REVIEW 3 major objections 5 minor 26 references
Dual RIS-Assisted Monostatic L-Band Radar Target Detection in NLoS Scenarios
T0 review · 3 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read Two large RISs beat one RIS for radar detection behind obstacles.
desk verdict Routine extension of a single-RIS radar model to two RISs, with a central crossover claim that is invalidated by phase inconsistencies, unphysical independent forward/return phase shifts, and simulations run outside the model's far-field validity. 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 central object is the two-hop channel Radar→RIS-1→RIS-2→Target→RIS-2→RIS-1→Radar. The derivation models each RIS as an array of tunable unit cells and introduces per-unit-cell complex weights Wj,k and Wm,n that absorb the reflection coefficient, unit-cell radiation pattern, path distance, and phase. The received power then equals a product of two fourth-power array-factor sums, |ΣWj,k|^4 |ΣWm,n|^4 (Eq. 13), which under far-field and maximum alignment collapses to the closed-form SNR of Eq. (15) with element counts entering as $J^{4}$$K^{4}$$M^{4}$$N^{4}$ and the three hop distances entering as $r1^{4}$ $r2^{4}$ $r_RIS^{4}$.
What would settle it
Compute the exact near-field sum for the RIS-1-to-RIS-2 link at L-band with r_RIS = 50 m, keeping every unit-cell distance distinct, and compare the resulting dual-RIS SNR against the single-RIS baseline for 37×37 and 46×46 elements; if the SNR advantage disappears, the far-field factorization in Eq. (13) is what failed.
Extended reading notes
Core claim
The paper's central claim is that the SNR of a perfectly aligned dual-RIS monostatic radar, with the signal bouncing radar→RIS-1→RIS-2→target and back, can exceed the SNR of a single-RIS system, provided each RIS is large enough. The evidence is the derived closed-form SNR expression (Eq. 15), which grows like the fourth power of the element count of each RIS divided by the fourth power of the radar-to-RIS-1, RIS-1-to-RIS-2, and RIS-2-to-target distances. Under the simulation parameters—L-band, an inter-RIS distance of 50 m, and unit-cell spacing of λ/2—the dual-RIS configuration gives higher SNR for RIS sizes of 37×37 and 46×46 elements, while 10×10 and 19×19 surfaces lose to the additional path loss of the longer double-hop route. The paper interprets this as an 'RIS effect' that turns positive only beyond a size threshold, and concludes that adding RISs helps when the cumulative array gain outweighs the multiplicative path-loss penalty.
Load-bearing premise
The derivation assumes the two RISs are in each other's far field, so all unit-cell-to-unit-cell distances and angles collapse to one inter-RIS distance r_RIS; in the simulations r_RIS is only 50 m while the large RIS sizes that produce the claimed advantage have far-field distances of about 147 m and 227 m.
Editorial extensions
If this is right
- At L-band, upgrading both RISs from 10×10 to 37×37 elements flips the comparison from single-RIS-favorable to dual-RIS-favorable, so the crossover is a design target rather than a universal property.
- Doubling the number of elements along each axis of both RISs multiplies the SNR by 2^16 according to Eq. (15), so the formula predicts extremely steep returns to RIS size.
- The fourth-power distance dependence means the inter-RIS and RIS-to-target distances dominate the SNR; a small increase in the gap between the RISs costs far more than the same increase in transmit power can recover.
- The model identifies a threshold number of unit cells per RIS for a given geometry; below it, a second RIS is counterproductive, above it, it improves detection.
Reading between the lines
- Beyond the paper: the far-field assumption between the two RISs is violated for the very sizes that produce the claimed crossover (4 m and 5 m RISs have far-field distances near 147 m and 227 m, but the simulation places them 50 m apart), so the factorized Eq. (13) and the crossover curves in Fig. 2 should be treated as predictions that need a near-field check.
- Beyond the paper: the same product-of-array-factors structure suggests the derivation extends naturally to more than two RISs; each additional RIS multiplies the SNR by another fourth-power array-gain factor divided by the fourth power of the new hop distance, so a chain of many small RISs might match one large RIS.
- Beyond the paper: the crossover element count should shrink at higher frequencies such as X-band because the same physical aperture contains more unit cells and the far-field distances are shorter, making dual-RIS setups practical for smaller surfaces.
