REVIEW 3 major objections 7 minor 36 references
Federated learning over physical channels: adaptive algorithms with near-optimal guarantees
T0 review · 3 major / 7 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read Over-the-air federated learning can match the accuracy of coded transmission.
desk verdict A genuinely new debiasing construction for over-the-air federated learning with a clean variance lemma, but Theorem 1 as stated needs a stronger synchronization condition than (9b), and the low-SNR experiments sit outside the proved feasibility region. 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
Three components carry the argument. First, a stochastic post-coding matrix $H$: the server solves a linear program over transition matrices so that $H\circ Q_C\circ C$ is unbiased on interior quantization levels and has worst-case variance $v^*$, and combining it with dithered randomized quantization $Q_D$ makes the whole channel unbiased. Second, a scale-adaptive transformation $(\beta_\omega,\Psi_\omega,A_\omega)$ that rescales each coordinate onto a dyadic grid before transmission, keeping signals inside the range where post-coding is unbiased and making the variance depend on $\|u\|^2$ rather than on the signal range. Third, a periodic synchronization schedule: the server broadcasts the global parameter over the coded channel at times $\tau_i$ spaced so that $T(\tau_i)-T(\tau_{i-1})\le 1/(2L)$, which controls the disagreement $D_k$ between worker and server models. Lemma 2 is the hinge: it turns the channel into an unbiased oracle satisfying the moment condition needed for stochastic first-order methods.
What would settle it
Solve the post-coding linear program for the low-SNR simulation parameters $\sigma_c=0.2$, $\Delta\approx 0.286$, $q=8$; if it is infeasible, or if a direct measurement of the composed channel on a fixed vector shows the received expectation differs from the transmitted value, the central unbiasedness claim fails in that regime.
Extended reading notes
Core claim
The central claim is that the composition of the over-the-air channel, the ADC/DAC quantization, and a carefully chosen random post-processing matrix can be made an unbiased stochastic gradient oracle, with mean squared error bounded by $(4v^*+\Delta^2)(4\|u\|^2+\omega^2 d)$ for a transmitted vector $u$. With this oracle, adaptive over-the-air SGD, using coded channels only for occasional synchronization of the global model, achieves a last-iterate risk bound of the form $e^{-\mu T(n)/2}\|\theta_0-\theta_*\|^2 + (c\eta_n/\mu)(\sigma_*^2/m + (v^*+\Delta^2)\omega^2 d)$ under strong convexity, and a matching gradient-norm bound in the non-convex setting. These bounds coincide with the centralized SGD rate up to the extra channel term $(v^*+\Delta^2)\omega^2 d$, which can be made negligible by choosing the scale parameter $\omega$ small at the price of more communication. The paper claims this is the first exact unbiased gradient oracle under quantization and hardware noise.
Load-bearing premise
The whole argument rests on the post-coding linear program being solvable for the channel actually used, and the paper proves solvability only when the channel noise is at most half the quantization step.
Editorial extensions
If this is right
- If the unbiased-oracle analysis is correct, over-the-air transmission can replace coded channels for gradient exchange in federated learning without changing the order of the convergence rate.
- Choosing $\omega$ small drives the extra statistical error $(v^*+\Delta^2)\omega^2 d$ toward zero, so the method can approach the centralized minimax rate $\sigma_*^2/(\mu m)$ while still using physical channels for the bulk of the traffic.
- The synchronization condition is mild with decreasing stepsizes: it permits geometrically spaced synchronization rounds, and with constant stepsize it requires synchronization every $O(1/(L\eta))$ steps.
- The non-convex guarantee gives the standard $O(\epsilon^{-4})$ sample complexity for finding an $\epsilon$-stationary point, with only $O(\sqrt{n})$ broadcasting steps over $n$ iterations under the $\eta_k\asymp 1/\sqrt{n}$ schedule.
- In the paper's MNIST experiments, the full method matches coded transmission in test accuracy, differing by 0.07%, while transmitting about one fifth of the symbols.
Reading between the lines
- Editorial extension: the proof of Theorem 1 in Section 6.2 invokes the stronger synchronization spacing $T(\tau_i)-T(\tau_{i-1}) < 1/[2L(v^*+\Delta^2)]$ to absorb the worker-model disagreement terms $D_k$, not merely the stated $1/(2L)$; whether Theorem 1 holds exactly as stated depends on closing that gap.
