Rongrong Qian

A Study on MVDR Beamforming Applied to an ESPAR Antenna

The adaptive beamforming algorithm-minimum variance distortionless response (MVDR) has been studied based on the electronically steerable parasitic array radiator (ESPAR) antenna. The ESPAR antenna uses a single radio-frequency (RF) front end, and its beamforming is achieved by adjusting reactance loads of parasitic elements coupled to the central active element. In the proposed beamforming method, the MVDR beamformer optimizes weights applied to outputs of beams. The optimization problem is formulated as a second-order-cone programming (SOCP) problem including a Euclidean distance metric to approximate the optimal equivalent weight vector to a feasible solution. Then the ESPAR beampattern design strategy iterates between the SOCP problem and a simple projection of reactance loads. The simulations show that the proposed MVDR beamforming method based on an ESPAR antenna gives a beam steering at the desired direction and placing nulls at the interfering directions, and it converges fast. However, when the desired source is close to the interferer, the output signal-to-interference-plus-noise ratio (SINR) degrades and where we use the interference-plus-noise sample covariance matrix to improve the beamforming performance.

Performance of the blind interference alignment using ESPAR antennas

Recently, a technique entitled “Blind Interference Alignment (BIA)” was proposed, which allows interference alignment to be achieved without the knowledge of channel state information at the transmitter. The key to realizing the BIA scheme is the use of a receive antenna capable of switching among multiple beampatterns. The ESPAR antenna, which uses only a single RF front-end, is capable of forming different directional beampatterns by the use of circular permutations of the reactive loads of the passive elements. We introduce the ESPAR antenna as a potential solution for the practical implementation of the BIA for broadcast channel as well as cellular. The BIA scheme is accomplished by the transmission strategy jointly coordinating with the ESPAR beampattern switching symbol-wise. The ESPAR beampattern switching provides the necessary channel diversity for receiving users. Simulation results demonstrate that the ESPAR beam steering can be designed to improve the performance of the BIA scheme by enhancing the receive signal-to-noise ratio (SNR). Furthermore, we study the proposed BIA scheme for a simple 1-dimensional cellular setting to illustrate that the ESPAR beamforming can improve the performance of the cell-edge users through further suppressing the remaining inter-cell interference.

Direction-of-arrival estimation with single-RF ESPAR antennas via sparse signal reconstruction

In this paper, a direction-of-arrival (DoA) estimation method based on compressive sensing is proposed for an electronically steerable parasitic array radiator (ESPAR) antenna, which uses only a single radio frequency (RF) chain, and is thereby suited for application in compact wireless terminals. Unlike a conventional multi-active antenna array, signals impinging on parasitic elements in an ESPAR array cannot be processed, and only the output of the sole active element can be processed. In this context, for an ESPAR array, a sparse representation of the DoA estimation problem is formulated by first using an overcomplete dictionary composed of samples from the array manifold and then projecting them onto a set of directional beampatterns. The projection matrix is designed to divide the angle space of the receive antenna array into sectors which are accessed via their corresponding sector beampatterns formed on a time division basis. The sparse signal spectrum is reconstructed by the l1-SVD (singular value decomposition) method [1], where the sparsity is enforced by the l1-norm penalty. Simulation results are presented to demonstrate the efficiency of the proposed method.

Directional spectrum sensing for cognitive radio using ESPAR arrays with a single RF chain

In this paper, we propose to use the electronically steerable parasitic array radiator (ESPAR) antenna, which relies on a single radio frequency (RF) front end coupled with a number of parasitic elements to steer beams in prescribed directions in the angular domain on a time division basis, to identify the directional spectrum sensing opportunities for cognitive radios. In particular, we propose a two stage spectrum sensing: First ESPAR signal measurements from different directional beampatterns are fed as input to the generalized likelihood ratio test (GLRT) algorithm to detect the existence of primary user signals. If signal existence is detected by the GLRT, then the directional measurements are used to obtain the direction of arrival (DoA) using a multiple signal classification (MUSIC) algorithm. We show that the DoA estimation performance using the ESPAR is comparable to that of the traditional uniform linear array when the signal-to-noise ratio (SNR) is larger than -15dB.

Design of ESPAR based Blind Interference Alignment for cellular systems

Recently, a technique entitled “Blind Interference Alignment (BIA)” was proposed, which allows interference to be aligned within a reduced subspace without knowledge of the channel state information at the transmitter. The key to realizing the BIA scheme is the use of a receive antenna that is capable of switching among multiple channel-states (with channel diversity) in a pre-determined way. In this paper, we exploit the ESPAR antenna, which uses only a single radio frequency chain as the receiver to provide the necessary beampattern switching for the BIA technique. Moreover, in the proposed ESPAR based BIA scheme, beam-steering of each ESPAR receiver is designed according to its position relative to the transmitter to increase the received SNR. Furthermore, we study the proposed BIA scheme by applying the ESPAR antennas to a 2-D cellular setting in order to illustrate the ESPAR beam pattern design and associated improvements in the performance of the BIA scheme. In order to keep the number of coordinating BSs at a reasonable level, we study the case where only three adjacent BSs operate the BIA scheme coordinatively. This is found to be reasonable since the other inter-cell interference is not significant due to the ESPAR beam-steering and the longer propagation distance. The simulation shows that our scheme provides comparable sum rate to that of BIA scheme operated in all cells.

