Abstract

With the paradigm shift from sub-6 GHz to millimeter-wave (mm-Wave) for wireless communications, beamforming becomes essential for mm-Wave access points to mitigate losses. Due to the small wavelength, a compact circuit could accommodate a large number of antenna elements. This favors the principle of beamforming to achieve high array-gain and spatial resolution through a large-scale N × M array. For such antenna frontends, full-digital beamforming circuitry requires N × M RF chains, which is unfeasible and energy inefficient. Likewise, a higher-order mm-Wave analog beamforming network is highly lossy to generate N × M beams. Hybrid beamforming addresses this dilemma by partitioning the beamforming between the analog and digital domains appropriately. For this purpose, the antenna frontend needs to be segmented into subarrays, such that the subarray-based analog beamspace patterns are digitally processed rather than processing element patterns individually. Thus, hybrid beamforming requires a suitable subarray-based N × M multibeam antenna frontend.

In this thesis, a study of the subarray antennas is presented for hybrid beamforming operation. A simplified model is considered in which the analog beam-switching is performed in the azimuth plane (H-plane) and the digital beamspace beamforming in the elevation plane (V-plane). This is to reduce the number of RF chains as well as to achieve fine-tuned digital beam-steering in V-plane along with predefined analog switched-beams in H-plane. In this research work, the frequency band of 28 – 32 GHz is considered for prototyping purposes. For practical use at mm-Wave, the microstrip line technology is augmented with the perfect magnetic conductor (PMC) packaging. The fixed-beam and switched-beam subarrays with an order of n × m = 1 × 4, 2 × 2, and 4 × 4 are investigated. A dual-polarized aperture-coupled magneto-electric dipole antenna is designed as a single element with 20% bandwidth, ports' isolation better than 35 dB, cross-polarization less than -25 dB, and gain of 8 dBi. Using this element, a fixed-beam 4 × 4 dual-polarized subarray is designed that maintains a bandwidth of 16.7% at 30 GHz with a maximum gain of 19.3 dBi and symmetrical radiation patterns. The fixed-beam limitation of the 2n × 2m subarray leads to building the efficient switched-beam subarray antennas for hybrid beamforming. For this purpose, a 2 × 2 dual-polarized analog beamforming network is designed for 28-32 GHz. Two identical PMC packaged microstrip line networks, one for each polarization, are designed on a single substrate surface. However, to be processed for beamspace digital beamforming, this topology exhibits physical layout and array factor problems. Thus, further designs are investigated to meet the hybrid beamforming frontend requirements.

To this end, as switched-beam subarrays for hybrid beamforming, two PMC packaged 4 × 4 Butler matrices are presented with a longitudinal layout and a folded layout for the end-fire and broadside radiation characteristics, respectively. The former design achieves a 5 GHz (28-33 GHz) bandwidth with return loss and isolation, both better than 15 dB. At 30 GHz, the insertion loss is 0.8 ± 0.3 dB, and antenna-ports' phase distributions are ±45° and ±135°. E-plane-flared horn antennas terminate the Butler matrix antenna-ports as a linear array. The double-ridge gap waveguide horn antenna is designed to reduce the scan loss within a subarray environment. The H-plane fan-beam switching covers ±42° with a maximum gain of 11.7 and 11.2 dBi for the inner (1R) and outer (2R) radiation beams. The latter novel topology of the folded Butler matrix is laid out for a compact tiled planar antenna frontend to accommodate a beamforming network beneath the antenna array's physical footprints. As compared to the conventional longitudinal layout, the size is reduced by more than 50 %. The PCB aperture-coupled antenna elements are integrated within the PMC packaged environment for a broadside radiation characteristic. The folded Butler matrix and antenna element are designed for a bandwidth of 4 GHz (28-32 GHz). The single antenna element's directivity is 5.22 dBi; whereas, for a 1 × 4 switched-beam subarray antenna, the directivities are 11.1 dBi and 10.6 dBi for 1R and 2R beams, respectively. Using Butler matrices-based 1 × 4 switched-beam subarrays, two types of multibeam antenna frontends with order N × M = 4 × 4 are constructed. Post-processing for the digital beamforming is applied over the subarray-based analog beamspaces. The first hybrid beamforming model maintains a scan range of ± 42o in the H-plane and ± 28o in the V-plane with BV × BH = 3 × 4 = 12 beams. Similarly, the second model maintains a scan range of ± 38o in the H-plane and ± 40o in the V-plane with BV × BH = 4 × 4 = 16 beams. As compared to full-analog two-dimensional (2-D) beamforming, these models are capable of fine-tuned beam-steering in the V-plane because the complex beamforming coefficients are not fixed but calculated digitally. Furthermore, compared to full-digital 2-D beamforming, it reduces the number of active RF chains from N × M to N.