Time-modulated Arrays Research Articles

Application of the Time Modulated Array in Satellite Communications

As a research hotspot in antenna arrays, the time modulated array (TMA) has advantages in pattern synthesis, direction finding, adaptive beamforming, and multi-beams, making it a potential application in satellite communications. This article illustrates how to utilize the TMA in satellite communications, to improve the performance of communication between the satellite and Earth. First, the overview of the TMA is provided. Second, direction finding by the TMA on the satellite is analyzed, which helps to detect and track users on Earth. Third, adaptive beamforming based on the TMA is discussed, which synthesizes the pattern of the generated harmonic component to point to mobile users. Fourth, the multibeam characteristic of the TMA is analyzed to simplify the complexity of a multi-beam system on the satellite. Finally, some conclusions are drawn, and areas of further research are put forward.

Direction Finding of Linear Frequency Modulation Signal With Time-Modulated Array

This communication proposes a novel method for finding the direction of the linear frequency modulation (LFM) signal, which has a bandwidth that is much larger than that of the modulated frequency, based on the time-modulated array (TMA). The relationship between the LFM signal and the modulated frequency is discussed and the constraint to avoid aliasing is also derived. To verify the proposed constraint, simulation signals are separately analyzed in the time domain, time–frequency domain, and frequency domain. Meanwhile, simulation results are provided to examine the validation of the proposed method with different factors such as incident angles and signal noise ratios. An S-band two-element TMA is constructed and implemented to experimentally verify the feasibility of the method.

A Novel Radar Based on Two-Element Time-Modulated Array

A novel radar is proposed and verified based on a two-element time-modulated array. The distance of the object is estimated from the time delay between the source signal and the return one, while the direction is calculated by analyzing the spectrum of the return signal after the time modulation. The proposed radar needs no rotary machinery or electronic scanning system to detect the direction of the object, and it can deal with multiple objects from different directions simultaneously, if their distance differences are larger than the range resolution. Numeric simulations are provided to verify the proposed method.

Time-Modulated Phased Array Controlled With Nonideal Bipolar Squared Periodic Sequences

Bipolar ( $\pm$ 1) sequences with no zero state suit particularly well for safeguarding the switched feeding network efficiency when applied to time-modulated arrays. During the zero state of a conventional time-modulating sequence, if a given array element is switched off , a certain amount of energy of the transmitted/received signal is wasted. We propose a novel single sideband time-modulated phased array (TMPA) architecture governed by realistic bipolar squared sequences in which the rise/fall time of the switches is considered. By using single-pole dual-throw switches and nonreconfigurable passive devices, the TMPA exploits, exclusively, the first positive harmonic pattern while exhibiting an excellent performance in terms of efficiency and control level of the undesired harmonics without using synthesis optimization algorithms (software simplicity).

Dual-Band Performance Evaluation of Time-Modulated Circular Geometry Array With Microstrip-Fed Slot Antennas

This paper applies time modulation to deal with the design of dual-band antenna arrays in a circular geometry using a micro-strip-fed slot antenna as an element in the frequencies of 2.4 and 5.5 GHz. The motivation for this paper is to exploit the properties and advantages of time-modulated arrays in a dual-band scenario considering a circular geometry. This design considers the optimization of the time switched sequences of the antenna elements in the circular geometry for both operation frequencies. The design process to find the optimal switch- ON intervals of the circular array is carried out by means of the method of differential evolution and particle swarm optimization. The main contribution to the field of this paper is the dual-band performance evaluation of circular antenna arrays considering time modulation and measurements of the antenna element pattern. The performance evaluation and the results and discussion are achieved in terms of the radiation pattern with the measured antenna element, sidelobe level, sideband levels, switching times and efficiency.

Time-Modulated Arrays for Physical Layer Secure Communications: Optimization-Based Synthesis and Experimental Assessment

The synthesis of time-modulated arrays (TMAs) for physical layer secure communications is carried out in this paper. The problem is cast as the transmission from the TMA of a signal undistorted within a desired angular region, while the signal distortion is maximized elsewhere. Toward this aim, a customized optimization strategy based on a binary genetic algorithm is proposed to define the optimal ON–OFF sequence controlling the radio-frequency switches modulating the signal transmitted by the elements of the TMA. An ad hoc fidelity factor metric is used to quantify the signal distortion as a function of the angular direction. A set of numerical examples is reported and discussed to illustrate the behavior of the proposed synthesis approach as well as to give some insights on its potentialities and achievable performance. Moreover, this paper shows, for the first time, the results of an experimental validation when considering a real TMA.

A Study on Harmonic Power Calculation for Nonuniform Period Linear Time Modulated Arrays

In this letter, sideband radiation (SR) power has been investigated for nonuniform time modulated array (NTMA) systems. It is suggested that the SR calculations already studied in the literature need to be reviewed for NTMAs. From this point of view, a so-called prime-distributed time-modulated array that is a special kind of NTMA is introduced and a closed-form harmonic power expression is presented. Finally, it is shown that the former expression needs to be reconsidered for more appropriate results.

