Steerable Parasitic Array Radiator Antenna Research Articles

RSS-Based DoA Estimation Using ESPAR Antennas and Interpolated Radiation Patterns

In this letter, it is shown how an algorithm, which employs received signal strength values in order to estimate direction of arrival (DoA) of impinging signals in wireless sensor network (WSN) nodes equipped with electronically steerable parasitic array radiator (ESPAR) antennas, can easily be improved by applying an interpolation algorithm to radiation patterns recorded in the calibration phase of the DoA estimation process. The proposed method allows one to measure ESPAR antenna's radiation patterns during the initial calibration phase with much coarser than 1° angular resolution, while the overall DoA estimation accuracy can be kept at the similar level. Measurements using a fabricated ESPAR antenna have indicated that anechoic chamber calibration procedure of WSN nodes equipped with such antennas can be done almost 50% faster, which in consequence will reduce system deployment costs in practical WSN applications where a number of WSN nodes can easily reach hundreds.

Simple 2-D Direction-of-Arrival Estimation Using an ESPAR Antenna

In this letter, it has been shown how an electronically steerable parasitic array radiator (ESPAR) antenna can be used for 2-D direction-of-arrival (DoA) estimation employing received signal strength (RSS) values only. The proposed approach relies on changes in RSS values recorded at the antenna output port observed for different vertical and horizontal directions, while antenna's main beam sweeps 360° area around the ESPAR antenna. Based on the observed diversity in RSS values, obtained for different ESPAR antenna radiation patterns and different directions of a signal impinging the antenna, a simple 2-D DoA estimator has been proposed. Measurements using the fabricated ESPAR antenna indicate that, by relying solely on RSS values and the proposed algorithm, it is possible to perform 2-D DoA estimation with the acceptable accuracy in realistic and inexpensive systems, where the phase of impinging signals cannot easily be measured.

On the Implementation of Blind Interference Alignment With Single-Radio Parasitic Antennas

This paper studies the electronically steerable parasitic array radiator (ESPAR) antenna as a solution for beam pattern switching to implement blind interference alignment (BIA). The ESPAR antenna uses a single radio-frequency (RF) chain, and its beamforming is achieved by simply tuning the reactance loads of parasitic elements. Furthermore, ESPAR's beam pattern can be designed to enhance the received signal-to-noise ratio (SNR), thereby improving the performance of BIA. The focus of this work is the design of the ESPAR beam pattern to realize and improve BIA. In the first approach, each receiver ESPAR antenna dynamically selects the proper beam patterns from among the sector beam patterns, which are pre-designed to access different angular sectors with constant optimal antenna efficiency. We also propose a singular value decomposition (SVD) beamforming method, by means of the available channel state information at the receiver and a dynamic matching network for a high antenna efficiency. The simulation results illustrate the improvement of BIA with proposed methods, especially in the low and moderate SNR region, due to ESPAR's beamforming capability.

Five‐element ESPAR antenna using the annular ring slot active element

ABSTRACTThis study proposes a five‐element electric steerable parasitic array radiator (ESPAR) antenna that uses annular ring slot feeding and can be applied to Beamspace MIMO communication. A typical dipole active element that can be applied to the ESPAR antenna usually has a cylindrical structure. Since the coaxial cable is located inside the cylinder, it is restricted by the diameter of the dipole. In the proposed annular ring slot active element, the annular ring slot is formed on the outer conductor, and then the inner conductor is put into the outer conductor to be fed. The typical dipole active element currently takes a cylindrical shape, and therefore has a omnidirectional pattern. Therefore, it generates identical radiation patterns, regardless of the number of parasitic elements. Moreover, in the proposed method, it is easy to undertake impedance matching that controls the length, diameter, and thickness of the slot, outer conductor, and inner conductor. Furthermore, since the dielectric cylinder is located between the outer and inner conductors, it is structurally solid. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:2800–2804, 2016

Design and Beamforming Performance of the Compact ESPAR Antenna for the Beam Space MIMO System

