Waveform Diversity Research Articles

Coherence optimised subarray partition for hybrid MIMO phased array radar

Hybrid MIMO phased array radar (HMPAR) is widely recognized in the research of radar systems owing to its capacity to provide a balance or trade-off between transmit coherent gain and waveform diversity gain. The balance or trade-off between these two gains is closely related to spatial coherence between different arrays, represented by the degree of cross-correlation between steering vectors over different angles. Proper subarray partitions can provide an extra degree of freedom for optimising the coherence of the array manifold. A coherence optimised subarray partition for HMPAR is proposed with an analysis of connections between coherence and subarray partitions. First, the definition of coherence of array manifold is presented, and its connection between beampattern and the signal to interference plus noise ratio (SINR) is analysed. Next, the problem of optimising array manifold coherence is transformed into the discrete coherence optimization of the steering vector codebook. By properly selecting different codewords with the proper degree of correlation between each other, coherence between the target and interference steering vectors is optimised. The original combinatorial problem is approximated by a series of subproblems, which is solved in iteratively. Numerical results revealed that the proposed subarray partition provides lower coherence, lower beampattern peak sidelobes and higher SINR compared with` other subarray configurations.

Radar waveform diversity using nonlinear chirp with improved sidelobe level performance

Radar system with high range-resolution can be obtained through discrete frequency-coding waveform (DFCW). In this paper, we investigate the design process of DFCW using nonlinear chirp (DFCW-NLC), with improved autocorrelation sidelobe peak (ASP) and cross-correlation peak (CCP) levels, to attain radar waveform diversity. In this context, we present a parameterized waveform design model and derive NLC functions for designing the required NLC waveform, thereby simplifying the waveform design process. We derive analytical expressions of autocorrelation and cross-correlation functions for the NLC waveform. To achieve a waveform with desired properties, we present an optimization process based on genetic algorithm, and the optimization of parameterized NLC waveforms attains improved ASP performance over existing designs. Subsequently, we propose the design of a set of DFCW-NLC, double the number as compared to the reported designs. We derive analytical expressions of ambiguity function to investigate waveform performance. We perform a joint optimization of frequency hopping sequence and associated chirp-pulse type of the DFCW-NLC. Numerical results prove that the designed DFCWs-NLC achieve significant improvement in ASP and CCP levels of ≈5.0 dB and ≈5.47 dB, respectively in comparison with the state-of-the-art designs, and makes the proposed waveforms a superior candidate for multiple-input-multiple-output (MIMO) and multiuser radar applications.

A reduced-dimension RML method for transmit beamspace-based MIMO radar

In this paper, the problem of low angle estimation for multiple-input multiple-output (MIMO) radar is considered. Though the refined maximum likelihood (RML) algorithm can be directly applied to low angle estimation in MIMO radar, its computational burden is huge due to the large virtual aperture. Moreover, the conventional MIMO radar employs full waveform diversity, which actually sacrifices the emission gain. To reduce the computational burden and improve the transmit gain, a reduced-dimension RML method for transmit beamspace-based (TB-based) MIMO radar is developed. Firstly, the transmit beamspace (TB) matrix is designed to focus the transmit energy within the desired spatial sector. Then, to reduce the computational complexity and retain the estimation precision of the direct RML (D-RML) method for TB-based MIMO radar, a transmit-receive separate RML (TRS-RML) method is developed by applying the RML algorithm to the transmit and receive side respectively. Moreover, we propose to fuse the estimation results of the transmit and receive side together to improve the estimation precision. Theoretical analysis shows the TRS-RML method requires much lower computational burden than D-RML method. Numerical experiments show the proposed method has higher estimation accuracy compared with the D-RML method for the conventional MIMO radar in most angle region.

Extended Target Estimation and Recognition Based on Multimodel Approach and Waveform Diversity for Cognitive Radar

This article proposes a cognitive radar (CR) framework to improve the performance of the estimation and recognition for range-extended targets. To overcome the angular uncertainty and exploit the temporal correlation of target signatures, we introduce the static multimodel approach and the exponential correlation model to describe the dynamics of target signatures. The estimate of target signatures and the probability distribution of target hypotheses are updated recursively in response to the latest echo, ruled by the Bayesian theory. A waveform optimization module is integrated into the framework, which designs the waveform for the next emission according to the knowledge extracted from the echo history. A design criterion based on mutual information (MI) is derived, which implies the average estimation capability for all target hypotheses and the mean between-hypotheses divergence. The proposed method is examined using both synthetic data and electromagnetic (EM) simulation data of realistic targets.

