Abstract: With the relatively recent realization that millimeter wave frequencies are viable for mobile communications, extensive measurements and research have been conducted on frequencies from 0.5 to 100 GHz, and several global wireless standard bodies have proposed channel models for frequencies below 100 GHz. Presently, little is known about the radio channel above 100 GHz where there are much wider unused bandwidth slots available. This paper summarizes wireless communication research and activities above 100 GHz, overviews the results of previously published propagation measurements at D-band (110–170 GHz), provides the design of a 140 GHz wideband channel sounder system, and proposes indoor wideband propagation measurements and penetration measurements for common materials at 140 GHz which were not previously investigated.

Abstract: A range of efficient wireless processes and enabling techniques are put under a magnifier glass in the quest for exploring different manifestations of correlated processes, where sub-Nyquist sampling may be invoked as an explicit benefit of having a sparse transform-domain representation. For example, wide-band next-generation systems require a high Nyquist-sampling rate, but the channel impulse response (CIR) will be very sparse at the high Nyquist frequency, given the low number of reflected propagation paths. This motivates the employment of compressive sensing based processing techniques for frugally exploiting both the limited radio resources and the network infrastructure as efficiently as possible. A diverse range of sophisticated compressed sampling techniques is surveyed, and we conclude with a variety of promising research ideas related to large-scale antenna arrays, non-orthogonal multiple access (NOMA), and ultra-dense network (UDN) solutions, just to name a few.

Abstract: Rydberg atoms, with one highly excited, nearly ionized electron, have extreme sensitivity to electric fields, including microwave fields ranging from 100 MHz to over 1 THz. Here, we show that room-temperature Rydberg atoms can be used as sensitive, high bandwidth, microwave communication antennas. We demonstrate near photon-shot-noise limited readout of data encoded in amplitude-modulated 17 GHz microwaves, using an electromagnetically induced-transparency (EIT) probing scheme. We measure a photon-shot-noise limited channel capacity of up to 8.2 Mbit s−1 and implement an 8-state phase-shift-keying digital communication protocol. The bandwidth of the EIT probing scheme is found to be limited by the available coupling laser power and the natural linewidth of the rubidium D2 transition. We discuss how atomic communication receivers offer several opportunities to surpass the capabilities of classical antennas.

Abstract: This work proposes a high-bandwidth white-light system consisting of a blue gallium nitride (GaN) micro-LED (μLED) exciting yellow-emitting CsPbBr1.8I1.2 perovskite quantum dots (YQDs) for high-speed real-time visible light communication (VLC). The packaged 80 μm × 80 μm blue-emitting μLED has a modulation bandwidth of ∼160 MHz and a peak emission wavelength of ∼445 nm. The achievable bandwidth of the white-light system is up to 85 MHz in the absence of filters and equalization technology. Meanwhile, the bandwidth of the YQDs as a color converter is as high as 73 MHz with the blue GaN μLED as the pump source. A maximum data rate of 300 Mbps can be achieved by taking advantage of the high bandwidth of the white-light system using the non-return-to-zero on–off keying (NRZ-OOK) modulation scheme. The resultant bit-error rate is 2.0 × 10–3, well beneath the forward error correction criterion of 3.8 × 10–3 required for error-free data transmission. In addition, the YQDs which we proposed as a color converter p...

Abstract: The focus of this paper is on coexistence between a communication system and a pulsed radar sharing the same bandwidth. Based on the fact that the interference generated by the radar onto the communication receiver is intermittent and depends on the density of scattering objects (such as, e.g., targets), we first show that the communication system is equivalent to a set of independent parallel channels, whereby precoding on each channel can be introduced as a new degree of freedom. We introduce a new figure of merit, named the compound rate , which is a convex combination of rates with and without interference, to be optimized under constraints concerning the signal-to-interference-plus-noise ratio (including signal-dependent interference due to clutter) experienced by the radar and obviously the powers emitted by the two systems: the degrees of freedom are the radar waveform and the aforementioned encoding matrix for the communication symbols. We provide closed-form solutions for the optimum transmit policies for both systems under two basic models for the scattering produced by the radar onto the communication receiver, and account for possible correlation of the signal-independent fraction of the interference impinging on the radar. We also discuss the region of the achievable communication rates with and without interference. A thorough performance assessment shows the potentials and the limitations of the proposed co-existing architecture.

