This thesis presents a simulation model of Stepped Frequency (SF) and Near Field BeamForming (NF BF) based stationary Through Wall Imaging (TWI) system to scan an object behind the wall for the reconstruction of 2D/3D image of it. The developed simulation model of TWI system requires neither the movement of the antenna array nor the object to reconstruct the image of the object behind the wall, thus overcoming the limitation of SAR/ISAR based TWI system. The simulation model of TWI system arrived at in this thesis facilitates the scan of the desired scenario in both azimuth and elevation to maximize the information available for more effective reconstruction of the Image of object behind the wall. The reconstruction of the image has been realized through conventional image processing algorithms which are devoid of inversion techniques to minimize the computational burden as well as the overall execution time of the TWI system. Contrary to the present TWI systems, the proposed simulation model has the capability for the reconstruction of the shape and contour of the object. In addition, the formulated simulation model of the TWI system overcomes the previously imposed constraints on the distances of separation between the object and the wall as well as the wall and the target. The simulation model of TWI of this thesis can handle arbitrary distances (far field or near field) between the antenna array and the wall as well as the wall and the object, which is not the case with the existing TWI systems. The thesis provides wave propagation analysis from the transmitting antenna array through the wall and the obstacle behind it and back to the receiver. Subsystems of TWI system like beamforming antenna arrays, wall and obstacles have been modeled individually. The thesis proposes a novel near field beamforming method that overcomes the usual requirement of 3D or volumetric near field radiation patterns of the beamforming array. Typical simulation results of
NF BF with linear and planar arrays reveal the beam formation at a distance of one wavelength from the aperture of the array and which corresponds to the ratio of observation distance to aperture of array to be 0.2334. As a supplement to the presented NF BF a generic and versatile procedure to compute near field radiation patterns of antennas with prior knowledge of its either field or current distribution over the radiating aperture is also proposed. Examples of reconstruction of images of typical 2D and 3D objects are also illustrated in the thesis.
Conclusion:
Chapter 7: Conclusion and Future Work
This chapter provides a summary of the inferences and technical conclusions derived from the
research undertaken in this thesis. Potential scope and academic avenues for further research
which emerge as logical extensions of the analytical formulations and simulations of this thesis
are also highlighted
7.1 Summary
The thesis proposes a new approach for the development of TWI system invoking near-field
beamforming techniques overcoming the limitations of SAR as well as ISAR based TWI systems.
The near-field beamforming technique does not require movement of the antenna array or the
objects to reconstruct the image of the object behind the wall. The proposed TWI system facilitates
the scan of the desired scenario in both azimuth and elevation. In the proposed approach, the
motion required by the conventional SAR and ISAR based TWI has successfully been replaced
by near-field beamforming technique involving electronic steering of the main beam of the
beamforming antenna array. Unlike the existing TWI systems reported in the literature, the
simulation model of TWI system of this thesis can treat any distance of separation(far field or near
field) between the antenna array and the wall as well as the wall and the object. The thesis provides
the design and development of a novel simulation model of a TWI system which uses Stepped
Frequency and Near Field — BeamForming (NF-BF) techniques to scan a through the wall scene
and reconstruct 2D image of it. The reconstruction of the image of the object behind the wall has
been realized through conventional digital image processing techniques which are devoid of
inversion techniques which are as explained in section 6.5.
The proposed new approach to image objects behind the wall is built upon the fundamental
principles of antennas, wave-propagation, beamforming, signal and image processing. Structures
like wall and objects behind the wall were modeled considering electromagnetic properties like
permeability, permittivity and conductivity. The objects behind the wall were modeled to be made
of Carbonyl metal, Rubber, Graphite, Borosilicate glass and Aluminium. The various optical
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phenomena like reflection, refraction, scattering and attenuation induced by the medium of wave
propagation were incorporated in the simulation model.
The advantageous features of adapting the stepped frequency waveform generation as well as the
application of image registration in the improvement of reconstructed image have also been
demonstrated through numerous case studies. Simulation results of TWI associated with Stepped
Frequency technique showed that images obtained at lower frequencies are of higher quality
compared to the one higher frequencies. Fusion of the images obtained from the desired range of
frequencies resulted in a high resolution image providing a detailed contour of the shape of the
imaged structure. The reconstructed image also indicates the variation in composition of the
material of the target by change in pixel intensity. Image reconstruction of curved bodies or 3D
surfaces behind the wall was carried out and the simulation results illustrated that the reconstructed
image was only due to field contributions from the lit region of the target with no contributions
from shadow region. The simulation results also demonstrated the satisfactory reconstruction of
the shape and contour of the imaged 3D structure using the image intensity as a parameter that
varies with the depth of the object.
