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arxiv: 2601.16543 · v2 · submitted 2026-01-23 · 📡 eess.SP

Cell-Free MIMO with Rotatable Antennas: When Macro-Diversity Meets Antenna Directivity

Xingxiang Peng , Qingqing Wu , Ziyuan Zheng , Yanze Zhu , Wen Chen , Penghui Huang , Ying Gao , Honghao Wang This is my paper

Pith reviewed 2026-05-16 12:05 UTC · model grok-4.3

classification 📡 eess.SP
keywords cell-free MIMOrotatable antennasmax-min rateantenna orientation optimizationbeamformingmacro-diversitysuccessive convex approximation
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The pith

Rotatable antennas in cell-free MIMO networks improve the worst-user rate by jointly optimizing beamforming and antenna orientations.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes that rotatable antennas provide an extra degree of freedom in cell-free networks by allowing the boresight to be steered toward dominant propagation directions, which strengthens weak links caused by user geometry or blockages. This leads to a max-min rate optimization problem solved through alternating updates: beamformers via second-order cone programming and orientations via successive convex approximation, with a lower-complexity two-stage alternative that first sets orientations for proportional fairness. Simulations show these designs deliver markedly higher minimum rates than beamforming-only baselines, and that higher directivity improves fairness only when orientations are chosen correctly.

Core claim

In an RA-enabled cell-free downlink, jointly optimizing transmit beamforming and antenna orientations via alternating SOCP and successive convex approximation yields substantially higher worst-user rates than conventional beamforming benchmarks; a two-stage scheme that first maximizes proportional-fair log-utility over orientations using manifold-aware Frank-Wolfe updates and then solves for beamformers via SOCP achieves similar gains at reduced complexity.

What carries the argument

Alternating optimization of beamforming vectors and antenna orientation angles, where each iteration aligns the antenna boresight with dominant channel directions to strengthen disadvantaged links before re-computing the beamformers.

Load-bearing premise

Antenna orientations can be adjusted in real time to align with dominant propagation directions and the resulting channel gains are known accurately enough for the joint optimization.

What would settle it

A measurement campaign in a distributed AP testbed that shows the minimum user rate remains unchanged or decreases when antenna orientations are varied, even with perfect instantaneous CSI, would falsify the claim.

Figures

Figures reproduced from arXiv: 2601.16543 by Honghao Wang, Penghui Huang, Qingqing Wu, Wen Chen, Xingxiang Peng, Yanze Zhu, Ying Gao, Ziyuan Zheng.

Figure 1
Figure 1. Figure 1: A cell-free MIMO network with rotatable antennas. [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 3
Figure 3. Figure 3: The convergence behavior of the proposed AO-based algorithm. [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: The min-user rate vs. the maximum zenith angle. [PITH_FULL_IMAGE:figures/full_fig_p009_4.png] view at source ↗
Figure 7
Figure 7. Figure 7: The min-user rate vs. the number of APs. [PITH_FULL_IMAGE:figures/full_fig_p010_7.png] view at source ↗
Figure 6
Figure 6. Figure 6: The min-user rate vs. the antenna directivity factor. [PITH_FULL_IMAGE:figures/full_fig_p010_6.png] view at source ↗
read the original abstract

Cell-free networks leverage distributed access points (APs) to achieve macro-diversity, yet their performance is often constrained by large disparities in channel quality arising from user geometry and blockages. To address this, rotatable antennas (RAs) add a lightweight hardware degree of freedom by steering the antenna boresight toward dominant propagation directions to strengthen unfavorable links, thereby enabling the network to better exploit macro-diversity for higher and more uniform performance. This paper investigates an RA-enabled cell-free downlink network and formulates a max-min rate problem that jointly optimizes transmit beamforming and antenna orientations. To tackle this challenging problem, we develop an alternating-optimization-based algorithm that iteratively updates the beamformers via a second-order cone program (SOCP) and optimizes the antenna orientations using successive convex approximation. To reduce complexity, we further propose an efficient two-stage scheme that first designs orientations by maximizing a proportional-fair log-utility using manifold-aware Frank-Wolfe updates, and then computes the beamformers using an SOCP-based design. Simulation results demonstrate that the proposed orientation-aware designs achieve a substantially higher worst-user rate than conventional beamforming-only benchmarks. Furthermore, larger antenna directivity enhances fairness with proper orientation but can degrade the worst-user performance otherwise.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper claims that adding rotatable antennas to cell-free MIMO networks provides an extra degree of freedom that, when jointly optimized with transmit beamforming, substantially improves the worst-user rate and fairness compared with conventional beamforming-only designs. It formulates a max-min rate problem and solves it via an alternating SOCP+SCA algorithm and a lower-complexity two-stage manifold Frank-Wolfe + SOCP scheme; simulations are used to demonstrate the gains.