Abstract: More people prefer to using rail traffic for travel or for commuting due to its convenience and flexibility. As the record of the maximum speed of rail has been continuously broken and new applications are foreseen, the high-speed railway (HSR) communication system requires higher data rate with seamless connectivity, and therefore, the system design faces new challenges to support high mobility. Millimeter-wave (mmWave) technologies are considered as candidates to provide wideband communication. However, mmWave is rarely explored in HSR scenarios. In this paper, channel characteristics are studied in the 5G mmWave band for typical HSR scenarios, including urban, rural, and tunnel, with straight and curved route shapes. Based on the wideband measurements conducted in the tunnel scenario by using the “mobile hotspot network” system, a 3-D ray tracer (RT) is calibrated and validated to explore more channel characteristics in different HSR scenarios. Through extensive RT simulations with 500-MHz bandwidth centered at 25.25 GHz, the power contributions of the multipath components are studied, and the dominant reflection orders are determined for each scenario. Path loss is analyzed, and the breakpoint is observed. Other key parameters, such as Doppler shifts, coherence time, polarization ratios, and so on, are studied. Suggestions on symbol rate, sub-frame bandwidth, and polarization configuration are provided to guide the 5G mmWave communication system design in typical HSR scenarios.

Abstract: This paper presents the design and characterization of a load modulated balanced amplifier for telecom base station applications adopting a novel mode of operation. The theory of operation is described explaining the main differences compared to Doherty amplifiers, in particular the RF bandwidth advantages and, on the other hand, the intrinsic nonlinear behavior. The specific design strategy that adopts prematching for back-off broadband matching is explained in detail. A prototype, based on 25-W GaN packaged devices, has been fabricated and measured with single tone CW and modulated signal stimulus. For CW conditions, on the 1.7–2.5-GHz band, the peak output power is between 63 and 78 W, with power added efficiency higher than 48%, 43%, and 39% at saturation, 6- and 8-dB output power back-off, respectively. With a modulated signal for Long Term Evolution the amplifier provides an average output power of around 10 W, with efficiency higher than 40%, and can be linearized by adopting a low complexity predistorter. If compared to previously published power amplifiers targeting similar power and bandwidth, the measurement shows very good performance, demonstrating the potential of this novel technique in the field of efficiency enhanced transmitters.

Abstract: The increasing demand for high-rate broadcast and multicast services over satellite networks has pushed for the development of high throughput satellites characterized by a large number of beams (e.g., more than 100). This, together with the variable distribution of data traffic request across beams and over time, has called for the design of a new generation of satellite payloads, able to flexibly allocate bandwidth and power. In this context, this paper studies the problem of radio resource allocation in the forward link of multibeam satellite networks adopting the digital video broadcasting-satellite-second generation standard. We propose a novel objective function with the aim to meet as close as possible the requested traffic across the beams while taking fairness into account. The resulting non-convex optimization problem is solved using a modified version of the simulated annealing algorithm, for which a detailed complexity analysis is presented. Simulation results obtained under realistic conditions confirm the effectiveness of the proposed approach and shed some light on possible payload design implications.

Abstract: Impedance matching is one of the most important practices in wave engineering as it enables one to maximize the power transfer from the signal source to the load in the wave system. Unfortunately, it is bounded by the Bode-Fano criterion which states that, for any passive, linear, and time-invariant matching network, there is a stringent trade-off between the matching bandwidth and efficiency, implying severe constraints on various electromagnetic and acoustic wave systems. Here, we propose a matching paradigm that overcomes this issue by using a temporal switching of the parameters of a metamaterial-based transmission line, thus revoking the time-invariance assumption underlying the Bode-Fano criterion. Using this scheme we show theoretically that an efficient wideband matching, beyond the Bode-Fano bound, can be achieved for short-time pulses in challenging cases of very high contrast between the load and the generator impedances, and with significant load dispersion, situations common in, e.g., small antenna matching, cloaking, and with applications for ultrawideband communication, high resolution imaging, and more.

Abstract: A design method for frequency selective surface (FSS) structure with high angular stability has been proposed in this letter. In the proposed method, bandwidth angular stability of FSS structure has been improved by adopting the bandwidth compensation technique, and structural parameters can be obtained with a curve-fitting method from the desired resonate frequency and bandwidth. Discussions on improving the bandwidth design accuracy have been presented. By taking bandwidth angular stability into consideration, the structure designed by the proposed method is more suitable to construct FSS radome. For verification, an FSS structure operating at 15 GHz with a bandwidth of 2 GHz has been designed, fabricated, and measured. Good agreements between the simulated and measured results can be observed.