- Editorial extension: the low-SNR simulation uses $\sigma_c=0.2$ with $\Delta\approx 0.286$, which falls outside the $\sigma_c\le\Delta/2$ regime where Lemma 1 proves the post-coding linear program is feasible, so the simulation implicitly assumes feasibility beyond the proven range; a direct feasibility check of the linear program at those parameters would settle it.
- Editorial extension: a natural testable prediction is that the same post-coding construction remains unbiased for sub-Gaussian or heavy-tailed channel noise with a modified variance bound, since the argument only uses symmetry and concentration of the noise through the transition matrix $P$.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies federated optimization over AWGN physical channels with low-resolution ADC/DAC quantization. It proposes a stochastic post-coding linear program that makes the quantized noisy channel unbiased on interior quantization levels, a scale-adaptive transformation that transmits normalized values over the air while sending scale indices over a coded channel, and periodic synchronization of worker models. For strongly convex and non-convex losses it proves convergence rates comparable to centralized SGD up to a channel-dependent term (v*+Δ²)ω²d, and it reports MNIST experiments with a CNN showing accuracy close to coded transmission at substantially lower symbol cost.
Significance. If the proofs are completed, this is a useful contribution: the paper gives a concrete construction (LP (6)) that turns a biased quantized AWGN channel into an approximately unbiased stochastic gradient oracle with an explicit variance bound, and the convergence analysis covers state-dependent gradient noise and periodic synchronization. The explicit constants in Lemmas 1 and 2 and the self-contained proof structure are strengths, as is the clear separation of the unbiasedness construction from the optimization analysis. The advertised near-optimal risk, matching centralized SGD up to the channel-dependent term (v*+Δ²)ω²d, would be of interest to the federated-learning-over-wireless-channels community. However, the main theorem as stated is not fully supported by the proof as written, and one simulation operating point lies outside the proved feasibility regime of the post-coding LP; these issues require correction before the central claims can be accepted.
major comments (3)
- [§4.2 and §6.2, Eq. (9b) and Eq. (13)] Theorem 1 is stated with the synchronization condition T(τ_i)-T(τ_{i-1}) ≤ 1/(2L), but the proof absorbs the disagreement term 6LΣη_kD_{k-1} only under the stronger condition T(τ_i)-T(τ_{i-1}) < 1/[2L(v*+Δ²)], together with a small-stepsize condition η_t ≤ 1/(24ec_1(L+ℓ²)). Lemma 1 provides only v* ≤ 4Δ², and with the natural spacing Δ=2/(q-1) one has v*+Δ² ≤ 5Δ² = 20/(q-1)², which exceeds 1 for q=4,5. Hence (9b) does not imply the proof's condition for general q≥4, and the stated bound in Theorem 1 is not established as written. Please state the channel-dependent synchronization condition in the theorem, or restrict the quantization resolution so that v*+Δ² ≤ 1, and reconcile the stepsize constants c0 and 1/(24ec_1). Separately, the proof concludes the bound at synchronization times τ_r, while the theorem statement promises the bound for any n≥1; a final partial-block argument for n between synchronization times should be supplied or explicitly referenced.
- [§5, low-SNR experiment] The low-SNR simulation uses σ_c = 0.2 and q = 8, which with Δ = 2/(q-1) = 2/7 gives σ_c ≈ 0.2 > Δ/2 ≈ 0.1429, so the feasibility guarantee of Lemma 1 does not cover this operating point. Since the 'Ours' and 'Postcode' schemes require an optimal solution of LP (6), the paper should either report numerically verified feasibility of (6) for this channel (for example, the optimal value and constraint slacks), or provide a feasibility condition that covers the low-SNR experiment. Without this, the empirical claim in the low-SNR regime is not fully supported.
- [§4.3, Theorem 2] The statement of Theorem 2 imposes the stepsize condition η_k ≤ c0/(ℓ²+L), but under Assumption 3 the quantity ℓ is not defined and the proof in §6.3 uses η_k ≤ c0/[L(1+λ)]. Please replace the displayed condition with the condition actually used in the proof, or define ℓ consistently; as stated, the non-convex result cannot be checked because the stepsize condition is ill-posed.
minor comments (7)
- [§3.2, Eq. (7a)] The definition of βω(x) is undefined for x = 0, since log2(ω^{-1}|x|) = -∞; please specify βω(0) = 0 or otherwise define the boundary case.
- [§4.2, Eq. (9b)] The condition (9b) uses T(τ_0), but T(0) is never defined; please state explicitly that T(0) = 0.
