Electronically Steerable Parasitic Array Radiator Research Articles

Spectrally Efficient Pulse Shaping for Beamspace Space Shift Keying in Single-RF ESPAR Systems

A novel spectrally efficient pulse shaping technique for space shift keying (SSK) in single radio frequency (single-RF) antenna systems is proposed. This technique is important since a key issue often overlooked in most single-RF systems, including SSK, is pulse shaping. The proposed pulse shaping technique is based on using an electronically steerable parasitic array radiator (ESPAR). It can avoid the bandwidth expansion arising from switching in SSK while enhancing spectral efficiency (SE) compared to conventional time-limited pulse shaping. In the proposed approach, we pulse shape the parasitic loads of the antennas in the ESPAR by using pulse shaped baseband control voltages to continuously vary the parasitic loads. This is enabled by developing an approximate linear mapping from the baseband control voltages to the ESPAR element currents. Simulation results of the proposed technique are provided, including the continuous-time ESPAR current and its power spectrum density, adjacent channel power ratio, symbol error rate (SER), SE, and energy efficiency (EE). It is shown that the proposed technique can perform spectrally efficient pulse shaping for beamspace SSK and achieve higher SE and EE than time-limited pulse shaping while maintaining the same SER.

Analog Beamforming Using ESPAR for Single-RF Precoding Systems

A novel analog beamforming technique that utilizes single radio frequency (single-RF) electronically steerable parasitic array radiator (ESPAR) antennas is proposed. The ESPAR beamforming techniques are aimed at precoding in multiple antenna wireless communication systems using the single or hybrid RF approach. One advantage of ESPAR analog beamforming compared to conventional analog beamforming using phase shifters is that arbitrary patterns can be straightforwardly formed. Therefore, ESPAR analog beamforming can provide performance almost identical to digital beamforming. More importantly, ESPAR analog beamforming does not require phase shifters and power splitters, overcoming implementation challenges of conventional analog beamforming. It also has a more compact size compared to conventional beamforming using uniform antenna arrays. We analyze and optimize ESPAR analog beamforming and propose a channel approach based on beamspace pilot transmission to acquire channel state information (CSI). In addition, we propose limited feedback ESPAR analog beamforming based on a beamforming selection strategy. Simulation results for average gain show that ESPAR analog beamforming has almost identical performance to digital beamforming and better performance than analog beamforming using phase shifters. Energy efficiency simulations show that the technique has the highest efficiency among all the beamforming techniques. Therefore, ESPAR analog beamforming has near optimal performance, high energy efficiency, low circuit complexity, low cost, compact size and can be an effective alternative to conventional analog beamforming.

Single-RF MIMO-OFDM Using ESPAR

A novel beamspace multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) transmission system using a single-radio-frequency (RF) electronically steerable parasitic array radiator (ESPAR) is described. It makes use of recent results for decomposing the patterns of any arbitrary ESPAR onto an orthogonal basis set using the Theory of Characteristic Modes (TCM). Leveraging this decomposition, OFDM symbols can be mapped into the beamspace domain so that the ESPAR can be used to transmit multiple streams of OFDM symbols using a single RF chain. In the proposed approach, we use the Linde-Buzo-Gray (LBG) algorithm to quantize the OFDM symbol vectors and form a codebook. A binary search tree is then utilized to efficiently find the reactive loads of ESPAR to excite the radiation patterns corresponding to the quantized symbols. It is shown by simulation that quantized codewords and the corresponding radiation patterns can be excited using ESPAR. In addition, the simulated bit error rate (BER) results of the proposed approach are shown to be close to that of conventional MIMO-OFDM systems, demonstrating the effectiveness of implementing a single-RF MIMO-OFDM transmitter.

ESPAR array design for the UHF RFID wireless sensor communication system

SummaryIn this paper, in order to improve the received signal strength (RSS) and signal quality, three arrays of electronically steerable parasitic array radiator (ESPAR) antennas are suggested for the ultra‐high frequency (UHF) radio frequency identification (RFID) communication and sensing system applications. Instead of the single antenna, the array antennas have recently been widely used in many communication systems because of their peak gains, better radiation patterns, and higher radiation efficiency. Also, there are some important issues to use the antenna array like high data rates in wireless communication systems and to better understand the many targets or sensors. In this article, a wireless sensor network (WSN) is being investigated to overcome multipath fading and interference by antenna nulling technology that can be achieved through beam control ESPAR array antennas. The proposed ESPAR array antennas exhibit higher gains like 9.63, 10.2, and 12 dBi and proper radiation patterns from one array to another. Moreover, we investigate the mutual coupling effect on the performance of array antennas with different spacing (0.5λ, 0.75λ, λ) and configurations. It is found that the worst mutual coupling reduced by −28 to −34 dB for 2 × 2 array, −3 to −43 dB for 2 × 3 array, and finally −42 dB to −51 dB due to the antenna spacing from 0.5λ to λ. Thus, these suggested antennas could effectively be applied in the WSN communication systems, internet of things (IoT) networks, and massive wireless and backscatter communication systems.