Abstract: This letter investigates an uplink power control problem for unmanned aerial vehicles (UAVs)-assisted wireless communications. We jointly optimize the UAV’s flying altitude, antenna beamwidth, UAV’s location, and ground terminals’ allocated bandwidth, and transmit power to minimize the sum uplink power subject to the minimal rate demand. An iterative algorithm is proposed with low complexity to obtain a suboptimal solution. Numerical results show that the proposed algorithm can achieve good performance in terms of uplink sum power saving.

Abstract: Antenna miniaturization has been the subject of numerous studies for almost 70 years [1]-[4]. Early studies showed that a decrease in the size of an antenna results in a direct reduction in its bandwidth and efficiency (hr) [1], [2]. The size limitation translates into a lower boundary on the achievable radiation quality factor (Q factor) and consequently on the maximum achievable impedance bandwidth. Recently, many new investigations have been conducted to reduce the form factor (or the overall size) of different types of antennas while trying to maintain acceptable matching properties and operating bandwidth. These miniaturization techniques are generally related to changing the electrical and physical properties of an antenna.

Abstract: A wideband $8 \times 8$ element slot antenna array based on ridge gap waveguide feeding network has been proposed for mmWave applications. The antenna subarray consists of four radiating slots which are excited by a groove gap cavity layer. Compared with previously published works, the proposed planar antenna array has quite wide impedance bandwidth. The antenna covers a wideband of 50–67.8 GHz with 30% impedance bandwidth (VSWR < 2). Also, the antenna has only 2.5 dB gain variation over the entire bandwidth which implies also good radiation characteristics for the proposed antenna. The maximum measured gain value is about 27.5 dBi with a total efficiency of 80% for the proposed antenna within the band of interest. With this performance, the proposed antenna array is a promising candidate for mmWave communication systems.

Abstract: This paper presents a novel technique for the design of broadband Doherty power amplifiers (DPAs), supported by a simplified approach for the initial bandwidth estimation that requires linear simulations only. The equivalent impedance of the Doherty inverter is determined by the value of the output capacitance of the power device, and the Doherty combiner is designed following this initial choice and using a microstrip network. A GaN-based single-input DPA designed adopting this method exhibits, on a state-of-the-art bandwidth of 87% (1.5–3.8 GHz), a measured output power of around 20 W with 6 dB back-off efficiency between 33% and 55%, with a gain higher than 10 dB. System-level measurements prove the linearizability of the designed Doherty amplifier when a modulated signal is applied.

Abstract: By placing active antennas in a 2D grid at a BS, 3D MIMO is considered as a promising and practical technique for 5G New Radio (NR). So far, 3D MIMO studies reported are mostly done with antenna elements from 32 up to 128 in the limited scenario at one frequency. To gain further insights into the 3D massive MIMO channel and performance, field measurements from 32 to 256 antenna elements at the transmitter and 16 antenna elements at the receiver are performed in three typical deployment scenarios, including outdoor to indoor, urban microcell, and urban macrocell at both 3.5 and 6 GHz frequencies with 200 MHz bandwidth. Based on the extracted channel information from measured data, power angle spectrum, root mean square angle spread, channel capacity, and eigenvalue spread have been studied. Several observations, including 3D MIMO channel spatial dispersive properties and multi-user performance varying with antenna number, scenario, and frequency are given. These findings can provide valuable experimental insights for efficient utilization of 3D MIMO with massive antenna elements.

Abstract: This paper presents a “fully-connected” hybrid beamforming receiver that independently weights each element in an antenna array prior to separate downconversion chains that output independent baseband streams. A receiver architecture is introduced, which implements RF-domain complex-valued Cartesian weighting, RF-domain combining, and multi-stream heterodyne complex-quadrature downconversion. Each RF-domain Cartesian weight is implemented by a pair of 5-bit digitally controlled programmable-gain amplifiers, whose outputs are combined with the weighted signals from other antennas prior to complex-quadrature downconversion. Signal combination is performed by a wideband small-footprint distributed active combiner. A 25–30 GHz hybrid beamforming receiver with eight antenna inputs and two baseband output streams is designed in 65-nm CMOS. In each antenna path, the receiver achieves 34-dB conversion gain, 7.3-dB minimum noise figure, and 5 GHz of RF bandwidth. The entire receiver consumes 340 mW (equivalent to 27.5 mW per antenna per stream) including low-noise amplification, RF-domain beamforming, multi-stream downconversion, and local oscillator generation and distribution circuitry. The receiver occupies 3.86 mm2 excluding pads, equivalent to 0.36 mm2 per antenna per stream. Single-element characterization results are presented, along with characterization of several spatial processing techniques including interference cancellation (20 dB peak to null for two elements), simultaneous two-stream reception, and adaptive-codebook-search-based beam acquisition.

