Channel Capacity Analysis of Partial-CSI SWIPT Opportunistic Amplify-and-Forward (OAF) Relaying over Rayleigh Fading
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis paper presents a comprehensive analytical framework for evaluating the ergodic channel capacity of SWIPT-enabled opportunistic AF relaying systems under partial CSI conditions. The authors extend their previous work on error performance to capacity analysis, deriving both upper and semi-lower bounds and proposing a mean-capacity approximation. The work is well-motivated, technically sound, and addresses a relevant gap in the literature. The analytical derivations are rigorous, and the simulation results convincingly validate the proposed
- Insufficient Discussion on the Limitations of the High-SNR Assumption：The derivation of key approximate formulas (e.g., Eq. (7)) relies on the high-SNR assumption (i.e., γr​≫1). However, in practical SWIPT systems, relay nodes often operate in the low-SNR regime (due to low energy harvesting efficiency), where this assumption may no longer hold.The approximation in Eq. (7), is valid at high SNR but may introduce non-negligible errors at low SNR.
2.Complex and Inconsistent Notation System
The paper extensively uses nested summations and omitted notations (e.g., ∑i,j,k,l​ in Eqs. (9), (10), and (12), and symbols like γ~il,β~ij​), and the definitions of these symbols are inconsistent across sections.
For example，Eq. (10) define , but subsequent equations (e.g., Eq. (12)) introduce γ~i′′γ~​i′′​ without clearly explaining its relationship to γ~ilγ~​il​.
The indices j,k,l are frequently "omitted for simplicity," making it difficult for readers to trace their physical meanings.
Add a notation table in the appendix or the beginning of the paper to clarify the definition and range of each variable.
3. Insufficient Comparison with Existing Capacity Analysis Works
The paper does not sufficiently compare its results with existing literature on SWIPT capacity analysis, especially recent studies on nonlinear energy harvesting models or partial CSI scenarios.
4.Neglect of Practical System Factors
The analysis overlooks several practical system factors, which may lead to discrepancies between the theoretical model and real-world scenarios.All noise components are assumed to be independent and identically distributed (i.i.d.), ignoring non-ideal factors such as RF chain nonlinearities and phase noise.The energy harvesting efficiency η is assumed constant, while in practice, it is often nonlinearly related to input power.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe presented paper is devoted to the issues related to the effective selection of a radio transmitters installed in a UAVs. Authors investigated the channel capacity performance of SWIPT OAF relaying systems over Rayleigh fading channels under partial CSI-based relay selection. Building on the previous analytical framework for generalized AF relays, approximate but accurate closed-form expressions for the capacity of indirect and combined links are derived. The proposed approximation of the mean capacity, falling between analytically feasible lower and upper bounds, is demonstrated through extensive simulations to closely reflect real-world capacity performance under various SNR regimes and channel conditions. Presented simulation results reveal that relay selection based on the SR link significantly outperforms RD-based selection in terms of capacity, owing to its direct impact on energy harvesting efficiency at the relay. Additionally, selection diversity gains manifest primarily as SNR improvements rather than diversity order increases, aligning with similar observations in error performance analyses. A brief literature review was also conducted, based on which specific channel capacity assessment methods were selected for comparison. The paper was written in an accessible manner, using relatively simple technical language, which will undoubtedly increase its readership. To improve the quality of the article, a few minor corrections and extensions are recommended:
- The paper title should be rewritten to be more meaningful and concise, with fewer abbreviations, especially those that are not further developed in the abstract.
- The abstract should remove the unnecessary, lengthy theoretical introduction and replace it with a presentation of calculation and simulation results.
- The calculation algorithm should be provided in pseudocode or a flowchart, highlighting the formulas presented in the analytical section of the article. This will improve the readability of the graphs in relation to the data contained in the tables.
- At the beginning of Chapter 5, the reasons for selecting specific parameter values ​​should be clearly described. Units should be provided, or if they are missing, references should be provided. This must be clear. For example, the channel capacity graphs do not use units, which is unusual for this type of transmission channel parameter. The situation is similar for the power shown in Table 1 (mW, or perhaps dBm?), and so on. This needs to be clarified.
