REVIEW 3 major objections 4 minor 35 references
This paper claims that adding an adaptive momentum term, gated by cosine similarity between the current mixed gradient and the previous update, lets distributed multichannel active noise control converge faster when communication delays for
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2026-08-01 18:48 UTC pith:3CKOYACS
load-bearing objection A plausible incremental ANC algorithm undermined by a sign inconsistency: Eq. (14) applies maximum momentum exactly when the text says it should attenuate. the 3 major comments →
Adaptive Momentum Enhanced Distributed Multichannel Active Noise Control for Faster Convergence under Communication Delays
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The central claim is that AMAS-MGDFxLMS—formed by adding an adaptive momentum term to the ASSS-MGDFxLMS update—provides faster convergence than its base algorithm while maintaining stable noise reduction under communication delays. The momentum factor at node k is βk(n) = min(β0 |ρk(n)|^p, β0), where ρk(n) is the cosine similarity between the mixed gradient gk(n) and the previous weight increment vk(n) = wk(n) − wk(n−1), with 0 < p < 1. Because |ρk| is typically small for high-dimensional vectors, the power-law scaling lifts small similarities, so the momentum is active when directions agree and attenuated when they disagree. The paper demonstrates in two simulated scenarios—sudden jumps in
What carries the argument
The key mechanism is the adaptive momentum factor βk(n) = min(β0 |ρk(n)|^p, β0), where ρk(n) is the cosine similarity between the mixed gradient and the momentum (previous weight update). This factor dynamically gates how much of the previous update is injected into the current weight update. The cosine similarity acts as a directional agreement probe: strong alignment (|ρ| near 1) enables maximal acceleration, while weak or anti-aligned directions reduce the momentum contribution, preventing the instability that a fixed momentum factor would cause under delayed gradients.
Load-bearing premise
The paper assumes that the adaptive momentum factor, especially when the gradient and previous update point in opposite directions (|ρ| = 1, giving the maximum momentum), preserves the stability guarantee that the auto-shrink step size provides, but this is not proven analytically and is only demonstrated in one simulation setup with hand-chosen hyperparameters (p = 0.25, β0 not reported).
What would settle it
A stability analysis or a simulation with a different delay pattern—for example, delays alternating rapidly between 0 and 1 second, or a larger network with 10 or more nodes and measured acoustic paths—that produces divergence of AMAS-MGDFxLMS (ANSE growing unboundedly) where ASSS-MGDFxLMS remains stable, or a measurement showing no convergence-speed improvement over ASSS-MGDFxLMS in such a setting, would refute the paper's central claim.
If this is right
- When communication delays change suddenly, AMAS-MGDFxLMS converges faster than ASSS-MGDFxLMS while keeping the error bounded, whereas the unmodified MGDFxLMS diverges and fixed-momentum variants can over-accelerate into divergence.
- Under a gradually fluctuating network delay, the adaptive momentum maintains stable and faster noise reduction, eliminating the need to manually tune a momentum factor for a given delay condition.
- The extra computation is modest—about 4Lw+2 multiplications and 6Lw−3 additions per node per iteration—making the algorithm practical for real-time distributed ANC.
- The design is a direct plug-in modification to ASSS-MGDFxLMS, so systems already using that algorithm can upgrade without changing the distributed gradient-exchange protocol.
Where Pith is reading between the lines
- The same cosine-similarity gating could be applied to other distributed adaptive filters (e.g., diffusion LMS or augmented diffusion FxLMS) that suffer from delayed information exchange, since the gating only uses local gradient and momentum vectors.
- The exponent p appears to control how aggressively small directional agreements are amplified; we infer that p likely needs to scale with the ratio of the delay spread to the filter length, though the paper does not investigate this relationship.
- The anti-aligned case (ρ ≈ −1) deserves scrutiny: because |ρ|^p = 1, the momentum is applied at full strength in the opposite direction of the gradient—a configuration that could destabilize the update if the delayed gradient is stale; the paper does not provide a theoretical bound for this case.
- One testable extension would be to replace βk(n) with a version that only engages momentum when ρk(n) > 0, ignoring negative similarities, and compare convergence and stability; this would isolate whether the anti-aligned full-momentum case is benign or harmful.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes AMAS-MGDFxLMS, an extension of the ASSS-MGDFxLMS distributed multichannel active noise control algorithm. The method adds an adaptive momentum term whose strength is set from the cosine similarity between the current mixed gradient and the previous update. The stated goal is to accelerate convergence under communication delays while preserving stability. The paper reports simulation results for a six-node ANC window setup showing that AMAS-MGDFxLMS converges faster than ASSS-MGDFxLMS and avoids the divergence seen with a fixed momentum factor.
