Antenna Options for High Altitude IMT Base Stations (HIBS) in Cellular Networks

Abstract: This thesis is the result of a collaboration between Ericsson AB and Luleå University of Technology. A feasibility study has been conducted to investigate antenna options for the HIBS access link. The study contains two parts. Firstly, a link budget investigating the gain required from the antenna. The metric of concern in the link budget was SNR. Secondly, a wide area coverage investigation that explored coverage feasibility over an area with a radius of 100 km. The metrics of concern in this investigation were antenna gain and beamwidth. Two types of antennas have been included: parabolic reflector and phased array. Seven frequency bands have been studied: 0.7, 1.9, 2.7, 3.5, 6, 10, and 26 GHz. The first three bands shared a bandwidth of 20 MHz, the next three shared a bandwidth of 80 MHz, and the last band had a bandwidth of 100 MHz. This bandwidth difference was found to have a meaningful effect on SNR. The feasibility condition for the link budget was -6 dB SNR for uplink and 6 dB SNR for downlink. The link budget concluded that the first three bands (0.7, 1.9, and 2.7 GHz) are feasible with reasonably sized antennas. This meant a parabolic reflector dish diameter of 0.6 m for all three bands, or a phased array antenna with 4, 32, and 64 elements, respectively, that all resulted in a roughly equal physical size of the array. The 3.5 GHz frequency band was found to be feasible with a much larger antenna (512 element array). The bands above 3.5 GHz were not deemed feasible. The wide area investigation limited the antenna to a phased array antenna. Two cell layouts were considered for coverage: a 7 cell layout with one nadir cell surrounded by 6 cells and a 19 cell layout which encapsulates the former with another layer of 12 cells. The feasibility condition was that the half power beamwidth is equal to the angular size of a cell from the HIBS for each cell layer while maintaining gain. Beamwidth was controlled through array tapering and altering element configurations. This investigation concluded that coverage is feasible for two bands. In the 0.7 GHz band, the chosen option was a 7 cell layout using a single element antenna for the nadir cell and 3 by 1 arrays for the outer cells. In the 1.9 GHz band, the chosen option was a 19 cell layout with a single element antenna for the nadir cell, 5 by 1 arrays for the cells in the middle layer, and 8 by 5 arrays for the outer layer. Higher frequency bands required higher gain antennas which in turn did not provide adequate beamwidth for coverage.