An ISAC-Assisted Beam Alignment Design for HAP-Based 6G Flying Ad-Hoc Networks

High altitude platforms (HAPs) can establish autonomous airborne networks, known as flying ad-hoc networks (FANETs), without relying on terrestrial infrastructure. A crucial aspect of these networks is maintaining robust links between HAPs, achievable through advanced multiple input multiple output (MIMO) techniques like beamforming. However, the unpredictable motion of HAPs necessitates frequent beam alignment. This paper proposes a novel two-step beam alignment method for inter-HAPs networks, leveraging the integrated sensing and communication (ISAC) concept. The first step involves a coarse beam alignment using sensing and triggering signals to extract channel parameters and activate a targeted HAP at low frequency bands. In the second step, a fine beam alignment is performed at high frequency bands to enhance the link connection within the narrowed area defined in the first step. Utilizing an antenna activation controller, the proposed method efficiently uses a single antenna array for dual-purpose signals across different frequency bands, reducing deployment costs and enhancing system efficiency. Mathematical analysis and numerical results demonstrate that this method significantly improves system performance by reducing alignment time overhead, while maintaining high communication capacity, beamforming gain, and detection rates for targeted HAPs.