Null-steering Beamformer Research Articles

A Novel Realistic Approach of Adaptive Beamforming Based on Deep Neural Networks

A new deep neural network (NN) approach applied to antenna array adaptive beamforming is presented in this article. A recurrent NN (RNN) based on the gated recurrent unit (GRU) architecture is used as a beamformer in order to produce proper complex weights for the feeding of the antenna array. The proposed RNN utilizes four hidden GRU layers and one extra layer for linear transformation. The produced weights are subsequently compared with respective weights derived by a null steering beamforming (NSB) technique in order to measure the accuracy of the RNN. The RNN training is performed by using a large dataset derived from an NSB technique applied to a realistic microstrip linear antenna array, in order to consider real-world effects, such as the nonisotropic radiation pattern of an array element and the mutual coupling between the array elements. The RNN performance is examined by using the root-mean-square error metric, whereas its beamforming performance is evaluated by estimating the mean value and the standard deviation of the divergences of the main lobe and nulls directions from their respective desired directions. A comparison between various NN structures and an overall study of the proposed RNN-based beamformer are also presented.

Improved Beamforming in 3D Space Applied to Realistic Planar Antenna Arrays by Using the Embedded Element Patterns

Most of beamforming methods rely on the proper estimation of the steering vectors of the incoming signals in order to calculate appropriate array feeding weights. However, this estimation is often performed on a theoretical basis, since most methods assume that the array elements radiate like isotropic sources and consider only the phase differences between them due to their different spatial positions. This is a rough estimation as it ignores both the non-isotropic radiation pattern of the array elements and the mutual coupling between them. To overcome these limitations, we use a different approach for the construction of steering vectors based on the embedded patterns of the array elements, extracted through full-wave analysis. This approach is used in this paper to modify two well-known beamforming methods, namely the null steering beamforming and the minimum variance distortionless response method. Both modified methods are applied to a realistic model of a microstrip planar antenna array, and thus, both the azimuth and polar angles of the main lobe and nulls directions are controlled (beamforming in 3D space). Extensive statistical analyses are performed by implementing several scenarios of escalating difficulty, in order to examine the performance and temporal response of the modified beamforming methods in comparison to the respective conventional methods and also in comparison to beamforming methods based on evolutionary techniques.

Null broadened–deepened array antenna beamforming for GNSS jamming mitigation in moving platforms

Abstract Steering vector and covariance matrix estimation mismatch along with moving jammer/platform, can affect the performance of null-steering beamformer. In this paper, by utilizing null-widening, null-deepening and diagonal loading shrinkage in space–time adaptive processing (STAP) structure, two methods for nullifying GNSS (global navigation satellite system) jamming/interference in cruise moving platforms are introduced. The enhanced performance of the proposed methods is investigated through numerical simulations. The results indicate improvement in the number of acquired GNSS satellites, in heavy jamming (GNSS denied) scenario, by reconstructing and acquiring at least 87% of satellites.

Null-Steering Beamformers for Suppressing Unknown Direction Interferences in Sidelobes

This paper proposes an efficient null-steering beamformer for adaptive pattern synthesis of uniform linear arrays based on binary bat algorithm (BBA) and the amplitude-only control of array excitation weights. The proposed beamformer is able to suppress unknown direction interferences in the sidelobes while simultaneously maintaining the main lobe and suppressing the sidelobes. The performance of the proposal has been investigated via a couple of scenarios including operation speed and adaptive null-steering ability with or without the effect of mutual coupling in half-wave dipole uniform linear arrays (DULA). The simulation results have proved that the proposed beamformer is a promising approach for adaptive pattern synthesis in the case of interference suppression. In addition, this proposal outperforms those using binary particle swarm optimization (BPSO) on the operation speed and null-steering efficiency.

