Quanjiang Zhu

4-D Arrays as Enabling Technology for Cognitive Radio Systems

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.

Directional Modulation Based on 4-D Antenna Arrays

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.

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

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.

A Compact Dual-Polarized Double

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 2.5 GHz - 2.7 GHz, while the return loss is less than - 15 dB within the band. Moreover, stable radiation pattern with a peak gain of 6.8 dBi - 7.4 dBi is obtained within the band.

{\rm E}

Time-modulated arrays (TMAs) are studied in the time domain and frequency domain in this paper. The studies are based on the “instantaneous” array factor as a function of time and “average” array factor as a function of frequency, respectively. The two analysis approaches are shown to be unified and complementary through the Fourier transform. Antenna parameters including pattern, directivity, and gain are all addressed. Moreover, an experimental setup for the study of signal transmission is built and measured. Experimental results are in good agreement with the theoretical study. Finally, the bandwidth of TMAs is taken into account, and amplitude modulation (AM) signal transmission through the TMA is studied experimentally. The results presented in this paper can be used to explain the fundamental working principle of TMAs in a clear and comprehensive way.

-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.

Unified Time- and Frequency-Domain Study on Time-Modulated Arrays

Traditional monopulse systems used for direction finding usually face the contradiction between high angle precision and wide angle-searching field, and a compromise has to be made. In this paper, the time modulation technique in four-dimensional (4D) antenna array is introduced into the conventional phase-comparison monopulse to form a novel direction-finding system, in which both high angle resolution and wide field-of-view are realized. The full 4D array is divided into two subarrays and the differential evolution (DE) algorithm is used to optimize the time sequence of each subarray to generate multibeams at the center frequency and low sidebands. Then the multibeams of the two subarrays are phase-compared with each other and multiple pairs of sum-difference beams are formed at different sidebands and point to different spatial angles. The proposed direction-finding system covers a large field-of-view of up to ±60° and simultaneously maintains the advantages of monopulse systems, such as high angle precision and low computation complexity. Theoretical analysis and experimental results validate the effectiveness of the proposed system.

Gain Improvement in Time-Modulated Linear Arrays Using SPDT Switches

An e-cient approach is presented for the design of a low sidelobe four-dimensional (4D) planar antenna array, taking into account mutual coupling and platform efiect. The approach is based on the combination of the active element patterns and the difierential evolution (DE) algorithm. Difierent from linear and circular arrays, the mutual coupling compensation in a planar array is more complicated since it requires numerous data of the active element patterns in difierent azimuth planes. In order to solve this problem, a useful interface program is developed to get these data from commercial software HFSS automatically. Also difierent from conventional low sidelobe arrays with tapered amplitude excitations, the low sidelobe in the 4D array is realized using time-modulation technique under uniform static amplitude and phase conditions. The DE algorithm is used to optimize the time sequences which are equivalent to the complex excitations in conventional arrays. Both computed results and simulated results in HFSS show that a i30dB sidelobe pattern can be synthesized in a 76-element planar array with an octagonal ground plane and a radome, thus verifying the proposed approach.

Direction Finding Using Multiple Sum and Difference Patterns in 4D Antenna Arrays

This paper introduces a novel approach for the synthesis of array patterns in four-dimensional (4-D) antenna arrays, taking into account practical element models. The approach is based on the combination of the differential evolution (DE) algorithm and full-wave simulation software HFSS. The DE algorithm is used to optimize the time sequences and static phase excitations, and then the equivalent amplitude and phase excitations are transferred to HFSS automatically by an interface program. The patterns at the center frequency and sideband frequencies are all obtained through full-wave simulation of practical antenna arrays in HFSS, thus many non-ideal effects such as mutual coupling can be taken into account. Numerical simulation results for the synthesis of low sidelobe sum-difference patterns in a 16-element printed dipole linear array and a 16-element microstrip patch conformal array are presented to demonstrate the effectiveness of the proposed approach.

DESIGN OF A LOW SIDELOBE 4D PLANAR ARRAY INCLUDING MUTUAL COUPLING

A general multiobjective optimization and design proce- dure of a Luneberg lens antenna (LLA) with a compact multiband multi-polarized feed-system for a broadband satellite communication terminal is presented. The LLA utilizes a compact multiband feed horn, consisting of an inner dielectric loaded circular horn for the K/Ka-band (dual-circular polarization) and a coaxial waveguide with axially corrugated ∞ange for the Ku-band (dual-linear polarization). Measurements show good agreements with simulations. Moreover, an e-cient multiobjective evolutionary algorithm based on decomposition (MOEA/D) with difierential evolution operator and objective normal- ization technique is flrstly coupled with the vector spherical wave func- tion expansions (VSWE) for the optimal design of a 7-layer 650mm diameter LLA, which provides higher aperture e-ciency at Ku/K/Ka- band simultaneously. The frequency dependence of the LLA is also investigated. Finally, the gain and sidelobe level of a 5-layer design are jointly evaluated and compared with previous works. The pro- posed design procedure provides much better radiation performances and greater design freedom to the designers, as a group of Pareto- optimal LLA solutions can be obtained with just one simulation.