Interference Reduction between HAPSs using Subarray Grouping and Nullforming Techniques for Cylindrical Massive MIMO Systems

High-altitude platform station (HAPS) is a promising technology for extending the coverage of cellular networks. For multi-HAPS scenarios in which multiple HAPSs use a single shared frequency band to communicate with terrestrial users in their service areas, the interference between neighboring HAPSs is a concerning issue that degrades the overall system capacity in service links. Accordingly, this study proposes a novel two-stage precoding scheme for massive multiple-input and multiple-output systems with a large-scale cylindrical array antenna to mitigate the interference with the neighboring service areas covered by other HAPSs. The proposed scheme comprises outer and inner precoders; the outer precoder reduces the interference with other service areas via subarray grouping and nullforming, whereas the inner precoder generates user-specific beams. Results show that the proposed precoders improve the signal-to-interference-plus-noise ratio of terrestrial users by up to 6.8 dB in a multi-HAPS scenario compared to a conventional fully-connected zero-forcing precoder without any interference reduction techniques.