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Overcoming BS Down-Tilt for Air-Ground ISAC Coverage: Antenna Design, Beamforming and User Scheduling

Lingyi Zhu, Zhongxiang Wei, Fan Liu, Jianjun Wu, Xiao-Wei Tang, Christos Masouros, Shanpu Shen

TL;DR

This work tackles the challenge of enabling full-space sensing and communication for low-altitude ISAC by introducing an air-ground OmniSteering antenna that replaces the traditional backlobe reflector with a tunable omni-steering plate. A unified sum-MI objective combines communication mutual information $I_{C,k}$ and sensing mutual information $I_S$, and the problem is decomposed into passive coefficient optimization for the plate and a joint scheduling/beamforming subproblem, solved via either a low-complexity Riemannian gradient ascent or an SDR-based benchmark. The joint scheduling and active beamforming subproblem is transformed into a sum weighted MMSE problem using a Lagrangian-based equivalence, enabling closed-form MMSE combiners and iterative updates for the scheduling variables and transmit beams. Numerical results show that the proposed US-AGO algorithms outperform baselines across sum-MI and sum-NMSE with 360-degree sensing coverage, and beampattern analyses confirm effective target alignment and user scheduling, highlighting the practical impact for dense ISAC deployments in urban air-ground scenarios.

Abstract

Integrated sensing and communication holds great promise for low-altitude economy applications. However, conventional downtilted base stations primarily provide sectorized forward lobes for ground services, failing to sense air targets due to backward blind zones. In this paper, a novel antenna structure is proposed to enable air-ground beam steering, facilitating simultaneous full-space sensing and communication (S&C). Specifically, instead of inserting a reflector behind the antenna array for backlobe mitigation, an omni-steering plate is introduced to collaborate with the active array for omnidirectional beamforming. Building on this hardware innovation, sum S&C mutual information (MI) is maximized, jointly optimizing user scheduling, passive coefficients of the omni-steering plate, and beamforming of the active array. The problem is decomposed into two subproblems: one for optimizing passive coefficients via Riemannian gradient on the manifold, and the other for optimizing user scheduling and active array beamforming. Exploiting relationships among S&C MI, data decoding MMSE, and parameter estimation MMSE, the original subproblem is equivalently transformed into a sum weighted MMSE problem, rigorously established via the Lagrangian and first-order optimality conditions. Simulations show that the proposed algorithm outperforms baselines in sum-MI and MSE, while providing 360 sensing coverage. Beampattern analysis further demonstrates effective user scheduling and accurate target alignment.

Overcoming BS Down-Tilt for Air-Ground ISAC Coverage: Antenna Design, Beamforming and User Scheduling

TL;DR

This work tackles the challenge of enabling full-space sensing and communication for low-altitude ISAC by introducing an air-ground OmniSteering antenna that replaces the traditional backlobe reflector with a tunable omni-steering plate. A unified sum-MI objective combines communication mutual information and sensing mutual information , and the problem is decomposed into passive coefficient optimization for the plate and a joint scheduling/beamforming subproblem, solved via either a low-complexity Riemannian gradient ascent or an SDR-based benchmark. The joint scheduling and active beamforming subproblem is transformed into a sum weighted MMSE problem using a Lagrangian-based equivalence, enabling closed-form MMSE combiners and iterative updates for the scheduling variables and transmit beams. Numerical results show that the proposed US-AGO algorithms outperform baselines across sum-MI and sum-NMSE with 360-degree sensing coverage, and beampattern analyses confirm effective target alignment and user scheduling, highlighting the practical impact for dense ISAC deployments in urban air-ground scenarios.

Abstract

Integrated sensing and communication holds great promise for low-altitude economy applications. However, conventional downtilted base stations primarily provide sectorized forward lobes for ground services, failing to sense air targets due to backward blind zones. In this paper, a novel antenna structure is proposed to enable air-ground beam steering, facilitating simultaneous full-space sensing and communication (S&C). Specifically, instead of inserting a reflector behind the antenna array for backlobe mitigation, an omni-steering plate is introduced to collaborate with the active array for omnidirectional beamforming. Building on this hardware innovation, sum S&C mutual information (MI) is maximized, jointly optimizing user scheduling, passive coefficients of the omni-steering plate, and beamforming of the active array. The problem is decomposed into two subproblems: one for optimizing passive coefficients via Riemannian gradient on the manifold, and the other for optimizing user scheduling and active array beamforming. Exploiting relationships among S&C MI, data decoding MMSE, and parameter estimation MMSE, the original subproblem is equivalently transformed into a sum weighted MMSE problem, rigorously established via the Lagrangian and first-order optimality conditions. Simulations show that the proposed algorithm outperforms baselines in sum-MI and MSE, while providing 360 sensing coverage. Beampattern analysis further demonstrates effective user scheduling and accurate target alignment.
Paper Structure (12 sections, 2 theorems, 55 equations, 7 figures, 3 tables, 3 algorithms)

This paper contains 12 sections, 2 theorems, 55 equations, 7 figures, 3 tables, 3 algorithms.

Table of Contents

  1. Introduction
  2. System Model
  3. Air-Ground OmniSteering Antenna Design
  4. S&C Channel Model
  5. Signal Model
  6. Joint User Scheduling and Air-Ground Omnidirectional Sensing and Communication Algorithm Design
  7. Problem Formulation
  8. Backward Passive Coefficient Optimization
  9. Joint Scheduling and Active Beamforming Optimization
  10. Complexity Analysis
  11. Numerical Results
  12. Conclusion

Key Result

Proposition 1

Let $\boldsymbol{b} \triangleq \boldsymbol{G} \boldsymbol{W} \mathrm{diag}(\boldsymbol{\alpha}) \boldsymbol{s} \in \mathbb{C}^{M \times 1}$, and define $\boldsymbol{C} \triangleq \mathrm{diag}(\boldsymbol{b})^H \boldsymbol{R}_H \mathrm{diag}(\boldsymbol{b}) \in \mathbb{C}^{M \times M}$. Then (P2) ca $\square$

Figures (7)

  • Figure 1: Conventional ISAC base station antenna structure and its sensing coverage limitations.
  • Figure 2: Illustration of the air-ground OmniSteering antenna structure.
  • Figure 3: The convergence behavior by the proposed US-AGO-R and US-AGO-S algorithms under various system configurations.
  • Figure 4: Sum-MI versus SNR by proposed algorithms and baselines.
  • Figure 5: Sum-NMSE versus SNR by proposed algorithms and baselines.
  • ...and 2 more figures

Theorems & Definitions (2)

  • Proposition 1
  • Proposition 2