Wenxing Li

A Reconfigurable Triple-Notch-Band Antenna Integrated with Defected Microstrip Structure Band-Stop Filter for Ultra-Wideband Cognitive Radio Applications

A printed reconfigurable ultra-wideband (UWB) monopole antenna with triple narrow band-notched characteristics is proposed for cognitive radio applications in this paper. The triple narrow band-notched frequencies are obtained using a defected microstrip structure (DMS) band stop filter (BSF) embedded in the microstrip feed line and an inverted π-shaped slot etched in the rectangular radiation patch, respectively. Reconfigurable characteristics of the proposed cognitive radio antenna (CRA) are achieved by means of four ideal switches integrated on the DMS-BSF and the inverted π-shaped slot. The proposed UWB CRA can work at eight modes by controlling switches ON and OFF. Moreover, impedance bandwidth, design procedures, and radiation patterns are presented for analysis and explanation of this antenna. The designed antenna operates over the frequency band between 3.1 GHz and 14 GHz (bandwidth of 127.5%), with three notched bands from 4.2 GHz to 6.2 GHz (38.5%), 6.6 GHz to 7.0 GHz (6%), and 12.2 GHz to 14 GHz (13.7%). The antenna is successfully simulated, fabricated, and measured. The results show that it has wide impedance bandwidth, multimodes characteristics, stable gain, and omnidirectional radiation patterns.

New Development of Parallel Conformal FDTD Method in Computational Electromagnetics Engineering

In this paper, we summarize the new development of hardware-acceleration techniques and a parallel conformal Finite-Difference Time-Domain (FDTD) method in computational electromagnetics engineering. We investigate the performance of a parallel conformal FDTD method on different hardware platforms, such as Intel and AMD processors, with vector-arithmetic logic unit (VALU) acceleration, a regular PC cluster, a high-performance server cluster, the use of a graphics-processing unit (GPU), and an IBM CELL processor. The FDTD method, which is parallel in nature, is one of the best candidate numerical methods for using multiple-core processors and computer clusters to efficiently simulate various electromagnetic problems. Several representative examples, such as a UWB (ultra-wideband) antenna array and reflector antennas, are employed to demonstrate the engineering applications of the parallel conformal FDTD method.

Sparse Adaptive Channel Estimation Based on -Norm-Penalized Affine Projection Algorithm

We propose an -norm-penalized affine projection algorithm (LP-APA) for broadband multipath adaptive channel estimations. The proposed LP-APA is realized by incorporating an -norm into the cost function of the conventional affine projection algorithm (APA) to exploit the sparsity property of the broadband wireless multipath channel, by which the convergence speed and steady-state performance of the APA are significantly improved. The implementation of the LP-APA is equivalent to adding a zero attractor to its iterations. The simulation results, which are obtained from a sparse channel estimation, demonstrate that the proposed LP-APA can efficiently improve channel estimation performance in terms of both the convergence speed and steady-state performance when the channel is exactly sparse.

A printed diversity Cantor set fractal antenna for ultra wideband communication applications

In this paper, a printed diversity Cantor set fractal antenna for ultra wideband (UWB) applications is proposed. The antenna consists of two identical wide slot antennas and the radiation patches are designed by using Cantor set fractal technology. The proposed antenna operates over a wide band spanning from 4.5 GHz to 10.6 GHz. The diversity antenna has a high isolation over 20dB. The high isolation is obtained using multiple slots etched in its CPW ground plane. The return loss and the isolation of the antenna are investigated by means of HFSS. The diversity performance of the antenna is assessed by isolation and radiation patterns. The results show that the proposed antenna provides stable patterns diversity, which enables the designed antenna to combat multi-path in wireless channels used in UWB systems.

Compact reconfigurable UWB antenna integrated with SIRs and switches for multimode wireless communications

A kind of reconfigurable wide slot UWB antenna integrated with stepped impedance resonators (SIRs) and ideal switches is presented. The proposed antenna 1 has dual notch band functions which are obtained by using two stepped impedance stub loaded SIRs. The three reconfigurable models are investigated and discussed. The measurement results with ideals switches demonstrate the performances of proposed reconfigurable antennas. The designed reconfigurable antennas can be suitable for future multi-mode wireless communications systems.

A cognitive radio antenna integrated with narrow/ ultra-wideband antenna and switches

A cognitive radio antenna integrated with narrow /ultrawide band (UWB) antenna is described in this paper. The proposed antenna consists of two port-fed wide-slot antennas sharing the same substrate. The proposed ultra-wideband (UWB) antenna which covers the entire UWB band from 3.1 GHz to 10.6 GHz is used for channel sensing. The integrated, narrowband wide-slot antenna, operating in the range of 8.2 GHz to 11.0 GHz is designed to communicate with other wireless devices. The reconfigurable function is realized by utilizing ideal switches. The isolation between the two ports is better than −10 dB. The design is suitable for cognitive radio applications where the UWB antenna is required for spectrum sensing and the narrow band antenna is used for reconfigurable operation, which can be tuned over a wide bandwidth by changing the dimensions of the triangular radiation patch using ideal ON/OFF switches.

