An Orthogonal Hybrid Analog–Digital Multibeam Antenna Array for Millimeter-Wave Massive MIMO Systems

A hybrid analog-digital multibeam antenna array (MBAA) consisting of two types of 1-D beamformers for two orthogonal planes, respectively, is proposed for massive multiple-input multiple-output (MIMO) communication systems at millimeter-waves. In the vertical direction, the multiple beams are generated by a passive beamformer, i.e., a Butler matrix, whereas the multibeam coverage in the horizontal plane is achieved digitally at the baseband. Comparing to a full-digital multibeam array, the digital beamforming (DBF) processing is simplified in the proposed scheme, significantly alleviating the computational burden of data processing. Since the radiation beams are generated by only a part of the radio frequency (RF) channels in the hybrid multibeam array, the power consumption could be reduced by powering down the idle channels. To verify the concept, a two-dimensional (2-D) array comprised by 16 columns of eight-beam tapered slot antenna subarrays is designed and fabricated, in which each subarray contains a pair of 4 × 8 modified Butler matrices based on the substrate-integrated waveguide technology. The measured results exhibit that the operating bandwidth is 13.45% for the subarray fed by the modified Butler matrices implemented by substrate-integrated waveguides. The RF receivers, intermediate frequency (IF) chains, and analog-to-digital converters (ADCs) are also implemented to enable the DBF in the horizontal plane. The maximum simulated directivity of the array is 27 dBi. A wide 2-D spatial angle is covered simultaneously by the proposed hybrid passive-DBF scheme. The demonstrated 2-D millimeter-wave hybrid array may find widespread potential applications in future communication systems.