A Method for Forming Distributed Beams in Time-Modulated Planar Arrays

The sidebands that originate in time-modulated arrays can be used for numerous applications including multipath signal separation and directional power transfer. In this paper, a method of simultaneously producing a series of beams that are steered toward unique 2-D directions is presented by developing the theory of time-modulated linear arrays to time-modulated planar arrays. A planar array is modeled as two linear arrays on orthogonal axes, and the binomial and Dolph–Chebyshev time-weighting distributions are applied to a $5\times 5$ element array. The output of the array is shown in both the time and frequency domains.

Beamforming Optimization for Time-Modulated Circular-Aperture Grid Array With DE Algorithm

This letter put forward a method of numerical optimization for the circular-aperture time-modulated grid array. The amplitude weight and the time modulation pulsewidth are optimized based on a differential evolution algorithm to get the needed array pattern. Adopting the weighting mode of the conventional rectangular array, the equivalent weighting of a circular-aperture time-modulated array is decomposed into two independent components, respectively, in the horizontal and vertical directions, so as to shorten the length of the optimization vector from M × N order to M + N order. Besides, the circular-aperture time-modulated array is symmetrically excited in the horizontal and vertical directions. The numerical experiments show that the differential evolution algorithm can realize a sidelobe level of −35 dB and a nulling level of −60 dB in the patterns of the center frequency and one chosen sideband, and the unexploited sidebands are also suppressed with their peak levels lower than −35 dB. Meanwhile, the dynamic range of the amplitude weighting and the time modulation pulsewidth can be limited to 10 dB. The overall efficiency of the time-modulated array is 0.28.

Time-Modulated Multibeam Phased Arrays With Periodic Nyquist Pulses

We present a single sideband time-modulated multibeam phased array governed by periodic Nyquist pulsed signals. A Nyquist pulse is a physically realizable approach to the ideal sinc function. Hence, its low-pass spectrum suits particularly well for time-modulated arrays (TMAs) to perform harmonic beam steering. Contrarily to switched TMAs and standard solutions based on variable phase shifters, the performance and complexity of the proposed time modulation scheme is rather robust when increasing the number of multibeams.

Dual-Signal Transmission Using RF Precoding and Analog Beamforming With TMAs

Time-modulated arrays (TMAs) receiving beamforming provide a noteworthy hardware simplicity given the ability of this multi-antenna technique to transform spatial diversity into frequency diversity. However, the dual behavior at transmission seems to be as simple as limited: a given signal is simultaneously transmitted over all the different TMA harmonic patterns. We investigate the efficient and simultaneous transmission of two different signals over the same physical antenna using two independent harmonic beam-patterns of the TMA. For that purpose, we propose an innovative dual-signal TMA transmitter based on two complementary operations: the complex mixing of the baseband signals and the TMA processing with quadrature and time-delayed periodic pulses.

High-Accuracy DOA Estimation Based on Time-Modulated Array With Long and Short Baselines

A novel direction-of-arrival estimation method based on the time-modulated array is proposed and verified. Long and short baselines are exploited to improve the accuracy of direction finding, in the meantime, to distinguish the true angle from the ambiguous ones. The idea is derived from the direction-finding method based on the phase interferometer, in which multibaselines are used to implement high-accuracy and unambiguous direction finding. The proposed method is analyzed in theory, and numeric simulations are provided to verify its effectiveness.

Beam steering for OAM beams using time‐modulated circular arrays

The theory of time-modulated circular arrays in respect of generating orbital angular momentum (OAM) beams at harmonic frequencies is analysed. Without using expensive phase shifters in phased arrays, beam steering for OAM beams is achieved using switches connected to array elements. The model of an eight-element time-controlled circular array is derived with the method of time-modulation beam steering techniques. Simulation results show that beam steering for OAM beams is realised, which indicates the effectiveness of the proposed method.

Joint radar-communications design based on time modulated array

The demand for coexistence of radar and communication systems is increasing rapidly in recent years. In this paper, we propose a joint radar-communications design based on time modulated array (TMA) to satisfy the need of electronic system integration in civil and military, and mitigate the tension of spectrum resource. First, we investigate the basic characteristics of the TMA. Then, we present a TMA-based waveform design algorithm for joint radar-communications. By using the derived results, a dual-function radar-communication (DFRC) system is established, which can detect targets in a large scanning range and maintain the communication direction static simultaneously, while avoiding the mutual interference. Moreover, the communication direction can be adjusted easily as needed. The validity of the theoretical derivations is demonstrated via simulations.