Beamforming with Cube Microphone Arrays Via Kronecker Product Decompositions

Microphone arrays combined with beamforming have been widely used to solve many important acoustic problems in a wide range of applications. Much effort has been devoted in the literature to microphone array beamforming, among which the Kronecker product beamforming method developed recently has demonstrated some interesting properties. Generally, this method decomposes the global beamforming filter into a Kronecker product of a number of sub-beamforming filters, each of which corresponds to a virtual subarray and can be designed individually. This decomposition not only reduces significantly the number of beamforming coefficients, but also can be explored to improve the robustness and flexibility of beamforming. This paper extends Kronecker product beamforming from two-dimensional arrays into three-dimensional cube arrays. We consider two decompositions, i.e., fully and partially separable ones. The former decomposes the entire array into three linear subarrays while the latter decomposes the entire array into a linear subarray and a planar one. Then, for each case, we derive the Kronecker product maximum white noise gain beamformer, the Kronecker product approximate maximum directivity factor (DF) beamformer, the Kronecker product null-steering beamformer, and the Kronecker product iterative maximum DF beamformer. Simulation results demonstrate the properties and advantages of the proposed beamformers.

Noninvasive measurement of sensory action currents in the cervical cord by magnetospinography

ObjectiveTo obtain magnetic recordings of electrical activities in the cervical cord and visualize sensory action currents of the dorsal column, intervertebral foramen, and dorsal horn. MethodsNeuromagnetic fields were measured at the neck surface upon median nerve stimulation at the wrist using a magnetospinography system with high-sensitivity superconducting quantum interference device sensors. Somatosensory evoked potentials (SEPs) were also recorded. Evoked electrical currents were reconstructed by recursive null-steering beamformer and superimposed on cervical X-ray images. ResultsEstimated electrical currents perpendicular to the cervical cord ascended sequentially. Their peak latency at C5 and N11 peak latency of SEP were well-correlated in all 16 participants (r = 0.94, p < 0.0001). Trailing axonal currents in the intervertebral foramens were estimated in 10 participants. Estimated dorsal–ventral electrical currents were obtained within the spinal canal at C5. Current density peak latency significantly correlated with cervical N13–P13 peak latency of SEPs in 13 participants (r = 0.97, p < 0.0001). ConclusionsMagnetospinography shows excellent spatial and temporal resolution after median nerve stimulation and can identify the spinal root entry level, calculate the dorsal column conduction velocity, and analyze segmental dorsal horn activity. SignificanceThis approach is useful for functional electrophysiological diagnosis of somatosensory pathways.

Echo Separation for Space-Time Waveform-Encoding SAR With Digital Scalloped Beamforming and Adaptive Multiple Null-Steering

Space-time waveform-encoding (STWE) synthetic-aperture radars (SARs) can realize waveform diversity in the space-time domain and effectively improve the system performance. However, the benefits of the STWE SAR are based on the accurate separation of the overlapped echoes. Digital beamforming (DBF) in elevation using the null-steering techniques has the potential for echo separation in the STWE SAR. However, the performance of the conventional null-steering beamformer deteriorates when it meets with the extended pulse. Constant time delay of the signal in each channel is usually used to relieve the influence of the extended pulse in the DBF SAR. However, for the STWE SAR in elevation with multiple subswaths, the used delay values are only compatible for one single imaging swath. This letter proposes an innovative echo separation beamformer with digital scalloped beamforming (DSBF) and adaptive multiple null-steering for the STWE SAR. The proposed beamformer can relieve the influence of the pulse-extension loss (PEL) for the echo to be extracted and much better suppress the interfering echoes compared with the conventional methods. Simulation results show the effectiveness of the proposed beamformer. The proposed beamformer enables perfect echo separation for the STWE SAR and makes the STWE SAR a more promising technique for future SAR missions.

Distributed Null-Steering Beamformer Design for Physical Layer Security Enhancement in Internet-of-Things Networks

With the rapid proliferation of the Internet of Things (IoT), small devices in various applications generate, process, and exchange security and safety-critical data as well as privacy-sensitive information. However, due to limited hardware capabilities of such small devices, the IoT networks are inherently vulnerable to security attacks. In this context, the analog cooperative beamforming (ACB)-based physical layer security (PLS) technique can be a suitable solution to guarantee security in the IoT networks by creating a virtual antenna array with multiple IoT devices. Nevertheless, such techniques suffer from poor secrecy performance, when an eavesdropper is located within the main beam toward the legitimate receiver. Motivated by this limitation, in this article, we propose a fully distributed null-steering beamformer, which is suitable for IoT networks following decentralized architecture. Through analysis and simulation, we show that the proposed PLS algorithm outperforms the existing ACB-based PLS scheme. Furthermore, we show that there exists an optimal degree of nulling that maximizes the secrecy performance, which can be estimated based on the analytical results.