- Beyond the paper: a direct experiment—two 4 m×4 m RISs at 50 m separation at L-band—would settle whether the predicted 0.84 dB RIS effect at 37×37 elements survives near-field coupling.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper derives closed-form expressions for the received power, SNR, and path loss of a monostatic L-band radar whose signal propagates through two RISs in an NLoS scenario (Radar–RIS-1–RIS-2–Target–RIS-2–RIS-1–Radar). The authors then compare the dual-RIS SNR with a single-RIS baseline from their prior work and claim, based on simulations, that dual RISs outperform a single RIS when the RIS sizes are sufficiently large, specifically for 37x37 and 46x46 elements at the configured geometry. The main contribution is the SNR formula in Eq. (15) and the crossover claim in Fig. 2.
Significance. If the derivation were sound, the paper would provide a useful first-order model for cascaded RIS-assisted radar and a concrete quantitative prediction about when adding a second RIS is beneficial. The problem is clearly motivated and the authors make their assumptions explicit, which is commendable. However, the central result rests on a far-field factorization that is not justified and is evaluated in a regime that contradicts the paper's own far-field distances. The internal phase inconsistency between Eqs. (10) and (11) further weakens the derivation. As it stands, the quantitative crossover claim is not established, so the paper does not yet provide a reliable design guideline.
major comments (3)
- [Section 2, Eqs. (10) and (11)] The factorization of the received power into |sum_{j,k} W_{j,k}|^4 |sum_{m,n} W_{m,n}|^4 is load-bearing for the entire paper, but it is not justified by the stated far-field approximation. The text immediately before Eq. (4) asserts that in the far field r_RIS,j,k = r_RIS,m,n for all elements, but equality of all element-pair distances is not a consequence of the far-field approximation; even for two finite apertures in the far field, the element-to-element distance contains a linear phase taper depending on both element indices. A standard far-field model can make this phase separable, and then the sums factor, but the paper does not present that derivation; it simply collapses every inter-RIS distance to a constant. Moreover, the simulation uses r_RIS = 50 m (Table 1) for RIS sizes whose far-field distances are 146.6 m and 226.6 m (Table 2), namely the 37x37 and 46x46 configurations that produce the claimed crossover. Thus Eq. (15) and Fig. 2 are evaluated in a regime where neither the stated assumption nor a separable far-field model is valid, and the central quantitative claim is unsupported.
- [Section 2, before Eq. (14)] There is an internal inconsistency in the core derivation. The first sum in Eq. (10) contains the phase term exp(-j(2*pi*r_r,j,k/lambda - phi_1,j,k + 2*pi*r_RIS/lambda)), while W_{j,k} in Eq. (11) is defined with 4*pi*r_r,j,k/lambda, and the final SNR expression in Eq. (13) and Eq. (15) uses W_{j,k}. Since the factor 4*pi*r_r/lambda represents the two-way propagation between the radar and RIS-1, Eq. (10) should already contain that factor if it is the basis for Eq. (13). As written, the derivation does not connect Eq. (10) to Eq. (13). This is not a notational quibble: the phase is what determines the maximum-alignment condition that produces Eq. (14).
- [Section 3, Tables 1-2 and Fig. 2] The maximum-alignment condition theta_t = theta_RIS and phi_t = phi_RIS + pi is asserted without proof. This condition is nontrivial because the sums in Eq. (13) include the radiation-pattern factors F, which depend on angles, and the target is modeled only through an RCS that is treated as a scalar. It is not obvious that this angle assignment simultaneously maximizes the two independent sums in Eq. (13). The authors should either prove this condition or provide a reference; without it, Eq. (14) and the subsequent SNR-maximization result rest on an unverified assumption.
minor comments (5)
- [Section 3, Fig. 2] The single-RIS baseline SNR equation from [17] is not reproduced in the manuscript. Since the central comparison in Fig. 2 depends on that baseline, the authors should state the baseline equation and the parameter mapping used for the comparison so that the reader can verify the crossover is not an artifact of differing model assumptions.
- [Table 2] The vertical lines in Fig. 2 are described as the minimum distance required for the far-field assumption, but it is unclear whether they refer to the target-RIS distance or the inter-RIS distance. Since r_RIS is fixed at 50 m in Table 1, the vertical lines cannot refer to the inter-RIS link for the larger RIS sizes; please clarify.
