Home > All Issues > PIER C
ISSN: 1937-8718

Vol. 101

Front:[PDF file] Back:[PDF file]
Latest Volume
All Volumes
All Issues

2020-05-16 PIER C Vol. 101, 261-273, 2020. doi:10.2528/PIERC20040701

UWB Microstrip-Fed Slot Antenna with Improved Bandwidth and Dual Notched Bands Using Protruded Parasitic Strips

Naser Ojaroudi Parchin, Haleh Jahanbakhsh Basherlou, and Raed A. Abd-Alhameed

In this research work, a new and simple design method of a compact slot antenna with dual notched bands is demonstrated for ultra-wideband (UWB) wireless networks. The presented antenna design is printed on a low-cost FR-4 substrate. Initially, an antenna with improved impedance bandwidth is designed. This is archived by employing the an extra slot with two T-shaped strips which increases the upper-frequency band of the design from 9 to 15 GHz. Later, undesirable bands including 4 GHz C-band, worldwide interoperability for microwave access (WiMAX) at 3.5/5.5 GHz (3.3 to 3.7 GHz and 5.15-5.85 GHz), wireless area network (WLAN) systems at 5-6 GHz (5.15-5.35 and 5.725-5.825 GHz) are eliminated by modifying the upper layer of the antenna using the protruded L-shaped strips inside the square radiation stub and the protruded E-shaped strip inside the feed-line. The proposed antenna offers quite good fundamental properties in terms of impedance bandwidth, gain, fidelity, radiation pattern, etc. A good agreement is observed between the measured and simulated results. Due to the simple structure and excellent performance of the design with controllable band-notch function, the presented microstrip antenna is useful for modern UWB wireless networksand can be an attractive

2020-05-12 PIER C Vol. 101, 247-259, 2020. doi:10.2528/PIERC20031604

Characterization Study of Mutual Coupling Between Monopole Antennas on Finite Ground Plane at Out of Band Resonant Frequencies

Prasobh Martin, Kamla Prasan Ray, and Nalli Naga Satya Sai Rama Krishna Prasad

When multiple antennas, operating at different frequencies, are installed on a single platform where the typical inter antenna spacing is a few wavelengths at the lowest frequency, the mutual coupling between the antennas can be optimized by the suitable selection of frequencies and the separation of adjacent antennas. This paper characterizes the dependency of mutual coupling between monopoles on the frequency and separation of the radiating/interfering monopole as well as on the size and shape of the ground plane. The out of band (off-band) characteristics of the monopoles are studied, and the effect of frequency offset between the adjacent monopoles on off-band mutual coupling is summarized. The off-band mutual coupling is reduced by more than 15 dB when the adjacent antenna frequency is selected to be near the fourth harmonic. In the case of smaller ground planes, better isolation of more than 20 dB is possible at intermediate antenna spacing than at the edges. The effect on radiation pattern of an antenna by the proximity of nearby antennas is also studied. The operating frequency/resonant length of the nearby antenna and the inter antenna spacing are found to be the key factors causing variation in radiation pattern. Lower off-band interfering antenna of bigger size is found to have significant effect on radiation pattern at spacing less than 2λ. Analysis has been carried out using FEKO, whose findings are validated using another software HFSS and measurements.

2020-05-11 PIER C Vol. 101, 233-245, 2020. doi:10.2528/PIERC20012801

Adaptive Polarimetric Detection for MIMO Radar and its Optimal Polarimetric Design in Compound-Gaussian Clutter

Zhikun Chen and Yinan Zhao

This study addresses the problem of adaptive polarimetric detector (APD) and optimal polarimetric design for the distributed multiple-input-multiple-output radar in compound-Gaussian clutter with inverse-gamma distributed texture component. We derive the APD by maximizing a posteriori estimation and performing a generalized likelihood ratio test. The false alarm probability for the detector is analyzed to validate the corresponding constant false alarm rate property. Furthermore, based on the concepts of game theory, we formulate an optimal polarimetric design as a two players zero-sum game, which further improves the performance of the proposed detector. Simulation results show that the proposed detector outperforms its counterparts, and the optimal polarimetric design algorithm can efficiently enhance the detection performance.