Significance. If the reported simulation gains survive realistic CSI acquisition and orientation-adjustment overhead, the work would establish a lightweight hardware enhancement that meaningfully augments macro-diversity in cell-free systems, improving uniformity of service without additional APs or power.

major comments (2)
  1. [§V] §V (Simulation results): the claimed 'substantially higher worst-user rate' is obtained under the assumption of perfect instantaneous knowledge of the effective channels (including dominant propagation directions) for both orientation and beamformer optimization; no Monte-Carlo trials with channel estimation error, pilot overhead, or orientation adjustment latency are reported, rendering the central performance claim load-bearing on an unverified modeling choice.
  2. [§III] §III (Problem formulation, Eq. (P1)): the max-min rate objective treats post-orientation channel gains as deterministic and perfectly known quantities that can be freely adjusted at each coherence block; the formulation contains no constraints on mechanical rotation speed, power consumption of the rotator, or estimation of the angle-of-arrival parameters that would be required in practice.
minor comments (2)
  1. [Abstract] The abstract states that 'larger antenna directivity enhances fairness with proper orientation but can degrade the worst-user performance otherwise,' yet the text does not provide a quantitative threshold or figure that isolates this transition point.
  2. [§II] Notation for the antenna directivity factor and the geometric channel model parameters is introduced without an explicit table of symbols, making cross-referencing between the system model and the optimization algorithms cumbersome.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments, which help clarify the scope and limitations of our work. We address each major comment below and indicate the revisions we will incorporate.

read point-by-point responses

  1. Referee: [§V] §V (Simulation results): the claimed 'substantially higher worst-user rate' is obtained under the assumption of perfect instantaneous knowledge of the effective channels (including dominant propagation directions) for both orientation and beamformer optimization; no Monte-Carlo trials with channel estimation error, pilot overhead, or orientation adjustment latency are reported, rendering the central performance claim load-bearing on an unverified modeling choice.

    Authors: We agree that the reported gains rely on perfect CSI. Our simulations are designed to isolate the fundamental benefit of the additional degree of freedom provided by rotatable antennas. In the revised manuscript we will add a dedicated subsection in §V that includes Monte-Carlo results under imperfect CSI (using standard MMSE estimation with pilot overhead) and a brief discussion of orientation adjustment latency, thereby qualifying the performance claims. revision: yes

  2. Referee: [§III] §III (Problem formulation, Eq. (P1)): the max-min rate objective treats post-orientation channel gains as deterministic and perfectly known quantities that can be freely adjusted at each coherence block; the formulation contains no constraints on mechanical rotation speed, power consumption of the rotator, or estimation of the angle-of-arrival parameters that would be required in practice.

    Authors: The formulation in (P1) deliberately omits hardware constraints to derive the theoretical performance limits of joint optimization. We will revise §III to explicitly state that orientations are intended to be updated on a slower time scale than beamforming (e.g., based on long-term angle-of-arrival statistics) and will add a short paragraph discussing mechanical rotation speed and power consumption as practical considerations left for future implementation studies. revision: partial

Circularity Check

0 steps flagged

Standard convex optimization applied to rotatable-antenna model; no self-referential reductions

full rationale

The derivation formulates a max-min rate problem and solves it via alternating SOCP/SCA or two-stage manifold Frank-Wolfe. These are standard convex techniques applied to the new RA channel model; no equations reduce claimed rate gains to fitted constants or self-citations by construction. Simulations provide the performance evidence under stated assumptions, with no load-bearing self-citation chains or ansatz smuggling visible in the text.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

Based on the abstract alone, the central claim rests on standard wireless channel modeling assumptions plus the new rotatable-antenna degree of freedom; no explicit free parameters or invented entities are named.

free parameters (1)
  • antenna directivity factor
    The abstract states that larger directivity enhances fairness when orientations are chosen properly, implying a model parameter whose value affects the reported gains.
axioms (1)
  • domain assumption Channel state information is perfectly known and orientations can be set to align with dominant paths
    Required to formulate and solve the max-min rate problem with joint beamforming and orientation variables.

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supports
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extends
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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. Two-Timescale Design for Rotatable-Antenna Systems With Imperfect CSI: Rate Analysis and Orientation Optimization

    eess.SP 2026-05 unverdicted novelty 6.0

    Two-timescale rotatable-antenna design derives closed-form rates for MRC and wZF under imperfect CSI and optimizes orientations via projected gradient, showing different preferred rotations for estimation error versus rate.

Reference graph

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