REVIEW 4 major objections 4 minor 35 references
Energy Efficiency Analysis of Active RIS-enhanced Wireless Network under Power-Sum Constraint
T0 review · 4 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read The paper derives an energy-efficiency crossover: active RIS wins below roughly 1024 elements, passive RIS wins above.
desk verdict A real crossover question and the right large-N frame, but the PA-factor derivation has a load-bearing algebraic error (q4 is dimensionally wrong and reused), so N0≈2^10 is not reproducible; worth refereeing as a major-revision candidate. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The argument is carried by a law-of-large-numbers asymptotic approximation: sums over the $N$ RIS reflection elements are replaced by their expectations, turning the SNR and total power consumption into deterministic closed forms, Eq. (25) for active RIS and Eq. (74) for passive RIS. With these forms, equality of the two efficiencies becomes a single nonlinear equation in $\alpha=1/N$, and the paper applies Newton's method, bisection, and simulated annealing to locate its root, which is the crossover $N_0$.
What would settle it
Run an exact Monte Carlo simulation that keeps the direct-path amplitude $|h|$ as a random variable (no mean replacement) for $N$ near $10^3$, with the paper's parameter set, and check whether the active-versus-passive EE crossover still lies at $N_0\approx 2^{10}$; a shift of more than a few hundred elements would falsify the threshold as stated.
Extended reading notes
Core claim
The central discovery is a quantitative threshold for the active-versus-passive RIS energy-efficiency trade-off. Under a power-sum constraint, where the base station and the RIS share a fixed total power, the paper shows that active RIS energy efficiency first rises and then falls as the number of elements $N$ grows, because the amplifier adds noise whose impact scales with $N$, while passive RIS efficiency rises monotonically. Solving $\mathrm{EE}_{\mathrm{active}}(N)=\mathrm{EE}_{\mathrm{passive}}(N)$ with $N$ as the variable yields a unique crossover $N_0\approx 2^{10}$; for $N<N_0$ the active RIS achieves higher EE, and for $N>N_0$ the passive RIS does. The comparison is made on the same total power budget, and the asymptotic EE expressions agree with exact Monte Carlo simulation for medium and large $N$.
Load-bearing premise
The derivation replaces the single direct-path channel amplitude $|h|$ by its mean value, but $|h|$ is one random draw and does not average out as $N$ grows, so the crossover location could shift if this averaging is inaccurate for medium-scale $N$.
Editorial extensions
If this is right
- Below roughly 1024 RIS elements, deploying an active RIS yields higher energy efficiency than a passive one on the same total power budget.
- Above the crossover, passive RIS becomes the greener choice, so RIS size alone can drive the active/passive decision.
- The optimal power allocation factor $\beta$ has a closed form derived from a first-order Taylor expansion and Ferrari's method, letting operators set BS-versus-RIS power splitting without search.
- Increasing total power $P_t$ beyond its optimum reduces EE, since capacity grows logarithmically while power grows linearly.
- As the noise variances at the RIS and the user shrink, EE saturates at a constant value rather than growing without bound.
Reading between the lines
- The crossover $N_0\approx 2^{10}$ is reported for one parameter set (noise variances at $-70$ dBm, specific path-loss exponents); under a different noise budget or power level the threshold would likely move, so the useful output is the crossing mechanism rather than the exact number.
- The same equal-EE equation could be solved for the total power $P_t$ or noise variance instead of $N$, giving complementary deployment thresholds.
- Because the law-of-large-numbers step averages the direct-path amplitude, a finite-$N$ exact analysis that avoids that averaging would be a sharper test of the crossover; the paper's own asymptotic-versus-exact agreement for medium and large $N$ suggests the error is small there but is not shown for the crossover region.
