Design of Beamforming Network for 28 GHz Bands Using Modified Maxwell’s Fisheye Lens Profile

Abstract

A beamforming network (BFN) for 28 GHz with the enhanced beam scanning angle is studied in this dissertation. One of the main goal of 5G NR communication is the real-time communication with a massive number of devices, by using millimeter-wave bands. However, the signal strength of the millimeter-wave bands is less than the conventional bands at the same physical distance. It is because the wavelength is inversely proportion to the signal frequency. To compensate for the weak signal strength with high antenna gain, array antennas are frequently used. Although the array antenna with a high gain has a narrow beamwidth, the communication coverage can be broadened by steering the beam using BFN. Lens type BFN is widely used for the broadband applications. The progressive phase shift is generated in the lens cavity by the optical properties of the RF wave. Among the lens type BFNs, the Rotman lens BFN is the most typical BFN. Luneburg lens BFN is another lens type BFN, which is recently proposed. To secure the wider beam scanning range compared to the existing BFNs, the design method of BFN using a half section of Maxwell’s fisheye lens (MFL) was studied. The conventional permittivity profile of MFL has a condition that the permittivity range have to be 1:4. However, most printed circuit boards (PCB) used for RF applications has the relative permittivity of 2 to 3, which cannot implement the conventional MFL profile. To avoid the problem by the lens permittivity profile, a method to design the modified profile is proposed. The modified profile can be designed by remaining the permittivity ratio between the discretized shells of the MFL. To realize the profile in a given permittivity range, permittivity jump is applied. The effective permittivity for the modified profile can be controlled by arranging the thru-holes as honeycomb. The permittivity formula on the thru-hole region is derived by the average permittivity of a unit cell. As a practical example, the modified MFL profile was designed, and the performance of the profile was appeared as similar to the conventional MFL. To verify the beamforming performance, a practical BFN system was realized using the modified MFL profile. The BFN system including an antenna array is realized to have a frequency band of 24 to 29 GHz, the beam scanning range of ±50°, and the steerable 8 beam directions. A microstrip slot array antenna is also realized to generate the beam pattern. The gain pattern of the BFN was measured in an anechoic chamber, and the result was well agreed as the expectations. Therefore, the proposed design method can be applied to design a BFN system with an enhanced beam scanning range.