- At the end of Chapter 5, please add, for example, a subsection entitled “Final discussion of results” and present there a summary discussion of the calculation and simulation results.
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsDependence on Prior Work ([36] and [37])
1) The paper assumes familiarity with [36] and [37], referencing them extensively for key derivations, definitions, and even simulation setups.
2) Critical expressions (e.g., for SNR approximations, relay selection probabilities, and capacity bounds) are not fully re-derived or explained in the current paper, making it difficult for readers unfamiliar with the prior work to follow the logic.
3) The relay selection algorithm, PS optimization strategy, and transformation from SWIPT to non-SWIPT models are all rooted in [36]/[37], but are only briefly
4) The paper could benefit from a dedicated background section that concisely revisits the key results from [36] and [37], especially the semi-lower bound derivation and the generalized approximation framework.
5) Including visual aids or flow diagrams to show how the current work extends the previous models would improve clarity.
6) The assumption of mutually independent Rayleigh fading channels is standard in theoretical wireless communication studies, but real-world channels often exhibit correlation, shadowing, or follow different fading distributions (e.g., Rician, Nakagami-m).
7) This limits the generalizability of the results to environments like urban or indoor IoT deployments where multipath effects and channel correlation are significant.
8) Fixed Initial Power Splitting Ratio (ρ = 0.5):
While analytically convenient, this uniform initialization may not reflect optimal or realistic energy harvesting conditions across heterogeneous relay nodes.
In practice, relays may have different energy harvesting capabilities, battery states, or channel conditions, making a fixed ρ suboptimal.
9) The model assumes two time slots for transmission and reception, which inherently reduces spectral efficiency.
10) Many modern systems are moving toward full-duplex relaying or hybrid schemes to overcome this limitation. The paper does not explore how its framework might adapt to such setups.
11) The selection is based on partial CSI and assumes ideal identification signaling and estimation. In real deployments, estimation errors, feedback delays, and hardware constraints could degrade performance.
Suggestions for Improvement:
1) Re-derive or summarize essential equations from [36] and [37] within the main text or appendices.
2) Provide a comparative table showing how the current approximations differ from or improve upon those in prior work.:
3) Clearly indicate which sections require prior knowledge and suggest reading order (e.g., “Readers unfamiliar with [36] are encouraged to review Section X of that paper before proceeding”).
4) Add illustrative examples or simplified scenarios to demonstrate the impact of the new approximation.
5) Include a conceptual diagram showing the relationship between SWIPT, non-SWIPT, and generalized models.
6) Consider alternative fading models (e.g., Rician, Nakagami-m) and correlated channels to better reflect real-world conditions.
7) Explore adaptive or distributed PS initialization strategies that reflect relay heterogeneity.
8) Discuss the potential extension of the framework to full-duplex systems, including the impact on capacity and relay selection.
9) Include a sensitivity analysis showing how deviations from the assumptions (e.g., ρ ≠ 0.5, imperfect CSI) affect performance.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for AuthorsThe manuscript comprehensively presents mathematical expressions for the for evaluating the channel capacity of simultaneous wireless information and power transfer (SWIPT) opportunistic amplify-and-forward (OAF) relaying systems over Rayleigh fading channels. Authors can consider the following improvements or betterment of the manuscript:
- Modify the abstract to include quantitative observation in your simulation
- Similar can be followed in Conclusion section.
- Authors can include a mathematical block diagram/ Pseudo-code/ flow diagram as a part of mathematical model/ background.
- References are not in sequence manner in the manuscript.
- ρ is optimized for asymptotic BER minimization and is used in a capacity study. Why not optimize ρ for capacity directly. Will it affect the outcomes?
There are some typo and repeat of subject themes, Authors can go thought the manuscript before final submission.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
Comments and Suggestions for AuthorsI am happy with the changes
