The recently proposed digital coding metasurfaces make it possible to control electromagnetic (EM) waves in real time, and allow the implementation of many different functionalities in a programmable way. However, current configurations are only space-encoded, and do not exploit the temporal dimension. Here, we propose a general theory of space-time modulated digital coding metasurfaces to obtain simultaneous manipulations of EM waves in both space and frequency domains, i.e., to control the propagation direction and harmonic power distribution simultaneously. As proof-of-principle application examples, we consider harmonic beam steering, beam shaping, and scattering-signature control. For validation, we realize a prototype controlled by a field-programmable gate array, which implements the harmonic beam steering via an optimized space-time coding sequence. Numerical and experimental results, in good agreement, demonstrate good performance of the proposed approach, with potential applications to diverse fields such as wireless communications, cognitive radars, adaptive beamforming, holographic imaging.

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Space-time-coding digital metasurfaces

The proliferation of wireless services is driving innovative phased array solutions that are able to provide better cost/performance tradeoffs. In this context, the use of irregular array architectures provides a viable solution. This paper reviews and highlights some of the most recent advances in this field, including clustered, thinned, sparse, and time-modulated arrays, and their proposed design methodologies.

Unconventional Phased Array Architectures and Design Methodologies—A Review

Harmonic manipulations are important for applications such as wireless communications, radar detection and biological monitoring. A general approach to tailor the harmonics involves the use of additional amplifiers and phase shifters for the precise control of harmonic amplitudes and phases after the mixing process; however, this approach leads to issues of high cost and system integration. Metasurfaces composed of a periodic array of subwavelength resonators provide additional degrees of freedom to realize customized responses to incident light and highlight the possibility for nonlinear control by taking advantage of time-domain properties. Here, we designed and experimentally characterized a reflective time-domain digital coding metasurface, with independent control of the harmonic amplitude and phase. As the reflection coefficient is dynamically modulated in a predefined way, a large conversion rate is observed from the carrier signal to the harmonic components, with magnitudes and phases that can be accurately and separately engineered. In addition, by encoding the reflection phases of the meta-atoms, beam scanning for multiple harmonics can be implemented via different digital coding sequences, thus removing the need for intricate phase-shift networks. This work paves the way for efficient harmonic control for applications in communications, radar, and related areas.

Independent control of harmonic amplitudes and phases via a time-domain digital coding metasurface.

Time and frequency modulated arrays have numerous application areas including radar, navigation, and communications. Specifically, a time modulated array can create a beampattern with low sidelobes via connecting and disconnecting the antenna elements from the feed network, while the frequency modulated frequency diverse array produces a range-dependent pattern. In this paper, we aim to introduce these advanced arrays to the signal processing community so that more investigations in terms of theory, methods, and applications, can be facilitated. The research progress of time/frequency modulated array studies is reviewed and the most recent advances are discussed. Moreover, potential applications in radar and communications are presented, along with their technical challenges, especially in signal processing aspects.

An Overview on Time/Frequency Modulated Array Processing

A compact dual-polarized printed dipole antenna for wideband base station applications is presented in this communication. The proposed dipole antenna is etched on three assembled substrates. Four horizontal triangular patches are introduced to form two dipoles in two orthogonal polarizations. Two integrated baluns connected with 50 Ω SMA launchers are used to excite the dipole antenna. The proposed dipole antenna achieves a more compact size than many reported wideband printed dipole and magneto-electric dipole antennas. Both simulated and measured results show that the proposed antenna has a port isolation higher than 35 dB over 52% impendence bandwidth (VSWR <; 1.5). Moreover, stable radiation pattern with a peak gain of 7 dBi - 8.6 dBi is obtained within the operating band. The proposed dipole antenna is suitable as an array element and can be used for wideband base station antennas in the next generation IMT-advanced communications.

A Compact Dual-Polarized Printed Dipole Antenna With High Isolation for Wideband Base Station Applications

Four-dimensional (4-D) antenna arrays are formed by introducing a fourth dimension, time, into traditional antenna arrays. In this paper, a time-modulated 4-D array with constant instantaneous directivity is proposed for directional modulation. The main idea is that the 4-D array transmits correct signal without time modulation in the desired direction, while transmitting time-modulated signals in other directions. As longs as the time modulation frequency is less than the bandwidth of the transmitted signal, the time-modulated signals cannot be demodulated correctly due to the aliasing effect, implying that time-modulated signals go distorted. Thus, the 4-D array can be used to transmit direction-dependent signals in secure wireless communications. The proposed idea is verified by experiments based on AM signal transmission through the 4-D array. Moreover, BPSK signal transmission through the 4-D array is studied and the bit error rate (BER) performance is investigated. Simulation results show that the BERs of time-modulated BPSK (TM-BPSK) signals transmitted through the sidelobes of the 4-D array are much higher than those of BPSK signals and almost keep unchanged even under higher SNR. Finally, two enhanced methods are presented to improve the feasibility of directional modulation by using random time sequences and random time modulation frequency.