Efficiency Enhancement of Time-Modulated Arrays With Optimized Switching Sequences

The conventional time-modulated arrays (TMAs) have a low radiation efficiency, since each antenna turns off at specific time slots. In this paper, first, the efficiency of TMA is investigated analytically. Next, the optimized switching sequences are proposed to enhance the radiation efficiency of a specific sideband, while preserving the level of the other sidebands. The practical hardware limitations are considered in the switching sequence design. Moreover, a low-cost flexible eight-element printed dipole array operating at 1.2-1.4 GHz is implemented to verify the proposed algorithms. The measured radiation patterns indicate that by applying the proposed sequences, the TMA efficiency improves by 8.3 dB for the fundamental frequency. The efficiency enhancement for the first and second sidebands is 2.9 and 2.95 dB, respectively. In addition, a fast and low-cost setup for TMA measurement using the commercial software defined radio dongle is implemented in this paper.

Direction Finding of BPSK Signals Using Time-Modulated Array

A direction-finding algorithm by the time-modulated array (TMA) is proposed for binary phase shift keying (BPSK) signals. In the proposed scheme, only two antenna elements and a single RF channel are required, and the switch period has no relation with the symbol period of the BPSK signal. Numeric simulations are provided to examine the performance of the proposed scheme, and a two-element TMA test platform is constructed to verify its effectiveness.

Advanced Pulse Sequence Design in Time-Modulated Arrays for Cognitive Radio

This work deals with the synthesis of time-modulated arrays (TMAs) for cognitive radio systems, in which the harmonic radiations arising from the periodic modulation of the elements in the time-domain are exploited as independent channels for multi-point-to-point communications. The descriptors of advanced time-modulating sequences using sum-of-weighted cosine waveforms are optimized through the particle swarm optimization with the aim of maximizing the overall signal-to-interference-plus-noise ratio on receiving and increasing the antenna directivity. Representative numerical results are presented and discussed in comparison with the ones obtained from classical TMA implementations, based on rectangular pulse sequences in order to highlight the achievable performance improvements.

Instantaneous Gain Optimization in Time Modulated Array Using Reconfigurable Power Divide/Combiner

Time modulated array (TMA) suffers from relatively unstable instantaneous gain due to the variation in both efficiency of feeding network and directivity as different number switches are in off state. In this letter, a TMA design using reconfigurable power divide/combiner (RPDC) is proposed to improve the stability of instantaneous gain. Since the power, which is always wasted in the absorbed switches during off state, can be reused by RPDC, the fluctuation of instantaneous efficiency of feeding network can be avoided, and the corresponding stability of instantaneous gain can be improved. According to the work characteristic of RPDC, the genetic algorithm is adopted to optimize the pulse sequences for controlling both the instantaneous gain and average radiation pattern. Representative results are presented to verify the effectiveness of the proposed design, and are also in comparison with previous works.

Time-Modulated Arrays: A Four-Dimensional Antenna Array Controlled by Switches

With the rapid development of modern electronic technologies, antenna arrays typically operate in very complex electromagnetic environments. However, owing to the various errors such as systematic errors and random errors, conventional antenna arrays have relatively high sidelobes. Time modulated arrays (TMAs), also known as four-dimensional (4-D) antenna arrays, introduce time as an additional dimension for generating ultra-low sidelobes at fundamental component and realizing real-time beam scanning by harmonic components. Recently, the harmonic components can also be developed for various new applications including wireless communications and radar systems. In this review, we introduce comprehensively the fundamental methodologies and recent applications of TMAs. This aims to stimulate continuing efforts for the understanding of TMAs and explore their applications in various aspects. The methods mentioned in this review include three aspects: sideband radiation suppression, power efficiency of TMAs, and applications of harmonic components. These methods either improve the existing TMAs or promote the practical applications of TMAs. First, to suppress the sideband radiation, a method using non-uniform periodical modulation is introduced. The proposed method has an advantage of low computation and can be easily used for synthesizing a real-time radiation pattern according to the environmental need. Next, a TMA structure using reconfigurable power dividers/combiner is introduced to improve the power efficiency of feeding network. Finally, three applications of harmonic component including direction finding, calibration method, and space division multiple access are separately introduced to illustrate the development potential of TMAs.

Analog Beamforming Using Time-Modulated Arrays With Digitally Preprocessed Rectangular Sequences

Conventional time-modulated arrays are based on the application of variable-width periodical rectangular pulses (easily implemented with radio frequency switches) to the individual antenna excitations. However, a serious bottleneck arises when the number of exploited harmonic beams increases. In this context, the modest windowing features of the rectangular pulses produce an inflexible and ineffective harmonic beamforming. The use of other pulses, such as sum of weighted cosines, partially solves these issues at the expense of introducing additional non-timing variables. We propose the discrete-time preprocessing of rectangular pulses before being applied to the antenna to accomplish an agile, efficient, and accurate harmonic beamforming, while keeping the simplicity of the hardware structure.