The developed simulation model of TWI radar system has planar array of circular waveguides as
antenna elements radiating in X- band (8 – 12GHz) to carryout signal transmission, beamforming,
reception and image processing. An evaluation of an estimate of total signal loss a typical TWI
system would undergo was carried out and it was found that only 1% of the actual signal
transmitted power is received back and the received signal with such an extremely low amplitude
is processed to realize the formation of the images. The received signal when subjected to a
combination of image translation and fusion methods resulted in improved reconstruction of
image of the target/object than possible with fusion method alone. The calculated visual accuracy
based on the difference in pixels with the actual scene of the image reconstruction was 81%.
In summary, this thesis is associated with the following original contributions: - · Mathematical and simulation model of near field beamforming based stationary TWI system with Two Dimensional beam steering capability to reconstruct the image of the object including its shape and contour
- · Novel near field Beamforming method that overcomes the usual requirement of 3D or volumetric near field radiation patterns of the beamformimg array
- · New imaging methodology involving fusion and translation methods to develop high resolution images of behind the wall scenarios with the amplitude of received signals
- · Generic and versatile new procedure to compute near field radiation patterns of antennas with prior knowledge of its either field or current distribution over the radiating aperture
- · Novel scheme which employs the hybrid combination of Geometrical Optics (Ray Technique) and Physical Optics techniques to reduce the computational burden in the wave propagation studies involved in the development of simulation model of TWI system 7.2 Conclusions This section provides with the observations, derived inferences and reasoned interpretations associated with the research findings in near field analysis of antennas, near field beamforming and mathematical modelling of TWI system. 7.2.1 Near Field Analysis of Antennas
- The following are the technical inferences and interpretations associated with near field analysis of antennas
- · A generalized procedure for the determination of near field radiation patterns of an antenna at any arbitrary distance is presented and the proposed approach is applicable to wide variety of antenna elements. The proposed procedure has a requirement of prior knowledge of either the current or field distribution on the radiating aperture
- · The underlying principle of the generalized procedure tantamount to considering the radiating aperture as an array of point electric and magnetic dipoles. The radiation pattern of the antenna at any arbitrary point of observation (valid for both near field as well as far field) is obtained by the vector summation of the fields radiated by the array of both electric and magnetic dipoles located on the aperture of the antenna.
- · The significance of the proposed analysis lies in the derivation of explicit expressions for the co- polar and cross polar components of the radiation pattern of both electric and magnetic dipoles.
- · With circular cylindrical waveguide and conical horn as radiating elements in the near field beamforming analysis, the generic and versatile features of proposed novel approach for the determination of near field radiation pattern of these antennas have been substantiated
- · The simulation results of the proposed near field analysis with the radial distance of observation of 1000 λ show excellent correlation with the results derived through analytical formulation of (Balanis C.A., 2005) which is exclusively valid for the far field
- · At a distance of 0.2 λ from the aperture, the radiation patterns of circular waveguide are vastly/drastically different from the corresponding conventional far field patterns. The broadside radiating feature of the circular waveguide is also absent
- · With the gradual increase in the radial distance of observation, one notices the increasing trend in the radiation patterns of circular waveguides to broadly conform to the profile of conventional far field patterns
- · The radiation patterns of circular waveguide at r=0.75λ for (Distance of Observation /Diameter) = 0.571 nearly resemble the corresponding conventional far field patterns
- · For a conical horn with a radius of 4.1 cm, the radiation patterns at r=2λ (Distance of Observation /Diameter)=0.781 show defined main lobe 7.2.2 Near Field Beamforming Analysis The following are the technical inferences and interpretations associated with the analysis of near field beamforming
- · A novel approach for near field beamforming is proposed that is also valid for far field. This feature can be attributed to the following:
- · Computation of beamforming weights directly at the specified arbitrary near
- field distance
- · Generalization of near-field array factor and near-field beam steering phase
- factor to overcome the assumption of planarity of the wave-front impinging on
- the radiating aperture of the array elements.
- · Unlike the method of radial reciprocity (Kennedy R.A. et al, 1999), the proposed method
- does not mandate the requirement of 3D near field radiation pattern of the beamforming
- array.
- · Simulation results derived through proposed near field beamforming technique with radial
- distance ‘RNF ‘ equal to or greater than conventional far field distance criterion show
- perfect correlation with the results obtained through alternative formulations in the open
- literature meant exclusively for far field beamforming or beamsteering technique.
