  The high altitude platform station (HAPS) technology is garnering significant
interest as a viable technology for serving as base stations in communication
networks. However, HAPS faces the challenge of high spatial correlation among
adjacent users' channel gains which is due to the dominant line-of-sight (LoS)
path between HAPS and terrestrial users. Furthermore, there is a spatial
correlation among antenna elements of HAPS that depends on the propagation
environment and the distance between elements of the antenna array. This paper
presents an antenna architecture for HAPS and considers the mentioned issues by
characterizing the channel gain and the spatial correlation matrix of the HAPS.
We propose a cylindrical antenna for HAPS that utilizes vertical uniform linear
array (ULA) sectors. Moreover, to address the issue of high spatial correlation
among users, the non-orthogonal multiple access (NOMA) clustering method is
proposed. An algorithm is also developed to allocate power among users to
maximize both spectral efficiency and energy efficiency while meeting quality
of service (QoS) and successive interference cancellation (SIC) conditions.
Finally, simulation results indicate that the spatial correlation has a
significant impact on spectral efficiency and energy efficiency in multiple
antenna HAPS systems.
