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arxiv: 2511.06199 · v2 · submitted 2025-11-09 · 📡 eess.SY · cs.SY

A Passive Software-Defined Radio-based mmWave Sensing System for Blind Integrated Communication and Sensing

Shiqi Liu , Hang Song , Bo Wei , Nopphon Keerativoranan , Jun-ichi Takada This is my paper

Pith reviewed 2026-05-18 00:20 UTC · model grok-4.3

classification 📡 eess.SY cs.SY
keywords passive mmWave sensingblind ISACsoftware-defined radiodifferential receiverDoppler spectrogramhuman activity recognitionambient signal sensing
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The pith

A passive mmWave sensing system uses two oppositely-oriented receivers to extract motion data from ambient communication signals without synchronization.

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

The paper presents a fully passive sensing system for millimeter-wave bands that performs sensing using existing communication signals rather than generating its own transmissions. By employing a differential structure with two receivers pointed in opposite directions, the system cancels out the effects of unknown transmitted signals and environmental distortions. This design enables blind integrated sensing and communication, where the receiver operates independently of any transmitter. Validation experiments with a moving metal plate produce Doppler spectrograms that match simulations, and further tests detect handwaving and single or multiple person movements in real scenarios.

Core claim

The central discovery is that a differential pair of oppositely-oriented passive receivers in a software-defined radio setup can mitigate unknown source signals and distortions, allowing reliable Doppler-based sensing of object and human motions from ambient mmWave communication signals without any need for synchronization or knowledge of the transmitted waveform.

What carries the argument

differential structure with two oppositely-oriented receivers that cancels influences of unknown source signals and distortions

If this is right

  • Ambient mmWave signals can be used for sensing without interrupting existing communication systems.
  • The system supports applications such as signal detection and dynamic human activity recognition in field deployments.
  • Measured Doppler spectrograms agree with simulations, confirming sensing accuracy for known motions.
  • Complex scenarios with handwaving and multi-person motion can be successfully detected.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • This passive approach could reduce the need for dedicated sensing hardware in environments with existing mmWave networks.
  • Similar differential techniques could be adapted for other passive sensing modalities beyond mmWave.

Load-bearing premise

The differential structure with two oppositely-oriented receivers sufficiently mitigates influences of unknown source signals and other distortions to enable reliable sensing in complex scenarios.

What would settle it

A direct comparison where the system is tested with a known transmitter signal that includes specific motion patterns and the resulting spectrogram fails to match independent simulation or ground truth measurements would falsify the claim of correctness.

Figures

Figures reproduced from arXiv: 2511.06199 by Bo Wei, Hang Song, Jun-ichi Takada, Nopphon Keerativoranan, Shiqi Liu.

Figure 1
Figure 1. Figure 1: Schematic diagram of the proposed passive mmWave ISAC sensing system. [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Deployment geometry of the differential architecture [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Through sampling, the received signal from reference [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Relative channel response Hrel(fq, ti) in time￾frequency domain. (a) Concept diagram. (b) Measurement result. process is based on the assumption that the signal bandwidth is smaller than the channel coherence bandwidth, allowing the channel to be approximated as a flat fading model. To justify this averaging, the relative channel response can be approximated with a simplified model as: Hrel(fq, ti) = H2(fq… view at source ↗
Figure 5
Figure 5. Figure 5: Experimental hardware and validation setup. (a) A [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Deployment for human activity sensing, illustrating [PITH_FULL_IMAGE:figures/full_fig_p008_6.png] view at source ↗
Figure 8
Figure 8. Figure 8: Doppler spectrograms generated from measurement [PITH_FULL_IMAGE:figures/full_fig_p008_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Experiment results from the continuous plate motion [PITH_FULL_IMAGE:figures/full_fig_p009_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Doppler Spectrograms under (a) static background scenario and (b) unidirectional walking scenario with one human [PITH_FULL_IMAGE:figures/full_fig_p010_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Doppler spectrograms under periodic motion scenarios. (a) Hand-waving of one human. (b) Back-and-forth walking [PITH_FULL_IMAGE:figures/full_fig_p010_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: Doppler spectrogram under two-person walking sce [PITH_FULL_IMAGE:figures/full_fig_p010_12.png] view at source ↗
read the original abstract

Integrated Sensing and Communication (ISAC) is considered as a key component of future 6G technologies, especially in the millimeter-wave (mmWave) bands. Recently, the performances of ISAC were experimentally evaluated and demonstrated in various scenarios by developing ISAC systems. These systems generally consist of coherent transmitting (Tx) and receiving (Rx) modules. However, actively transmitting radio waves for experiments is not easy due to regulatory restrictions of radio. Meanwhile, the Tx/Rx should be synchronized and Rx need the information of Tx. In this paper, a fully passive mmWave sensing system is developed with software-defined radio for blind ISAC. It only consists of a passive Rx module which does not depend on the Tx. Since the proposed system is not synchronized with Tx and has no knowledge of the transmitted signals, a differential structure with two oppositely-oriented receivers is introduced to realize the sensing function. This structure can mitigate the influences of unknown source signals and other distortions. With the proposed sensing system, the ambient mmWave communication signals are leveraged for sensing without interrupting the existing systems. It can be deployed for field applications such as signal detection and dynamic human activity recognition since it does not emit signals. The efficacy of the developed system is first verified with a metallic plate with known motion pattern. The measured Doppler spectrogram shows good agreement with the simulation results, demonstrating the correctness of the sensing results. Further, the system is evaluated in complex scenarios, including handwaving, single- and multi-person motion detection. The sensing results successfully reflect the corresponding motions, demonstrating that the proposed sensing system can be utilized for blind ISAC in various applications.