2020-05-11 PIER C Vol. 101, 219-232, 2020. doi:10.2528/PIERC20031401

Analysis and Optimization of Double-Side Hybrid Excitation Flux-Switching Motor

Yunyun Chen, Tongle Cai, Jiahong Zhuang, and Xiaoyong Zhu

In this paper, a double-side hybrid excitation flux-switching (DSHE-FS) motor employing a double stator structure with special multi-excitations is presented. The high space utilization improves the torque density and power density of DSHE-FS motor. The addition of non-rare-earth permanent magnet material reduces the consumption of rare-earth permanent magnet material. The double-side field windings enable the motor to have more flexible magnetic modulation properties. To investigate the principle of motor operation and flux regulation, the equivalent magnetic circuit method is employed. In order to achieve higher operation performances of the motor in different driving modes, the multi-objective optimization with coupled multi-physical field calculation is carried out. The multi physical comprehensive sensitivity function is defined which couples the electromagnetic performance optimization objective and mechanical performance objective. Then multi-objective genetic algorithm (MOGA) method was used to find a feasible solution set. Response surface (RS) method and parameter scan method are used to further determine the five important dimensions. The electromagnetic characteristics of optimized DSHE-FS motor are evaluated and compared in detail. Moreover, the mechanical analysis is conducted for the cupped rotor of DSHE-FS motor to validate the operation security. Theoretical analysis and simulation results verify the rationality of the DSHE-FS motor and the proposed optimization design method.

2020-05-08 PIER C Vol. 101, 203-218, 2020. doi:10.2528/PIERC20031003

A Novel Complementary Slotted Split Ring Resonator Loaded Truncated Arc Patch Antenna with Enhanced Performance

Shailesh Maroli Rao and Prabhugoud Iranna Basarkod

This paper proposes a truncated arc patch antenna loaded with a novel complementary slotted split ring resonator (CSlSRR) in the ground plane. The antenna achieves wide bandwidth, circular polarisation (CP), and omnidirectional radiation pattern in the S-band. The electrical size of the antenna is 0.36λ0 × 0.31λ0, and the radiating metal dimension is 0.18λ0 × 0.21λ00 corresponds to f0 = 2.45 GHz). Truncated corners with a semi-circular arc produce CP with the inset feed. The CSlSRR helps in improving the bandwidth and miniaturisation of the antenna. The design achieves a size reduction of 61%. The fabricated antenna exhibits 12.3% impedance bandwidth (IBW), 4.07% axial ratio bandwidth (ARBW), and a maximum gain of 2.476 dBi at 2.75 GHz. The antenna prototype is characterised in an anechoic chamber. The paper carries out a comparison of the measured and simulated results and other reported works in literature.

2020-05-07 PIER C Vol. 101, 187-202, 2020. doi:10.2528/PIERC19122601

A Hybrid Magnetic Couplers of Wireless Charging System for Electric Vehicles

Lei Zhang, Wei Tian, Hao Ding, Kai Lu, Wei Hong, and Rongming Liu

Coupling coefficient of a magnetic coupler is a key factor that affects the efficiency of wireless charging system. DD-type couplers have the most common topology in the literature. However, they have low coupling coefficients. In order to obtain high coupling coefficient of magnetic coupler, firstly, the magnetic circuit models of DD-type and solenoid-type magnetic couplers commonly adopted in electric vehicles are built in this paper. Secondly, a hybrid DD-solenoid type coil winding is proposed based on the analytical model, and the optimized design of the magnetic core and shielding structure are also introduced in this paper. Thirdly, an optimization design method for magnetic coupler is proposed. 3-D finite-element analysis (FEA) and experimental results verify the theoretical analysis. It is shown that the performance of the hybrid winding method proposed in this paper is significantly improved compared to the traditional DD winding method, and it can also keep the high offset tolerance characteristics of DD winding. In the meantime, the proposed method can increase the coupling coefficient and decrease the cost through optimization of magnetic core, and the shielding structure can effectively reduce the electromagnetic interference.