Significance. If the claims hold, the algorithm would address a real practical limitation of delay-robust DMCANC, where the auto-shrink step size trades convergence speed for stability. The incremental contribution is modest but relevant to the ANC community. The paper gives an explicit cost estimate for the added computations, which is useful. However, the central adaptive mechanism is internally inconsistent with its stated motivation, and the empirical evidence is thin: no theoretical stability analysis, unstated hyperparameter values, and single-trace simulations. The significance is therefore conditional on correcting these issues.
major comments (3)
- [III, Eq. (14)] The adaptive momentum rule contradicts the stated purpose. The text says that when gradient and momentum directions are inconsistent, the momentum contribution is attenuated. But beta_k(n) = min(beta0 |rho_k(n)|^p, beta0) uses the absolute value: as rho approaches -1, |rho|^p approaches 1 and the momentum factor is at its maximum. Anti-aligned updates, which are the main risk under communication delays, therefore receive full momentum. The ASSS step-size scaling in Eq. (9) is not shown to compensate, and no stability bound is given. The central claim of stable faster convergence is not established for this case. Please either change the rule to a signed measure that actually attenuates anti-aligned momentum, or provide a rigorous argument and targeted simulations showing that applying maximum momentum for anti-aligned updates is stable.
- [IV A/B and Table I] The two new hyperparameters are not fully reported. p is fixed to 0.25 for all simulations, and beta0 is said to be 'estimated under ideal network conditions [35]' but the value and estimation procedure are not given. The fixed-momentum comparator FMAS-MGDFxLMS is also not specified (which beta0 value was chosen and why). The reported acceleration may therefore depend on hand-tuned or unreported parameter choices. Please report the actual beta0 value, give the estimation procedure, and include a sensitivity study over p and beta0 to show the improvement is not a tuning artifact.
- [IV, Figs. 3 and 4] The simulation evidence consists of single traces from one configuration, with no statistical replicates, error bars, or quantitative convergence metrics. The visual claim of faster convergence is not robustly supported. Please run multiple independent trials (different noise realizations, initializations, and delay profiles) and report mean/median curves with spread, or provide quantitative values such as time to reach a target ANSE and final steady-state ANSE for each algorithm.
minor comments (4)
- [II A, around Fig. 1] The text says 'shown in 1' but should say 'shown in Fig. 1'.
- [IV] The ANSE metric is mentioned but not defined. Please provide the formula or a citation with the exact definition used.
- [III] The claim that cosine similarity values are 'typically small' because the vectors are high-dimensional needs justification; v_k(n) is a difference of consecutive filters and may have structured small norm. A remark with a bound or reference would help.
- [III, after Eq. (15)] The computational cost statement ('approximately 4L_w+2 multiplications and 6L_w-3 additions per node') is not derived. A short breakdown of the operations would improve verifiability.
Circularity Check
No significant circularity; the absolute-value momentum formula is a correctness concern, not a circularity.
full rationale
The paper's core algorithm is a direct modification of the previously proposed ASSS-MGDFxLMS update: Eq. (15) adds an adaptive momentum term v_k(n) scaled by beta_k(n) from Eq. (14). The claimed faster convergence is supported by simulations comparing AMAS-MGDFxLMS against MGDFxLMS, ASSS-MGDFxLMS, and FMAS-MGDFxLMS, rather than being derived by construction from the adaptive-momentum definition. The self-citations (e.g., [31], [35]) supply the baseline algorithm and momentum-factor estimation, but the reported acceleration is not forced by those citations; it is an empirical observation over the tested scenarios. No parameter is fitted to a subset of data and then presented as a prediction of a closely related quantity; beta0 and p are chosen hyperparameters, and their values (or the omission of beta0) create a reproducibility/rigor gap, not circularity. One genuine internal inconsistency warrants note but is not circular: Eq. (14) uses |rho_k(n)|^p, so when rho is near -1 (anti-aligned gradient and momentum), the momentum factor is at its maximum, contradicting the text's statement that 'otherwise, the momentum contribution is attenuated to enhance stability.' This threatens the stability claim under delayed communication, but the update rule is not equivalent to its inputs by construction. Overall, the derivation chain is not circular.
Axiom & Free-Parameter Ledger
free parameters (3)
- beta0 (maximum momentum factor) =
not reported
- p (power-law exponent) =
0.25
- mu0 (initial step size) =
1e-7
axioms (4)
- domain assumption The estimated self-secondary path s_hat_kk(n) and compensation filters c_mk(n) are accurate enough that the mixed-gradient update approximates centralized MEFxLMS performance.
- ad hoc to paper The stability and performance guarantees of ASSS-MGDFxLMS under communication delays carry over when the adaptive momentum term is added.
- ad hoc to paper Cosine similarity values between the high-dimensional gradient and momentum vectors are small, so raising |rho| to a power p<1 is a valid way to amplify the signal.
- domain assumption Communication delays are integer sample counts and their maximum per node is known to the node.
read the original abstract
Distributed multichannel active noise control (DMCANC) reduces the computational burden of centralized ANC systems by distributing processing tasks across multiple nodes, while requiring information exchange to achieve satisfactory global noise reduction. To improve robustness under communication delays, the auto-shrink step size mixed-gradients filtered reference LMS (ASSS-MGDFxLMS) algorithm has been proposed. However, the reduced step size inevitably slows convergence. In this work, an adaptive momentum term is introduced to accelerate convergence, where cosine similarity is used to evaluate the alignment between the instantaneous gradient and the momentum component and dynamically adjust the momentum parameter. This design accelerates convergence when the directions are consistent while preserving stability under delayed communication. Simulation results demonstrate that the proposed adaptive momentum ASSS-MGDFxLMS (AMAS-MGDFxLMS) algorithm achieves faster convergence than ASSS-MGDFxLMS while maintaining stable and effective noise reduction performance.
Figures
Reference graph
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discussion (0)
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