- The framework could extend to discrete phase shifters, imperfect channel knowledge, or multi-antenna nodes, where the amplification-noise penalty of active RIS would interact with those effects.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper analyzes the energy efficiency (EE) of an active RIS-assisted wireless network under a power-sum constraint. Using the law of large numbers, it derives an asymptotic EE expression as a function of the power allocation factor β, the number of RIS elements N, total power Pt, and noise variances σr² and σu². It then studies the effect of each parameter, provides a closed-form expression for the optimal β via Taylor expansion and Ferrari's method, and defines a crossover equation between the EE of active and passive RIS systems. Three root-finding algorithms (Newton, bisection, simulated annealing) are proposed, and the paper claims that active RIS outperforms passive RIS for N below about 2^10 and is worse above that threshold.
Significance. If the crossover result is correct, it would provide a simple and practically useful deployment rule for choosing between active and passive RIS. The paper has genuine strengths: the derivation is not circular, no parameter is fitted to force the threshold, exact and asymptotic EE expressions are compared in simulation, and the proposed numerical methods are standard and clearly described. However, the central claim is built on approximations whose validity conditions are violated at the paper's own parameters, and several coefficient definitions in the derivation are internally inconsistent. These issues prevent the reported N0≈2^10 from being reproduced from the manuscript as written, so the significance of the claimed threshold cannot yet be assessed reliably.
major comments (4)
- [Eqs. (27)–(28)] The definition q4 = -A5 N Pt in Eq. (28) is internally inconsistent. In the total-power denominator q4β + q9 of Eq. (27), the β coefficient must be -A5 N Pt, as follows from Eq. (23). But inside the square root q4β² + q5β + q6, the β² coefficient extracted from Eq. (25) is -A5 N Pt². A single symbol q4 cannot have both values. Because the subsequent coefficients c1, c2, l1–l5 in Eqs. (31)–(36) use q4 through q3q4/(2√q6), the closed-form optimal β is wrong. This inconsistency must be corrected and all derived coefficients re-derived before the crossover computation can be trusted.
- [Eqs. (29)–(30)] The Taylor expansions are not valid at the paper's operating point. With Pt = 26 dBm, σr² = σu² = -70 dBm, and the stated path-loss exponents, the corrected small parameter satisfies Δx ≈ -39.3β² + 38.3β, which is about 4 at the reported optimum β ≈ 0.85; the argument of the logarithm is γ ≈ 10^4, so the first-order expansion log2(1+γ) ≈ γ/ln2 is grossly invalid. The closed-form β obtained from Ferrari's method is therefore not justified, and any 'Taylor approximate' curve in Fig. 3 is not a controlled approximation. Since the crossover equation (56) depends on β, the claimed threshold N0 ≈ 2^10 is not established by the manuscript. In addition, Eq. (29) as printed drops the factor √q6, so the equality '√q6(1+…) = 1+…' is dimensionally wrong; the correct form is √q6 + (q4β² + q5β)/(2√q6).
- [Eq. (50)] The coefficient m2 in Eq. (50) omits the factor βPt that multiplies the square-root term in Eq. (25). In Eq. (49), the cross-product term is βPt A4√(N X)/Dden, with X = A5βPt²(1−β) − βPtσr² + Ptσr², but m2 as printed is A4√X/Dden. This gives m2 the dimension of 1/W while m1 and m3 are dimensionless, and it underweights the cross term by a factor βPt in the EE comparison (56), so the computed root N0 shifts even if all other equations are corrected.
- [Section VI / Fig. 8] The parameters needed to reproduce the claimed N0 ≈ 2^10 are not given. Section VI specifies geometry, path-loss exponents, and μ, but not the bandwidth B, the hardware power constants Pc,n, P0,RIS, P^p_c,n, and P^p_0,RIS, nor the value of β used when evaluating EEa(N) in Eq. (56). Since the active-RIS total power in Eq. (23) and the crossover equation depend on these quantities, a reader cannot verify Fig. 8 or the central threshold claim from the manuscript as written.
minor comments (4)
- [Eq. (22)] The single direct-path amplitude |h| is replaced by its expectation, although there is only one realization of this Rayleigh random variable and the law of large numbers does not apply to it. For the stated geometry and N ≈ 1024 this term is very small relative to the RIS term, so this is not the main defect, but the approximation should be justified or bounded, especially for smaller N.