2020-04-30 PIER C Vol. 101, 173-186, 2020. doi:10.2528/PIERC20012105

2-D Analytical Model for Slotless Double-Sided Outer Armature Permanent-Magnet Linear Motor

Alireza Ghaffari, Farzaneh Khalili, Amir Abbas Vahaj, Hamidreza Ghaffari, and Amin Mahmoudi

Slotless double-sided outer armature permanent-magnet (PM) linear motors (SDOPMLs) have high efficiency and low detent force. Despite their simple control strategy and easy manufacturing process, finding an accurate model of these motors to calculate the machine quantities is challenging. It is particularly critical for obtaining the optimum design of these machines which may include too many iterations in a short time. To overcome this challenge, a 2-D analytical model based on the sub-domain method is presented to determine the magnetic flux density components for the motor under the study. According to this analytical procedure, the motor cross-section is divided to 11 sub-regions, then the superposition theorem is utilized to analyze the flux density distribution in all sub-regions due to various magnetization patterns, (i.e., parallel, two-segment Halbach, ideal Halbach, and bar magnet in shifting directions) as well as armature reaction current, respectively. According to the calculated magnetic flux density components, machine quantities like flux linkage, induced voltage, inductances and electromagnetic force components are explained. Also, the obtained analytical results are compared with those of the finite-element method (FEM) to confirm the accuracy of the proposed model. The proposed model can be used in the design and optimization stage of the linear slotless motor against the numerical model to save time. Finally, a comparative study between the performance of the single-sided and double-sided slotless PM linear motors in the same volume is implemented. This comparison shows the advantage of the double-sided motorin terms of the unbalanced magnetic force (UMF).

2020-04-28 PIER C Vol. 101, 159-172, 2020. doi:10.2528/PIERC20022603

28/38 GHz Dual-Band Yagi-Uda Antenna with Corrugated Radiator and Enhanced Reflectors for 5G MIMO Antenna Systems

Asmaa Elsayed Farahat and Khalid Fawzy Ahmed Hussein

A novel design of an enhanced Yagi-Uda antenna is introduced for dual-band operation at 28/38 GHz. The antenna is constructed by a corrugated dipole strip and a capacitively end-coupled extension strip as the driving element, two reflectors, and one director. Periodic parasitic elements are added in front of the reflectors to enhance the antenna gain and improve the impedance matching. The driving dipole is fed through a coplanar strip line, and in order to facilitate the experimental measurements using a coaxial feed line, a microstrip to coplanar strip (CPS) line transition is employed. A four-port MIMO antenna system is constructed using the proposed Yagi-Uda antenna arranged at the edges of the mobile handset. Numerical and experimental investigations are achieved to assess the performance of both the single-element antenna and the four-port MIMO antenna system. It is shown that the simulation results agree with the experimental measurements, and both show good performance of the single antenna as well as the MIMO antenna system. The bandwidths achieved around 28 GHz and 38 GHz are about 3.42 GHz and 1.45 GHz, respectively, using the microstrip feed line. Each antenna has a maximum gain of about 9 dB. The four antenna configuration shows radiation pattern diversity required for MIMO system. The envelope correlation coefficient (ECC) and diversity gain (DG) are calculated, and the results show that the proposed MIMO antenna system is suitable for the forthcoming 5G mobile communications.

2020-04-24 PIER C Vol. 101, 147-158, 2020. doi:10.2528/PIERC20031402

CPW Fed Wideband Bowtie Slot Antenna on PET Substrate

Manjurul Ahsan Riheen, Tuan Nguyen, Tonmoy Kumar Saha, Tutku Karacolak, and Praveen Kumar Sekhar

In this article, a new wideband bowtie shaped slot antenna is realized on a flexible polyethylene terephthalate (PET). The slotted bowtie design is implemented with an asymmetric bow-tie flare angle and a larger feeding neck with a metal strip inside the bowtie slot to achieve a wider bandwidth and a higher gain. The designed free space antenna is fabricated using inkjet printing and tested. The fabricated antenna operates over 2.1-4.35 GHz frequency range (69.77% fractional bandwidth) which covers WLAN, WiMax, and most of the 3G and 4G frequency bands. Further, the antenna exhibits an omnidirectional radiation pattern with a peak gain of 6.3 dBi at 4.35 GHz. The bending test of the fabricated device reveals adequate flexibility without significant antenna performance degradation. Moreover, the antenna tunability for any mounting structure application is also investigated by simulating another version of the parent antenna (free space antenna) for drywall mounting applications. The tuned antenna covers a similar frequency band as a free space antenna maintaining the desired radiation performances. The compact size, higher bandwidth, omnidirectional pattern with a higher peak gain and flexible properties make the antenna design suitable for mounting structure for Internet of Things (IoT) applications.

