Weixing Sheng

Modified projection approach for robust adaptive array beamforming

A novel modified projection method is proposed in this paper, which is robust against the signal steering vector mismatch and covariance matrix uncertainty. First, an enhanced covariance matrix estimate based on a shrinkage method is obtained. Then, the desired signal subspace is estimated from the eigenvectors of the enhanced covariance matrix, and a calibrated steering vector of the desired signal is obtained in sequence by projecting the presumed one onto the new estimated desired signal subspace. Finally, the robust adaptive beamformer weight is obtained from the estimated covariance matrix and the calibrated steering vector. Compared with the traditional projection method, the proposed method can work well even at low signal-to-noise ratio. Also, it is not necessary to estimate the number of sources. Simulation results demonstrate the efficiency of the proposed robust method compared with some of the existing ones.

Improved switching CFAR detector for non-homogeneous environments

In order to mitigate the excessive false alarm rate of the switching constant false alarm rate (S-CFAR) detector at the clutter edge, an improved switching CFAR (IS-CFAR) detector is proposed in this paper. In IS-CFAR, a comparison threshold is generated by multiplying the amplitude of the test cell by a scaling factor. Then in the leading and lagging reference windows, the numbers of reference cells whose amplitudes are smaller than the comparison threshold are countered and compared with a threshold integer. Based on the comparison result, the detection threshold is generated by selecting appropriate reference cells for background noise/clutter power estimation. The detection probability of IS-CFAR in various environments is derived in a closed-form expression. The performance is evaluated and compared with other CFAR detectors. It is shown that IS-CFAR exhibits a low CFAR loss in a homogeneous environment and almost the same detection performance as S-CFAR in a multiple targets situation; at a clutter edge, the false alarm rate of IS-CFAR is much lower than that of S-CFAR and approximates to the greatest-of CFAR (GO-CFAR). Experimental results from a linear frequency modulated continuous wave (LFMCW) radar system are given to demonstrate the efficiency of IS-CFAR.

Robust adaptive beamforming for large-scale arrays

For a large-scale adaptive array, heavy computational load and high-rate data transmission are two challenges in the implementation of an adaptive digital beamforming system. Moreover, the large-scale array becomes extremely sensitive to array imperfections. First, based on a restructured recursive linearly constrained minimum variance algorithm and a gradient-based optimization method, a new robust recursive linearly constrained minimum variance (RRLCMV) algorithm is proposed in this paper. The computational load of the RRLCMV algorithm is on the order of o(N), which is less than that of the conventional gradient-based robust adaptive algorithm. Then, a new efficient parallel robust recursive linearly constrained minimum variance (PRRLCMV) adaptive algorithm is proposed by appropriately partitioning the RRLCMV algorithm into a number of operational modules. It can be easily executed in a distributed-parallel-processing fashion, sequentially and in parallel. As a result, the PRRLCMV algorithm provides an effective solution that can alleviate the bottleneck of high-rate data transmission and reduce the computational cost. Finally, an implementation scheme of the PRRLCMV algorithm based on a distributed-parallel-processing system is also proposed. The simulation results demonstrate that the new PRRLCMV algorithm can significantly reduce the degradation due to various array errors.

Super-resolution DOA estimation in switch beam smart antenna

Base transceiver stations (BTS) with antenna arrays employing narrow beam patterns pointing to each user rather than omnidirectional or directional antenna covering a large number of users or areas can provide better network performance. It is the fundamental theory of smart antenna systems (SAS) designed for a modern cellular network, which is more challenged nowadays by network capacity and service quality due to the increased Internet traffic volume. In this paper, super-resolution algorithms are applied to the switched beam smart antenna system, and a higher angle resolution and performance are achieved. This paper presents super-resolution algorithms specifically developed for the switched beam smart antenna system. The particular research interest is focused on the performance of a multiple signal classification (MUSIC) algorithm and a RELAX algorithm. The study results show a higher angle resolution and a significant overall system benefit by using these two algorithms. We also discuss the non-uniform geometry arrangement of the array and its application to improve the capacity of direction finding further. Simulation results are presented.

High-sum-rate beamformers for multi-pair two-way relay networks with amplify-and-forward relaying strategy

In multi-pair two-way relay networks, the control over inter-pair-interference is a challenging problem. When mobile stations (MSs) cooperate fully, to achieve the sum capacity of networks and at the same time completely remove the inter-pair-interference, a singular value decomposition (SVD) based beamforming scheme at relay station (RS) is established where SVD beamformers are applied over both uplink and downlink channels, respectively. To avoid any cooperation among users, an efficient beamforming scheme is proposed. This scheme uses the Max-SINR based receive beamformer at RS in uplink channel and the Max-SLNR based transmit precoder at RS in downlink channel. It omits interference (uplink) and leakage (donwlink) inside each pair of users due to their abilities of self-interference cancellation. This beamforming scheme provides an analytical solution like some well-known linear beamformers such as zero-forcing, block diagonalization, etc. From simulation, we find: the proposed SVD shows an excellent sum-rate performance at the expense of a large cooperation overhead at MSs and can be used as a tighter upper bound of sum-rate; the proposed scheme of Max-SINR beamformer plus Max-SLNR precoder performs much better on sum-rate than the ZF-based and the BD-based schemes by using almost the same computational amount.