Miniaturized Electronically Steerable Parasitic Array Antenna

In this article we propose a miniaturized dual-band electronically steerable parasitic array radiator (ESPAR) antenna. The antenna can generate up to two steerable beams. The beam-steering range of the proposed antenna is 360˚ in the azimuth plane. The antenna’s eual-band coverage includes the frequency ranges from 2.3 GHz to 2.53 GHz and from 2.9 GHz to 3.7 GHz. The antenna consists of six folded parasitic monopole elements surrounding an active conical element. The folded monopole element design offers three times lower antenna height than that of the conventional ESPAR antennas. The active element has conical shape and it is larger in length than the parasitic monopole elements, this enables the dual-band operation. Thus, the proposed design is not only smaller than the conventional ESPAR antennas but it also achieves dual-band operation. Despite its compact design, the antenna has a peak gain of 6.3 dBi, which is equivalent to the gain of conventional ESPAR antennas. These characteristics make the antenna a good candidate for next generation communication systems.

A Pattern Correlation Decomposition Method for Analysis of ESPAR in Single-RF MIMO Systems

A systematic and general method for obtaining orthogonal far-field radiation patterns using any electronically steerable parasitic array radiator (ESPAR) embedded in a propagation environment with an arbitrary power angular spectrum (PAS) is described. The method is useful in designing single-RF multiple-input multiple-output (MIMO) systems where the orthogonal patterns can be utilized as a basis set for beamspace MIMO. The method utilizes the correlation matrix of the open-circuit radiation patterns and is termed the pattern correlation decomposition method (PCDM). PCDM is applicable to any antenna type (including realistic antennas with losses), array configuration, PAS and overcomes the limitations of previously proposed techniques. PCDM also provides closed-form solutions for the antenna currents to generate the orthogonal patterns. It also provides an estimate of the effective aerial degrees-of-freedom of the ESPAR. In addition, efficient optimization approaches to finding the optimal reactive loads that excite the required orthogonal radiation patterns are provided. Simulation examples using a 5-element and a 8-element planar inverted-F antenna array including material losses in various PAS are provided. These show that the proposed approach increases system capacity by up to 7 bits/s/Hz or equivalently reduces the required transmit power by up to 73% compared with previous approaches.

Direction of Arrival Estimation Based on Received Signal Strength Using Two-Row Electronically Steerable Parasitic Array Radiator Antenna.

In this paper, we present a novel approach to direction-of-arrival (DoA) estimation using two-row electronically steerable parasitic array radiator (ESPAR) antenna which has 12 passive elements and allows for elevation and azimuth beam switching using a simple microcontroller, relying solely on received signal strength (RSS) values measured at the antenna output port. To this end, we thoroughly investigate all 18 available 3D antenna radiation patterns of the antenna measured in an anechoic chamber with respect to radiation coverage in the horizontal and vertical direction and propose a generalization of the power-pattern cross-correlation (PPCC) algorithm involving a high number of multiple calibration planes (MCP) as well as specific combinations of radiation pattern sets. Additionally, a new way of RSS-based DoA estimation accuracy assessment, which involves thorough testing conducted along the elevation direction when RF signals impinging on the antenna arrive from arbitrary θ angles, has been reported in this paper to verify the overall algorithm’s performance. The results obtained for different signal-to-noise ratio (SNR) levels indicate that two-row ESPAR antenna can produce, even for low SNR values, accurate DoA estimation in the horizontal plane without prior knowledge about the elevation direction of the unknown RF signals by using appropriate combinations of only 12 3D antenna radiation patterns.