- [Abstract] The quantity labeled 'RIS effect' is not defined anywhere in the paper. Please state its formula and explain how it is computed from Eq. (15) or the single-RIS baseline; otherwise the positive values for the 37x37 and 46x46 configurations are not interpretable.
- [Throughout] The abstract claims that 'the required accuracy in target localization can be achieved' by controlling the number of RISs and unit cells, but the paper contains no localization accuracy analysis; it derives SNR and path loss only. Please rephrase to avoid overclaiming.
- [Throughout] There are several typographical and notation issues, including 'rtn,m' in Eq. (6) instead of a properly subscripted distance, the P6/P7 label mismatch around Eqs. (9)-(10), and inconsistent use of 'r_RIS' versus 'r_RIS,j,k' in the derivation. A careful proofreading pass is needed.
Circularity Check
No significant circularity: the dual-RIS SNR result is derived from Friis-style power-transfer equations, not fitted to or assumed from the single-RIS conclusion.
full rationale
The central dual-RIS SNR expression (Eq. 15) follows from a step-by-step power-transfer derivation (Eqs. 2-13). The inter-RIS and RIS-target sums become magnitude sums only under the explicitly stated far-field and maximum-alignment assumptions; no constant is fitted to the dual-RIS SNR curve. The crossover against the single-RIS system (Fig. 2, Table 2) is obtained by comparing Eq. 15 with the single-RIS formula from the authors' prior work [17], used as an input baseline rather than as the target conclusion. The dual-RIS expression does not assume dual superiority; its M^4N^4/r_RIS^4 factor is a derived consequence of coherent summation over the second RIS. The manuscript's own far-field collapse r_RIS_j,k = r_RIS_m,n stated before Eq. (4) and the use of r_RIS = 50 m with RIS sizes whose Table 2 far-field distances are 146.6 m and 226.6 m is a substantive validity/correctness concern, but it is not circularity: the factorization in Eq. (13) is not obtained by defining the comparison to be true or by renaming a fitted parameter. No circular step is present.
Assumptions & free parameters
assumptions (5)
- domain assumption The two RISs are in each other's far field, so r_RIS,j,k = r_RIS,m,n and the inter-RIS angles coincide.
- ad hoc to paper Each RIS unit cell can impose independent phase shifts phi and phi' on the forward and return passages.
- domain assumption The target behaves as a point scatterer with radar cross section sigma and reradiates isotropically.
- ad hoc to paper The maximum received power condition is theta_t = theta_RIS and phi_t = phi_RIS + pi.
- domain assumption The radar antenna has identical transmit and receive radiation patterns and gains (monostatic reciprocity).
Cite this review
Pith. "Pith review of Dual RIS-Assisted Monostatic L-Band Radar Target Detection in NLoS Scenarios." pith.science (2026). https://pith.science/paper/PJQEFORH
@misc{pith2026250711036,
author = {Pith},
title = {Pith review of: Dual RIS-Assisted Monostatic L-Band Radar Target Detection in NLoS Scenarios},
year = {2026},
howpublished = {\url{https://pith.science/paper/PJQEFORH}},
note = {Machine review of arXiv:2507.11036}
}
read the original abstract
The use of a single Reconfigurable Intelligent Surface (RIS) to boost the signal-to-noise ratio (SNR) at the radar offers significant improvement in detecting targets, especially in non-line-of-sight (NLoS) scenarios. However, there are scenarios where no path exists between the radar and the target, even with a single RIS-assisted radar, due to other present obstacles. This paper derives an expression for SNR in target detection scenarios where dual RISs assist a monostatic radar in NLoS situations. We calculate the power received at the radar through a dual RIS configuration. We show that the SNR performance of RIS-assisted radars can improve with known locations of the radar and RISs. Our results demonstrate that the required accuracy in target localization can be achieved by controlling the number of RISs, the number of unit cells in each RIS, and properly selecting the locations of RISs to cover the desired region. The performance of dual RIS-assisted radar systems can surpass that of single RIS-assisted radar systems under favourable alignment and sufficiently large RIS sizes.
Figures
Reference graph
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Reviewed August 6, 2026 · model on record in the stance chip above.
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