2020-04-22 PIER C Vol. 101, 133-146, 2020. doi:10.2528/PIERC20020706

High-Gain and Circularly Polarized Fractal Antenna Array for Dedicated Short Range Communication Systems

Deven G. Patanvariya, Anirban Chatterjee, and Kalyan Sundar Kola

In this paper, a low-profile fractal antenna and its array for DSRC-band applications have been proposed. The proposed single element is a newly designed fractal antenna which is right-handed circularly polarized (RHCP) and derived from the Koch-snowflake 1st-iteration. Moreover, a diagonal slot defect in the ground plane has been implemented for resonating the structure at the desired frequency and, to get a low cross-polarization over the operating frequency. The compact feed-network of the array is designed using s Wilkinson power-divider. A single element and a 4 × 1 antenna array are designed, prototyped and verified. The antenna array is designed by a single-layer microstrip structure with a compact size of 151.70 × 43.50 mm2. According to the experimental results, the single element and the antenna array have S11 of -15.27 dB and -13.95 dB, and RHCP gain of 6.14 dBic and 11.98 dBic, respectively. Moreover, the computed radiation efficiencies of single element and array are 78.17% and 71.50%, respectively, while CP bandwidths of single element and array are 49.00 MHz and 58.00 MHz, respectively. The performance of the proposed RHCP antenna is suitable for the DSRC-band application.

2020-04-22 PIER C Vol. 101, 119-132, 2020. doi:10.2528/PIERC20011701

Inset-Feed Frequency Reconfigurable Compact E-Shape Patch with DGS

Rashmi A. Pandhare and Mahesh Pandurang Abegaonkar

In this paper, a new miniaturized switchable band microstrip patch antenna array using PIN-diode is presented for WLAN/WiMax applications. In the first stage DGS has been employed to miniaturize a dual band microstrip patch antenna array simultaneously resonating at 2.2 GHz and 3.8 GHz. Further in second stage RF PIN-diodes has been used to achieve the frequency reconfigurability to serve for different communication systems. The designs are verified through both simulation and measurement of fabricated prototype. The measured results were in good agreement with simulated results.

2020-04-20 PIER C Vol. 101, 105-118, 2020. doi:10.2528/PIERC19091105

Thermal Characteristics of Hybrid Excitation Double Stator Bearingless Switched Reluctance Motor

Qian Wen Xiang, Liyun Feng, Yanjun Yu, and Kunhua Chen

In order to research the temperature distribution of a hybrid excitation double stator bearingless switched reluctance motor (HEDSBSRM), the finite element method (FEM) is used to conduct thermal modeling and analysis. First, 2D FEM is used to calculate the losses of the motor, including the core losses and copper losses of the windings. Then, in the thermal analysis module of ANSYS Workbench software, losses are used for calculation and analysis as the thermal load. Furthermore, in order to enhance the accuracy of modeling, this paper also considers the equivalent thermal conductivity of each part of the motor, and the equivalent insulation of the windings and surface convection heat transfer coefficient are also considered. Finally, the simulation results of motor temperature field distribution are analyzed and studied in detail. The thermal characteristic is also of guiding significance to the optimal design of the motor.

2020-04-17 PIER C Vol. 101, 95-104, 2020. doi:10.2528/PIERC20030408

Compact Dual Polarised Branch-Line Printed Inverted-F Antenna Covering Both Cellular and Non-Cellular Bands with Independent Tuning

Anupa Chatterjee, Manas Midya, Laxmi Prasad Mishra, and Monojit Mitra

In this paper a novel branch-line printed inverted-F antenna (IFA) loaded with a rectangular complementary split-ring resonator (CSRR) is proposed, designed and experimentally studied. The proposed antenna shows four operating frequencies and can be used for various cellular and wireless applications (900 MHz/3.5 GHz/4.2 GHz/5.5 GHz). The antenna is compact in size having dimensions 0.059λ0 × 0.053λ0 × 0.002λ0 at the lowest resonance frequency. Each of the bands is independently tunable and shows circular polarisation (CP) in the WLAN band with linear polarization (LP) in the other three bands. The axial ratio (AR) bandwidth is 1.82% in WLAN band. The simulated and fabricated results are reported in terms of S-parameters and radiation pattern. The prototype of the antenna has been fabricated and measured using VNA and simulation done in ANSYS HFSS.