- [Eq. (12)] The total power model subtracts the received power Pin from the reflected power Pout and then adds μβPt + Pc. For some parameter values this expression can become negative, so the physical interpretation of the power model should be clarified.
- [Fig. 2] The legend contains the typo 'Asympototic'; it should read 'Asymptotic'.
- [References] The simulated-annealing method is attributed to reference [24], which is a paper on power allocation and beamforming; a standard numerical-analysis textbook reference would be more appropriate for this classical method.
Circularity Check
No significant circularity: the active/passive EE crossover N0 is solved from independently derived formulas, not fitted or defined into existence.
full rationale
The derivation chain is self-contained. Section III derives the asymptotic SNR and EE from the system model (Eqs. (1)-(14)) by applying the law of large numbers and the Rayleigh moment formulas (Eqs. (19)-(24)), so Eq. (25) is a closed-form consequence of the stated model rather than an imported prediction. The PA factor beta in Section IV-A is produced by optimizing the paper's own Taylor-approximated EE through Ferrari's method, not by calibrating a parameter to the threshold N0. The crossover in Section V is obtained by equating two separately derived EE functions, EEa(N) in Eq. (49) and EEp(N) in Eqs. (51)-(54), and solving Eq. (56); no parameter in m1-m9 is fitted to make the root equal 2^10. The only notable self-citation is Ref. [26] for the uniform active-RIS amplification factor in Eq. (13), but that assumption is explicitly stated and comes from a peer-reviewed prior work; the threshold is not forced by that citation. Reproducibility weaknesses (e.g., the Taylor expansion in Eq. (29), omitted hardware-power constants, and the unspecified beta used in Fig. 8) are correctness concerns, not circularity.
Assumptions & free parameters
free parameters (4)
- per-element RIS circuit power P_c,n =
not specified
- static power constants P_0 and P_0,RIS (combined as A7) =
not specified
- bandwidth B =
not specified
- power allocation factor beta in the crossover simulation =
not specified for Figs. 7-8
assumptions (7)
- standard math Sums of N independent channel products converge to their expectation as N grows (weak law of large numbers).
- domain assumption All channels are Rayleigh fading with parameters alpha_f, alpha_g, alpha_h and are mutually independent.
- domain assumption Continuous phase shifters at the RIS perfectly align all phases, and the direct-path phase is set to zero.
- domain assumption All active RIS elements share the same amplification factor lambda given by Eq. (15).
- ad hoc to paper The direct-path amplitude |h| in the SNR numerator can be replaced by its mean E|h| even though it is a single random variable.
- ad hoc to paper The Taylor expansions of the square root and the logarithm are valid because Delta x and gamma are small.
- domain assumption Total power consumption of the active RIS system is P_out - P_in + mu beta P_t + P_c, so received power reduces consumption.
Cite this review
Pith. "Pith review of Energy Efficiency Analysis of Active RIS-enhanced Wireless Network under Power-Sum Constraint." pith.science (2026). https://pith.science/paper/OO7BO7Y7
@misc{pith2026250602823,
author = {Pith},
title = {Pith review of: Energy Efficiency Analysis of Active RIS-enhanced Wireless Network under Power-Sum Constraint},
year = {2026},
howpublished = {\url{https://pith.science/paper/OO7BO7Y7}},
note = {Machine review of arXiv:2506.02823}
}
read the original abstract
Recently, as a green wireless technology, active reconfigurable intelligent surface (RIS) attracts numerous research activities due to its amplifying ability to combat the double-fading effect compared to passive one. How about its energy efficiency (EE) over passive one? Below, the EE of active RIS-aided wireless network in Rayleigh fading channels is analyzed. Using the law of large numbers, EE is derived as a function of five factors: power allocation factor, the number (N) of RIS elements, the total power, the noise variances at RIS and at user. To evaluate each factor's impact, the simple EE function for the concerning factor is given with others fixed. To assess the impact of N on EE, we establish an equation with the EE of active RIS equaling that of passive one, and three methods, bisection, Newton's method, and simulated annealing, are designed to find the roots of this equation. Simulation results show that as N tends to medium-scale or large-scale, the asymptotic performance formula is consistent with the exact EE expression well. As N varies from small-scale to large-scale, the active RIS intersects passive one at some point. When N< N_0, active RIS performs better than passive one in terms of EE. Otherwise, there is a converse conclusion.