Relay-Aided Wireless Sensor Network Discovery Algorithm for Dense Industrial IoT Utilizing ESPAR Antennas

Industrial Internet-of-Things (IIoT) applications require reliable and efficient wireless communication. Assuming dense wireless sensor networks (WSNs) operating in a harsh environment, a concept of a time-division multiple access (TDMA)-based WSN enriched with electronically steerable parasitic array radiator (ESPAR) antennas is proposed and examined in this work. The utilized antenna provides one omnidirectional and 12 directional radiation patterns that can be electronically switched by the sensor node. We introduce a relay discovery algorithm, which selects those sensor nodes with an ESPAR antenna capable to act as relay. The selection of the relay nodes is based on a certain link quality threshold that algorithm uses as input. The outcome is a reduction in the number of layers or hops with a guaranteed Quality of Service (QoS). To emphasize the physical aspect of the wireless propagation, we introduce the measured antenna radiation patterns and consider two different path-loss propagation models representing blockage-free and blockage-prone industrial environments. A number of network simulations were performed and signal-to-noise ratio (SNR) as a link quality measure was examined with respect to the network density and different measured radiation pattern settings. The main outcomes show a tradeoff between SNR per link and the percentage of nodes that can serve as relays. As a result, we propose network design guidelines that take under consideration the QoS range with respect to SNR together with an optimal number of antenna radiation patterns that should be selected as a tradeoff between latency, energy consumption, and reliability in a network.

Calibration-Free Single-Anchor Indoor Localization Using an ESPAR Antenna.

In this paper, we present a novel, low-cost approach to indoor localization that is capable of performing localization processes in real indoor environments and does not require calibration or recalibration procedures. To this end, we propose a single-anchor architecture and design based on an electronically steerable parasitic array radiator (ESPAR) antenna and Nordic Semiconductor nRF52840 utilizing Bluetooth Low Energy (BLE) protocol. The proposed algorithm relies on received signal strength (RSS) values measured by the receiver equipped with the ESPAR antenna for every considered antenna radiation pattern. The calibration-free concept is achieved by using inexpensive BLE nodes installed in known positions on the walls of the test room and acting as reference nodes for the positioning algorithm. Measurements performed in the indoor environment show that the proposed approach can successfully provide positioning results better than those previously reported for single-anchor ESPAR antenna localization systems employing the classical fingerprinting method and relying on time-consuming calibration procedures.

Digital pattern synthesis with a compact MIMO antenna of half-wavelength diameter

The paper presents a compact multiport antenna, with beamforming capabilities and a realized gain of 4.31 dBi with the size of λ/2. The antenna allows digital beamforming with independent control over three parameters: main-beam direction, beamwidth, and null-depth. Those parameters can be dynamically adjusted within a continuous range, create multiple beams for different frequency channels, and simultaneously generate multiple digital patterns. Due to digital beamforming capabilities, the proposed antenna offers much greater flexibility over switch-based analogue beamforming of Electronically Steerable Parasitic Array Radiator (ESPAR) antennas. The reconfiguration properties are achieved by using superposition of three dipole-like radiation patterns, where each radiator generates different spherical mode, i.e. with different angular phase distribution in the horizontal plane. This allows beamsteering with compact size, while the use of different modes ensures low Envelope Correlation Coefficient (ECC) from 10−4 to 3.2 × 10−5 for the operating frequency of 2.4 GHz. The proposed technique allows for significantly greater pattern reconfiguration than any reconfigurable antenna or array of comparable size and is a perfect candidate for Software Defined Radios.

Slot-Based Pattern Reconfigurable ESPAR Antenna for IoT Applications

In this article, a slot-based electronically steerable parasitic array radiator (ESPAR) antenna suitable for Internet-of-Things (IoT) applications is proposed. The antenna can provide 12 directive and two slightly different omnidirectional radiation states. The 12 directive patterns allow the antenna to focus the radiated energy toward six different directions uniformly distributed in the azimuth plane and with two different beamwidths. The reconfiguration of the radiation pattern is based on the use of low-capacitance low-power consuming diodes that can be digitally controlled. This article details the design of the antenna structure and the practical implementation of the reconfiguration mechanism, discussing the effects of real electronic components on the antenna performance. The antenna effectiveness is numerically and experimentally assessed.