- [§3.3] The text says the algorithms provide 'last-iterate and average-iterate guarantees', but only last-iterate results are stated and proved; please either add the average-iterate statement or rephrase the sentence.
- [§3.1 and §6.1.1] The same symbol H is used both for the stochastic post-coding mapping in (5) and for the matrix variable in LP (6); using a different symbol for the matrix (for example W) would avoid confusion in the proof of Lemma 1.
- [§6.2, after Eq. (13)] The proof introduces an additional stepsize condition η_t ≤ 1/(24ec_1(L+ℓ²)) that is not listed in (9a); please make the relationship between this constant and c0 explicit so that the theorem statement and proof are consistent.
- [§5] There is a typo 'syncrhonization', and the sentence 'the test accuracy of our method and the coded transmission results differs by only 0.07%' should be reworded for grammatical agreement.
- [§4.2 and §4.3] The notation eO(·) is used without definition; please define it or replace it with standard big-O notation with explicit logarithmic factors.
Circularity Check
No circularity found: the post-coding unbiasedness is a constructive design, and the convergence theorems are derived from stated assumptions.
full rationale
The derivation chain is self-contained. Lemma 2's unbiasedness is not imported from a fitted parameter or from the theorem being proved; it is enforced directly by the LP constraint (6c) (e[j]^T P H z = z_j for interior levels) and by the unbiased dithering quantizer Q_D, while the variance bound (4v*+Δ^2)(4||u||^2+ω^2d) follows from the LP variance objective (6d) and elementary inequalities. The convergence results in Theorem 1 and Theorem 2 are then obtained from standard smoothness/strong-convexity arguments, Lemma 2, and the stated moment assumptions; no parameter is fitted to data and then relabeled as a prediction. There are also no load-bearing self-citations: the references are to prior work by other groups, and no uniqueness theorem or ansatz is imported from the authors' own earlier papers. The one significant concern is a proof-to-statement mismatch in Theorem 1: the proof in Section 6.2 requires the stronger synchronization condition T(τ_i)-T(τ_{i-1}) < 1/[2L(v*+Δ^2)] together with a small-stepsize condition to absorb the disagreement terms D_{k-1}, whereas the theorem statement only assumes (9b), namely T(τ_i)-T(τ_{i-1}) ≤ 1/(2L). This is a correctness gap in the written proof, not a circularity: it does not make the claimed bound equivalent to an input by construction. Accordingly, the circularity score is 0.
Assumptions & free parameters
free parameters (2)
- ω (scale-adaptive tuning parameter) =
not reported in experiments
- stepsize ηk =
η=0.01 in experiments; theory requires ηk≤c0/(L+ℓ²)
assumptions (5)
- domain assumption Population loss is L-smooth and μ-strongly convex (Assumption 1)
- domain assumption State-dependent stochastic gradient noise bound (Assumptions 2 and 3)
- domain assumption AWGN channel with known variance σc and uniform quantization grid with known Δ
- domain assumption Coded channel is error-free for scale indices and synchronization messages
- ad hoc to paper Post-coding LP (6) is feasible for the operating SNR
Cite this review
Pith. "Pith review of Federated learning over physical channels: adaptive algorithms with near-optimal guarantees." pith.science (2026). https://pith.science/paper/63R5JDJV
@misc{pith2026250902538,
author = {Pith},
title = {Pith review of: Federated learning over physical channels: adaptive algorithms with near-optimal guarantees},
year = {2026},
howpublished = {\url{https://pith.science/paper/63R5JDJV}},
note = {Machine review of arXiv:2509.02538}
}
read the original abstract
In federated learning, communication cost can be significantly reduced by transmitting the information over the air through physical channels. In this paper, we propose a new class of adaptive federated stochastic gradient descent (SGD) algorithms that can be implemented over physical channels, taking into account both channel noise and hardware constraints. We establish theoretical guarantees for the proposed algorithms, demonstrating convergence rates that are adaptive to the stochastic gradient noise level. We also demonstrate the practical effectiveness of our algorithms through simulation studies with deep learning models.