Figures
Figures from the paper (5 more)
Reference graph
Works this paper leans on
-
[26]
Asymptotic performance analysis of large-scale active IRS-aided wireless network,
Y . Wang, F. Shu, Z. Zhuang, et al., “Asymptotic performance analysis of large-scale active IRS-aided wireless network,” IEEE Open J. Commun. Soc., vol. 4, pp. 2684–2696, Oct. 2023. 25
work page 2023
-
[27]
Active RIS versus passive RIS: Which is superior with the same power budget?,
K. Zhi, C. Pan, H. Ren, et al., “Active RIS versus passive RIS: Which is superior with the same power budget?,”IEEE Commun. Lett., vol. 26, no. 5, pp. 1150–1154, May. 2022
work page 2022
-
[31]
R. K. Fotock, A. Zappone and M. D. Renzo, “Energy efficiency optimization in RIS-aided wireless networks: Active versus nearly-passive RIS with global reflection constraints,”IEEE Trans. Commun., vol. 72, no. 1, pp. 257–272, Jan. 2024
work page 2024
-
[1]
J. Li, L. Xu, P. Lu, et al., “Performance analysis of directional modulation with finite-quantized RF phase shifters in analog beamforming structure,”IEEE Access, vol. 7, pp. 97457–97465, Jul. 2019
work page 2019
-
[2]
Performance analysis of massive hybrid directional modulation with mixed phase shifters,
R. Dong, B. Shi, F. Shu et al., “Performance analysis of massive hybrid directional modulation with mixed phase shifters,” IEEE Trans. on Veh. Technol., vol. 71, no. 5, pp. 5604–5608, May. 2022
work page 2022
-
[3]
Overview of IRS for 6G and industry advance,
R. Liu, K. Katsanos, Q. Wu, et al., “Overview of IRS for 6G and industry advance,”Intelligent Surfaces Empowered 6G Wireless Network, pp. 83–115, 2024
work page 2024
-
[4]
6G wireless communications networks: A comprehensive survey,
M. Alsabah, M. Naser, B. Mahmmod, et al., “6G wireless communications networks: A comprehensive survey,”IEEE Access, vol. 9, pp. 148191–148243, Jan. 2024
work page 2024
-
[5]
Q. Wu, R. Zhang, “Intelligent reflecting surface enhanced wireless network: Joint active and passive beamforming design,” 2018 IEEE Global Communications Conference, pp. 1–6, Feb. 2018. 24
work page 2018
Show all 35 references
-
[6]
Reconfigurable intelligent surface aided amplitude and phase-modulated downlink transmission,
Q. Li, M. El-Hajjar, I. Hemadeh, et al., “Reconfigurable intelligent surface aided amplitude and phase-modulated downlink transmission,”IEEE Trans. on Veh. Technol., vol. 72, no. 6, pp. 8146–8151, Jun. 2023
2023
-
[7]
Reconfigurable intelligent surfaces for energy efficiency in wireless communication,
C. Huang, A. Zappone, G. C. Alexandropoulos, et al., “Reconfigurable intelligent surfaces for energy efficiency in wireless communication,”IEEE T. Wirel. Commun., vol. 18, no. 8, pp. 4157–4170, Aug. 2019
2019
-
[8]
Towards smart and reconfigurable environment: Intelligent reflecting surface aided wireless network,
Q. Wu and R. Zhang, “Towards smart and reconfigurable environment: Intelligent reflecting surface aided wireless network,” IEEE Commun. Mag., vol. 58, no. 1, pp. 106–112, Jan. 2020
2020
-
[9]
Reflection resource management for intelligent reflecting surface aided wireless networks,
Y . Gao et al., “Reflection resource management for intelligent reflecting surface aided wireless networks,”IEEE Trans. on Commun., vol. 69, no. 10, pp. 6971–6986, Oct. 2021
