Height-Dependent Spectrum Activity Measurements and Modeling: A Case Study with FM Radio Bands

TL;DR

This paper identifies a clear transition from non-line-of-sight (NLoS) to line-of-sight (LoS) regimes at a specific altitude threshold and proposes an altitude-dependent path loss model that incorporates this transition, providing a more accurate framework for altitude-aware spectrum prediction and management across emerging aerial wireless technologies and bands.

Abstract

The increasing demand for wireless connectivity necessitates advanced spectrum modeling to enable efficient spectrum sharing for next-generation aerial communications. While traditional models often overlook vertical variations in signal behavior, this paper proposes a height-dependent propagation model using a helikite-mounted software-defined radio (SDR). We collected extensive measurement data across the 88 MHz to 6 GHz range in both urban and rural environments. As a case study to validate our methodology, we focus on the FM radio band, which allows us to use publicly available transmitter locations and transmit power levels to facilitate comparisons between analytical with measurement results. We identify a clear transition from non-line-of-sight (NLoS) to line-of-sight (LoS) regimes at a specific altitude threshold and propose an altitude-dependent path loss model that incorporates this transition. Our results demonstrate that the proposed model significantly outperforms the standard free space path loss (FSPL) model in complex urban topologies, providing a more accurate framework for altitude-aware spectrum prediction and management across emerging aerial wireless technologies and bands.

Figures (6)

• Figure 1: Photo of helikite flying over the experiment site, altitude and 2D distance change during the experiment.
• Figure 2: Spectrum activity of the FM radio frequency band ($87$ - $108$ MHz) from the helikite measurement. Occupied spectrum for the WKNC, WNCB, WQDR, and WBBB FM radio stations are also highlighted.
• Figure 3: Location and coverage of the FM radio stations and the helikite experiment site location at the NC State Main campus.
• Figure 4: Power of the signals from different FM radio stations and corresponding altitude is marked for every spectrum sweep in the urban area. We obtain the LoS altitude threshold of the helikite as $50$ m based on the trend of the measurement points in the urban area.
• Figure 5: Power of the signals from different FM radio stations and corresponding altitude is marked for every spectrum sweep in the rural area. The path loss model fits either the free space path loss model or the altitude-dependent path loss model based on the location of FM radio stations.