Range Enhancement of a 60 GHz FMCW Heart Rate Radar Using Fabry–Perot Cavity Antenna

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

electronics-3875287

Authors presented a method of range extension in a bistatic 60 GHz FMCW radar system for non-contact heart rate monitoring. They increased the antenna gain by about 4 dB using a Fabry–Perot cavity (FPC). With the increased gain of the antenna, authors were able to increase the radar detection range by about 2.3 times.

My comments are as follows.

1. Contribution of the paper:

1) Authors need to enhance the contribution of the proposed idea to the technical advancement. Gain enhancement using an FPC is known and the radar range increase with an increased antenna gain is also known. So what is the point of the authors' idea?

2) Maybe authors could place the main point of the manuscript on the design of an FPC antenna for 60-GHz radar applications.

2. Equation 1:

1) Equation 1 simplifies to D = (1+Γ)/(1−Γ) when the phase condition is met. Please add some more comments on the limitation and application of Equation 1. Please add some comments regarding the following issues.

- If you use Γ = 0.9, D will be increased by 19 times. Is this possible?

- Γ_PRS, φ_PRS and φ_GND all need to be determined by simulation. The PRS is in the near field of the antenna and the ground plane is in the near field of the wave reflected from the PRS.

- What are the parameters controlling Γ_PRS  and φ_PRS? The dielectric constant and the thickness of the PRS dielectric material?

- It appears that the FPC antenna needs to be designed by parametric analysis.

3. Figures 2a and 5a:

The rectangular patch has a different aspect ratio (width to height). Please check and make corrections if applicable.

4. Lines 107 to 110: " To implement the antenna along with the RF circuit realized on the PCB, both antennas were fabricated on an FR4 substrate, which is known to have a relatively high loss tangent of 0.02. Due to this material characteristic, the realized gain of both the Tx and Rx antennas is limited to approximately 4 dBi at operation frequency, even though each antenna is implemented as a 2×1 patch array."

The PCB in Figure 5a does not look like being made of an FR4 substrate. Please check and make corrections if applicable.

5. Please add S11 and S21 with the PRS similar to Figure 2.
6. Figure 5 and Lines 111-113: "To improve the antenna performance without expanding the physical size of the antenna or increasing the number of elements, we introduced the PRS with a dielectric constant of 4.3, forming an FPC antenna structure."

Please specify the material (commercial?) type, the loss tangent, and the thickness.

7. Lines 114 to 117: "Since the PRS has a reflection phase of −150°, it was placed above the patch antenna with a spacing of 2.7 mm, which was calculated according to Equation (2). Moreover, since the PRS has a reflection coefficient of 0.6, a gain enhancement of approximately 6 dB can be theoretically and ideally predicted according to Equation (1)."

Please add some comments on how to calculate the reflection phase and magnitude of the PRS.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this manuscript, the author proposes a new kind of FPC antenna to increase antenna gain, which is beneficial for the vital sign detection at longer distance. I have some comments showing as follows.

[1] Can author clarify what each symbol stands for in Equation (1) in the manuscript? such as T(PRS).

[2] In the manuscript, the author points out the S-parameters shows nearly same w/o PRS. However, there are no results shown to support this claim. Could author add the corresponding results?

[3] In addition, Figure "5(b)" not "5(a)" presents the FPC Tx and Rx antennas with a 2.7 mm spacing. Please correct the typo in the manuscript.

[4] In the manuscript, the author points out the measured distance increased from 1.1 m to 2.5 m by using FPC antennas. However, No results support this claim. Could author add more results to show this detail? For example, the different heartbeat rate vs the ground truth under different distances when using conventional antennas and FPC antennas for comparison.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The paper proposes and experimentally demonstrates a bistatic 60 GHz FMCW radar for non-contact heart-rate monitoring that replaces 2×1 patch arrays at Tx and Rx with Fabry–Pérot cavity (FPC) antennas. The authors report simulated realized gains increasing from 4 dBi to 8 dBi per side and an overall link-budget improvement of 8 dB, which they show increases reliable detection distance.

The novelty is quite poor. Similar approaches in literature (M. S. Rabbani, J. Churm and A. P. Feresidis, "Fabry–Perot Beam Scanning Antenna for Remote Vital Sign Detection at 60 GHz," in IEEE Transactions on Antennas and Propagation, vol. 69, no. 6, pp. 3115-3124, June 2021, doi: 10.1109/TAP.2021.3049233; Mingle, S.; Kampouridou, D.; Feresidis, A. Multi-Layer Beam Scanning Leaky Wave Antenna for Remote Vital Signs Detection at 60 GHz. Sensors 2023, 23, 4059. https://doi.org/10.3390/s23084059).

The following points should be discussed by the authors:

• Use of FR-4 at 60 GHz is questionable for high-gain claims. FR-4 has high loss tangent and poorly controlled ε at 60 GHz; realized efficiency and absolute gain are very sensitive to substrate loss and PRS material loss/tolerance. The paper notes FR-4 was used but does not provide measured radiation efficiencies, measured aperture efficiency, or measured realized gain vs. angle (only stated realized dBi numbers). Without efficiency and pattern plots (measured 2D cuts, sidelobe levels, beamwidth), the gain claims are incomplete.

• Limited experimental detail: The manuscript reports detection-range improvement and shows waveform traces, but omits important measurement/statistics: number of subjects, repetitions, SNR as a function of range, noise figure of receiver chain, ADC specs, exact transmit power, and measured EIRP, environmental conditions (clutter reflectors, orientation), and measurement uncertainty. A correlation with the true heart rate pulse shall be shown.

• The use of the simplified formula for directivity enhancement (Equation (1) in the manuscript) is fine as first order, but the paper should discuss finite aperture effects, PRS reflection amplitude & phase dispersion across the bandwidth, and off-broadside behavior — these matter for real FMCW chirps (bandwidth) and for human targets with slight misalignment or movement. Multi-frequency behavior of FPC (gain vs frequency across chirp) should be provided.

• The system uses bistatic FMCW, but the paper does not quantify advantages/ versus disadvantages vs common modern options for vital signs: CW Doppler with beam scanning, MIMO FMCW with digital beamforming, or multi-antenna coherent combining. Recent works show MIMO FMCW and advanced algorithms (e.g., MUSIC/DR-MUSIC, wavelet decomposition, HRV extraction methods) can significantly improve robustness and range — combining high-gain antennas with such methods could provide larger gains than antenna gain alone.

• The paper lacks any geometrical dimension of the proposed antennas and PRS, consequently, the presented results are not reproducible.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

In my mind, the table 2 in the revised manuscript belongs to summary, which is not sufficient enough to support the outstanding performance of the proposed antenna compared with conventional antennas. Authors need to show more solid experiment results to support their conclusion. Furthermore, Figure including S-parameters w/o PRS won’t make the figure complicated. However, it will be clear and intuitive to make the reader to understand the author’s claim.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors, thanks for accepting the given suggestions and for the clarifications expressed in the reviewed version of the manuscript. 

Comments on the Quality of English Language

no specific comments about quality of english language

Author Response

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Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

Thanks for author's effort. I don't have more comments on the revised manuscript.