Salp swarm optimization in hybrid beamforming for MIMO radar

The Multi Input Multi Output (MIMO) radar waveform diversity Significantly improves parameter identifiably than phased-array radar performance. Precoding, combining and spatial multiplexing techniques improves the data throughput and reliability of the transmission in MIMO systems. But increment in transmit and receive elements in MIMO antenna array induces considerable increase in required power for hardware and computation cost. Hybrid beamforming employs fewer RF-to-baseband chains. With conscious selection of the weights for pre-coding and combining, hybrid beamforming establishes perfect trade-off between complexity, performance, cost, and power consumption in practical applications. Performance of MIMO radar system can be improved using newly developed bio inspired metaheuristic algorithms as compared to conventional and adaptive beamforming algorithms. In this work the Salp Swarm algorithm (SSA) is implemented to optimize the performance of hybrid beamforming using Raleigh channel and considering the bit error rate and normalized array power parameters. The swarming behavior of salps when navigating and foraging in oceans is the inspiration behind the SSA optimization algorithm. The obtained results are compared with the conventional phase-shift as well as adaptive linearly constrained minimum variance beamforming algorithms on simulation platform with standard considerations. It is observed that this new approach of Salp swarm algorithm is having improved and much better performance with the considered parameters.

An Advanced Phase Synchronization Scheme Based on Coherent Integration and Waveform Diversity for Bistatic SAR

In the bistatic synthetic aperture radar (BiSAR) system, the deviation between two oscillators in different platforms will cause an additional modulation of BiSAR echoes. Therefore, phase synchronization is one of the key issues that must be addressed for the BiSAR system. The oscillator phase error model and the principle of phase synchronization are firstly described. The waveform diversity technology has been widely used in many fields, for example, the hearing aids device and the recognition of auditory input source in cocktail party problem. Inspired by this, an advanced phase synchronization scheme based on coherent integration and waveform diversity is proposed. The synchronization signal and radar signal are orthogonal signals which can be separated by using waveform diversity technique. After extracting the synchronization signal, the phase synchronization accuracy can be further improved by coherent integration. The transmission of synchronization signals between two synchronization antennas is analyzed, followed by the theoretical error analysis. Then, the processing of separating the echo signal and synchronization signal is described in detail. The simulation experiments are performed. The accuracy of phase synchronization can reach 1 degree, which verifies the effectiveness of the proposed synchronization scheme.

Transmit-Receive Subarrays for MIMO Radar Array Antenna

The phased multiple-input multiple-output (PMIMO) radar uses subarrays in transmit array that overlap one another to exploit the main advantage of the phased array (PA) radar, i.e., high directional coherent gain, and the main advantage of the MIMO radar, i.e., high waveform diversity gain. This paper has derived the radar formula that utilizes overlapping subarrays on transmit (Tx) and receive (Rx) array simultaneously called the transmit-receive subarrays of MIMO (TRSM) radar. The use of overlapping subarrays in Tx-Rx establishes the TRSM radar has high flexibility to configure the number of Tx-Rx subarrays that use the performance of all the gains simultaneously, so that produce Tx-Rx beampattern and signal-to-noise-plus-interference ratio (SINR) more than those achieved by PMIMO radars. The approach aims to overcome the beam shape loss, increase the transmit-receive gain, minimize the maximum peak sidelobe levels, narrow the half-power beamwidth, increase directivity, and increase SINR. The effectiveness of this radar's performance is compared to the PMIMO radar in various methods such as equal subarrays, unequal subarrays, and optimum partitioning, the PA, and the MIMO radar. The numerical simulation and evaluation results show that the proposed radar has several advantages such as lowest the peak sidelobe level, narrow the half-power beamwidth, and high directivity, so it is very resilient to interferences on target locations. The effect of the number of subarrays as a function of performance parameters on the Tx-Rx array that is ready to adjust to the detected target conditions is also presented.

Thinned Virtual Array for Cramer Rao Bound Optimization in MIMO Radar

By transmitting multiple independent waveforms at the transmit side and processing echoes of spatial targets at the receive side, Multiple Input Multiple Output (MIMO) radar enjoys virtual array aperture expansion and more degree of freedom (DOF), both of which favors the application of direction finding or estimation of direction of arrival (DOA). The expanded virtual aperture provides higher angular resolution which also promotes the precision of DOA estimation, and the extra DOF brought by waveform diversity can be leveraged to focus energy in certain spatial region for better direction-finding capacity. However, beamspace methods which match certain beampatterns suffer from deteriorated performance and complexity in implementation, and the advantage of virtual array aperture is limited by its virtual element redundancy. As an important performance indicator of DOA estimation, Cramer–Rao Bound (CRB) is closely connected to the array configuration of the system. To reduce the complexity of the system and improve CRB performance at the same time, in this paper, the virtual array of MIMO radar is designed directly by selecting outputs from matched filters at the receive side. For the sake of fair comparison, both scenarios with and without priori directions are considered to obtain optimized virtual array configuration, respectively. The original combinatorial problems are approximated by sequential convex approximations methods which produce solutions with efficiency. Numerical results demonstrate that the proposed method can provide thinned virtual arrays with excellent CRB performance.