2021
-
[10]
MIMO transmission through reconfigurable intelligent surface: System design, analysis, and implementa- tion,
W. Tang et al., “MIMO transmission through reconfigurable intelligent surface: System design, analysis, and implementa- tion,”IEEE J. Sel. Areas Commun., vol. 38, no. 11, pp. 2683–2699, Nov. 2020
2020
-
[11]
Performance analysis and simulation of IRS-aided wireless networks communication,
O. Dikmen, “Performance analysis and simulation of IRS-aided wireless networks communication,”Symmetry, vol. 16, no. 2, Mar. 2024
2024
-
[12]
Performance analysis of wireless network aided by discrete-phase-shifter IRS,
R. Dong, Y . Teng, Z. Sun, et al., “Performance analysis of wireless network aided by discrete-phase-shifter IRS,”J. Commun. Netw., vol. 24, no. 5, pp. 603–612, Aug. 2022
2022
-
[13]
Secrecy Performance Analysis of RIS-Aided Wireless Communication Systems,
L. Yang, J. Yang, W. Xie, et al., “Secrecy Performance Analysis of RIS-Aided Wireless Communication Systems,”IEEE Trans. on Veh. Technol., vol. 69, no. 10, pp. 12296–12300, Oct. 2020
2020
-
[14]
Performance analysis of discrete-phase-shifter IRS-aided amplify-and-forward relay network,
R. Dong, Z. Xie, F. Shu, et al., “Performance analysis of discrete-phase-shifter IRS-aided amplify-and-forward relay network,”J. Election. Inf. Techn., vol. 47, no. 1, pp. 138–146, 2025
2025
-
[15]
Network deployment with energy efficiency optimization in IRS-assisted cell-free MIMO networks,
H. Liu, N. Qi, K. Wang, et al., “Network deployment with energy efficiency optimization in IRS-assisted cell-free MIMO networks,”Physical Commun., vol. 63, Mar. 2024
2024
-
[16]
Enhanced secrecy rate maximization for directional modulation networks via IRS,
F. Shu et al., “Enhanced secrecy rate maximization for directional modulation networks via IRS,”IEEE Trans. Commun., vol. 69, no. 12, pp. 8388–8401, Dec. 2021
2021
-
[17]
Terahertz-band MIMO systems: Adaptive transmission and blind parameter estimation,
H. Sarieddeen et al., “Terahertz-band MIMO systems: Adaptive transmission and blind parameter estimation,”IEEE Commun. Lett., vol. 25, no. 2, pp. 641–645, Feb. 2021
2021
-
[18]
A path to smart radio environments: An industrial viewpoint on reconfigurable intelligent surfaces,
R. Liu, Q. Wu, M. Di Renzo, et al., “A path to smart radio environments: An industrial viewpoint on reconfigurable intelligent surfaces,”IEEE Wireless Commun., vol. 29, no. 1, pp. 202–208, 2022
2022
-
[19]
Physics-based modeling and scalable optimization of large intelligent reflecting surfaces,
M. Najafi, V . Jamali, R. Schober, et al., “Physics-based modeling and scalable optimization of large intelligent reflecting surfaces,”IEEE Trans. Commun., vol. 69, no. 4, pp. 2673–2691, Apr. 2021
2021
-
[20]
RIS-assisted green secure communications: Active RIS or passive RIS?,
W. Lv, J. Bai, Q. Yan, et al., “RIS-assisted green secure communications: Active RIS or passive RIS?,”IEEE Wireless Commun. Lett., vol. 12, no. 2, pp. 237–241, Feb. 2023
2023
-
[21]
Active RIS vs. passive RIS: Which will prevail in 6G?,