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 manuscript presents a fully passive mmWave sensing system using software-defined radio for blind integrated communication and sensing (ISAC). It employs a differential structure with two oppositely-oriented receivers to leverage ambient mmWave communication signals for sensing without synchronization or knowledge of the transmitted waveform. The system is validated experimentally: Doppler spectrograms for a metallic plate with known motion show qualitative agreement with simulations, and further tests demonstrate motion detection for handwaving, single-person, and multi-person scenarios.

Significance. If the differential cancellation proves robust, the work could enable non-intrusive sensing in existing mmWave networks, addressing regulatory restrictions on active transmission and synchronization challenges in 6G ISAC. The experimental demonstrations of motion detection indicate practical applicability for dynamic human activity recognition, though the current evidence base is primarily qualitative.

major comments (2)
  1. Abstract and § on experimental validation: The claim that the measured Doppler spectrogram shows 'good agreement' with simulations and demonstrates 'correctness of the sensing results' rests on qualitative visual comparison only. No quantitative metrics (e.g., correlation coefficient, MSE, or error bars) or ablation studies (single-receiver vs. differential) are provided, which is load-bearing for substantiating the central sensing claims.
  2. Section describing the differential structure: The assertion that the two oppositely-oriented receivers 'mitigate the influences of unknown source signals and other distortions' lacks supporting analysis or measurements, such as common-mode rejection ratio, residual power after subtraction, or bounds under phase/gain mismatch. This is central to reliable performance in complex multipath or multi-person scenarios flagged in the paper.
minor comments (2)
  1. Introduction: A few additional citations to recent passive or blind ISAC works would better contextualize the contribution relative to active coherent ISAC systems.
  2. Figure captions: Ensure all spectrogram figures include explicit scale bars, axis units, and a brief note on processing parameters (e.g., STFT window) for reproducibility.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the thorough and constructive review. The comments highlight important areas where additional quantitative support and analysis can strengthen the presentation of our results. We address each major comment below and will revise the manuscript to incorporate the suggested improvements.

read point-by-point responses

  1. Referee: Abstract and § on experimental validation: The claim that the measured Doppler spectrogram shows 'good agreement' with simulations and demonstrates 'correctness of the sensing results' rests on qualitative visual comparison only. No quantitative metrics (e.g., correlation coefficient, MSE, or error bars) or ablation studies (single-receiver vs. differential) are provided, which is load-bearing for substantiating the central sensing claims.

    Authors: We agree that the current validation relies on qualitative visual comparison. In the revised manuscript, we will add quantitative metrics by computing the correlation coefficient and mean squared error between the measured and simulated Doppler spectrograms. We will also report error bars derived from repeated measurements to indicate variability. For the ablation aspect, we will include a direct comparison of sensing performance using a single receiver versus the proposed differential configuration, using the existing experimental data to demonstrate the improvement in mitigating unknown source signals. revision: yes

  2. Referee: Section describing the differential structure: The assertion that the two oppositely-oriented receivers 'mitigate the influences of unknown source signals and other distortions' lacks supporting analysis or measurements, such as common-mode rejection ratio, residual power after subtraction, or bounds under phase/gain mismatch. This is central to reliable performance in complex multipath or multi-person scenarios flagged in the paper.

    Authors: We acknowledge that additional quantitative characterization of the differential structure would better support the claims. In the revision, we will include experimental measurements of the common-mode rejection ratio and the residual power levels after subtraction. We will also provide an analytical derivation of performance bounds under phase and gain mismatch between the receivers, accompanied by supporting experimental results from our testbed to address robustness in multipath and multi-person environments. revision: yes

Circularity Check

0 steps flagged

No circularity: experimental hardware demonstration with external validation

full rationale

The paper describes a passive mmWave sensing system using SDR hardware and a differential receiver structure to enable blind ISAC without synchronization or knowledge of the transmitter. Claims rest on empirical results: measured Doppler spectrograms for a metallic plate and human motions show visual agreement with independent simulations, plus successful detection in handwaving and multi-person scenarios. No mathematical derivations, fitted parameters, self-citations of uniqueness theorems, or ansatzes are invoked that reduce outputs to inputs by construction. The work is self-contained against external benchmarks (simulations and physical motions) and contains no load-bearing steps that qualify as circular under the defined patterns.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

No free parameters, axioms, or invented entities are introduced in the provided abstract; the work relies on standard assumptions of signal reflection and Doppler processing.

pith-pipeline@v0.9.0 · 5618 in / 1003 out tokens · 49558 ms · 2026-05-18T00:20:09.458360+00:00 · methodology

discussion (0)

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