Multiple-input multiple-output (MIMO) system is essential to wireless and mobile communication for the higher channel capacity. However, this MIMO system requires many active RF (radio frequency) stages, and is very difficult to be used in mobile devices. It produces the high energy consumption at RF stage as well as the high hardware complexity. Especially, it will be more serious in the massive MIMO system that is strongly considered as an enabling technology for the 5G mobile communication system. In order to solve this problem, beam-space MIMO (BS-MIMO) systems were proposed using the electronically steerable parasitic array radiator (ESPAR) antenna, which work as the MIMO system using single RF stage with many passive parasitic elements. However, there are some limits in this ESPAR based BS-MIMO system because of this many passive parasitic elements. The conventional BS-MIMO system basically uses two parasitic elements for one dimension of MIMO. So, in this paper we like to propose a design method for reducing the passive elements number in this BS-MIMO system. In the proposed system, we cut down the number of the parasitic elements into half. So, the proposed BS-MIMO system uses one parasitic element for one dimension of MIMO. To design 8 ? 8 MIMO system, only eight elements of parasitic elements are needed instead of 16 elements in conventional design. Since this proposed ESPAR based BS-MIMO system requires only the half number of parasitic elements, we can design more compact ESPAR antenna design. Also, we can easily increase the MIMO dimension with compact size by this proposed ESPAR antenna. Simulation results can confirm that the proposed ESPAR BS-MIMO system with half number of elements keeps the very similar performance of the conventional system and acceptable beamforming performance as well. This proposed system could be very useful for the massive MIMO system design for the 5G mobile communication.

Random Aerial Beamforming for Underlay Cognitive Radio With Exposed Secondary Users

In this paper, we introduce the exposed secondary-user (SU) problem in underlay cognitive radio systems, where both the secondary-to-primary and primary-to-secondary channels have a line-of-sight (LoS) component. Based on a Rician model for the LoS channels, we show, both analytically and numerically, that LoS interference hinders the achievable SU capacity when interference constraints are imposed at the primary user (PU) receiver. This is caused by the poor dynamic range of interference channel fluctuations when a dominant LoS component exists. To improve the capacity of such a system, we propose the use of an electronically steerable parasitic array radiator (ESPAR) antenna at the secondary terminals. An ESPAR antenna involves a single radio frequency (RF) chain and has a reconfigurable radiation pattern that is controlled by assigning arbitrary weights to $M$ orthonormal basis radiation patterns via altering a set of reactive loads. By viewing the orthonormal patterns as multiple virtual dumb antennas , we randomly vary their weights over time, creating artificial channel fluctuations that can perfectly eliminate the undesired impact of LoS interference. This scheme is termed as random aerial beamforming (RAB) and is well suited for compact and low-cost mobile terminals as it uses a single RF chain. Moreover, we investigate the exposed-SU problem in a multiuser setting, showing that LoS interference hinders multiuser interference diversity and affects the growth rate of the SU capacity as a function of the number of users. Using RAB, we show that LoS interference can, in fact, be exploited to improve multiuser diversity by boosting the effective number of users .

Single-Anchor Indoor Localization Using ESPAR Antenna

In this letter, a new single-anchor indoor localization concept employing electronically steerable parasitic array radiator (ESPAR) antenna has been proposed. The new concept uses a simple fingerprinting algorithm adopted to work with directional main beam and narrow minimum radiation patterns of ESPAR antenna that scans 360 ° area around the base station, while the signal strength received from a mobile terminal is being recorded for each configuration. The letter describes the antenna design and necessary fingerprinting algorithm expansion and shows measurements of the proof-of-concept prototype performed within the experimental setup. Localization results obtained from indoor measurements indicate that the proposed concept can provide better results than the similar approach based on a switched-beam antenna introduced by Giorgetti (IEEE Commun. Lett., vol. 13, no. 1, pp. 58-60, Jan. 2009).

Achieving Arbitrary Signals Transmission Using a Single Radio Frequency Chain

Electronically steerable parasitic array radiator (ESPAR) antenna has been proposed to obtain multiple antenna functionality with only a single radio frequency (RF) chain. However, for some signals, the input resistance of the ESPAR antenna (EA) becomes negative. A negative input resistance would lead to oscillatory/unstable behavior of an EA to transmit signals. In this work, we address this issue of an EA by obtaining approximate signals for transmission, which are close to the ideal signals in the mean-squared-error (MSE) sense, and satisfy the requirement that the input resistance is positive. Specifically, we formulate and solve an optimization problem to minimize the MSE between the ideal and the approximate signals taking into account the constraint on the input resistance. Closed-form expressions to calculate the approximate signal vector from ideal signal vector are also obtained, resulting in simplified processing at the transmitter. We denote the EA utilizing the novel closed-form expressions as EA with preprocessing (EA-P) and the EA without any preprocessing as the standard EA (EA-S). The performance of EA-P is compared with the performance of EA-S and the standard multiple antenna system, with multiple RF chains, under various communication scenarios. Our simulations show that the performance of EA-S significantly degrades compared with the standard multiple antenna system and an error floor is obtained. However, using the proposed EA-P scheme, the error floor is avoided and nearly the same performance as that of the standard multiple antenna system is achieved.