Acoustic Feedback Suppression for Multi-Microphone Hearing Devices Using a Soft-Constrained Null-Steering Beamformer

Acoustic feedback occurs in hearing aids due to the coupling between the hearing aid loudspeaker and microphone(s). In order to reduce the acoustic feedback, adaptive filters are commonly used to estimate the feedback contribution in the microphone(s). While theoretically allowing for perfect feedback cancellation, in practice the adaptive filter typically converges to a biased optimal solution due to the closed-loop acoustical system of the hearing aid. Previously it has therefore been proposed to suppress the acoustic feedback contribution for an earpiece with multiple integrated microphones and loudspeakers using a fixed null-steering beamformer and hence avoiding a biased adaption. While previous null-steering beamforming approaches aimed at perfect preservation of the incoming signal using its relative transfer function (RTF), in this article we propose to use a soft constraint that allows to trade off between incoming signal preservation and feedback suppression. We formulate the computation of the beamformer coefficients both as a least-squares optimization procedure, aiming to minimize the residual feedback power, and as a min-max optimization procedure, aiming to directly maximize the maximum stable gain of the hearing aid. Experimental evaluations were performed using measured acoustic feedback paths from a custom earpiece with two microphones in the vent and a third microphone in the concha. Results show that the proposed fixed null-steering beamformer using the RTF-based soft constraint provides a reduction of the acoustic feedback by 7–8 dB compared to the previously proposed RTF-based hard constraint while limiting the distortions of the incoming signal in the beamformer output.

Diagnosis of C8 radiculopathy by magnetospinogram

It is difficult to detect lesion proximal to Erb’s point in peripheral nerve by conventional electroneurophysiological study. Here, we report two cases of C8 radiculopathy diagnosed by our prototype magnetospinogram (MSG) system with highly sensitive SQUID sensors. Case 1: A 57 year-old-male presented with weakness and sensory disturbance in the right upper limb for 5 months. Case 2: A 56 year-old-male presented with weakness and sensory disturbance in the left upper limb for 2 and half years. In both cases, neurological findings, nerve conduction test and needle electromyogram findings suggested a disturbance between the proximal part to C8 dorsal root ganglion and the spinal cord. The neuromagnetic fields were measured from the brachial plexus and cervical cord in response to electrical supramaximal stimulation of the ulnar nerve at the elbow by using a newly developed 132-channel MSG system. The action currents were reconstructed from the evoked magnetic field by a spatial filter, recursive null steering beamformer and superimposed on the X-ray image. Virtual electrodes were set along nerve travel and equivalent current waveforms at each site were determined and compared. [Results] In both cases no conduction failure was pointed out in the affected side brachial plexus and the estimated electrical currents in the affected side C8 intervertebral foramen were smaller than that on the non-affected side. Because MSG can evaluate nerve activity sequentially with high spatial resolution, we believe it will be useful in clinical diagnosis.

Null-Steering Beamformer-Based Feedback Cancellation for Multi-Microphone Hearing Aids With Incoming Signal Preservation

In hearing aids, acoustic feedback occurs due to the coupling between the hearing aid loudspeaker and microphone(s). In order to reduce the acoustic feedback, adaptive filters are commonly used to estimate the feedback contribution in the microphone(s). While theoretically allowing for perfect feedback cancellation, in practice the adaptive filter converges to an optimal solution that is typically biased due to the closed-loop acoustical system of the hearing aid. In order to avoid the adaptation to a biased optimal solution, in this paper we propose to use a fixed beamformer to cancel the acoustic feedback contribution for an earpiece with multiple integrated microphones and loudspeakers. By steering a spatial null in the direction of the hearing aid loudspeaker, we show that theoretically perfect feedback cancellation can be achieved. While previous null-steering beamforming approaches did not control for distortions of the incoming signal, in this paper we propose to incorporate a constraint based on the relative transfer function (RTF) of the incoming signal, aiming to perfectly preserve this signal. We formulate the computation of the beamformer coefficients both as a least-squares optimization procedure, aiming to minimize the residual feedback power, and as a min–max optimization procedure, aiming to directly maximize the maximum stable gain of the hearing aid. Experimental results using measured acoustic feedback paths from a custom earpiece with two microphones in the vent and a third microphone in the concha show that the proposed fixed null-steering beamformer using the RTF-based constraint provides a reduction of the acoustic feedback and substantially increases the added stable gain while preserving the incoming signal. This can even be achieved for unknown acoustic feedback paths and incoming signal directions.

