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Dynamic Downlink-Uplink Spectrum Sharing between Terrestrial and Non-Terrestrial Networks

Sourav Mukherjee, Bho Matthiesen, Armin Dekorsy, Petar Popovski

TL;DR

This work tackles interference in dense terrestrial and non-terrestrial network coexistence by introducing dynamic downlink-uplink band assignment (spin) for two fixed frequency bands. It develops a joint optimization framework over dynamic band assignment, user scheduling, and power allocation, and solves it using a sequence of transform-based steps (Lagrangian dual and Quadratic transforms) with alternating optimization, including an exhaustive search over spin configurations. The results show that dynamic spin-enabled FDD can yield up to 94% throughput gains in high-interference, dense LEO deployments, significantly improving spectrum efficiency. The approach is practical for deployment, leveraging standard solvers like MOSEK and requiring only readily computable channel statistics and beamforming terms.

Abstract

6G networks are expected to integrate low Earth orbit satellites to ensure global connectivity by extending coverage to underserved and remote regions. However, the deployment of dense mega-constellations introduces severe interference among satellites operating over shared frequency bands. This is, in part, due to the limited flexibility of conventional frequency division duplex (FDD) systems, where fixed bands for downlink (DL) and uplink (UL) transmissions are employed. In this work, we propose dynamic re-assignment of FDD bands for improved interference management in dense deployments and evaluate the performance gain of this approach. To this end, we formulate a joint optimization problem that incorporates dynamic band assignment, user scheduling, and power allocation in both directions. This non-convex mixed integer problem is solved using a combination of equivalence transforms, alternating optimization, and state-of-the-art industrial-grade mixed integer solvers. Numerical results demonstrate that the proposed approach of dynamic FDD band assignment significantly enhances system performance over conventional FDD, achieving up to 94\% improvement in throughput in dense deployments.

Dynamic Downlink-Uplink Spectrum Sharing between Terrestrial and Non-Terrestrial Networks

TL;DR

This work tackles interference in dense terrestrial and non-terrestrial network coexistence by introducing dynamic downlink-uplink band assignment (spin) for two fixed frequency bands. It develops a joint optimization framework over dynamic band assignment, user scheduling, and power allocation, and solves it using a sequence of transform-based steps (Lagrangian dual and Quadratic transforms) with alternating optimization, including an exhaustive search over spin configurations. The results show that dynamic spin-enabled FDD can yield up to 94% throughput gains in high-interference, dense LEO deployments, significantly improving spectrum efficiency. The approach is practical for deployment, leveraging standard solvers like MOSEK and requiring only readily computable channel statistics and beamforming terms.

Abstract

6G networks are expected to integrate low Earth orbit satellites to ensure global connectivity by extending coverage to underserved and remote regions. However, the deployment of dense mega-constellations introduces severe interference among satellites operating over shared frequency bands. This is, in part, due to the limited flexibility of conventional frequency division duplex (FDD) systems, where fixed bands for downlink (DL) and uplink (UL) transmissions are employed. In this work, we propose dynamic re-assignment of FDD bands for improved interference management in dense deployments and evaluate the performance gain of this approach. To this end, we formulate a joint optimization problem that incorporates dynamic band assignment, user scheduling, and power allocation in both directions. This non-convex mixed integer problem is solved using a combination of equivalence transforms, alternating optimization, and state-of-the-art industrial-grade mixed integer solvers. Numerical results demonstrate that the proposed approach of dynamic FDD band assignment significantly enhances system performance over conventional FDD, achieving up to 94\% improvement in throughput in dense deployments.

Paper Structure

This paper contains 10 sections, 2 theorems, 19 equations, 5 figures, 1 algorithm.

Table of Contents

  1. Introduction
  2. System Model & Problem Formulation
  3. Formulation of the Optimization Problem
  4. Solution to $(\mathrm{P0})$
  5. Update $\bm \chi^{\mathrm{dl}}$ and $\bm \chi^{\mathrm{ul}}$
  6. Update $\bm \xi^{\mathrm{dl}}$ and $\bm \xi^{\mathrm{ul}}$
  7. Update Jointly $\bm d, \bm u, \bm p^{\text{dl}},\bm p^{\text{ul}}$
  8. Numerical Results
  9. Proof of Proposition 1
  10. Proof of Proposition 2

Key Result

Proposition 1

Consider a solution to $(\mathrm{P2})$ given by $(\bm d^\dagger, \bm r^\dagger, \bm u^\dagger, \bm p^{\mathrm{dl}\dagger}, \bm p^{\mathrm{ul}\dagger}, \bm \chi^{\mathrm{dl}\dagger}, \bm \chi^{\mathrm{ul}\dagger}, \bm \xi^{\mathrm{dl}\dagger}, \bm \xi^{\mathrm{ul}\dagger})$. Then, the tuple $(\bm d^\

Figures (5)

  • Figure 1: Motivation for dynamic downlink–uplink (DL–UL) band selection, shown for two satellites: (a) Fixed-band allocation, each satellite uses predetermined DL and UL bands, leading to strong interference. (b) Dynamic band selection, satellites flexibly assign bands for DL or UL, reducing inter-satellite interference through spatial separation and directive antennas.
  • Figure 2: A system of $J$leo satellites in orbit serving $K$ue over region.
  • Figure 3: Introduced spin variable for a satellite $j$.
  • Figure 4: Monotonic increase of $f_2$ with iterations for the proposed Algorithm \ref{['algo:1']}.
  • Figure 5: CDF of the sum-rate $f_0$ with and without spin.

Theorems & Definitions (4)

  • Proposition 1
  • proof
  • Proposition 2
  • proof