Abstract: A wideband omnidirectional dielectric resonator antenna (DRA) with filtering response is investigated. The DRA is placed on a circular patch and excited by a hybrid feed that consists of a probe and a metallic disk. Two omnidirectional DR modes (TM $_{01\delta }$ and TM013 modes) along with a patch mode and a probe mode are simultaneously excited by the hybrid feed, providing a broad bandwidth of 52.8%. The feeding scheme also establishes a cross-coupled structure in the DRA, which introduces a radiation null at the upper edge of the passband. A ring slot and four shorting pins are loaded on the patch to provide another radiation null at the lower edge of the passband. Consequently, a compact wideband filtering DRA (FDRA) with quasi-elliptic bandpass response is obtained without requiring specific filtering circuits. This wideband design is also modified to realize a dual-band FDRA. The metallic disk of the hybrid feed is moved to touch the upper DR face of the cylindrical hole. It divides the wide passband into two separate bands, giving a flat stopband between them. Four additional rectangular ring slots and shorting pins are fabricated on the ground plane to further widen the bandwidth and improve the filtering performance of the second band. As a result, a dual-band FDRA with the bandwidths of 10.1% and 3.73% is achieved. In each design, both the measured and simulated out-of-band suppression levels are about 15 dB.

Abstract: Total bandwidth of existing wireless communication technologies covers a wide frequency range of over one octave. But most existing power amplifier configurations cannot meet this requirement while at the same time maintaining a high efficiency. Therefore, a new structure that can achieve multioctave bandwidth is proposed in this paper together with the design methodology. The difficulty in realizing a bandwidth larger than one octave lies in the overlapping of fundamental and harmonic frequencies. Regarding this problem, the continuous class-F mode is extended to allow a resistive second harmonic impedance, rather than the pure reactive one. With the relaxed design requirements and overlapping design space of fundamental and second harmonic frequencies, harmonic tuning and fundamental frequency matching networks can be designed separately. More importantly, broadband matching for fundamental frequencies can be implemented simply by considering only three fundamental frequency points using the multiple frequencies matching method. To verify the validity of the proposed methodology, a multioctave power amplifier was designed, fabricated, and measured. Measured results verify a wide bandwidth of 128.5% from 0.5 to 2.3 GHz. Over this frequency range, drain efficiency was larger than 60% with output power greater than 39.2 dBm and large signal gain larger than 11.7 dB.

Abstract: Access to the electromagnetic spectrum is an ever-growing challenge for radar. Future radar will be required to mitigate RF interference from other RF sources, relocate to new frequency bands while maintaining quality of service, and share frequency bands with other RF systems. The spectrum sensing, multioptimization (SS-MO) technique was recently investigated as a possible solution to these challenges. Prior results have indicated significant improvement in the signal-to-interference plus noise ratio at the cost of a high computational complexity. However, the optimization computational cost must be manageable in real time to address the dynamically changing spectral environment. In this paper, a bioinspired filtering technique is investigated to reduce the computational complexity of SS-MO. The proposed technique is analogous to the processing of the thalamus in the human brain in that the number of samples input to SS-MO is significantly decreased, thus, resulting in a reduction in computational complexity. The performance and computational complexity of SS-MO and the proposed technique are investigated. Both techniques are used to process a variety of measured spectral data. The results indicate a significant decrease in computational complexity for the proposed approach while maintaining performance of the SS-MO technique.

Abstract: Radio reception relies on antennas for the collection of electromagnetic fields carrying information, and receiver elements for demodulation and retrieval of the transmitted information. Here we demonstrate an atom-based receiver for AM and FM microwave communication with a 3-dB bandwidth in the baseband of $\sim$100~kHz that provides optical circuit-free field pickup, multi-band carrier capability, and inherently high field sensitivity. The quantum receiver exploits field-sensitive cesium Rydberg vapors in a centimeter-sized glass cell, and quantum-optical readout of baseband signals modulated onto carriers with frequencies ranging over four octaves, from C-band to Q-band. Receiver bandwidth, dynamic range and sideband suppression are characterized, and acquisition of audio waveforms of human vocals demonstrated. The atomic radio receiver is a valuable receiver technology because it does not require antenna structures and is resilient against electromagnetic interference, while affording multi-band operation in a single compact receiving element.