Constant false alarm rate detector based on the maximal reference cell

In order to improve the detection performance of constant false alarm rate (CFAR) detectors in multiple targets situations, a CFAR detector based on the maximal reference cell (MRC) named MRC-CFAR is proposed. In MRC-CFAR, a comparison threshold is generated by multiplying the amplitude of MRC by a scaling factor. The number of the reference cells left, whose amplitudes are smaller than the comparison threshold, is counted and compared with a threshold integer. Based on the comparison result, proper reference cells are selected for detection threshold computation. A closed-form analysis for MRC-CFAR in both homogeneous and non-homogeneous environments is presented. The performance of MRC-CFAR is evaluated and compared with other CFAR detectors. MRC-CFAR exhibits a very low CFAR loss in a homogeneous environment and performs robustly during clutter power transitions. In multiple targets situations, MRC-CFAR achieves a much better detection performance than switching CFAR (S-CFAR) and order-statistic CFAR (OS-CFAR). Experiment results from an X-band linear frequency modulated continuous wave radar system are given to demonstrate the efficiency of MRC-CFAR. Because ranking reference cells is not required for MRC-CFAR, the computation load of MRC-CFAR is low; it is easy to implement the detector in radar system in practice.

A single-feed tri-band circularly polarized dual-annular slot antenna for wireless applications

A novel single-feed tri-band circularly polarized (CP) dual-annular slot antenna is presented. The antenna structure is composed of two nonconcentric annular slots fed by a novel inverted h-shaped microstrip line configuration. Compared to most of the previously reported tri-band CP antenna structures, the proposed antenna has low profile, compact in size, and easy to fabricate on the single-layer PCB. It is easy for the antenna to match all the three frequency bands by employing an h-shaped feeding line. The antenna also can provide a conical radiation pattern at the third band. A sample design of a tri-band CP antenna operated at 1.58, 2.42, and 3.0 GHz is realized. Details of the experimental results compared with simulated results are presented and discussed. The proposed compact tri-band CP antenna design is suitable for mounting onto the mobile communication devices and portable positioning systems.

Tri-band circularly polarized annular slot antenna for GPS and CNSS applications

A novel single-feed tri-band circularly polarized (CP) annular slot antenna is presented. The antenna structure is composed of two nonconcentric annular slots that are fed by an L-shaped series step impedance microstrip line configuration. The proposed antenna is designed to operate at the L 1 and L 2 bands of the global positioning system and the operating band of the compass navigation satellite system. Compared to the most of the previously reported tri-band CP antenna structure, the proposed antenna has low profile, compact size, and easy fabrication on the single-layer PCB. The measured results show that the impedance bandwidths are good for all bands. The axial ratio bandwidth and radiation patterns are also measured at the three resonant frequencies. The proposed compact tri-band CP antenna design is suitable for mounting onto the mobile communication devices and portable positioning systems.

A W-Band Low-Loss and Wideband LTCC Transition From Waveguide to Microstrip

A broadband transition structure from rectangular waveguide (RWG) to microstrip line (MSL) is presented for the realization of the low-loss packaging module using Low-temperature co-fired ceramic (LTCC) technology at W-band. In this transition, a cavity structure is buried in LTCC layers, which provides the wide bandwidth, and a laminated waveguide (LWG) transition is designed, which provides the low-loss performance, as it reduces the radiation loss of conventional direct transition between RWG and MSL. The design procedure is also given. The measured results show that the insertion loss of better than 0.7 dB from 86 to 97 GHz can be achieved.

An improved CFAR detector for non-homogeneous clutter environment

An improved CFAR detector for non-homogeneous clutter environment is proposed. At LFMCW radar receiver, after moving target detection (MTD) and linear-law envelop detector, CFAR detector is usually adopted to detect radar targets, the threshold of CFAR detector should be adaptive to the local background noise/clutter environment. Interfering targets often exist in the leading and lagging windows of the test cell which results in an increased detection threshold and a degraded probability of detection (PD). In the improved CFAR detector, an order statistic VI CFAR detector (OSVI-CFAR) is proposed to adaptively remove the interfering targets from the reference windows and increase the PD. Simulation and experiment results are given to prove the feasibility of the OSVI-CFAR detector.