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Movable-Antenna Index Modulation (MA-IM): System Framework and Performance Analysis

Bang Huang, Shunyuan Shang, Mohamed-Slim Alouini

Abstract

This paper proposes a movable-antenna-based index modulation (MA-IM) framework that exploits the spatial mobility of a single reconfigurable antenna to create additional information-bearing dimensions for next-generation wireless systems. By discretizing the continuous movable region into a dense set of candidate sampling points and selecting representative anchors for indexing, the proposed framework converts spatial degrees of freedom into a practical modulation resource. Building on this framework, we develop a family of anchor-selection strategies with different levels of channel awareness, including geometry-based, SNR-based, max--min channel-domain, and joint constellation-aware designs. For the resulting MA-IM schemes, joint maximum-likelihood (ML) detectors are derived, along with a low-complexity two-stage detector, and unified analytical upper bounds on the average bit error probability (ABEP) are established based on the joint index--modulation constellation. The results reveal that directly indexing all sampling points is generally unreliable, highlighting the necessity of anchor optimization. The performance of MA-IM is shown to depend on key system parameters, including channel richness, spatial correlation, the number of index states, and the modulation order. In particular, increasing the number of index states and increasing the QAM order affect MA-IM in fundamentally different ways, even under the same transmission rate. Among the proposed schemes, the joint constellation-aware anchor design achieves the best error performance, demonstrating that optimizing channel-domain separation alone is insufficient and that effective MA-IM design must account for the geometry of the joint signal constellation. Simulation results further show that, with properly designed anchors, MA-IM can approach or even outperform same-spectral-efficiency QAM baselines.

Movable-Antenna Index Modulation (MA-IM): System Framework and Performance Analysis

Abstract

This paper proposes a movable-antenna-based index modulation (MA-IM) framework that exploits the spatial mobility of a single reconfigurable antenna to create additional information-bearing dimensions for next-generation wireless systems. By discretizing the continuous movable region into a dense set of candidate sampling points and selecting representative anchors for indexing, the proposed framework converts spatial degrees of freedom into a practical modulation resource. Building on this framework, we develop a family of anchor-selection strategies with different levels of channel awareness, including geometry-based, SNR-based, max--min channel-domain, and joint constellation-aware designs. For the resulting MA-IM schemes, joint maximum-likelihood (ML) detectors are derived, along with a low-complexity two-stage detector, and unified analytical upper bounds on the average bit error probability (ABEP) are established based on the joint index--modulation constellation. The results reveal that directly indexing all sampling points is generally unreliable, highlighting the necessity of anchor optimization. The performance of MA-IM is shown to depend on key system parameters, including channel richness, spatial correlation, the number of index states, and the modulation order. In particular, increasing the number of index states and increasing the QAM order affect MA-IM in fundamentally different ways, even under the same transmission rate. Among the proposed schemes, the joint constellation-aware anchor design achieves the best error performance, demonstrating that optimizing channel-domain separation alone is insufficient and that effective MA-IM design must account for the geometry of the joint signal constellation. Simulation results further show that, with properly designed anchors, MA-IM can approach or even outperform same-spectral-efficiency QAM baselines.

Paper Structure

This paper contains 20 sections, 1 theorem, 36 equations, 12 figures, 1 table, 4 algorithms.

Table of Contents

  1. Introduction
  2. System Model and Movable Antenna Discretization
  3. Channel model
  4. Port discretization
  5. Index modulation scheme
  6. Random activation (Scheme 1)
  7. Geometry-based anchoring (Scheme 2)
  8. SNR-based selection (Scheme 3)
  9. Distance-based selection (Scheme 4)
  10. Channel-domain max--min anchor design (Scheme 5)
  11. Joint constellation–distance anchoring (Scheme 6)
  12. Computational Complexity Discussion
  13. Proposed detector
  14. ML Detector
  15. Low-complexity two-stage detector for scheme 1
  16. ...and 5 more sections

Key Result

Lemma 1

Consider a rich-scattering environment where the spatial correlation between two antenna positions separated by distance $\Delta r$ is approximated by jakes1994microwaveNew2024AnInformation where $J_0(\cdot)$ denotes the zeroth-order Bessel function. Given a target correlation level $\rho_{\mathrm{tar}}\in[0,1)$, the maximum spacing $\Delta r_{\max}$ that guarantees $\rho(\Delta r)\ge \rho_{\mathr

Figures (12)

  • Figure 1: System model of the proposed MA-IM framework.
  • Figure 2: Illustration of the spatial angle domain corresponding to the transmit region
  • Figure 3: Illustration of the sampling layout in the MA region under the target channel similarity constraint $\rho_{\mathrm{tar}} = 0.9$.
  • Figure 4: Example of MA indexing: each block corresponds to one index value, and the colored point denotes the selected physical transmit coordinate for that index.
  • Figure 5: Example of MA indexing under Scheme 3. Black dots denote candidate sampling positions, while colored circles indicate the selected representative ports in each cell.
  • ...and 7 more figures

Theorems & Definitions (2)

  • Lemma 1
  • proof