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Enhanced Time Division Duplexing Slot Allocation and Scheduling in Non-Terrestrial Networks

Alessandro Traspadini, Marco Giordani, Michele Zorzi

TL;DR

The paper tackles the inefficiency of Time Division Duplexing in non-terrestrial networks due to long propagation delays and large guard periods. It introduces Enhanced Synchronized Slot Allocation (ESSA), which enables scheduling multiple downlink slots during guard periods without interference, and combines it with two UE scheduling strategies, MG and MS, to maximize cell capacity. Simulation results show that ESSA dramatically reduces guard-period overhead and, when paired with MS scheduling, can achieve up to about three times higher capacity than traditional TA-based approaches. These findings support more efficient NTN-TN integration and point to future work on fairness and NTN-FDD/TN-TDD coexistence.

Abstract

The integration of non-terrestrial networks (NTNs) and terrestrial networks (TNs) is fundamental for extending connectivity to rural and underserved areas that lack coverage from traditional cellular infrastructure. However, this integration presents several challenges. For instance, TNs mainly operate in Time Division Duplexing (TDD). However, for NTN via satellites, TDD is complicated due to synchronization problems in large cells, and the significant impact of guard periods and long propagation delays. In this paper, we propose a novel slot allocation mechanism to enable TDD in NTN. This approach permits to allocate additional transmissions during the guard period between a downlink slot and the corresponding uplink slot to reduce the overhead, provided that they do not interfere with other concurrent transmissions. Moreover, we propose two scheduling methods to select the users that transmit based on considerations related to the Signal-to-Noise Ratio (SNR) or the propagation delay. Simulations demonstrate that our proposal can increase the network capacity compared to a benchmark scheme that does not schedule transmissions in guard periods.

Enhanced Time Division Duplexing Slot Allocation and Scheduling in Non-Terrestrial Networks

TL;DR

The paper tackles the inefficiency of Time Division Duplexing in non-terrestrial networks due to long propagation delays and large guard periods. It introduces Enhanced Synchronized Slot Allocation (ESSA), which enables scheduling multiple downlink slots during guard periods without interference, and combines it with two UE scheduling strategies, MG and MS, to maximize cell capacity. Simulation results show that ESSA dramatically reduces guard-period overhead and, when paired with MS scheduling, can achieve up to about three times higher capacity than traditional TA-based approaches. These findings support more efficient NTN-TN integration and point to future work on fairness and NTN-FDD/TN-TDD coexistence.

Abstract

The integration of non-terrestrial networks (NTNs) and terrestrial networks (TNs) is fundamental for extending connectivity to rural and underserved areas that lack coverage from traditional cellular infrastructure. However, this integration presents several challenges. For instance, TNs mainly operate in Time Division Duplexing (TDD). However, for NTN via satellites, TDD is complicated due to synchronization problems in large cells, and the significant impact of guard periods and long propagation delays. In this paper, we propose a novel slot allocation mechanism to enable TDD in NTN. This approach permits to allocate additional transmissions during the guard period between a downlink slot and the corresponding uplink slot to reduce the overhead, provided that they do not interfere with other concurrent transmissions. Moreover, we propose two scheduling methods to select the users that transmit based on considerations related to the Signal-to-Noise Ratio (SNR) or the propagation delay. Simulations demonstrate that our proposal can increase the network capacity compared to a benchmark scheme that does not schedule transmissions in guard periods.

Paper Structure

This paper contains 11 sections, 1 theorem, 9 equations, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. System Model
  3. Channel Model
  4. TDD Frame Structure
  5. Enhanced Synchronized Slot Allocation and Scheduling
  6. essa
  7. Scheduling
  8. Simulation results
  9. Slot Allocation Results
  10. Scheduling Results
  11. Conclusions and Future Works

Key Result

Lemma 1

In TDD, if a transmission is completed at time $t_0$, additional concurrent transmissions scheduled by the satellite between $t_0 + T_{th}$ and $t_0 + 2\tau_M$ do not create interference, and can be successfully received by all within the coverage area of the satellite.

Figures (5)

  • Figure 1: Illustration of the scenario for $N_{\rm UE}=2$ (top), and the TDD frame structure with (middle) and without (bottom) timing advance.
  • Figure 2: Illustration of the proposed essa mechanism.
  • Figure 3: Average guard period ($\Bar{t}_{\rm GP}$) and channel usage ($\Bar{\rho}$) vs. $\alpha_{min}$ and $h$.
  • Figure 4: SNR ($\gamma$) and throughput ($\Bar{C}$) of the selected vs. $h$ and the slot pattern.
  • Figure 5: Example of a slot pattern for a direct-to-satellite connection at $h=500$ km and $\alpha_{min}=80^\circ$. We compare TA vs. ESSA.

Theorems & Definitions (2)

  • Lemma 1
  • proof