2020-04-17 PIER C Vol. 101, 81-93, 2020. doi:10.2528/PIERC20020501

Optimization of a Rewritable Narrowband Filter in a SBN:75 Crystal

Luis Alberto Rubio-Saavedra, Birger Seifert, Pedro A. Márquez Aguilar, and Adalberto Alejo-Molina

We propose a rewritable optical frequency filter based on a volume Bragg grating recorded by holography on an SBN:75 photorefractive crystal. The theoretical results show the possibility of implementing a narrow-band filter whose reflectance is total for the characteristic wavelength of the third harmonic of the infrared for both TE and TM polarizations by optimizing the size of the interference fringes and the angle of incidence of the beam to be filtered, which must be close to 80 degrees.

2020-04-17 PIER C Vol. 101, 71-80, 2020. doi:10.2528/PIERC20013001

A Closely Spaced Dual-Band MIMO Patch Antenna with Reduced Mutual Coupling for 4G/5G Applications

Naser Ojaroudi Parchin, Yasir I. A. Al-Yasir, Haleh Jahanbakhsh Basherlou, and Raed A. Abd-Alhameed

This study proposes a low-profile dual-band MIMO patch antenna array with improved isolation for 4G-LTE and 5G wireless communications. The proposed antenna design contains two closely-spaced coaxial-fed patch antennas with U-shaped slots to generate dual-band operation at 2.6/3.6 GHz 4G/5G bands. The mutual coupling between MIMO elements can be reduced simultaneously at both operation bands by employing a pair of C-shaped parasitic structures with different sizes between the radiating patches. The results show that the isolation between the antenna ports has been enhanced by about 13 dB and 10 dB at the operation frequencies with the presence of the proposed parasitic structures. The simulation and measurements of the proposed antenna design have been provided to verify the performance of the design.

2020-04-14 PIER C Vol. 101, 53-69, 2020. doi:10.2528/PIERC19122204

Receiving UWB Antenna for Wireless Capsule Endoscopy Communications

Chaïmaâ Kissi, Mariella Särestöniemi, Timo Kumpuniemi, Sami Myllymäki, Marko Sonkki, Juha-Pekka Mäkelä, Mohamed Nabil Srifi, Heli Jantunen, and Carlos Pomalaza-Raez

Wireless capsule endoscopy systems utilize a combination of hardware and software devices to ensure the healthcare of a human being. In praise of involved antennas in the overall medical system design, UWB (Ultra-Wideband) range occupies highest ranks in the literature. The low-band of UWB is regarded as the best frequency range, within the approved standards, to realize the better transmission of captured medical images by the capsule inside the SI tract, in terms of high resolution and low-path loss. A variety of passive capsules have been designed and made available in the literature, while the accurate design of the corresponding on-body antenna is lagging. For this purpose, this paper provides an extended study of a recently published on-body antenna operating at 3.75-4.25 GHz band. The measured antenna realizes good directivity of 5.78 dBi and 9.50 dBi towards the body without and with the cavity, respectively. The direction of the proposed on-body antenna beam is targeted to be mounted on the body surface. On-body simulations were run with CST Microwave Studio by involving an abdominal multi-layer model, and followed by navel and back areas of the voxel model to predict the antenna behavior close to different lossy body environments. Later, the antenna structure was measured next to a real human abdomen. Simulation results reveal that the proposed antenna with or without the cavity enables enhanced in-body communication when mounted on the abdomen with less path loss. This is supported by the low power totaling 20 dB at the SI (Small Intestine) tract. Furthermore, on-body measurements confirm the good antenna performance. Consequently, the planar compact antenna is regarded as a good on-body candidate for wireless capsule endoscopy systems.