Characteristic Mode Analysis of ESPAR for Single-RF MIMO Systems

A systematic method based on the Theory of Characteristic Modes (TCM) for finding orthogonal radiation patterns of any electronically steerable parasitic array radiator (ESPAR) is described. This method can be useful for designing single-RF front-end multiple-input multiple-output (MIMO) systems in which orthogonal patterns can be utilized as a basis set for space modulation (SM) or full multiplexed MIMO systems. The method is based on the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N$ </tex-math></inline-formula> -port formulation of TCM and is not restricted to antenna type or array configuration. It can also provide closed formed solutions for the antenna element currents required to generate the orthogonal patterns. In addition an approach to finding the required load reactances in ESPAR for generating the orthogonal patterns is provided and is based on the quasi-Newton method utilizing a closed form expression for the initial approximate solution. Approximate estimation of the effective aerial degrees of freedom of the ESPARs is also discussed. Two simulation examples of ESPARs, a 4-element linear dipole array and an 8-element rectangular planar inverted-F antenna array, using SM as well as full multiplexed MIMO are provided, demonstrating the effectiveness of the proposed method.

Low-Complexity Implementation of Channel Estimation for ESPAR-OFDM Receiver

Electronically steerable parasitic array radiator (ESPAR) antenna is a novel low-cost technique to gain comparable diversity order to that of using multiple antennas while maintaining one radio frequency (RF) front-end, low power consumption, and simple wiring at the mobile receiver. It has huge potential to realize high energy efficiency for multiple and massive antennas systems. The main drawback of the ESPAR based OFDM system is that the channel estimation (CE) usually should be realized in time-domain with a huge computational complexity (CC) because the received signal to each parasitic element is overlapped each other both in frequency and time domain. This article will propose three ways to reduce the complexity of CE for the ESPAR-OFDM system. The parallel multi-column compressive sensing (CS) algorithm was first introduced to detect all locations of channel impulse response simultaneously using one small segment of sensing matrix. Furthermore, by exploiting the symmetrical properties of Digital Fourier transform (DFT), the size of matrix to decrease the CC of matrix operation. Finally, by selecting a small set of row vectors of sensing matrix at the receiver side, the CC involved in CS algorithm can be reduced. From the analysis and simulated results, the proposed method can obtain more than 90% CC reduction for CE of ESPAR-OFDM receiver can be confirmed.

A 20-Element Cavity-Backed Slot Electronically Steerable Parasitic Array Radiator (ESPAR) With 2-D Beamsteering and Minimized Beam Squint

A 20-element cavity-backed slot antenna array is presented herein based on an electronically steerable parasitic array radiator (ESPAR) cross subarray. This ESPAR array is able to steer the main beam within ±45° in the E-plane and ±40° in the H-plane without grating lobes. Inside each subarray, the beamsteering is realized by tuning the reactance of the loads at the four parasitic elements. Therefore, only four phase shifters are needed for this 20-element array to achieve 2-D beamsteering, corresponding to an 80% reduction on the number of phase shifters compared to a classic phased array. The measured impedance matching is maintained below -10 dB from 6.0 to 6.4 GHz (6.4% fractional bandwidth) at all scan angles. The radiation patterns are measured at five different scan angles in E- and H-planes. At each scan angle, the measured radiation patterns at five frequencies across the impedance-matching bandwidth exhibit very small beam squint. The measured peak gain is found to be 14.0 dBi at the boresight direction.

Low-Complexity Adaptive Spatial Processing of ESPAR Antenna Systems

Accessing the spatial domain of wireless environments through antenna arrays has become key to improve the use of energy and, in light of this, there is an increasing need for low cost multi-antenna architectures. As one alternative, the Electronically Steerable Parasitic Array Radiator (ESPAR) was proposed as a coupling-based inexpensive option; allowing to significantly reduce the amount of required radio frequency (RF) front-ends. As a caveat, due to its inherent non-linear behavior, the required computational complexity can be prohibitive. Additionally, the unavailability of precise mutual coupling and channel state information (CSI) becomes a further issue difficult to avoid in practical setups. In this regard, the current work has two main contributions: a) it deals with the computational complexity by proposing the linearization of ESPAR’s system model through the truncated Taylor expansion of the admittance matrix; facilitating the spatial processing algorithm of interest. Also, the authors propose b) to face the mutual coupling and CSI unavailability issue via the joint estimation of the channel-ESPAR parameters as directly observed through the single RF front-end. Relying on the optimization of the linearized system model, the pilot-based algorithm to be introduced allows ESPAR to track the configuration that synthesizes coherent combination at a significantly low complexity. Particularly, as shown through Monte-Carlo simulation, the latter allows to obtain 4 dB of array gain with a single RF front-end via a 5-element ESPAR.