The characteristics of broadband ESPAR antenna with coupled multiconductor strips

This article suggests a new structure for a five-element broadband electrically steerable parasitic array radiator (ESPAR) antenna that can be used in the 2.1–2.6 GHz band. Previous monopole ESPAR antennas are unlikely to have broadband characteristics because they use cylindrical elements. However, antennas can have broadband characteristics when active elements are replaced with a crossed monopole shape. Moreover, the new structure, where a number of neighboring conductors are arranged on both sides of the parasitic element's conductor, has the advantage of increasing the antenna's bandwidth. The method proposed by Pues may increase 10–20% of the bandwidth of the antenna. Finally, the fabrication was made easier due to the realization of the active and parasitic elements on the same flat surface of PCB. The crossed monopole ESPAR antenna with coupled multiconductor strips suggested in this article possesses broadband characteristics with 520 MHz band at a return loss of 15 dB and antenna gain about 4.7–5.7 dBi. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:210–215, 2016

Extended reactance domain algorithms for DoA estimation onto an ESPAR antennas

ABSTRACTBased on an extended reactance domain (RD) covariance matrix, this article proposes new alternatives for directions of arrival (DoAs) estimation of narrowband sources through an electronically steerable parasitic array radiator (ESPAR) antennas. Because of the centro symmetry of the classic ESPAR antennas, an unitary transformation is applied to the collected data that allow an important reduction in both computational cost and processing time and, also, an enhancement of the resolution capabilities of the proposed algorithms. Moreover, this article proposes a new approach for eigenvalues estimation through only some linear operations. The developed DoAs estimation algorithms based on this new approach has illustrated a good behaviour with less calculation cost and processing time as compared to other schemes based on the classic eigenvalues approach. The conducted simulations demonstrate that high-precision and high-resolution DoAs estimation can be reached especially in very closely sources situation and low sources power as compared to the RD-MUSIC algorithm and the RD-PM algorithm. The asymptotic behaviours of the proposed DoAs estimators are analysed in various scenarios and compared with the Cramer-Rao bound (CRB). The conducted simulations testify the high-resolution of the developed algorithms and prove the efficiently of the proposed approach.

철도 무선통신을 위한 단일 RF 체인을 사용하는 고이득 빔포밍 패치 ESPAR 안테나 설계 및 분석

본 논문에서는 철도 통신 환경에서 신뢰성 있는 초고속 통신을 위해 3개의 소자로 구성된 패치형 ESPAR(Electronically Steerable Parasitic Array Radiator) 안테나를 기반으로 한 배열 안테나 구조를 설계하였다. ESPAR 안테나는 단일 RF-체인을 가지는 능동소자와 능동소자를 둘러싼 기생소자들로 구성되며, 기생소자들의 리액턴스를 조절하여 빔포밍이 가능하다. 패치형 ESPAR 안테나 기반의 수직 배열 안테나 구조를 제안하고, 안테나 행간 거리와 안테나 수에 따라 시뮬레이션을 하였다. 시뮬레이션 결과, 안테나 행간 거리가 <TEX>${\lambda}$</TEX>일 때 가장 큰 빔의 이득과 지향성을 가지는 것을 확인할 수 있다. In this paper, we design an array antenna structure based on a patch ESPAR(Electronically Steerable Parasitic Array Radiator) antenna with three elements for reliable communication in high-speed railway wireless communication. The ESPAR antenna consists of the active element with a single RF-chain and the parasitic elements surrounding an active element. The ESPAR antenna is capable of beamforming by adjusting the reactance of the parasitic element. We propose a vertical array antenna structure based on the patch ESPAR antenna and simulate it according to the change of the number of antennas and the distance between antenna rows. The simulation results show that we can get the maximum beam gain and highest directivity when the distance between antenna rows is <TEX>${\lambda}$</TEX>.

