High-Altitude Platform Stations (HAPSs) enable wide-area coverage in 6G networks but introduce interference when sharing spectrum with terrestrial networks (TNs). Null-forming is a technique to mitigate this interference by directing low-power beams (nulls) toward terrestrial users. Traditional nullforming methods, such as Null-Sweeping, rely on changing null directions across the resource blocks (RB) to improve the impact of nullforming. Yet these null directions are predefined for uniform user distributions and may not fully account for nonuniform deployments. We propose a user-aware nullforming approach that leverages K-means clustering to adapt null positions to dense user regions in the terrestrial cells, while time-frequency resources are allocated proportionally to user density. Simulations show that our method reduces HAPS interference for terrestrial users and improves fairness in interference distribution.

In direct air-ground communications, high-altitude platform stations (HAPSs) are attracting considerable attention for their potential to extend the coverage of cellular networks significantly. Nullforming schemes for HAPSs have been proposed, enabling spectrum sharing between HAPSs and terrestrial networks by reducing interference with terrestrial base stations (BSs). However, nulls toward terrestrial BSs significantly degrade the received power of nearby HAPS users, creating a coverage hole around every BS. To overcome this issue, this study proposes a resource-allocation-aided nullforming scheme. Here, a HAPS stops nullforming toward terrestrial BSs on specific time-frequency resources to remove coverage holes, and each terrestrial BS performs user scheduling considering the presence of nulls to mitigate the impact of HAPS' interference on signal-to-interference-plus-noise ratios (SINRs). Simulation results show that the proposed scheme outperforms the conventional nullforming scheme with respect to coverage and achieves an SINR of 0 dB in 99.9% of the target service area covered by a single HAPS.

Interference management for aerial platforms is an essential technique for the spectrum sharing with terrestrial base stations (TBSs). In contrast to the extensive academic researches on the spectrum sharing, few studies have evaluated the spectral efficiency of an actual system. This study proposes a nullforming scheme for aerial base stations (ABSs) having a cylindrical array antenna (CAA). The proposed scheme mitigates the interference with TBSs and automatically adjusts the null directions based on the current position of an ABS. Moreover, a 5G-based spectrum sharing system is implemented, where an ABS with a 36-element CAA and a single TBS use a common frequency band of 1.8 GHz. The results measured in field trials demonstrate that the proposed scheme reduces the interference power by approximately 18 dB. Even though the ABS on a tethered balloon flying at an altitude of 250 m moves constantly, the measured throughput of the TBS increases up to 2.6 times that of a conventional scheme without nullforming because of the significant interference reduction.

Spectrum sharing with terrestrial base stations (BSs) is a challenging topic in the field of wireless communications using high-altitude platform stations (HAPSs). In an existing approach based on a coordinated nullforming technique, interference signals of a HAPS are reduced through multiple nulls toward each terrestrial cell. However, a huge number of control parameters are shared between the HAPS and terrestrial BSs so that they can calculate the interference of HAPSs for resource allocation, which causes heavy load on backhaul networks. This study proposes an interference estimation model (IEM) for reducing the number of control parameters in the conventional coordinated nullforming scheme. In addition, a novel resource allocation algorithm for the terrestrial BSs based on the IEM is developed. Simulation results show that the signal-to-interference-plus-noise ratio of users in terrestrial cells achieved using the proposed scheme is equivalent to that of the conventional scheme. The proposed scheme reduces the number of control parameters shared with the terrestrial BSs to approximately 1/400thof the conventional scheme.

Spectrum sharing with terrestrial networks (TNs) is a challenging task in wireless communications using high-altitude platform stations (HAPSs). Various methods for HAPS-terrestrial spectrum sharing have been proposed. However, few studies have verified their effectiveness using a real payload system for HAPSs. This study proposes a novel interference reduction scheme based on nullforming techniques for a cylindrical array antenna, which uses angular information of HAPS users instead of their channel state information. In addition, a 5G-based service link evaluation system, which includes a 36-element cylindrical array antenna and the proposed precoder, has been implemented, and a field trial using this system was conducted to evaluate the interference reduction performance of the proposed scheme in a real rural environment. The experimental results show that the proposed scheme can mitigate the interference with TNs by approximately 20 dB relative to the main lobe level of user-specific beams in a rural scenario.

To achieve wide coverage in sixth-generation (6G) wireless communication systems, high-altitude platform stations (HAPSs) have received considerable research attention. Interference management is required between HAPSs and terrestrial networks to achieve high spectral efficiency in their coexisting environments. Some studies have proposed the combination of these systems with well-established multiple-input multiple-output (MIMO) techniques, forming a null toward each ground base station (BS). However, this approach is inadequate for protecting the entire ground cell, as a large-scale antenna can only form narrow nulls. The formation of multiple nulls around the ground BS could enhance the communication quality of the ground cell, but at the expense of the HAPS service area. To avoid this tradeoff, we propose a null-sweeping scheme, which dynamically handles the directions of nulls around ground BSs over time and/or frequency domains. By coordination between the HAPS and terrestrial systems, the scheme controls residual interference and improves the overall communication quality of ground cell users. The simulation results demonstrate that the proposed scheme significantly enhances the received signal-to-interference-plus-noise ratio (SINR) in the ground cells. Regardless of the spatial relationship between HAPS and ground cells, the proposed scheme can increase the sum capacity of the terrestrial systems by up to 15% without any quality degradation to HAPS users.

A modified hypercube with multiple buses used as bypass routes is proposed. This is achieved by partitioning all processor elements into subsets using coding theory and by connecting all PEs in each subset to a bus. The basic structure for small systems reduces the diameter to two (one on a bus plus one on a link). In the generalized structure for large systems, it is reduced to less than about one-third that of the regular hypercube.>