The range-angle dependent beampattern synthesis method allows the FDA to transmit energy over a desired range or angle sector. This provides a potential to suppress range-dependent clutter and interference, which is not accessible for conventional phased-arrays. The system performance of the proposed FDA is evaluated by the output signal-to-interference-plus-noise ratio (SINR). The effectiveness is verified by comprehensive numerical simulation results.

Phased-MIMO Radar With Frequency Diversity for Range-Dependent Beamforming

Phased-multiple-input and multiple-output (MIMO) radar is a flexible technique which enjoys the advantages of MIMO radar without sacrificing the main advantage of phased-array radar. However, the phased-MIMO radar is limited to range-independent directivity; this limits the radar performance to mitigate non-desirable range-dependent interferences. In this paper, we propose a new phased-MIMO radar with frequency diversity for range-dependent beamforming. The essence of the proposed technique is to divide the frequency diverse array transmit array into multiple subarrays, each subarray coherently transmits a distinct waveform with a small frequency increment across the array elements. Each subarray forms a directional beam and all beams are independently steerable by tuning the frequency increment. The subarrays jointly offer flexible operating modes and range-dependent beamforming. The beamforming performance as compared to phased-MIMO radar is examined by analyzing the transmit-receive beampatterns and the output signal-to-interference-plus-noise ratio. The effectiveness of the proposed technique is verified by numerical simulation results.

Mitigating Range Ambiguities in High-PRF SAR With OFDM Waveform Diversity

Range-ambiguity suppression is a technical challenge for high-pulse-repetition-frequency (PRF) synthetic aperture radar (SAR). This letter proposes a practical approach to mitigate the range ambiguities in high-PRF SAR by using the orthogonal frequency-division multiplexing (OFDM) waveform diversity. The system scheme, waveform design, and range-ambiguity-to-signal-ratio performance are detailed. The approach eliminates the ambiguities, instead of just suppressing them like other techniques. The proposed OFDM chirp diverse waveform has a large time–bandwidth product. It is validated by computer-simulation results. Although OFDM radar has received much attention in recent years, there appears to be little work done in applying OFDM concepts to mitigate high-PRF radar range ambiguities, as is the subject of this letter.

Range-Angle Localization of Targets by A Double-Pulse Frequency Diverse Array Radar

Phased-array radar is widely used in localizating non-cooperative targets, but the range and angle of targets cannot be directly estimated from its beamforming output due to an inherent range ambiguity, i.e., two-dimensional localization of non- cooperative targets cannot be obtained directly from conventional linear phased-array radar beamforming peaks. This paper proposes a simple range-angle localization of targets by uniform linear array (ULA) double-pulse frequency diverse array (FDA) radar. The FDA transmits two pulses with zero and non-zero frequency increments, respectively. The azimuth angle and slant range of targets are then estimated directly from the beamforming output peaks. This approach can be interpreted as detecting the targets in angle dimension and then localizing them in range dimension by properly choosing the frequency increment. Moreover, multiple FDA radars can work as netted radar networks, in which distinct frequency increments or orthogonal waveforms are employed. The localization performance is examined by analyzing the Cramér-Rao lower bound (CRLB) and numerical mean square error (MSE). The effectiveness is demonstrated by simulation results.

Waveform-Diversity-Based Millimeter-Wave UAV SAR Remote Sensing

To integrate a synthetic aperture radar (SAR) into an operational unmanned airborne vehicle (UAV), it should be as small as possible to meet stringent limitations of size, weight, and power consumption. It appears that the novel combination of millimeter-wave frequency-modulated continuous-wave (FMCW) technology and SAR techniques can provide an optimal solution. However, some efficient techniques should be applied to resolve range/Doppler ambiguities in FMCW UAV SAR systems. As such, a technique of waveform-diversity-based millimeter-wave UAV SAR imaging is presented in this paper. Along with the described system concept and signal model, the performance of the diversified waveforms evaluated by their cross correlations is detailed. As the conventional stop-and-go approximation is not valid for FMCW SAR, a modified wavenumber-domain algorithm with a consideration of continuous antenna motion during transmission and reception is derived. This imaging algorithm is validated with computer simulations. Furthermore, one parallel direct-digital-synthesizer-driven phase-locked-loop synthesizer with adaptive nonlinearity compensation, which has been validated by the experimental results, is proposed to obtain a millimeter-wave FMCW signal with fine frequency linearity.

Virtual Antenna Array Analysis for MIMO Synthetic Aperture Radars

Multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) that employs multiple antennas to transmit orthogonal waveforms and multiple antennas to receive radar echoes is a recently proposed remote sensing concept. It has been shown that MIMO SAR can be used to improve remote sensing system performance. Most of the MIMO SAR research so far focused on signal/data models and corresponding signal processing algorithm. Little work related to MIMO SAR antenna analysis can be found. One of the main advantages of MIMO SAR is that the degrees of freedom can be greatly increased by the concept of virtual antenna array. In this paper, we analyze the virtual antenna array for MIMO SAR high-resolution wide-swath remote sensing applications. The one-dimensional uniform and nonuniform linear antenna arrays are investigated and their application potentials in high-resolution wide-swath remote sensing are introduced. The impacts of nonuniform spatial sampling in the virtual antenna array are analyzed, along with a multichannel filtering-based reconstruction algorithm. Conceptual system and discussions are provided. It is shown that high operation flexibility and reconfigurability can be obtained by utilizing the virtual antenna arrays provided by the MIMO SAR systems, thus enabling a satisfactory remote sensing performance.

MIMO SAR OFDM Chirp Waveform Diversity Design With Random Matrix Modulation

Multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) has received much attention due to its interesting application potentials, but effective waveform diversity design is still a technical challenge. In a MIMO SAR, each antenna should transmit a unique waveform, orthogonal to the waveforms transmitted by other antennas. The waveforms should have a large time-bandwidth product, low cross-correlation interferences, and a low peak-average ratio. To reach these aims, this paper proposes an orthogonal frequency division multiplexing (OFDM) chirp waveform with random matrix modulation. The designed waveforms are time-delay and frequency-shift decorrelated. Referring to MIMO SAR high-resolution imaging, the proposed OFDM chirp waveform parameters are optimally designed, and their performances are analyzed through the ambiguity function and range-Doppler-based MIMO SAR imaging algorithm. Extensive and comparative simulation results show that the waveforms have the superiorities of high range resolution, constant time domain and almost constant frequency-domain modulus, large time-bandwidth product, low peak-average ratio, and low time-delay and frequency-shift correlation peaks. More importantly, this scheme can easily generate over three orthogonal waveforms with a large time-bandwidth product.

Nonuniform Frequency Diverse Array for Range-Angle Imaging of Targets

Although phased-array antennas are widely used in communication, radar, and navigation systems, its beampattern is a function of angle only and thus there is no range information. To circumvent this limitation, we design a frequency diverse array (FDA) antenna system for range-angle imaging of targets. Our approach exploits the nonuniform FDA as the transmitter to provide range-dependent beampattern and the uniform phased-array as the receiver which results in angle-dependent beampattern. Range-angle imaging of targets is achieved from the cooperative transmit-receive beamforming. The imaging performance measures including spatial resolution and system processing gain are analyzed. In addition, several design specifications are discussed. The effectiveness of the proposed approach is verified by simulation results.