A frequency‐reconfigurable electronically‐steerable parasitic array radiator using microstrip patch antennas

AbstractA microstrip patch electronically‐steerable parasitic array radiator (ESPAR) with continuous frequency reconfigurability is presented in this paper. Loading varactors are strategically placed on the radiating edge of all antenna elements to achieve a 15% continuous frequency tuning range, from 0.87 to 1.02 GHz. At each operating frequency, this ESPAR is able to scan from −20 to 20° in the H plane. Therefore, this ESPAR is able to provide spectrum diversity in addition to the spatial diversity inherent in a frequency‐fixed ESPAR. The underlying physics and design procedures of this ESPAR are presented. The return losses at different operating frequencies and scan angles, radiation patterns, realized gains, and third‐order input intercept point (IIP3) are also measured.

A Circular Beam-Steering Antenna With Parasitic Water Reflectors

In this letter, a salty water antenna with beam-steering characteristics is proposed. The antenna combines the idea of the electronically steerable parasitic array radiator (ESPAR) antenna and the water antenna to realize a 360° beam steering. The unique features of water antennas, such as transparency, reconfigurability, and liquidity, are fully utilized. Both the computer simulations and measurements are conducted to validate the performance. The proposed design has a frequency range from 334 to 488 MHz ( S 11 60%), 13 different working states, and a maximum gain of 5.8 dB. Compared with existing pattern-reconfigurable liquid antennas, this new antenna has a wider bandwidth, more working states, and better invisible ability. It is, therefore, a promising candidate for a range of very high-frequency or ultrahigh-frequency applications, such as Internet of Things and maritime applications.

λ/64‐spaced compact ESPAR antenna via analog RF switches for a single RF chain MIMO system

In this study, an electronically steerable parasitic array radiator (ESPAR) antenna via analog radio frequency (RF) switches for a single RF chain MIMO system is presented. The proposed antenna elements are spaced at λ/64, and the antenna size is miniaturized via a dielectric radome. The optimum reactance load value is calculated via the beamforming load search algorithm. A switch simplifies the design and implementation of the reactance loads and does not require additional complex antenna matching circuits. The measured impedance bandwidth of the proposed ESPAR antenna is 1,500 MHz (1.75 GHz–3.25 GHz). The proposed antenna exhibits a beam pattern that is reconfigurable at 2.48 GHz due to changes in the reactance value, and the measured peak antenna gain is 4.8 dBi. The reception performance is measured by using a 4 4 BPSK signal. The measured average SNR is 17 dB when using the proposed ESPAR antenna as a transmitter, and the average SNR is 16.7 dB when using a four‐conventional monopole antenna.

An Electronically Steerable Parasitic Array Radiator (ESPAR) Using Cavity-Backed Slot Antennas

The electronically steerable parasitic array radiator (ESPAR) is an inexpensive way to achieve beam steering. Planar ESPARs usually exhibit narrow fractional bandwidth (FBW) in terms of both impedance matching and beam squinting. Herein, an E -plane three-element ESPAR using cavity-backed slot antennas is presented to demonstrate a wider FBW, i.e., 4%, by investigating not only S 11 but also beam squinting. It is found that a tunable load which uses a fixed inductor, a varactor, and a delay line can provide a much larger impedance tuning range compared to that without the inductor. This tunable load is critical to enhance the FBW by considering beam squinting. This ESPAR is able to scan from −26° to 20° in the E -plane, operating around 5.15 GHz. The measured peak gain is between 5.52 and 6.05 dBi at all scan angles. The radiation patterns are measured at different scan angles in the E -plane. In addition, they are measured at different frequencies to demonstrate the beam squinting performance.

RSS-Based DoA Estimation for ESPAR Antennas Using Support Vector Machine

In this letter, it is shown how direction-of-arrival (DoA) estimation for electronically steerable parasitic array radiator (ESPAR) antennas, which are designed to be integrated within wireless sensor network nodes, can be improved by applying support vector classification approach to received signal strength (RSS) values recorded at an antenna's output port. The proposed method relies on ESPAR antenna's radiation patterns measured during the initial calibration phase of the DoA estimation process. These patterns are then used in the support vector machine (SVM) training process adapted to handle ESPAR antenna-based DoA estimation. Measurements using a fabricated ESPAR antenna indicate that the proposed SVM approach provides more accurate results than available RSS-based estimation algorithms relying on power pattern cross-correlation method.