Z. Zhang, L. Dai, X. Chen, et al., “Active RIS vs. passive RIS: Which will prevail in 6G?,”IEEE Trans. Commun., vol. 71, no. 3, pp. 1707–1725, Mar. 2023
2023
-
[22]
Joint transmit and reflective beamforming design for active IRS-aided SWIPT systems,
W. Shi, Q. Wu, F. Shu, et al., “Joint transmit and reflective beamforming design for active IRS-aided SWIPT systems,” Chinese J. Electron., vol. 33, no. 2, pp. 536–548, Mar. 2024
2024
-
[23]
Power allocation for artificial-noise secure MIMO precoding systems,
S. Tsai, and H. Poor, “Power allocation for artificial-noise secure MIMO precoding systems,”IEEE Trans. on Signal Process., vol. 62, no. 13, pp. 3479–3493, Jun. 2014
2014
-
[24]
Joint power allocation and beamforming design for active IRS-aided secure directional modulation systems,
Y . Zhao, X. Wang, F. Shu, et al., “Joint power allocation and beamforming design for active IRS-aided secure directional modulation systems,”IEEE Open J. Commun. Soc., vol. 6, pp. 2853–2865, Nov. 2025
2025
-
[25]
Beamforming and transmit power design for intelligent reconfigurable surface-aided secure spatial modulation,
F. Shu et al., “Beamforming and transmit power design for intelligent reconfigurable surface-aided secure spatial modulation,”IEEE J. Sel. Topics Signal Process, vol. 16, no. 5, pp. 933–949, Aug. 2022
2022
-
[28]
Energy-efficient encoding for RIS-assisted communication system under measurement- based power consumption: Method and field trials,
S. Jian, L. Jifeng, L. Xiao, et al., “Energy-efficient encoding for RIS-assisted communication system under measurement- based power consumption: Method and field trials,”China Communications, vol. 22, no. 4, pp. 281–295, Apr. 2025
2025
-
[29]
Energy efficiency in RIS-assisted wireless networks: Impact of phase shift and deployment,
D. Jia, Y . Zhong, X. Zhou, et al., “Energy efficiency in RIS-assisted wireless networks: Impact of phase shift and deployment,”IEEE Wireless Communications and Networking Conference, pp. 1–6, 2024
2024
-
[30]
DRL-based energy efficient resource allocation for STAR-RIS assisted coordinated Multi-cell networks,
J. Chen, Z. Ma, Y . Zou, et al., “DRL-based energy efficient resource allocation for STAR-RIS assisted coordinated Multi-cell networks,”IEEE Global Communications Conference, pp. 4232–4237, 2022
2022
-
[32]
All of statistics: A concise course in statistical inference,
L. Wasserman, “All of statistics: A concise course in statistical inference,”New York, NY, USA: Springer, 2004
2004
-
[33]
Two enhanced-rate power allocation strategies for active IRS-assisted wireless network,
Q. Cheng, R. Dong, W. Cai, et al., “Two enhanced-rate power allocation strategies for active IRS-assisted wireless network,” International Conference on Computer Communication and Artificial Intelligence, pp. 458–463, 2024
2024
-
[34]
Numerical analysis,
G. Walter, “Numerical analysis,” Dec. 2013
2013
-
[35]
Current minimizing torque control of the IPMSM using Ferrari’s method,
S.-Y . Jung, J. Hong, and K. Nam, “Current minimizing torque control of the IPMSM using Ferrari’s method,”IEEE Trans. Power Electron., vol. 28, no. 12, pp. 5603–5617, Dec. 2013
2013
Reviewed August 7, 2026 · model on record in the stance chip above.
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