Directional Modulation Based on 4-D Antenna Arrays

Time-modulation (TM) in four-dimensional (4-D) arrays is implemented by using a set of radio-frequency switches in the beam forming network to modulate, by means of periodic pulse sequences, the static excitations and thus control the antenna radiation features. The on-off reconfiguration of the switches, that can be easily implemented via software, unavoidably generates harmonic radiations that can be suitably exploited for multiple channel communication purposes. As a matter of fact, harmonic beams can be synthesized having different spatial distribution and shapes in order to receive signals arriving on the antenna from different directions. Similarly, the capability to generate a field having different frequency and spatial distribution implies that the signal transmitted by time-modulated 4-D arrays is direction-dependent. Accordingly, such a feature is also exploited to implement a secure communication scheme directly at the physical layer. Thanks to the easy software-based reconfigurability, the multiple harmonic beamforming, and the security capability, 4-D arrays can be considered as an enabling technology for future cognitive radio systems. In this paper, these potentialities of time-modulated 4-D arrays are presented and their effectiveness is supported by a set of representative numerical simulation results.

4-D Arrays as Enabling Technology for Cognitive Radio Systems

A novel approach for the design of low sidelobe time modulated linear arrays (TMLAs) with uniform amplitude excitations and suppressed sidebands is presented. The approach is based on the division of the time modulation period Tp into several time steps with variable lengths. In each time step, the switch-on and switch-off times are optimized via the differential evolution (DE) algorithm. As compared to previous approaches, such as the variable aperture sizes (VAS), pulse shifting, and binary optimized time sequences (BOTS), the proposed approach has more flexibility in the design of time sequences in TMLAs. Numerical results show that a -30 dB sidelobe pattern with uniform excitations can be synthesized, while the sideband level (SBL) is suppressed to -27.8 dB. Experimental results based on a 16-element printed dipole linear array agree with the theoretical results, thus verified the proposed approach.

Design of a Low Sidelobe Time Modulated Linear Array With Uniform Amplitude and Sub-Sectional Optimized Time Steps

A compact dual-polarized double E-shaped patch antenna with high isolation for pico base station applications is presented in this communication The proposed antenna employs a stacked configuration composed of two layers of substrate Two modified E-shaped patches are printed orthogonally on both sides of the upper substrate Two probes are used to excite the E-shaped patches, and each probe is connected to one patch separately A circular patch is printed on the lower substrate to broaden the impedance bandwidth Both simulated and measured results show that the proposed antenna has a port isolation higher than 30 dB over the frequency band of 25 GHz - 27 GHz, while the return loss is less than - 15 dB within the band Moreover, stable radiation pattern with a peak gain of 68 dBi - 74 dBi is obtained within the band

A Compact Dual-Polarized Double ${\rm E}$ -Shaped Patch Antenna With High Isolation

Time-modulated linear arrays (TMLAs) based on single-pole-single-throw (SPST) switches usually have higher reduction in gain due to the sideband radiation and power absorption of the off-state absorptive switches. In this letter, a novel approach is proposed to improve the gain of TMLAs using single-pole-double-throw (SPDT) switches. The gain of the proposed TMLA can be improved significantly by controlling two array elements with each SPDT switch. Moreover, the proposed TMLA has a constant instantaneous directivity, which is favorable for some applications. Numerical results are presented to illustrate the proposed approach by considering a 16-element linear array with a -30 dB sidelobe level.

Quanjiang Zhu

Papers

Directional Modulation Based on 4-D Antenna Arrays

4-D Arrays as Enabling Technology for Cognitive Radio Systems

Design of a Low Sidelobe Time Modulated Linear Array With Uniform Amplitude and Sub-Sectional Optimized Time Steps

A Compact Dual-Polarized Double ${\rm E}$ -Shaped Patch Antenna With High Isolation

Gain Improvement in Time-Modulated Linear Arrays Using SPDT Switches