MIMO Radar Array Antenna with Transmit-Receive Subarrays

Unlike the PMIMO radar, the transmit-receive (Tx-Rx) subarrays MIMO (TRSM) radar uses overlaping subarrays in Tx and Rx array so that it simultaneously combines the main advantage of the phased array radar (PA) i.e., high directional coherent gain, and the main advantage of the MIMO radar i.e., high waveform diversity gain. This paper has derived the radar performance formula such that Tx-Rx gain and SINR. The approach aims to overcome the beam shape loss, increase the transmit-receive gain, minimize the maximum peak sidelobe levels, narrow the half power beamwidth, increase directivity, and increase signal-to-noise-plus-interference ratio (SINR). This radar's performance is compared to the PMIMO radar in various methods such as equal subarrays, unequal subarrays, and optimum partitioning, the PA radar, and the MIMO radar. The numerical simulation and evaluation results show that the proposed radar has several advantages such as lowest the peak sidelobe level, narrow the half power beamwidth, and high directivity, so it is very robust against interference effects. When compared to the OPPM radar, as representative of the best-performing radars, this radar has an average performance improvement of MPSLL, directivity, and HPBW which are 21.5 dB, 2 dB, and 0.45 deg, respectively.

Parameter Estimation and Target Detection of Phased-MIMO Radar Using Capon Estimator

Phased-Multiple Input Multiple Output (PMIMO) radar is multi-antenna radar that combines the main advantages of the phased array (PA) and the MIMO radars. The advantage of the PA radar is that it has a high directional coherent gain making it suitable for detecting distant and small radar cross-section (RCS) targets. Meanwhile, the main advantage of the MIMO radar is its high waveform diversity gain which makes it suitable for detecting multiple targets. The combination of these advantages is manifested by the use of overlapping subarrays in the transmit (Tx) array to improve the performance of parameters such as angle resolution and detection accuracy at amplitude and phase proportional to the maximum number of detectable targets. This paper derives a parameter estimation formula with Capon's adaptive estimator and evaluates it for the performance of these parameters. Likewise, derivation for expressions of detection performance such as the probability of false alarm and the probability of detection is also given. The effectiveness and validation of its performance are compared to conventional estimator for other types of radars in terms of the effect of the number of target angles, the RCS of targets, and variations in the number of subarrays at Tx of this radar. Meanwhile, the detection performance is evaluated based on the effect of Signal to Noise Ratio (SNR) and the number of subarrays at Tx. The evaluation results of the estimator show that it is superior to the conventional estimator for estimating the parameters of this radar as well as the detection performance. Having no sidelobe makes this estimator strong against the influence of interference and jamming so that it is suitable and attractive for the design of radar systems. Root mean square error (RMSE) on magnitude detection from LS and Capon estimators were 0.033 and 0.062, respectively. Meanwhile, the detection performance for this radar has the probability of false alarm above 10-4 and the probability of detection of more than 99%.

Simultaneous Excitation Systems for Ultrasonic Indoor Positioning

The ultrasonic technology is a tool for indoor positioning systems (IPSs) and has been extensively used in many applications. In ultrasonic IPSs (UIPSs), the use of a chirp signal (in which the frequency varies with time) is widespread due to its capability to obtain high-range resolution through its time-frequency characteristic. It also provides an opportunity to design effective waveform diversity which has always been the key to mitigating multiple-access interference (MAI) in multi-user UIPSs. To explore this, we analyze the chirp signal from the signal design perspective, with the goal of developing a precise and efficient UIPS for multi-user environments. To achieve this, three waveform diversity design schemes are proposed in which all the benefits of the classical chirp, such as high-range resolution, are retained while all the transmitters can transmit chirp signals simultaneously. In each scheme, a linear chirp is divided into two linear sub-chirps with diverse durations and/or bandwidths. This process is optimized by selecting the concatenated sub-chirps that generate a waveform which has a high-range resolution and relatively low interference in the same scheme. Initially, the effectiveness of the proposed schemes is evaluated for five simultaneous excitation signals using several metrics and experimental results are then presented for the ultrasonic indoor positioning.