- · Numerous simulations confirmed that the proposed technique yields reasonably
- encouraging beamforming results even at a distance of RNF=1λ from the aperture of (5×1)
- linear or (5×5) planar array. This corresponds to (RNF/Aperture of Array)=0.2334
- · At distances greater than RNF=1λ, the radiation patterns exhibit more pronounced structure
- of sidelobes
- · Below the distance of RNF=1λ, simulation results reveal the radiation pattern featured with
- less directive main beam
- · The significant differences in the radiation patterns of the near field and far field
- beamformers lie in the beamwidth of the mainlobe and the asymmetry in the radiation
- pattern in case of beamsteering off the boresight direction
- · Typical simulation studies on the radiation characteristics of planar array to illustrate the
- realizable two dimensional near field beamsteering have also been presented to validate
- the concept
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- 7.2.3 Mathematical Modelling of TWI
- The following are the technical inferences and interpretations associated with mathematical
- modelling of a TWI system
- · This thesis proposed a new approach for the development of TWI system invoking nearfield
- beamforming techniques overcoming the limitations of SAR as well as ISAR based
- TWI systems
- · The near-field beamforming technique does not require movement of the antenna array or
- the objects to reconstruct the image of the object behind the wall. The motion required by
- the SAR and ISAR has successfully been replaced by near-field beamforming technique
- involving electronic steering of the main beam of the beamforming antenna array
- · Unlike the previously reported research work on TWI, the novelty of the proposed TWI
- system in capturing the shape and contour of the object behind the wall has been
- demonstrated through extensive simulation studies
- · Subjecting the received signal (data) to translation and then fusion method showed more
- accurate reconstruction of the imaged object compared to fusion method alone. The
- calculated visual accuracy based on the difference in pixels with the actual scene of the
- image reconstruction is 81%
- · The advantageous features of adapting the stepped frequency waveform generation as well
- as the application of image registration in the improvement of reconstructed image have
- also been demonstrated through numerous case studies
- · Simulation results reveal that the reconstructed images obtained at lower frequencies are
- of higher quality compared to the one obtained at higher frequencies
- · Mean Square Error (MSE) and Peak Signal to Noise Ratio (PSNR) of reconstructed image
- clearly indicated that MSE progressively decreases with increase in frequency while its
- PSNR degrades
- · The simulated results of the reconstructed image of the object behind the wall derived
- through the proposed near-field beamforming technique shows a superior performance
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- over an existing system (Aftanas, 2010) which conclusively establishes the potential utility
- as well as the novelty of the proposed approach
- 7.3 Recommendation for Future Work:
- Suggestions for potential avenues to augment the existing body of knowledge are a significant
- feature of research philosophy.
- TWI is relatively a new and an emerging technology showing prominent promise for rescue and
- security applications. The analytical formulations and simulation studies of the thesis have clearly
- proved that the accuracy and precision of reconstructed image of the behind the wall scenario
- depends on the effectiveness of invoked analysis of electromagnetic wave propagation as well
- as algorithms of advanced signal and image processing techniques.
- Following are the suggestions/recommendations for the further pursuance of the research topic of
- this thesis.
- · The proposed model of TWI did overcome the requirement of motion of the radar(antenna
- array) and the object for the formation of image thereby circumventing one of the
- limitations of the SAR/ISAR based TWI system. The thesis has considered only the
- stationary target. However, an extension of the proposed analysis and simulation to deal
- with the non-stationary (moving) object can be a next logical step to pursue to enhance the
- scope of this thesis
- · In the presented analysis of the thesis, multiple reflections or the scattering between the
- target and the surrounding wall have not been considered. An analysis of improvement in
- the accuracy and resolution of reconstructed image through the inclusion of the multiple
- scattering in the analysis will be a significant academic contribution
- · The Image processing algorithms in the thesis did consider only the amplitude of the signal
- at the receiver points. Further analysis and simulation to consider the utility of phase
- information of the received signal in reconstruction process of the image can be of interest
- both from system and academic perspective
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- · Extension of the analysis for the estimation of the distance between the TWI radar and the
- target as well as the analysis to determine the size of the object behind the wall can be a
- considerable academic value to the TWI system
- · Formulation of an analysis and modelling of a near field beamforming based TWI system
- has been the principal theme of this thesis. In the formulation as well as in the simulation.
- ‚practicability of the implementation of the proposed TWI has always been a consideration
- throughout course of this research study. It will be of great practical utility to undertake an
- experimental investigation of the same topic to substantiate the novelty and to analyse the
- efficacy of implementation
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