MIMO-assisted HAPS as IMT Base Station for Downlink Spectrum Efficiency Enhancement

The High Altitude Platform Station (HAPS), as a network node in the Non-Terrestrial Network (NTN), has evolved as an important component of the 6G wireless network. Acting as an IMT base station (HIBS) in mobile services, HAPS is a promising approach to achieve vertical heterogeneous coverage enhancement. HIBS can operate in the stratosphere up to 20 km high to provide more coverage, however, it is limited by the ability to accommodate more users in scarce spectrum resources. In our study, a MIMO-assisted HIBS approach based on a 7-cell network structure is proposed to improve the spectral efficiency (SE) of the downlink. HIBS is configured with three different combinations of MIMO antenna arrays, including single antenna, 2x2MIMO+2x4MIMO, and 4x4MIMO+4x8MIMO. A Monte Carlo method is used to calculate the system SE for each configuration of the CDF curve, which brings the simulation results closer to the actual deployment. The simulation results show that the downlink SE of the system may be significantly improved by using MIMO-assisted HIBS based on a 7-cell scheme, and that the downlink SE can be further improved by scaling up the MIMO array.