On the Target Detection in OFDM Passive Radar Using MUSIC and Compressive Sensing

The passive radar also known as Green Radar exploits the available commercial communication signals and is useful for target tracking and detection in general. Recent communications standards frequently employ Orthogonal Frequency Division Multiplexing (OFDM) waveforms and wideband for broadcasting. This paper focuses on the recent developments of the target detection algorithms in the OFDM passive radar framework where its channel estimates have been derived using the matched filter concept using the knowledge of the transmitted signals. The MUSIC algorithm, which has been modified to solve this two dimensional delay-Doppler detection problem, is first reviewed. As the target detection problem can be represented as sparse signals, this paper employs compressive sensing to compare with the detection capability of the 2-D MUSIC algorithm. It is found that the previously proposed single time sample compressive sensing cannot significantly reduce the leakage from the direct signal component. Furthermore, this paper proposes the compressive sensing method utilizing multiple time samples, namely l1-SVD, for the detection of multiple targets. In comparison between the MUSIC and compressive sensing, the results show that l1-SVD can decrease the direct signal leakage but its prerequisite of computational resources remains a major issue. This paper also presents the detection performance of these two algorithms for closely spaced targets.

Direction-of-arrival estimation with espar antennas using Bayesian compressive sensing

This paper presents a novel approach of direction-of-arrival (DoA) estimation for the electronically steerable parasitic array radiator (ESPAR) antennas, using only a single radio-frequency (RF) chain. Starting from the problem formulation in the Bayesian compressive sensing (BCS) framework, the CS measurements are projected onto the beamspace of the unique configuration of the ESPAR antenna. In this work, measurements collected at multiple snapshots are considered. First, we propose to solve the sparse recovery problem by the multi-task BCS [1]. Then, the DoAs are estimated by employing a noise filter on the recovered sparse signal. In this method, the number of sources need not be known a priori, and computation complexity is reduced by avoiding computing the correlation matrix of measurements unlike the traditional DoA estimation techniques. Simulations show that the proposed method can recover closely spaced sources using a small number of noisy snapshots, and it performs better with more sources than other state-of-the-art algorithms.

Interference mitigation in femtocell networks using single-radio parasitic antennas

The femtocell networks provide a promising solution to increase system capacity and improve indoor coverage. However, inter-cell interference becomes enormous in femtocells due to dense deployment and cell-size reduction. This paper proposes the interference mitigation method for the downlink of femtocells, using the electronically steerable parasitic array radiator (ESPAR) antenna at both femto-base stations (FBSs) and user terminals (UTs), which relies on a single radio-frequency (RF) chain; thus meets the demanding low-power, low-cost and small-size requirements of modern wireless terminals - FBSs and UTs. We first exploit the ESPAR antenna as a switched-beam array capable of predefining directional beampatterns accessing to different angular sectors. Then each FBS/UT dynamically selects an appropriate beampattern according to its measurement of desired direction. We also consider an optimal way to employ the ESPAR antenna by adaptively designing the beampattern steering to desired direction while placing nulls to interferers. The results show significant gains obtained by using ESPPAR antennas at both FBSs and UTs.

Antenna selection for multi-user MIMO at millimeter-wave spectrum with lens antenna arrays

The recent concept of beamspace multiple-input-multiple-output (MIMO) enables the millimeter-wave (mmWave) MIMO system to reduce the radio-frequency chains by utilising beam selection and yet achieve near-optimal sum-rate performance. In this work, we study the lens antenna array as the transmit antenna at the base station for mmWave multiuser MIMO (MU-MIMO) system. Due to the direction-based energy focusing property of the array, the beam selection is able to reduce to antenna selection without significant performance degradation as that using traditional antenna selection. For this mmWave MU-MIMO system, direction-based antenna selection method is proposed, where the inter-user interference, resulting from the case that the same antenna is selected for different users with high probability, is considered and solved by a re-selection based on minimization of sum-rate loss. Simulation results show that spectral and energy effectiveness of the proposed antenna selection method for the mmWave MU-MIMO using lens antenna array.

Energy efficient switched parasitic array antenna for 5G networks and IoT

This paper includes design and implementation result of an adaptive beam forming antenna for upcoming 5G and Internet of Things (IoT). Switched parasitic array antennas are low cost, small sized and compact circular array antennas that steer beam in a desired direction by variation in switching pattern of parasitic elements. The proposed antenna design has an active center element, which is surrounded by several symmetrically placed parasitic elements. The designed antenna has a gain of 8 dB and is capable of 360 degrees beam steering in steps of 60 degrees each. Simulations are validated with results of the fabricated antenna. Antenna beam is steered by controlling parasitic elements. Future application of Electronically Steerable Parasitic Array Radiator (ESPAR) antennas and switched parasitic array antennas in next generation communication networks and methods for reducing size of the antenna are also highlighted.