2020-04-14 PIER C Vol. 101, 43-52, 2020. doi:10.2528/PIERC19100102

Simplified Polarization Estimation Using Co-Located Antennas

Hui Li, Miao Wu, Yibo Cheng, and Fei Wu

Estimating polarization information using vector antennas is of great significance in signal processing. However, the antenna patterns are normally assumed ideal without considering practical factors, such as cross polarization. Moreover, pattern calibration is required in data processing. In this work, we first illustrate the polarization estimation method, taking into account the cross polarization of antennas. To simplify the estimation, we introduce a practical co-located antenna pair comprising a sleeve monopole and a windmill loop, which share mostly identical radiation patterns but orthogonal polarizations. The cross polarizations of both antennas are below -20 dB. Besides, the phase and amplitude patterns of both antennas are almost omnidirectional in the azimuth plane, avoiding complicated calibrations. Attributed to orthogonal polarizations, good isolation is achieved, and the envelope correlation coefficient is below 0.01. With the proposed antenna, the axis ratio and phase difference of the incoming wave are reasonably estimated without pattern calibration and compensation. The co-located antenna pair was fabricated, using which the polarization information of a commercial WLAN antenna has been measured.

2020-04-13 PIER C Vol. 101, 29-42, 2020. doi:10.2528/PIERC19102403

Integration of 0.1 GHz to 40 GHz RF and Microwave Anechoic Chamber and the Intricacies

Sellakkutti Suganthi, Deepal Deepak Patil, and Elisha Chand

The aim of this paper is to highlight and elaborate the construction and establishment of a rectangular anechoic chamber (AC) of dimensions 7 m × 4 m × 3 m working from 0.1 GHz to 40 GHz. It is an informative checklist giving an insight on the reckoning of chamber dimensions and selection of appropriate absorbers as per the required specifications. It briefs the key features of validation of an anechoic chamber, namely, shielding effectiveness and reflectivity (quiet zone). It describes the intricacies of the integration of systems such as vector network analyzer (VNA), antenna mounting stands, three-axes motorized antenna rotation control circuitry and customized software. The validation of the established chamber is accomplished for overall shielding effectiveness of -80 dB and reflectivity of -40 dB in one cubic meter area at the receiving antenna or antenna under test (AUT) region far away from transmitter say, at 5.5 m separation. This paper covers the measurement results of three broadband horn antennas which can be used as reference antennas for characterization of other antennas in the chosen frequency range. The entire report will certainly be a guideline for any reader or aspirant who is interested in the development of a similar anechoic chamber and looking for complete intricacies.

2020-04-03 PIER C Vol. 101, 13-28, 2020. doi:10.2528/PIERC19123104

Tuneable Frequency Selective Surface

Yukti Anand and Ashok Mittal

This paper is presented to provide an overview on frequency selective surfaces and techniques to achieve tune-ability in frequency selective surface (FSS). FSS array element with specific arrangement on the dielectric surface either transmits (pass-band) or reflects (stop-band) partially or completely with resonance of the structure in tune with the frequency of electromagnetic wave. Tuning devices like PIN or Varactor incorporated in the structure tune the performance. The recent researches on FSS structures classifying them into structural classification and mechanisms to change the operating resonance frequency dynamically by changing the bias of the tuning devices like PIN or Varactor diode have been studied and detailed in this review article. Tune-ability allows the FSS layer filter to adapt to spectral changes and to compensate for the best performance in terms of bandwidth, gain, and directivity. We also focused important performance parameters, particularly on how development in this field could facilitate invention in advanced electromagnetics.

2020-04-09 PIER C Vol. 101, 1-12, 2020. doi:10.2528/PIERC20012003

A Design of Antenna Array with Improved Performance for Future Smartphones

Naser Ojaroudi Parchin, Haleh Jahanbakhsh Basherlou, Yasir I. A. Al-Yasir, and Raed A. Abd-Alhameed

In this study, a new multiple-input-multiple-output (MIMO) antenna array is introduced for fifth-generation (5G) smartphones. Its schematic contains eight planar inverted-F antenna (PIFA) elements placed at edges of the mobile-phone mainboard with a 75×150×0.8 mm3 FR-4 substrate. The ground plane and antenna resonators are etched on the back layer of the mainboard. By employing arrow strips between the adjacent elements, the frequency bandwidth and isolation level of the PIFA radiators are improved. The proposed smartphone antenna array is designed to support the spectrum of commercial sub 6 GHz 5G communication and cover the frequency range of 3.25-3.85 GHz with isolation levels better than -15 dB. Due to compact size and corner placements of the PIFAs, the presented MIMO antenna array occupies a small part of the board. In addition, the proposed smartphone antenna array provides not only sufficing radiation coverage supporting different sides of the mainboard but also the polarization diversity. The MIMO performance and characteristics of the proposed smartphone antenna design in the presence of the user phantom are also discussed.