Transmit beamspace design for FDA–MIMO radar with alternating direction method of multipliers

Hybridization of a frequency diverse array (FDA) generated range-angle-time dependent beampattern with multiple input multiple output (MIMO) radar waveform diversity, namely, FDA-MIMO radar, provides more degrees-of-freedom to improve overall system performance. As the beampattern optimization needs an efficient method to approximate the desired beampattern and simultaneously minimize the cross-correlation sidelobes, we optimize the transmit beampattern over the beamspace matrix in this paper. That is, the transmit FDA antennas are divided into multiple overlapping subarrays and each subarray transmits an orthogonal waveform towards the target, whereas the weight vectors of all subarrays jointly form a beamspace matrix. Since the optimization problem turns to be a non-convex problem with a constant energy constraint, we propose an efficient algorithm based on alternating direction method of multipliers (ADMM) to solve it, which has a very fast convergence speed. The effectiveness and superiorities of the proposed method over existing methods are verified by extensive simulation results.

Doppler Resilient Complementary Waveform Design for Active Sensing

Active sensing systems prefer probing waveforms with good auto-correlation properties. Faced with the difficulty in getting individual sequences with impulse-like aperiodic auto-correlation functions, we resort to the temporal waveform diversity based on complementary sets of sequences (CSS). It seems an ideal solution, but has rarely been used due to the extreme sensitivity of CSS to Doppler shifts. This paper is devoted to applying numerical methods to the design of Doppler resilient complementary waveforms (DRCW) whose ambiguity functions are almost free of range sidelobes along modest Doppler shifts. We formulate the problem as minimizing the worst-case peak sidelobe level (PSL) with low peak-to-average power ratio (PAR) constraints, and then approximate the min-max problem by a ${l}_{p}$ -norm minimization problem. A hierarchical strategy is developed to tackle the phase optimization and the amplitude-phase optimization respectively. Correspondingly, two algorithms based on phase-gradient and Majorization-Minimization are proposed, both of which can be efficiently computed via FFT. Numerical experiments show our method is effective and superior to existing ones. The DRCWs thus obtained have their range sidelobes at all lags suppressed below −60dB, and are expected to bring significant performance improvements in active sensing. Two application examples, detecting weak targets in heavy clutter and estimating the high-resolution range profile of an extended target by active radars, are given.

Golay Pairs Having Larger Swap Sets

Binary Golay pairs are examined with an interest in finding examples, in which members of the pair can be replaced with other codes sharing the same autocorrelation, but not due to negation and reversal. These are referred to as shared-autocorrelation (or shared-ACS) Golay pairs. Golay pairs can always formed from other Golay pairs by reversing and/or negating codes in those pairs; the discovery of other possibilities offers the promise of enhanced unpredictability by waveform diversity. A 1998 paper by Djokovic, which lists such pairs at lengths 32 and 40, is used as evidence that such pairs not only exist, but also likely exist in significant numbers. A common structure in these codes is determined, and conditions found for half-codes that can be employed to form larger examples. Furthermore, a necessary condition is found for code lengths allowing these shared-ACS Golay pairs.

The intrapulse beamsteering radar based on OFDM Chirp signals

The OFDM (Orthogonal Frequency Division Multiplexing) technique is widely used in communication field due to the great waveform diversity and anti-fading ability of OFDM signals. Because of the similarity of radar system and communication system, The OFDM radar has gradually emerged as a new system radar in recent years. At the same time, Intrapulse beamsteering system is a kind of multi-dimensional waveform encoding, which can improve the transmitting efficiency and multi-mission performance of radar. So, this paper proposes a new radar operating mode with the combination of the OFDM chirp signals and the intrapulse beamsteering in elevation, that is the combination of waveform orthogonality and beam orthogonality, we can also call it “double insurance”, which can greatly improve the emission efficiency and the system performance. Especially the interference suppression and the robustness to terrain undulation. The simulation results at the end of the paper prove the feasibility and advantages of the proposed operating mode.

Waveform Diversity-Based Generation of Convergent Beam Carrying Orbital Angular Momentum

The orbital angular momentum (OAM) in radio frequency is expected to provide a new dimension to the OAM mode and benefit the degree of freedom of the electromagnetic wave. However, a central energy hollow exists in the OAM-carrying beam, and its radius varies with the OAM mode values, which is the main bottleneck in the radio frequency OAM applications. This article proposes a solution to eliminate the central hollow, by fundamentally changing the transmitted signal of the uniform circular array (UCA). To generate the unidirectional beam pattern, defined as the beaming OAM (BOAM), the orthogonal waveforms are radiated by UCA with an equidifferent initial phase. The BOAM wave can be received by one single antenna, and the separate OAM mode demodulation is then achieved based on the waveform diversity. This article studies the theoretical model of the BOAM field distribution and shows that the waveform diversity among distributed units can not only alter the doughnut-like shape into the closed beam, but also maintain the intact vortex characteristic. On this basis, the extended OAM mode multiplexing system is designed with one single UCA. A 16-element UCA is shown to be capable of generating 15 OAM modes simultaneously, with all modes superposing on the same direction. Furthermore, it is verified by the simulation that the vortex phase of each single mode can be precisely extracted from the OAM-mode-multiplexing signal.