The characteristics of the dipole espar antenna using the cross‐coplanar waveguide feed

ABSTRACTThis article suggests a dipole electrically steerable parasitic array radiator (ESPAR) antenna that uses a crossed coplanar waveguide (CPW) transmission line that can be applied to beamspace multi‐input‐multioutput communication. The dipole antenna, which uses a vertically standing cylindrical structure, requires a feeding method that uses coaxial cable as the cable is connected inside the cylinder. As a result, it has the shortcoming of having to use cable with a limited diameter due to the cylindrical diameter as well as weak structure because of the flexible characteristic of coaxial cable. The antenna suggested in this article is rigid because it has a crossed PCB structure and it also has easier impedance matching than the cable feeding method because has a CPW transmission line. Moreover, the crossed CPW dipole ESPAR antenna turned out to have a radiation pattern similar to a dipole ESPAR antenna composed of a cylindrical structure. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:2238–2242, 2015

빔공간 MIMO 시스템에 기반한 OFDM 전송방법

Beamspace Multiple-Input Multiple Output(MIMO) system can transmit multiple data by using Electronically Steerable Parasitic Array Radiator(ESPAR) antenna which has single Radio Frequency(RF)-chain. Beamspace MIMO system can reduce complexity of the system and size of antenna in comparison with the conventional MIMO system because of characteristic of ESPAR antenna using the single antenna and the RF-chain. Heretofore, only the research of transmitting single-carrier has been conducted by the use of beamspace MIMO system. Therefore, in this paper, we propose beamspace MIMO system based on Orthogonal Frequency Division Multiplexing(OFDM) for transmitting the multi-carrier and analysis the performance of this system. We find a proper reactance value which has good performance because proposed system changes the performance by the reactance values of parasitic elements. and we confirm that performance of the proposed system is similar to conventional MIMO system based on OFDM. 논문 15-40-03-02 The Journal of Korean Institute of Communications and Information Sciences '15-03 Vol.40 No.03 http://dx.doi.org/10.7840/kics.2015.40.3.425 425 ※ 이 논문은 2012년도 정부(교육과학기술부)의 재원으로 한국연구재단의 지원을 받아 수행된 기초연구사업임(No. 2012017339) 그리 고 이 논문은 2013년도 정부(교육과학기술부)의 재원으로 한국연구재단의 지원을 받아 수행된 기초연구사업임(No.2013R1A2A2A01 005849) 그리고 본 연구는 미래창조과학부 및 정보통신기술진흥센터의 정보통신·방송 연구개발 사업의 일환으로 수행하였음(14-000 -04-001, 고집적 Compact MIMO 안테나 개발). First Author : Smart Radio Research Team, Electronics and Telecommunications Research Institute, jkchoi@etri.re.kr, 정회원 ° Corresponding Author : Department of electronics engineering, Chungbuk National University, ecomm@cbu.ac.kr, 정회원 * Department of electronics engineering, Chungbuk National University, acy890217@naver.com, 학생회원 논문번호:KICS2014-10-438, Received October 30, 2014; Revised March 2, 2015; Accepted March 2, 2015 The Journal of Korean Institute of Communications and Information Sciences '15-03 Vol.40 No.03

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.

Beamspace MIMO System Using Chaos BPSK Modulation

Multiple input and multiple output (MIMO) system is evaluated significantly to increase the channel capacity, but conventional MIMO system has disadvantages such as high-complexity and high-power-consumption. Beamspace MIMO system using electronically steerable parasitic array radiator (ESPAR) antenna with single radio frequency chain has been studied actively in order to solve these problems. In this paper, we design binary phase shift keying (BPSK) modulated beamspace MIMO system using ESPAR antenna for the low complexity and low power consumption. Also, for the first time, chaos communication algorithm is applied into the beamspace MIMO system for security improvement, and we evaluate the performance of this proposed system. From the simulation results, BPSK modulated chaos beamspace MIMO system using ESPAR antenna is able to reduce the complexity and the power consumption. Also, it is possible to improve the security without bandwidth spread and performance degradation.