A Miniaturized Frequency Selective Surface Based on Square Loop Aperture Element

We propose a miniaturized band-pass frequency selective surface (FSS) with periodic unit cell structure. The proposed FSS is realized by symmetrically bending the edges of the square loop aperture element, by which our proposed FSS increases the resonant length, and, hence, reduces its size. In this FSS, each unit cell has a dimension of 0.0538λ × 0.0538λ, where λ represents the wavelength of the corresponding resonant frequency. Both the theoretical analysis and simulation results demonstrate that our proposed FSS, having high polarization stability and angle stability, can achieve smaller size in comparison with the previously proposed structures.

A CPW-fed circular wide-slot UWB antenna with wide tunable and flexible reconfigurable dual notch bands.

A coplanar waveguide (CPW)-fed circular slot antenna with wide tunable dual band-notched function and frequency reconfigurable characteristic is designed, and its performance is verified experimentally for ultra-wideband (UWB) communication applications. The dual band-notched function is achieved by using a T-shaped stepped impedance resonator (T-SIR) inserted inside the circular ring radiation patch and by etching a parallel stub loaded resonator (PSLR) in the CPW transmission line, while the wide tunable bands can be implemented by adjusting the dimensions of the T-SIR and the PSLR. The notch band reconfigurable characteristic is realized by integrating three switches into the T-SIR and the PSLR. The numerical and experimental results show that the proposed antenna has a wide bandwidth ranging from 2.7 GHz to 12 GHz with voltage standing wave ratio (VSWR) less than 2, except for the two notch bands operating at 3.8–5.9 GHz and 7.7–9.2 GHz, respectively. In addition, the proposed antenna has been optimized to a compact size and can provide omnidirectional radiation patterns, which are suitable for UWB communication applications.

Small Antennas: Miniaturization Techniques and Applications

Although small antenna designs and miniaturization techniques have been widely studied in recent years, there are still many challenging issues in small antenna applications and research. In this special issue, several small antennas have been proposed and discussed for wireless local area networks (WLANs), UHF band, multiple-input-multipleoutput (MIMO), long-term-evolution (LTE), and other wireless communication applications. These designed antennas are not only small in size but also able to provide high performance to meet the practical engineering requirements with respect to the impedance bandwidth and the radiation patterns. On the other hand, two horn antennas with enhanced performance were developed for modern wireless communication applications. One of the horn antennas was designed to obtain high gain, which was realized by the integration of a horn antenna into a microstrip antenna. The other horn antennawas presented for ultra-wideband (UWB) applications by the combination of a TEM horn antenna with a Vivaldi antenna together. Both of the proposed improved horn antennas have been verified that they have low profile and good performance. In addition, several compact antennas have been proposed and investigated to provide high efficiency antenna and low SAR, which are suitable for on-body and mobile communications.Motivated by the rapid developments of the microwave imaging, a right angle-bent monopole antenna has been built to obtain small size and low profile for microwave tomographic imaging application. In addition, a compact frequency selective surface (FSS) has been proposed based on the square loop aperture element. The designed FSS can give high polarization stability, angle stability, and smaller size in comparison with the previously proposed FSS structures. Beyond that, frequency reconfigurable antennas have also been proposed in this special issue formultifunction communication applications. Since each narrowband system should be provided a different antenna, a reconfigurable antenna can be operated in several narrowband by controlling its reconfigurable states, which is flexible and can reduce the cost for practical engineering applications. A reconfigurable antenna has been investigated by using MEMS to realize multiband antenna. Furthermore, the MEMS technique has been also exploited for minimizing the antenna by means of tunable capacitors. As for the optimization method and measurement techniques, an orthogonal design method was proposed to optimize roughly designed antennas. Additionally, a wideband differential-mode current injection testing techniquewas also presented based on directional coupling device. In this special issue, two adaptive signal processing algorithms have been developed for sparse channel estimation and virtual array antenna applications. Both of the proposed adaptive algorithms can provide higher performance compared with previous classical algorithm and can be further developed for applying in the antenna arrays.

High Performance Robust Adaptive Beamforming in the Presence of Array Imperfections

A high performance robust beamforming scheme is proposed to combat model mismatch. Our method lies in the novel construction of interference-plus-noise (IPN) covariance matrix. The IPN covariance matrix consists of two parts. The first part is obtained by utilizing the Capon spectrum estimator integrated over a region separated from the direction of the desired signal and the second part is acquired by removing the desired signal component from the sample covariance matrix. Then a weighted summation of these two parts is utilized to reconstruct the IPN matrix. Moreover, a steering vector estimation method based on orthogonal constraint is also proposed. In this method, the presumed steering vector is corrected via orthogonal constraint under the condition where the estimation does not converge to any of the interference steering vectors. To further improve the proposed method in low signal-to-noise ratio (SNR), a hybrid method is proposed by incorporating the diagonal loading method into the IPN matrix reconstruction. Finally, various simulations are performed to demonstrate that the proposed beamformer provides strong robustness against a variety of array mismatches. The output signal-to-interference-plus-noise ratio (SINR) improvement of the beamformer due to the proposed method is significant.