Time-modulated Arrays Research Articles

A multibeam subarrayed time-modulated linear array

In conventional time-modulated arrays TMAs , because of the usage of the RF-switch, harmonics are generated at multiples of the modulation frequency. In this study, the synthesis of time-modulated arrays has been analyzed for cognitive radio CR systems, in which these harmonics are suitably exploited for more efficient utilization of the spectrum. In order to accomplish the desired pattern at requested harmonic frequencies, a new excitation strategy with sinusoidal signals is proposed. The use of sinusoidal waveforms creates independent beams, allowing independent steering capability. Moreover, by utilizing the subarray structure, it is possible to have a smaller number of excitation functions in the hardware. It is shown by means of explanatory examples that the effectiveness of the proposed approach is due to free-adjustable beamforming, beam steering, and low complexity.

Target Localization Based on Intermodulation Feedback for Multisine Wireless Power Transmission Using a Time-Modulated Array

To achieve accurate power feeding to a terminal (e.g. rectenna) in the far-field wireless power transmission (WPT), a new method is proposed to locate the terminal so that the base station can adaptively adjust its beam steering and transmitting power to satisfy the high-efficiency power conversion conditions of the rectifier. In this work, the generated intermodulation (IM) signal at rectenna under multisine incidence is employed to establish the feedback link with the base station. A linear time-modulated array (TMA) is exploited to generate two-tone multisine wave by the optimized modulation sequence thanks to its multi-beam radiation behavior at the base station side. In the terminal end, the IM signal is spontaneously produced due to the nonlinearity of power rectifying under two-tone incidence and employed as the feedback. To highly isolate the two tones and IM signal, a dual linear polarized (DLP) antenna is designed to ensure a compact system design for both sides. By this way, the feedback IM signal carrying the terminal's direction and distance information is sensed by orthogonally polarized antenna pairs. Then the direction of terminal is obtained by the proposed high-accuracy direction of arrival (DoA) estimation method using a multi-baseline TMA, which is realized by a two-stage single-pole-double-throw (SPDT) switching network. Regarding the imperfection of RF switches, a compensation method is proposed. With estimated DoA, the distance between terminal and base station is evaluated based on the received signal strength indicator (RSSI) method. To verify the feedback mechanism, a prototype is fabricated and tested.

Microwave Modulated Scatter Active Array Distortion Sidelobes

A signal processing antenna consisting of an array of modulated scatter elements is described. Time-modulation of array elements for radiation pattern and sidelobe control was initially described more than a half-century ago. Recent decades have produced many advances and extensive documentation for both transmit and receive arrays with time modulated elements. The new article presented here includes both receive and transmit functions in an array of modulated scattering elements. This new approach to time-modulated array experiments greatly simplifies both the physical array elements and the measurement apparatus. An incoming continuous wave (CW) signal in space arrives at the array, is time-modulated by the elements, and then re-radiated in space. The only electrical connections to the individual array elements are low-frequency time-modulation signals. The measurement apparatus consists of a spectrally pure and frequency stable CW source, an audio sine wave modulator with different phases for each element, a microwave receives down-converter to a baseband IF from dc to 24 kHz, and an audio frequency power spectrum analyzer running on a laptop computer. Models that incorporate both the CW signal arrival angle and radiation angles for each modulation and distortion product are presented, along with measurements of 16 element arrays in the anechoic chamber at 2.3 GHz using time-modulation frequencies near 1 kHz. The individual array element time-modulators are nearly ideal RF switches, and the agreement between the new time-modulated scatter array models and measurements is remarkable.

Wideband Hybrid Precoding Using Time Modulated Arrays

Hybrid digital-analog precoding is a cost effective solution for transmitting over the large bandwidths and huge antenna arrays available in the millimeter wave frequency bands. In this work we focus on the transmission of wideband signals over hybrid precoders that utilize Time-Modulated Arrays in the analog domain. We consider analog precoders constructed with Single-Pole-Double-Throw switches which are both flexible and efficient. We pose two Orthogonal Frequency-Division Multiplexing symbol configurations to cope with the harmonic interference introduced by the Time-Modulated Array. One does not take advantage of the wireless channel frequency diversity and the other does. We optimize digital and analog precoders to maximize the achievable rate in both symbol configurations. Optimization takes into account the switching devices efficiency and the inherent losses of this antenna technique. Finally, we show the excellent performance obtained with the proposed approach, in terms of achievable rate, compared to that of conventional phased arrays.