Abstract: Two novel microstrip patch antennas with multiple parasitic patches and shorting vias have been presented for the bandwidth enhancement. Based on the conventional triangular patch antenna, two more resonances can be obtained with the introduction of multiple parasitic patches, and consequently, the antenna bandwidth can be broadened. Parametric analysis of the patches has been studied for the verification of bandwidth enhancement. An example of the proposed antenna with multiple parasitic patches is designed, fabricated, and tested. The measured bandwidth with $\vert S_{11}\vert dB ranges from 5.46 to 6.27 GHz (13.8%), and good far-field radiation patterns can be obtained within the frequency band. In addition, two shorting vias are inserted into the above proposed antenna to decrease the input impedance, resulting in further bandwidth enhancement of the antenna. This antenna is fabricated and tested as well, which achieves a measured 10-dB impedance bandwidth of 17.4% from 5.5 to 6.55 GHz.

Abstract: This letter proposes a novel wideband rectifier circuit for RF energy harvesting. The proposed circuit can collect signals efficiently over broad bandwidth spanning from 0.87 to 2.7 GHz which includes UHF ISM 900 MHz, GSM 900 and 1800 MHz, wireless communication, PCS, and ISM 2.4 GHz. In order to obtain sufficiently large rectifier bandwidth, a matching circuit based on high-pass type L-section for lower band impedance matching and inductive L-section for higher band impedance matching is proposed. The rectifier circuit is constructed using voltage doubler configuration with Schottky diode SMS7630-005LF. The circuit is optimized and refabricated to compensate the undesired parasitic and obtain the required rectifier performance. Two prototypes were simulated, fabricated, and characterized. The rectifier has a measured conversion efficiency exceeding 30% from 870 MHz to 2.5 GHz at 0 dBm input power and a load terminal of 2 $\text{k}\Omega $ and a dc output voltage equal to 1 V. The circuit sensitivity may reach up to −20 dBm with dc output voltage 40 mV and 8% conversion efficiency. The maximum measured efficiency is 63% from 1.1 to 1.35 GHz.

Abstract: Recent publications show that the potential of using orthogonal frequency division multiplexing waveforms as radar signals. Since the range resolution is proportional to the RF bandwidth, the major obstacle that obstructs the practical use in automotive and other low-cost radars is the requirement to sample the received signal at sampling rates that span the whole RF signal bandwidth requiring ADCs with sampling rates in the order of GHz. This paper presents a method to achieve the high range resolution induced by a large RF bandwidth, but with a much lower baseband bandwidth, consequently requiring a much slower ADC while at the same time delivering a velocity profile for each subcarrier. In addition, the processing scheme induces a range migration compensation, independent of the number of targets. This is achieved with barely increased computational effort. The scheme is verified with simulations and measurements at 77 GHz.

Abstract: This letter proposes an implantable antenna with ultrawide bandwidth operating in the medical device radio communications service band (401–406 MHz) for the wireless capsule endoscopy (WCE). The simulation and experimental results show the proposed antenna has a good performance in terms of the return loss and hence the bandwidth from 284 to 825 MHz. The maximum realized gain of this antenna is –31.5 dBi at 403 MHz. The maximum simulated input power is <1.7 mW in order to satisfy the specific absorption rate (SAR) regulations in the IEEE standard. The tolerance of the antenna owing to bendability and different WCE shell thicknesses is investigated. These indicate that the proposed antenna is a good candidate for the WCE.

Abstract: This paper presents the design of $8 \times 8$ multiple-input multiple-output (MIMO) antennas for future 5G devices, such as smart watches and dongles. Each antenna of the MIMO configuration occupies $3 \times 4$ mm2 and is printed on the top layer of the substrate in the form of a rotated H-shaped patch. The substrate used for the design is a $31.2 \times 31.2 \times 1.57$ mm3, Rogers RT-5880 board, with a dielectric constant of 2.2. The top layer of the substrate has eight MIMO antennas, whereas the bottom layer is composed of ground plane. The ground plane is an electromagnetic bandgap-based structure designed for the enhancement of gain and efficiency. Each antenna is fed from the bottom layer of the substrate through vias to avoid any spurious radiation. The MIMO antennas resonate at 25.2 GHz with a 6-dB percentage bandwidth of 15.6%. The gain attained by the antennas in the entire bandwidth is above 7.2 dB with a maximum value of 8.732 dB at the resonant frequency. Likewise, the value of efficiency attained by the antennas in the entire bandwidth is above 65% with a maximum value of 92.7% at the resonant frequency. The simulation and measurement results have substantiated a good performance of the MIMO antennas, thus making them suitable for compact 5G devices.