Strong coherent interference suppression based on second- order cone null steering beamforming

For active sonar systems, coherent interference, such as direct-path wave and strong multi-path, will shield the weak target echo signal, which is one of the main factors affecting the target detection performance. Null steering beamforming is an important signal processing method that suppresses strong interference. In this paper, the second-order cone programming (SOCP) method is used to perform null steering beamforming on the uniform linear array. Beam null is caused in the direction of interference to suppress strong coherent interference in the active sonar system during detection. However, there are two limitations that can causes decline of the algorithm performance. A norm-control method is proposed to limit the response near the end-fire direction of linear array, and an array manifold compensation method is introduced to solve the array manifold mismatch. Simulated and experimental data was used to evaluate the performance and verify the feasibility of the methods.

Null-Steering Beamforming for Enhancing the Physical Layer Security of Non-Orthogonal Multiple Access System

This paper addresses enhancing the physical layer security of a downlink non-orthogonal multiple access systems. The proposed scheme consists of a base station, multiple legitimate users, and an eavesdropper. In each transmission slot, the base station communicates with two paired users, under the malicious attempts of the eavesdropper to intercept the information messages. In order to impair the eavesdropper's channel without affecting the legitimate paired users, a jamming signal is injected into the system by means of null-steering beamforming. In null-steering precoding, the jamming signal is directed toward the malicious node while being suppressed in the legitimate users' directions. Null-steering jamming is exploited in two different mainstreams, namely, self-cooperative and nonself-cooperative jamming. In the self-cooperative strategy, the base station implements the null-steering beamformer to transmit the jamming signal during the information exchange with the legitimate users, whereas in the nonself-cooperative jamming scheme, idle legitimate users (helpers) utilize the harvested energy in the first phase to transmit the jamming signal toward the eavesdropper in the second phase during the base station communication with the legitimate users. The outage behavior in the two considered scenarios is investigated, and the secrecy outage probability for both paired users is derived and provided in closed-form expressions. Numerical simulations are performed to validate the derived analytical results and to compare the two considered strategies secrecy performance. Comparisons yield that self-cooperative jamming has better outage behavior unless the number of helpers in the system is large enough.

A Game-Theoretical Modelling Approach for Enhancing the Physical Layer Security of Non-Orthogonal Multiple Access System

This paper investigates the physical layer security of a downlink non-orthogonal multiple access (NOMA) communication system, wherein a base station is communicating with two paired active users in the presence of an eavesdropper and multiple idle nodes (helpers). In order to enhance the secrecy performance, a two-phase harvest-and-jam null-steering jamming technique is deployed. In the first phase, the base station provides the helper with power in addition to active users and eavesdropper’s information via simultaneous wireless information and power transfer technique. The helpers exploit the harvested energy and the information received in the first phase to build a null-steering beamformer and jam the eavesdropper, during the information exchange between the base station and the legitimate users in the second phase. A game theory is introduced to the proposed scheme, and the base station-helpers interactions are modeled as a Stackelberg game, where the helpers play the leader role and the base station is the follower. The utility functions of both the leader and follower are formed, and the Stackelberg equilibrium is reached by means of the backward induction technique. The proposed scheme demonstrates better secrecy performance when compared with the artificial noise-aided secure NOMA system.

Methods of suppressing tow ship noise with a horizontal linear array

Tow ship noise seriously influence the detection and direction of arrival (DOA) estimation performance of towed array. Null steering beamforming(NSBF), inverse beamforming(IBF) and non-data based beam-space transformation(NDBBT) are applied to suppress interference of tow ship in this paper. These methods are evaluated according to the improvement of signal interference noise ratio and DOA estimation precision. Simulation and experimental results show that all three methods can suppress interference of tow ship effectively so as to improve DOA estimation precision. In addition, IBF and NDBBT do better in suppressing interference than NSBF.