ESPAR 안테나를 사용하는 카오스 QPSK 변조 빔 공간 MIMO 시스템

최근 대용량, 초고속 통신 서비스의 비중이 높아짐에 따라 배열 안테나를 사용하는 MIMO(Multi-Input Multi-Output) 시스템의 활용이 중요하게 평가되고 있다. 하지만 기존의 MIMO 시스템은 다수의 안테나를 사용하기 때문에 안테나의 개수만큼 RF(Radio Frequency) 체인이 요구되며, 이로 인해 복잡도와 전력 소모가 크다는 단점을 가진다. 이러한 기존 MIMO 시스템의 단점을 해결하기 위해서 단일 능동 소자와 다수의 기생 소자를 이용하는 방식의 ESPAR(Electronically Steerable Parasitic Array Radiator) 안테나를 사용하는 빔 공간 MIMO 시스템에 대한 연구가 활발히 진행되고 있다. ESPAR 안테나를 사용하는 빔 공간 MIMO 시스템은 단일 RF 체인으로 구성되어 있기 때문에 기존 MIMO 시스템의 단점을 해결할 수 있다. 본 논문에서는 시스템의 보안성을 향상시키기 위해 QPSK(Quadrature Phase Shift Keying) 변조 방식의 ESPAR 안테나를 사용하는 빔 공간 MIMO 시스템에 비예측성, 비주기성, 구현의 용이성, 민감한 초기조건 등의 특징을 가지고 있는 카오스 통신 알고리즘을 적용하여 QPSK 변조 방식의 ESPAR 안테나를 사용하는 카오스 빔 공간 MIMO 시스템을 구성하고 성능을 평가한다. Recently, utilization of MIMO(Multi-Input Multi-Output) system using array antennas is evaluated significantly according to the extension of high-capacity and high-speed communication services. However, MIMO system has disadvantages such as high-complexity and high-power-consumption, because RF(Radio Frequency) chain is required as antenna number, and several array antenna is used in conventional MIMO system. In order to solve these problems, research about beamspace MIMO system using ESPAR(Electronically Steerable Parasitic Array Radiator) antenna that has single RF chain by using one active antenna and several parasitic elements has been studied actively. Beamspace MIMO system using ESPAR antenna is possible to solve the problems of conventional MIMO system, because this system is composed by single RF chain. In this paper, in order to improve the system security, chaos communication algorithm that has characteristics such as non-periodic, non-predictability, easy implementation and initial condition is applied to QPSK (Quadrature Phase Shift Keying) modulated beamspace MIMO system. We design the chaos QPSK modulated beamspace MIMO system, and evaluate SER performance of this system.

ESPAR 안테나에서 M×M MIMO 송신방식의 설계와 성능 평가

본 논문에서는 ESPAR(Electronically Steerable Parasitic Array Radiator) 안테나를 이용한 <TEX>$M{\times}M$</TEX> BS-MIMO(beam space multiple input multiple output) 시스템을 제안한다. 기존 MIMO 시스템은 전송 데이터 신호를 다수의 안테나에 맵핑시켜 전송하기 때문에 다수의 RF 체인이 필요한 문제점이 있다. 다수의 RF 체인은 하드웨어의 비용을 및 RF 회로의 전력 소모를 증가 시킨다. 또한, 휴대폰과 같이 공간적인 제약이 큰 모바일 장비에서 MIMO 시스템을 사용하기 어렵다. 이러한 문제를 해결하기 위해서 단일 RF 체인을 가지는 ESPAR 안테나를 사용하여 빔 공간에서 신호를 맵핑시키는 BS-MIMO 시스템이 제안되었다. 본 논문에서는 BS-MIMO 시스템 기법에 대해서 설명하고 이를 확장한 <TEX>$M{\times}M$</TEX> BS-MIMO 전송 기법의 설계 및 성능을 분석한다. 컴퓨터 모의실험을 통한 성능 확인 결과 제안된 BS-MIMO 전송기법은 기존 MIMO 기법과 비교하여 거의 동일한 수신 BER 성능을 얻는다. 따라서 다수의 RF 체인을 가지는 기존 MIMO 시스템과 비교하여 BS-MIMO 시스템은 단일 RF 체인을 가지고 MIMO 전송이 가능하며, 이로 인해서 하드웨어 비용 및 RF 회로의 전력 소모를 획기적으로 줄일 것으로 예상된다. In this paper, we propose a <TEX>$M{\times}M$</TEX> beam-space multiple input multiple output (BS-MIMO) system using electronically steerable parasitic array radiator (ESPAR) antenna. Conventional MIMO method required multiple RF chains because it map the transmission signals onto multiple antennas. So, conventional MIMO system has high cost for design and high energy consumption at RF circuit. Also, It is difficult to use MIMO system in battery based mobile terminals with limited physical area. In order to solve these problems, BS-MIMO technique which map the data signal onto bases in beam space domain was proposed using ESPAR antenna with single RF chain. This paper, we design and analyze the performance of extended <TEX>$M{\times}M$</TEX> BS-MIMO technique. Simulation results show that the proposed BS-MIMO system has similar BER performance compare to conventional MIMO scheme. Therefore, BS-MIMO system with single RF chain will has a low RF power consumption and low cost for RF hardware design as compared with conventional MIMO technique with multiple RF chains.