Abstract: Historically, Vivaldi arrays are known to suffer from high cross-polarization when scanning in the nonprincipal planes—a fault without a universal solution. In this paper, a solution to this issue is proposed in the form of a new Vivaldi-type array with low cross-polarization termed the Sliced Notch Antenna (SNA) array. For the first proof-of-concept demonstration, simulations and measurements are comparatively presented for two single-polarized $19 \times 19$ arrays—the proposed SNA and its Vivaldi counterpart—each operating over a 1.2–12 GHz (10:1) band. Both arrays are built using typical vertically integrated printed-circuit board cards, and are designed to exhibit VSWR $\theta \!=\!45 {^{\circ }}$ polarization purity improvement at the high frequency. Moreover, the SNA element also: 1) offers better suppression of classical Vivaldi E-plane scan blindnesses; 2) requires fewer plated through vias for stripline-based designs; and 3) allows relaxed adjacent element electrical contact requirements for dual-polarized arrangements.

Abstract: The optimal filters with minimal bandwidth are highly desirable in many applications such as communication and biomedical signal processing In this study, we design optimally frequency localized orthogonal wavelet filters and evaluate their performance using electroencephalogram (EEG) signals for automated detection of the epileptic seizure The paper presents a novel method for designing optimal orthogonal wavelet filter banks (OWFB) with the objective of minimizing their frequency spreads The designed wavelet filter also possesses the desired degree of regularity The regularity condition has been imposed analytically so as to satisfy the constraint accurately We propose a novel semi-definite programming (SDP) formulation which does not involve any parametrization The solution of the SDP yields optimal orthogonal wavelet filter for the given length of the filter We have developed an automated diagnosis system that identifies epileptic seizure EEG signals using the features obtained from the designed minimally mean squared frequency localized (MMSFL) OWFB We have tested the performance of the proposed model using two independent EEG databases in order to ensure the consistency and robustness of the model Interestingly, the proposed MMSFL-OWFB feature-based model exhibits ceiling level of performance, with classification accuracy ≥ 99% in classifying seizure (ictal) and seizure-free (non-ictal) EEG signals for both databases Our developed system can be employed in hospitals and community cares to aid the epileptologists in the accurate diagnosis of seizures

Abstract: A photonics-based multiple-input-multiple-output (MIMO) radar is proposed and demonstrated based on wavelength-division-multiplexed broadband microwave photonic signal generation and processing. The proposed radar has a large operation bandwidth, which helps to achieve an ultra-high range resolution. Compared with a monostatic radar, improved radar performance and extended radar applications originated from the MIMO architecture can be achieved. In addition, low-speed electronics with real-time signal processing capability is feasible. A photonics-based 2 × 2 MIMO radar is established with a 4-GHz bandwidth in each transmitter and a sampling rate of 100 MSa/s in the receiver. Performance of the photonics-based multi-channel signal generation and processing is evaluated, and an experiment for direction of arrival (DOA) estimation and target positioning is demonstrated, through which the feasibility of the proposed radar system can be verified.

Abstract: A wideband linearly polarized transmitarray antenna operating in 7–16 GHz is presented in this paper. First, a wideband element with three metallic layers is proposed, which is composed of a split circular ring connected by a narrow strip in the middle layer, and two polarizers in the upper and bottom layers. This element features a low transmission loss and approximately linear phase curves in a wide frequency band over one octave. Then, an optimization method is introduced to design the transmitarray that follows the bandwidth definition of 1 dB gain drop. The first key point of this method is determining the transmitarray element distribution with the weighted reference phases. The second is realizing wideband performances by controlling the calculated directivities and radiation patterns of the modified wideband transmitarray model at all operating frequencies. In the experiment, a $25\times 25$ -element horn-fed transmitarray with a $240 \times 240$ mm2 aperture area is designed and fabricated. The simulated results show that its bandwidths of 0.5, 1.5, and 3 dB gain drop are 41%, 56%, and 71%, respectively. Moreover, a peak aperture efficiency of 40.7% is achieved. Measured results agree reasonably well with the simulated ones. These results validate the proposed wideband element and the optimization method.