With the demanding system requirements for the fifth-generation (5G) wireless communications and the severe spectrum shortage at conventional cellular frequencies, multibeam antenna systems operating in the millimeter-wave frequency bands have attracted a lot of research interest and have been actively investigated. They represent the key antenna technology for supporting a high data transmission rate, an improved signal-to-interference-plus-noise ratio, an increased spectral and energy efficiency, and versatile beam shaping, thereby holding a great promise in serving as the critical infrastructure for enabling beamforming and massive multiple-input multiple-output (MIMO) that boost the 5G. This paper provides an overview of the existing multibeam antenna technologies which include the passive multibeam antennas (MBAs) based on quasi-optical components and beamforming circuits, multibeam phased-array antennas enabled by various phase-shifting methods, and digital MBAs with different system architectures. Specifically, their principles of operation, design, and implementation, as well as a number of illustrative application examples are reviewed. Finally, the suitability of these MBAs for the future 5G massive MIMO wireless systems as well as the associated challenges is discussed.

A novel array antenna with a flat-shoulder shaped radiation pattern is proposed as the transmitting antenna for 77 GHz automotive radar application. When it is used in an automotive radar comprising one transmitter and multiple identical receivers, it can meet the demands of both long-range and medium-range detections without switching the operation mode back and forth between the long-range radar scenario and the medium-range radar scenario. The proposed antenna concept is fully introduced to explain its mechanism. An unconventional array was synthesized to achieve the desired flat-shoulder shaped radiation pattern. Prototype of the proposed array antenna was also designed according to the array synthesis result, which consists of several linear series-fed patch arrays and a substrate-integrated waveguide power and phase distributing network. Fabrication and measurement of the prototype array antenna were completed, showing a good agreement between the measured data and the synthesized as well as the simulation results, thereby validating the design.

In this communication, a corporate stacked microstrip and substrate integrated waveguide (SIW) feeding structure is reported to be used to broaden the impedance bandwidth of a 4 ×4 patch array antenna. The proposed array antenna is based on a multi-layer printed circuit board (PCB) structure containing two dielectric substrates and four copper cladding layers. The radiating elements, which consist of slim rectangular patches with surrounding U-shaped parasitic patches, are located on the top layer. Every four radiation elements are grouped together as a 2 ×2 subarray and fed by a microstrip power divider on the next copper layer through metalized blind vias. Four such sub-arrays are corporate-fed by a SIW feeding network underneath. The design process and analysis of the array antenna are discussed. A prototype of the proposed array antenna is fabricated and measured, showing a good agreement between the simulation and measurement, thus validating the correctness of the design. The measured results indicate that, the proposed array antenna exhibits a wide |S 11|<-10dB bandwidth of 17.7%, i.e. 25.3~30.2 GHz, a peak gain of 16.4 dBi, a high radiation efficiency above 80%, a good orthogonal polarization discrimination of higher than 30dB, etc. In addition, the use of low profile substrate in the SIW feeding network makes this array antenna easier to be integrated directly with millimeter-wave front-end integrated circuits (ICs). The demonstrated array antenna can be a good candidate for various Ka-band wireless applications, such as 5G, satellite communications, etc.

In the community of automotive millimeter wave radar, the recently developed concept of four-dimensional (4D) radar can provide high-resolution point clouds image with enhanced imaging performance. Currently, the density of point clouds for single-frame image is usually too sparse to satisfy the demands of target classification and recognition due to the limitation of Doppler and angle resolutions. To address the aforementioned issues, a novel algorithm is proposed for 4D high-resolution imagery generation of point clouds with extremely high Doppler and angle resolutions in this paper. For high Doppler resolution with high-dynamic, a novel velocity ambiguity resolution algorithm is proposed using a dual pulse repetition frequency (dual-PRF) waveform design embedded in an innovative time-division multiplexing & Doppler-division multiplexing MIMO (TDM-DDM-MIMO) framework. Meanwhile, an attractive complex-valued deep convolutional network (CV-DCN) of super-resolution direction-of-arrival (DOA) estimation is proposed only using single-frame data. To be specific, a spatial smoothing operator on array data is applied as input of the network, and a CV-DCN is designed to learn the transformation of the spatial spectrum from the end-to-end to effectively protect the spectrum extraction. Furthermore, experimental analysis is performed to confirm the effectiveness of the proposed super-resolution DOA estimation algorithm. Finally, the 4D high-resolution imagery of point clouds is obtained by experiments in the parking lot.

A novel full-digital multibeam array with wide scanning angle and enhanced beam gain is proposed for millimeter-wave massive multiple-input multiple-output applications. The array consists of 16 dual exponentially tapered slot antenna (DETSA) elements. Each element is integrated with a radio frequency receiver, an intermediate frequency chain, and an analog-to-digital converter (ADC). The H-plane radiation pattern of the proposed DETSA is very wide to enable the beam scanning in the azimuth plane. The beam gain is further enhanced by employing a 1-D multilayered planar lens that transforms the spherical wave into a plane wave in the E-plane for increasing the beam gain and keeping the phase compensation unaltered in the H-plane. Meanwhile, the planar lens is optimized to guarantee the wide scanning angle. In addition, all the signals received by the 16 independent channels are converted to a digital signal via ADC simultaneously and then used for synthesizing the multibeam patterns in the digital domain. Furthermore, the calibration and the verification of the synthesizing weights are discussed in detail. The measured results show that a scanning coverage of ±40° in the H-plane (or in horizontal plane) and an estimated maximum gain of 24.8 dBi with a gain tolerance of 3 dB can be achieved by the proposed 16-element array with a 1-D lens.