Multibeam Single-Sideband Time-Modulated Arrays

The design of efficient time-modulated arrays (TMAs) for multi-beam steering purposes must be based on both a type of periodical sequences that guarantees an adequate windowing of exploited harmonics, and a feeding network capable of providing low hardware insertion losses. Compliance with these two requirements is not simple and is aggravated by the fact that, as the number of exploited harmonic beams increases, not only drastically decreases the antenna efficiency, but also the beam steering flexibility. In this work, we propose three different single sideband (SSB) feeding networks for multibeam TMAs constructed considering different combinations of single-pole multiple-throw (SPMT) basic modules. The proposed feeding networks simultaneously accomplish excellent levels of modulation efficiency, reasonable hardware insertion losses, and beam scanning flexibility. In addition, a comparison with standard phased arrays in terms of performance, phase sensitivity, complexity, and cost-effectiveness is carried out.

Power Efficient Scheduling and Hybrid Precoding for Time Modulated Arrays

We consider power efficient scheduling and precoding solutions for multiantenna hybrid digital-analog transmission systems that use Time-Modulated Arrays (TMAs) in the analog domain. TMAs perform beamforming with switches instead of conventional Phase Shifters (PSs). The extremely low insertion losses of switches, together with their reduced power consumption and cost make TMAs attractive in emerging technologies like massive Multiple-Input Multiple-Output (MIMO) and millimeter wave (mmWave) systems. We propose a novel analog processing network based on TMAs and provide an angular scheduling algorithm that overcomes the limitations of conventional approaches. Next, we pose a convex optimization problem to determine the analog precoder. This formulation allows us to account for the Sideband Radiation (SR) effect inherent to TMAs, and achieve remarkable power efficiencies with a very low impact on performance. Computer experiments results show that the proposed design, while presenting a significantly better power efficiency, achieves a throughput similar to that obtained with other strategies based on angular selection for conventional architectures.

Total Harmonic Power of Arbitrarily Switched Nonuniform Period Time-Modulated Arrays

In this letter, nonuniform period time-modulated arrays (NTMAs) are investigated and analyzed in terms of radiated power at harmonics, and a more general and accurate closed-form expression is derived. Although there are some studies on the radiated power of NTMAs, it is claimed that the derived expressions in these studies have restrictions to be used as final forms. To overcome these restrictions, a new method is proposed and hence a closed-form power expression is achieved for uniform/nonuniform switching. Moreover, the obtained results are discussed over comparative examples to emphasize the differences among the recent findings. As a result, it is concluded that the proposed method is an effective solution that can eliminate the limitations of the studies in the literature.

UAVs-based antenna arrays using time modulation

This paper applies time modulation to deal with the design of antenna arrays for UAVs formation flight using a rectangular micro-strip as antenna element in the frequency of 2.4 GHz. The motivation for this paper is to exploit the properties and advantages of time-modulated arrays in an interesting application: UAVs formation flight. This design considers the optimization of the position of the drones (or antenna elements) and the time switched sequences of the elements in a linear geometry. The design process to find the optimal positions of the drones for flight formation and the switch-on interval of each element is carried out by means of the methods of Particle Swarm Optimization and Invasive Weed Optimization. The main contribution to the field of this paper is a performance evaluation of the design problem for UAVs-based linear antenna arrays considering time modulation in terms of side lobe level (SLL) and sideband levels (SBLs). Simulation results are provided in terms of SLL, SBLs, switching time and positions of the drones. These simulation results illustrate that the time modulation could provide a SLL improvement with respect to previous results in literature.

Multiple Access using Time Modulated Array with Single RF Chain

Background: A new wireless multiple access technology enabled by using Time Modulated Arrays (TMAs) is proposed in this paper. Methods: It benefits due to the requirement of only a single Radio Frequency (RF) chain, compared with other multiple-RF-chain schemes. Results: As a result, it is able to greatly reduce the system cost, energy consumption, and complexity. Conclusion: In addition, the signal through the single RF chain is narrow-band modulated, reducing the signal Peak-to-Average-Power-Ratio (PAPR), thus, further enhancing the power efficiency of the RF chain, especially for power amplifiers. The operation principle and synthesis approach are elaborated in this paper, and are demonstrated with two examples.

