Review Reports - Hybrid Radio-Frequency-Energy- and Solar-Energy-Harvesting-Integrated Circuit for Internet of Things and Low-Power Applications

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript presents a compact hybrid energy-harvesting integrated circuit (IC) that combines radio-frequency (RF) and solar energy sources for low-power Internet of Things (IoT) applications. Developed using a 0.18 µm CMOS process, the design integrates multiple critical blocks including a cross-coupled differential drive rectifier (CCDDR), a Dickson charge pump, a DC voltage combiner, an overvoltage protection circuit, and a low-dropout (LDO) voltage regulator. Among the paper’s strengths are the minimal chip area (0.050625 mm²), which is among the smallest reported in comparable literature, and a reasonably high RF rectification efficiency of 41.6% across a broad input range from −1 to 20 dBm.
The manuscript also includes a well-structured architectural breakdown and simulation-based validation of the system’s key components. The dual-source approach (RF + solar) is relevant and timely in the context of self-sustaining, battery-free embedded electronics. In particular, the modular and integrated nature of the design makes it well-suited for miniaturized IoT devices with limited access to conventional power sources.
However, despite the solid architectural basis, the work would benefit from several improvements to broaden its scientific and practical impact. Specifically, the authors must address the following observations:
1) The paper is entirely simulation-based. Do you intend to build and test a working prototype to experimentally verify the behaviour and efficiency of the system?
2) The manuscript does not evaluate how variation in light intensity, temperature or RF noise affects the operation of the system. Could you provide data or a planned methodology to evaluate the robustness of the system under real environmental conditions?
3) The dynamic interaction between RF and solar inputs is not explored. Would it be possible to characterise the contribution of each source under fluctuating input conditions or switching scenarios?
4) Although RF rectifier efficiency is competitive, more recent work has achieved 76.1% using DC adder topologies. Have you considered alternative combination techniques to improve the overall efficiency of power conversion?
5) No evaluation of system performance under variable or intermittent load conditions, a critical issue for IoT devices, is provided. Have you explored adaptive control mechanisms or feedback loops to maintain stability under dynamic loads?
6) The paper mentions a 10% loss due to the balun but does not quantify its impact on system efficiency. Could you elaborate on this and possibly present an analysis of simulated loss versus practical loss?
7) Given the proximity of wearable IoT devices to human tissue, it is crucial to assess electromagnetic safety. Have you considered integrating SAR impact and temperature analysis? The authors should include this issue in the paper and cite the study doi:10.3390/app15052439 in the bibliography because it is important to assess the feasibility of the system for wearable or implantable use.
8) The manuscript could benefit from a discussion of intelligent energy management (e.g., soft computing or adaptive control). Is the inclusion of optimisation algorithms to maximise energy harvesting in unpredictable environments envisaged?
9) No cost model or industrial scalability assessment is provided. What are the estimated costs per unit in a production scenario? Can the project be scaled to large installations or consumer markets?

Comments on the Quality of English Language

While the manuscript is generally understandable, it contains numerous grammatical inconsistencies, awkward phrasing, and structural issues that affect clarity. For example:

Sentences often have redundant or unclear technical wording.

There are several instances of poorly constructed transitions and overly long technical explanations.

Terminology is sometimes misused or inconsistently applied (e.g., switching between “conversion efficiency” and “power efficiency” without clarification).

Some figures and captions lack precise technical labeling, which creates ambiguity.

These issues do not obscure the overall content but do hinder the smooth communication of the research. A thorough language and style edit would enhance readability and professionalism.

Author Response

Please see the attached file.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors, in the next paragraphs my comments about your manuscript.

Although not very clear, the opinion suggests that the paper might have a legitimate technical contribution, particularly an experimental realization of a high-efficiency DC-DC converter. The simulated results, as well as experimental tests, are given in the paper, an advantage in electronic engineering work, as theory is confronted with experimental practice. Somewhat importantly, converter efficiency is a topic relevant to present-day applications, especially embedded systems and IoT, power management being of particular interest.

The paper follows a classical scientific structure (introduction, methodology, results, and conclusions), which means that the authors are aware of the conventions of scientific communication.

Points for improvement

The comments hold that authors do not provide a technical background regarding the converter topology choice. A technical comparison with alternatives (e.g., Boost, Flyback, SEPIC) should be given to elucidate why such an approach was selected. By not engaging in such a discussion, the scientific contribution of the work is significantly weakened.
The result analysis, whether simulated or experimental, is uncritical and lacks depth. For example: the efficiency calculation is not entirely clear; any deviation between simulation and practice has not been accounted for; moreover, the early presentation of data (in tables) is very confusing and poorly organized. The simulated data are there and so forth, but the critique thereof is missing.
Benchmarks have been omitted: the developed converter was never compared with those already existing in the literature or in the market. Such comparisons would be relevant to justify its technical advancement.
Without a proper description of the circuit design, including topology, components, and operating parameters, it becomes impossible to reproduce the work, which is an essential criterion in a scientific manner.
There should be improvements made in the layout of wording, clarity of expression with a technical bent, and the layout of tables. These issues, although formal, greatly affect the readability and comprehension of the article.
The experimental work is a strong point but yet must be better contextualized.
Lacking comparison with others' works varies in justification for relevance and progress.

Author Response

Please see the attached file.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This paper proposes a hybrid energy-harvesting chip that integrates an RF energy acquisition system with a solar photovoltaic cell, and demonstrates the effectiveness of the design. The following comments are provided for improvement:

The abstract currently only describes the system architecture and simulation results, without highlighting the key technical innovations. It should first emphasize the significance of the study and the limitations of existing research, followed by how the proposed system addresses these issues. The technical contributions of this work and its potential applications should also be clearly stated.
The introduction lacks a clear logical flow. The authors begin by discussing applications, then shift to the fundamentals of RF integrated circuit design, followed by a discussion of HEHSs, CMOS systems, and RF power amplifiers, before introducing the hybrid RF/solar system. This sequence is confusing. After explaining the fundamentals of RF ICs, the authors should first elaborate on RF circuits, and then transition to systems built upon them, such as HEHSs and CMOS-based solutions.
The introduction should include a critical review of prior work. The authors are encouraged to assess the limitations of existing studies and explain how their system addresses these challenges. Moreover, the introduction should include a brief discussion of the proposed system’s potential applications.
In Equations (2) and (3), the term Re{Y_in} should be clarified. The authors should define the meaning of the real part operator “Re” in this context and specify how it is computed.
In Equations (5) and (6), the symbols B₁, B_L, and G_L require clarification. The authors should explain what these parameters represent and provide the method for calculating them.
In Figure 4, numerical values such as 0.427, 3.102, 28.71, and 154.2 are presented without explanation. The rationale behind choosing these specific values should be clarified, and their derivation process should be briefly discussed.
In Figure 6, further explanation is needed to show how the threshold voltage V_th is canceled. The mechanism should be illustrated clearly.
Figures 20–23 require improvement in readability. The font sizes should be increased to enhance visibility. Additionally, for Figures 22 and 23, the x-axis labels and corresponding units should be included.
In Table 1, a new row should be added to include comments or remarks. This could provide a concise evaluation of the pros and cons of each listed work or configuration.
The conclusion should include a discussion of the limitations of the proposed system. Furthermore, the authors should briefly suggest directions for future improvements or possible solutions to the identified challenges.

Author Response

Please see the attached file.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I thank the authors for their replies to the comments. I have no further comments to ask.