Low‐cost intelligent antenna with low profile and broad bandwidth

In this paper, a low-cost intelligent antenna, which has a low profile, high gain and broad bandwidth, is presented. The proposed antenna is an extension of the conventional Electronically Steerable Parasitic Array Radiator (ESPAR) antenna. It consists of one short monopole and twelve folded monopoles. The short monopole is placed in the centre and serves as the driven element, and the twelve folded monopoles serve as parasitic elements. The coupling between the driven and parasitic elements provides capacitive loading to the driven element, leading to a low profile antenna. The use of twelve PIN diodes enables the beamforming being performed by using Transistor-transistor logic (TTL) supply. When interference signals exist, an adaptive beamforming algorithm is employed to choose a radiation pattern with the best signal-to-noise ratio (SNR). The front-to-back ratio (FBR) and SNR is improved by introducing a reconfigurable chock ring (RCR) surrounding the antenna. To validate the concept, a prototype at S band is developed. Both measured and simulated results are presented. The antenna has a height of 24 mm only (0.16 at 2.0 GHz). The measurement results confirm that the antenna is capable of beamforming and beam scanning over 360°range in horizontal plane. The antenna has a broadband bandwidth from 2.0 to 2.5 GHz. Finally, the antenna is employed in a wireless communication system in an office environment and the system link quality is tested. It is shown that the antenna can boost the throughput of wireless system significantly.

Improved reactance domain unitary propagator algorithms for electronically steerable parasitic array radiator antennas

In this study, the authors propose four algorithms for directions of arrival (DoAs) estimation of multiple incoming narrowband plane waves onto electronically steerable parasitic array radiator antennas. The constraints on the proposed algorithms are the same as those imposed onto the classic subspace methods allowing superior high-resolution and localisation capabilities even for correlated sources scenarios but with more reduced computations load and processing time than existing schemes. We also demonstrate that estimating the propagator operator through both introduced real-valued orthogonal decompositions techniques not only yields to a faster DoAs estimation with a reduced computational complexity, but also it improves the robustness of the developed algorithms to noise as compared with the classic propagator algorithm. The Cramer–Rao bound on the variance of the estimated DoAs by the proposed algorithms is analysed. The achieved performance by the developed methods is studied and compared with conventional antenna arrays. The simulation results confirm that high-resolution DoAs estimation can be achieved with the developed algorithms and prove the validity of the proposed approach.

Compact MIMO Antenna With Frequency Reconfigurability and Adaptive Radiation Patterns

A frequency-reconfigurable antenna for a multiple-input-multiple-output (MIMO) system with electronically steerable radiation patterns in azimuth plane is proposed in this letter. For MIMO systems, the steerable radiation patterns are able to reduce the correlation between signals and increase the channel capacity. There are two folded-monopole electronically steerable parasitic array radiator (FM-ESPAR) antennas in the proposed MIMO antenna system in this letter. The radiation pattern of each FM-ESPAR antenna is steerable in azimuth plane from 0 to 360 . Its frequency is reconfigurable between 2.3 and 2.6 GHz. A top-disk loaded monopole and folded monopoles are employed to reduce the height of each FM-ESPAR antenna to 12 mm (λ/10 at 2.4 GHz). The FM-ESPAR antennas are separated from each other by using a 30-mm-long sleeve ground plane. The measured results confirm that the MIMO antenna comprising two FM-ESPAR antennas achieves good isolation at any angle between the main lobes of the FM-ESPAR antennas. The proposed MIMO antenna has compact size, electronically steerable beams, and frequency-agile capability.