Time modulated array sideband suppression for joint radar-communications system based on the differential evolution algorithm

To satisfy the need of electronic system integration in civil and military, and mitigate the tension of spectrum resource, the demand for coexistence of radar and communication systems is increasing rapidly. As an important branch of radar-communications integration, the time modulated array (TMA) based coexistence method has been proposed recently. The TMA has an inherent drawback that there are many undesired sideband signals generated simultaneously. However, the existing TMA sideband suppression algorithms do not suit the radar-communications integration scenarios. In this paper, a time modulated array sideband suppression method for joint radar-communications system based on the differential evolution (DE) algorithm is proposed. We first introduce some facts of the time modulated array. Then, the radar-communication coexistence strategy is presented, and the form of integration signal is derived. Moreover, combined with a positive and negative alternating periodic modulation function, the TMA sideband suppression for radar-communications coexistence scenarios based on the DE algorithm is proposed. The sideband patterns of the TMA can be significantly lowered by optimizing the opening and closing moments of each element. Finally, the validity of the theoretical derivations is demonstrated via simulations.

A Figure of Merit in a Time-Modulated Array

In this letter, we use G/T as a figure of merit to evaluate the performance of time-modulated arrays (TMA). The noise performance of a receiver, which is periodically switched ON and OFF, is quite different from that of a conventional receiver. We analytically investigate the noise performance of a TMA. Based on this analysis, the figure of merit, G/T, of the TMA structure is investigated, including the details of the receiver hardware. Moreover, the G/T of the TMA is compared with that of a phased array. The comparison indicates that the TMA structure provides the same or even better performance compared to a phased array system, which demonstrates the capability of TMA in providing reasonable G/T as a low-cost beamforming structure.

An Effective Procedure for Nonlinear Dynamic Optimization of Time-Modulated Arrays

A complete framework for the accurate design of time-modulated arrays is presented in this letter. The proper control sequence synthesis originates, for the first time, from the deep knowledge of both the signal level and the switches dynamic, i.e., of the nonlinear regime of these complex radiating systems. The adopted two-nested loop approach relies on the Harmonic-Balance technique and a stochastic algorithm for the accurate estimation of the actual array regime and for an efficient optimization of the time-dependent design parameters, respectively. The effectiveness of the proposed tool is demonstrated through the control of a realistic 16-planar-monopoles array for maximum instantaneous directivity performance.

Direction Finding of Linear Frequency Modulation Signal in Time Modulated Array With Pulse Compression

A novel direction-finding method with time-modulated array (TMA) is proposed for modulating the echo linear frequency modulation (LFM) signal and analyzing its harmonic characteristics. Unlike conventional analysis method where each coefficient of harmonic components is analyzed by discrete Fourier transform, in the proposed method, the harmonic coefficients of modulated signals are calculated by using the pulse compression technology. After the modulated LFM signal is compressed by the matched filter, a closed-form expression of the corresponding output signal is derived and the harmonic coefficients of the input modulated signals can also be obtained. Meanwhile, in order to avoid being affected by other harmonic components in the process of analyzing each harmonic coefficient, the constraints to acquire independent harmonic coefficients are highlighted. Numerical simulations are provided to verify the performance of the proposed method, and a simple S-band eight-element TMA is constructed to experimentally verify its feasibility.

A Time-Modulated Array With Digitally Preprocessed Rectangular Pulses for Wireless Power Transmission

A cooperative design of the advanced time-modulated array (TMA) and the retrodirective (RD) array is introduced to act as a transmitting array for multiuser wireless power transmission (WPT) applications. The conventional rectangular-pulse based TMA is lack of harmonic beamforming flexibility limited by its frequency spectral properties. By preprocessing the rectangular pulses with the sinusoid-based control signal, harmonic beams are independently steerable. The proposed array is enabled with self-phasing capability by taking advantage of the RD technique. Besides, to avoid the unexploited radiation caused by unscannable fundamental beam and specular harmonic beams, the single sideband (SSB) modulator is utilized to enable the transmitting array with high-efficiency beamforming capacity. The numerical results verify that the presented array performs well in various charging scenarios, which brings a highly potential insight for far-field WPT.

Research on Side Lobe Suppression of Time-Modulated Sparse Linear Array Based on Particle Swarm Optimization

An efficient pattern synthesis approach is proposed for the synthesis of a time-modulated sparse linear array (TMSLA) in this paper. Due to the introduction of time modulation, the low/ultralow side lobe level can be obtained with a low amplitude dynamic range ratio. Besides, it helps reduce the difficulty of antenna feeding system effectively. Based on particle swarm optimization (PSO) and convex (CVX) optimization, this paper proposes a hybrid optimization method to suppress the grating lobes of the sparse arrays, peak side lobe level (PSLL), and peak sideband level (PSBL). Firstly, the paper utilizes the CVX optimization as a local optimization algorithm to optimize the elements’ switch-on duration time, which reduces the side lobe of the array. Secondly, with the PSBL as the objective function, the paper adopts the PSO as a global optimization algorithm to optimize the elements’ positions and switch-on time instant, which helps reduce the loss of sideband power caused by time modulation. With respect to the time modulation model, variable aperture sizes (VAS) and more flexible pulse-shifting (PS) schemes are used in this paper. Owing to the introduction of time modulation and CVX optimization, the proposed method is much more feasible and efficient than conventional approaches. Furthermore, it has better array pattern synthesis performance. Numerical examples of the TMSLA and comparisons with the reference are presented to demonstrate the effectiveness of the proposed method.