Magnetospinography visualizes electrophysiological activity in the cervical spinal cord

Diagnosis of nervous system disease is greatly aided by functional assessments and imaging techniques that localize neural activity abnormalities. Electrophysiological methods are helpful but often insufficient to locate neural lesions precisely. One proposed noninvasive alternative is magnetoneurography (MNG); we have developed MNG of the spinal cord (magnetospinography, MSG). Using a 120-channel superconducting quantum interference device biomagnetometer system in a magnetically shielded room, cervical spinal cord evoked magnetic fields (SCEFs) were recorded after stimulation of the lower thoracic cord in healthy subjects and a patient with cervical spondylotic myelopathy and after median nerve stimulation in healthy subjects. Electrophysiological activities in the spinal cord were reconstructed from SCEFs and visualized by a spatial filter, a recursive null-steering beamformer. Here, we show for the first time that MSG with high spatial and temporal resolution can be used to map electrophysiological activities in the cervical spinal cord and spinal nerve.

Full‐duplex MIMO system based on antenna cancellation technique

The performance of an antenna cancellation technique for a multiple-input-multiple-output (MIMO) full-duplex system that is based on null-steering beamforming and antenna polarisation diversity is investigated. A practical implementation of a symmetric antenna topology comprising three dual-polarised patch antennas operating at 2.4 GHz is described. The measurement results show an average of 60 dB self-interference cancellation over 200 MHz bandwidth. Moreover, a decoupling level of up to 22 dB is achieved for MIMO multiplexing using antenna polarisation diversity. The performance evaluation of the proposed technique was performed by simulation and measurement and the results have good agreement.

Source ambiguity resolution of overlapped sounds in a multi-microphone room environment

When several acoustic sources are simultaneously active in a meeting room scenario, and both the position of the sources and the identity of the time-overlapped sound classes have been estimated, the problem of assigning each source position to one of the sound classes still remains. This problem is found in the real-time system implemented in our smart-room, where it is assumed that up to two acoustic events may overlap in time and the source positions are relatively well separated in space. The position assignment system proposed in this work is based on fusion of model-based log-likelihood ratios obtained after carrying out several different partial source separations in parallel. To perform the separation, frequency-invariant null-steering beamformers, which can work with a small number of microphones, are used. The experimental results using all the six microphone arrays deployed in the room show a high assignment rate in our particular scenario.

Echo Separation in Multidimensional Waveform Encoding SAR Remote Sensing Using an Advanced Null-Steering Beamformer

In order to reap the potential benefits that waveform diversity can provide for spaceborne synthetic aperture radar remote sensing, echoes from different subpulses constituting a complete transmit waveform should be effectively separated at first. This paper presents a new separation approach implemented by an advanced null-steering beamformer on satellite. Compared with common null-steering beamforming, our approach will take into account the characteristics of echo signal from the scene and accordingly embed the finite-impulse response (FIR) filtering process into the modified null-steering beamformer to deal with the issue of pulse extension. In this paper, echo signals generated by multidimensional encoded waveform will be analyzed in detail; based on this analysis, FIR filter and the new null-steering beamformer are derived. Simulation results show that much better separation performance can be obtained by our approach than by conventional null-steering beamforming.

Performance Tradeoffs Offered by Beamforming in Cognitive Radio Systems: An Analytic Approach

This paper studies the design of beamforming weights for a multi-antenna secondary transmitter in an underlay cognitive setting that simultaneously maximizes the secondary received-power while limiting the primary interference to some threshold ∈. With perfect channel state information (CSI), a closed-form expression for the maximum secondary received-power is found. Under imperfect CSI and when the beamforming weights are computed using the channel estimates, the actual secondary received-power, G, and the actual primary interference-power, I, are derived. We show that the mean E[G] has a term that grows linearly with the number of secondary antennas, N, and additional terms dependent on ∈. Consequently, we obtain tradeoffs between E[G] and c. Under perfect CSI, we show that small increases in c from zero lead to moderate enhancements in E[G] for small N. However, increasing N reduces the enhancements. Under imperfect CSI, the gain in E[G] is less compared to the perfect CSI case. Furthermore, we show that the dominant parts of E[I] are independent of N. Thus, we conclude that there is no significant loss for the secondary to perform null-steering beamforming instead. Moreover, it can employ additional antennas to improve E[G] without generating significant extra interference on the primary.