Opportunistic Scheduling for Single-downlink Satellite-based Quantum Key Distribution

TL;DR

The paper tackles the challenge of scheduling in single-downlink satellite-based QKD with multiple satellites and ground stations to maximize both fairness and total key throughput under dynamic atmospheric and cloud conditions. It introduces a two-phase opportunistic framework: Phase 1 assigns satellites to ground stations using phase-aware, fairness-oriented policies (single-satellite minimum-performance guarantees and multi-satellite 2D assignment with adaptive weights), and Phase 2 allocates inter-ground-station keys via iterative max-min optimization using the per-link key pools $K_{s,g}$. The authors show that opportunistic scheduling (notably Op-RR) achieves superior tradeoffs between minimum ground-station key size and total key size across global and regional deployments, even under cloud coverage and seasonal variations modeled with MODTRAN-based atmospheres and weather data. The work demonstrates significant performance gains over pure Max-Min/Max-Sum baselines and highlights practical considerations such as cloud filtering and the reduced averaging effect in regional settings, underscoring the framework’s scalability and practical relevance for global QKD infrastructure.

Abstract

Satellite-based quantum key distribution (QKD), leveraging low photon loss in free-space quantum communication, is widely regarded as one of the most promising directions to achieve global-scale QKD. With a constellation of satellites and a set of ground stations in a satellite-based QKD system, how to schedule satellites to achieve efficient QKD is an important problem. This problem has been studied in the dual-downlink architecture, where each satellite distributes pairs of entanglements to two ground stations simultaneously. However, it has not been studied in the single downlink architecture, where satellites create keys with each individual ground station, and then serve as trusted nodes to create keys between pairs of ground stations. While the single downlink architecture provides weaker security in that the satellites need to be trusted, it has many advantages, including the potential of achieving significantly higher key rates, and generating keys between pairs of ground stations that are far away from each other and cannot be served using the dual-downlink architecture. In this paper, we propose a novel opportunistic approach for satellite scheduling that accounts for fairness among the ground station pairs, while taking advantage of the dynamic satellite channels to maximize the system performance. We evaluate this approach in a wide range of settings and demonstrate that it provides the best tradeoffs in terms of total and minimum key rates across the ground station pairs. Our evaluation also highlights the importance of considering seasonal effects and cloud coverage in evaluating satellite-based QKD systems. In addition, we show different tradeoffs in global and regional QKD systems.

Figures (12)

• Figure 1: Single-downlink satellite-assisted QKD system. A satellite runs a QKD protocol with individual ground stations, and then uses classical channels and one-time pad to establish secret keys for ground station pairs.
• Figure 2: Opportunistic scheduling framework. Phase 1 schedules satellites to establish keys with individual ground stations, leading to key bits, $K_{s,g}$ between satellite $s$ and ground station $g$, $\forall s$, $\forall g$, colored in the figure based on the satellites. Phase 2 uses iterative optimization to establish keys among the ground station pairs.
• Figure 3: Bipartite graph representing the relationship between satellites and ground stations in one time slot. A satellite is connected to a ground station if it can serve the ground station in that slot. A thicker line connecting satellite $s$ and ground station $g$ represents a higher weight, $w_{s,g}^t$. In (a) and (b), we show two examples that illustrate the coupling of the satellites; a general case is shown in (c).
• Figure 4: Global QKD setting. Top row: satellite view, i.e., histogram of the number of ground stations that can be served by a satellite in a slot. Bottom row: ground station (GS) view, i.e., histogram of the number of satellites that can serve a ground station in a slot. Both rows consider all the slots for the day in September.
• Figure 5: Key size (in bits) obtained for each satellite and ground station pair at the end of one day (i.e., Phase 1 results), global QKD setting, $A=500$km, for the day in September.