Downlink Outage Probability Analysis in Cooperative LEO-LAP Enabled Space-Air-Ground Vehicular Networks

Traditional vehicular network communication infrastructure is limited by road obstructions and insufficient deployment density, making it difficult to efficiently ensure communication performance in complex traffic scenarios. To address this issue, this paper proposes a space-air-ground integrated vehicular network architecture, which constructs a three-dimensional network by integrating low earth orbit (LEO) satellites, low-altitude platforms (LAPs), and ground vehicles. Additionally, non-orthogonal multiple access (NOMA) and simultaneous wireless information and power transfer (SWIPT) technologies are combined to enable the efficient cooperative transmission of information and energy. However, the multi-tier and multi-node network architecture increases interference between links, making it difficult to directly quantify the downlink outage probability. To overcome this challenge, this paper first establishes a system model based on NOMA transmission and a hybrid time-switching/power-splitting (TS/PS) protocol, and proposes an antenna selection criterion with the objective of maximizing energy harvesting at LAP nodes. Based on this, a dynamic power factor allocation mechanism is designed, and the downlink transmission performance is analyzed. Furthermore, this paper derives a closed-form expression for the downlink outage probability based on the Rayleigh fading and co-channel interference channel models of the satellite-LAP-vehicle link. By applying the central limit theorem, a Gaussian approximation solution is proposed. Simulation results show that the approximation solution matches well with the closed-form expression, with an average error of 6.2 %. In addition, the proposed scheme outperforms state-of-the-art schemes in terms of the downlink outage probability under various network conditions.