A Method of Improving the Correlation Properties of OFDM Chirp Waveform Diversity

In this letter, an orthogonal frequency-division multiplexing (OFDM) chirp waveform diversity design scheme is proposed to achieve better correlation properties. First, the subchirp duration and bandwidth are optimized simultaneously to increase the degree of freedom in waveform diversity. Second, an optimization procedure is proposed based on the idea of Gram-Schmidt orthogonalization. To reduce the problem complexity, the design problem is separated into several subproblems. Each subproblem corresponds to designing an OFDM chirp composite waveform and is optimized by the Pareto optimization framework. Simulation results show that the proposed method outperforms the conventional methods in terms of both autocorrelation peak sidelobe level (APSL) and cross correlation peak level (CPL) under the circumstance of the same mainlobe width.

Mismatched filter bank design for radio frequency interference suppression in multiple-input multiple-output over-the-horizon radar

Radio frequency interference (RFI) is a major problem in Multiple-Input Multiple-Output Over-the-Horizon Radar (MIMOOTHR). Selecting working frequencies and clean bandwidth in the complex electromagnetic environment is challenging using traditional frequency selection methods. We note that MIMOOTHR usually transmits Time-Staggered Linear Frequency Modulated Continuous Wave (TS-LFMCW) waveforms, so the target echo that occurs before matched filtering is a broadband (about tens of KHz) signal, while the RFI after frequency selection is usually a narrowband (about a few KHz) signal. To suppress the narrowband RFI and complete the waveform diversity process, the mismatched filter bank is designed to replace the original matched filter bank without changing the TS-LFMCW waveforms. First, we establish the mismatched filtering optimization problem for MIMOOTHR. We employ the output signal-to-interference-plus-noise ratio maximization criterion of adaptive RFI suppression and two constraints for adjustable target signal attenuation and TS-LFMCW waveforms diversity performance. Then, two analytic solutions to this constrained optimization problem are derived to guarantee the computational efficiency of the mismatched filtering. The proposed mismatched filtering methods can detect the target successfully in the presence of narrowband RFI. The RFI suppression performance, the waveform diversity performance and computational efficiency of the two mismatched filter banks are analysed using numerical results.

Time-Varying Nadir Echo Suppression for Spaceborne Stripmap Range Sweep Synthetic Aperture Radar via Waveform Diversity

The spaceborne stripmap range sweep synthetic aperture radar (SS-RSSAR) is a new operation mode for efficiently imaging the regions squinted with satellite orbit. By nonuniformly implementing azimuth sampling, the SS-RSSAR is able to avoid the problem of transmission blockage when the slant range varies seriously. However, this advantage is achieved at the cost of inducing time-varying nadir echo and makes the valid and the nadir echoes inseparable in the time domain. In this study, we propose to suppress the nadir echo of the SS-RSSAR via waveform diversity on transmission. The key mechanism is to always make the valid and the nadir echoes in the same pulse interval have different waveforms characterized by high autocorrelation but low cross correlation performances. In this way, the nadir echo can be effectively suppressed by performing matched filtering with respect to the valid echo. The main contributions of this study are twofold: the analyses for the lowest required waveform number and the strategy of achieving sufficient waveform patterns. The validity of the presented approaches is evaluated via the extended-target computer simulations.

Research on target detection technology of MIMO technology in vehicle collision avoidance

MIMO (multiple input multiple output) vehicle anti-collision radar refers to the application of MIMO system radar in vehicle anti-collision. MIMO radar is a new type of radar which has been put forward in recent years and attracted great attention. It is a kind of radar with multi transmitting and multi receiving. The characteristics of wave form diversity make it have many performance advantages, such as strong anti-jamming ability and strong resolution, compared with traditional radar. In recent years, many researchers have studied the application of MIMO Radar in the field of aviation and land transportation and made remarkable achievements. Compared with the traditional radar, the advantage of MIMO radar is due to the orthogonal performance of the transmitting waveform. The related problems of the pattern are deeply related to the resolution. As an important reference index in radar, the resolution is worthy of further study. In view of the above problems, the main work of this paper is as follows.