Time-modulated array beamforming with periodic stair-step pulses

Time-modulated arrays (TMAs) are able to improve the side-lobe level of the radiation pattern at the fundamental mode but cannot steer the beam at such a mode towards a given direction. Beam-steering is possible in a TMA, but only at the harmonic patterns and at the expense of a severe TMA efficiency reduction. In this work we propose a TMA approach that simultaneously performs both features over the same beam by using two sets of switches: (1) single-pole four-throw switches to generate periodic stair-step pulses suitable for efficiently synthesizing a uniform steerable beam over the first positive harmonic, and (2) single-pole single-throw switches to reconfigure the side-lobe level of the previous beam. Performance, small size, cost-effectiveness, and performance invariability with the carrier frequency are features that make this TMA approach a competitive solution for analog beamforming. Accordingly, the structure is an attractive proposal for the design of multibeam transceivers.

Chirp 신호의 특성을 이용한 LoRa 기기의 TMA 방향탐지

This study examines a direction finding system for a LoRa™ Internet of Things(IoT) device using a time-modulated array(TMA). LoRa IoT technology uses a spread spectrum to increase the communication range, thus widening the frequency bandwidth. Thus, to construct a general TMA direction finding system, a high-speed radio frequency switch that is faster than the signal bandwidth is required. In this study, we implemented a TMA-based direction finding system with a low-speed switch based on the characteristic whereby the instantaneous bandwidth of the chirp signal is narrower than the signal bandwidth. Our system consists of two commercial monopole antennas, a self-designed switch board, and a software defined radio. The system is controlled using a LabVIEW program in real-time. The measured results verify that our system can exactly detect the incident angle within a maximum error of 15° in the range of ±50o. These results can be applied to various cases in which the instantaneous bandwidth is smaller than the signal bandwidth.

Time-Modulated OFDM Directional Modulation Transmitters

In this paper, directional modulation (DM) transmitters, designed for orthogonal frequency-division multiplexing (OFDM) wireless data transfer are proposed using time-modulated arrays. It is shown that by properly designing the switch controlling time sequences the proposed time-modulated OFDM DM systems exhibit promising features, such as 1) requires only one radio frequency chain, 2) the conventional efficient OFDM signal construction approach, i.e., by using inverse fast Fourier transform modules, can be adopted, 3) it is “DM synthesis-free,” meaning that there is no need to resynthesize the array excitation vectors for different secure communication directions, different modulation schemes, and different DM power efficiencies, and 4) the tradeoff between secrecy performance and power efficiency can be flexibly adjusted.

Harmonic power losses in time-modulated arrays with non-uniform period modulation

In this paper, firstly, the theory of time-modulated array with non-uniform period modulation, named as non-uniform time modulated array (NTMA) where the antenna elements are modulated with different modulation frequencies has been examined. Then, it is investigated that, based on the selection of modulation frequencies, radiation of harmonic signals from such time-modulated array elements either can be spread in the space without overlapping with any other harmonic frequencies or can be accumulated in the space because of the radiation at the same frequency. Accordingly, the NTMA is classified into two categories as non-degenerate NTMA (ND-NTMA) and degenerate NTMA (D-NTMA) respectively. The expressions of total harmonic power losses for both ND-NTMA and D-NTMA have been derived. Through representative examples the harmonic power losses associated with different types of TMAs is calculated and their variations with inter-element spacing is presented.

Pattern Synthesis of Time-Modulated Array Antenna Based on an Improved Invasive Weed Optimization Method

In this paper, pattern synthesis through time-modulated linear array is studied, and a novel strategy for harmonic beamforming in time-modulated array is proposed. The peak side lobe level is designed as optimization objective function, and the switch-on time sequence of each element is selected as optimization variable. An improved invasive weed optimization (IWO) algorithm is developed in order to determine the optimal parameters describing the pulse sequence used to modulate the excitation weights of array elements. Representative results are reported and discussed to point out potentialities and advantages of the proposed approach, which can obtain lower objective function values.