Figures
Reference graph
Works this paper leans on
-
[1]
T. C. Aysal, M. J. Coates, and M. G. Rabbat. Distributed average consensus with dithered quantization. IEEE Transactions on Signal Processing , 56(10):4905--4918, 2008
work page 2008
-
[2]
F. Ang, L. Chen, N. Zhao, Y. Chen, W. Wang, and F. R. Yu. Robust federated learning with noisy communication. IEEE Transactions on Communications , 68(6):3452--3464, 2020
work page 2020
-
[3]
M. M. Amiri and D. G \"u nd \"u z. Federated learning over wireless fading channels. IEEE Transactions on Wireless Communications , 19(5):3546--3557, 2020
work page 2020
-
[4]
M. M. Amiri and D. G \"u nd \"u z. Machine learning at the wireless edge: Distributed stochastic gradient descent over-the-air. IEEE Transactions on Signal Processing , 68:2155--2169, 2020
work page 2020
-
[5]
M. M. Amiri, D. Gunduz, S. R. Kulkarni, and H. V. Poor. Federated learning with quantized global model updates. arXiv preprint arXiv:2006.10672 , 2020
arXiv 2006
-
[6]
M. M. Amiri, D. G \"u nd \"u z, S. R. Kulkarni, and H. V. Poor. Convergence of federated learning over a noisy downlink. IEEE Transactions on Wireless Communications , 21(3):1422--1437, 2021
work page 2021
-
[7]
Communication complexity of distributed convex learning and optimization
Yossi Arjevani and Ohad Shamir. Communication complexity of distributed convex learning and optimization. Advances in Neural Information Processing Systems , 28, 2015
work page 2015
-
[8]
E. Agrell and M. Secondini. Information-theoretic tools for optical communications engineers. In 2018 IEEE Photonics Conference (IPC) , pages 1--5. IEEE, 2018
work page 2018
Show all 36 references
-
[9]
S. Boyd, N. Parikh, E. Chu, B. Peleato, and J. Eckstein. Distributed optimization and statistical learning via the alternating direction method of multipliers. Foundations and Trends in Machine learning , 3(1):1--122, 2011
2011
-
[10]
Chang, M
T.-H. Chang, M. Hong, H.-T. Wai, X. Zhang, and S. Lu. Distributed learning in the nonconvex world: From batch data to streaming and beyond. IEEE Signal Processing Magazine , 37(3):26--38, 2020
2020
-
[11]
Gupta, A
S. Gupta, A. Agrawal, K. Gopalakrishnan, and P. Narayanan. Deep learning with limited numerical precision. In International conference on machine learning , pages 1737--1746. PMLR, 2015
2015
-
[12]
W. Guo, R. Li, C. Huang, X. Qin, K. Shen, and W. Zhang. Joint device selection and power control for wireless federated learning. IEEE Journal on Selected Areas in Communications , 40(8):2395--2410, 2022
2022
-
[13]
Has rc o g lu and D
B. Has rc o g lu and D. G \"u nd \"u z. Communication efficient private federated learning using dithering. In ICASSP 2024-2024 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP) , pages 7575--7579. IEEE, 2024
2024
-
[14]
Revision of IEEE Std 802.3-2015
IEEE Standard for Ethernet , 2018. Revision of IEEE Std 802.3-2015
2018
-
[15]
Ilandarideva, A
S. Ilandarideva, A. Juditsky, G. Lan, and T. Li. Accelerated stochastic approximation with state-dependent noise. Mathematical Programming , pages 1--42, 2024
2024
-
[16]
S. P. Karimireddy, S. Kale, M. Mohri, S. Reddi, S. Stich, and A. T. Suresh. Scaffold: Stochastic controlled averaging for federated learning. In International conference on machine learning , pages 5132--5143. PMLR, 2020
2020
-
[17]
K \"o ster, T
U. K \"o ster, T. Webb, X. Wang, M. Nassar, et al. Flexpoint: An adaptive numerical format for efficient training of deep neural networks. Advances in neural information processing systems , 30, 2017
2017
-
[18]
M. Li, D. G. Andersen, A. Smola, and K. Yu. Communication efficient distributed machine learning with the parameter server. Advances in neural information processing systems , 27, 2014
2014
-
[19]
J. Li, Z. Chen, K. F. E. Chong, B. Das, T. Q. S. Quek, and H. H. Yang. Robust federated learning over the air: Combating heavy-tailed noise with median anchored clipping. In 2025 23rd International Symposium on Modeling and Optimization in Mobile, Ad Hoc, and Wireless Networks...
2025
-
[20]
T. Li, A. K. Sahu, A. Talwalkar, and V. Smith. Federated learning: Challenges, methods, and future directions. IEEE signal processing magazine , 37(3):50--60, 2020
2020
-
[21]
Moulines and F
\' E . Moulines and F. Bach. Non-asymptotic analysis of stochastic approximation algorithms for machine learning. Advances in neural information processing systems , 24, 2011
2011
-
[22]
McMahan, E
B. McMahan, E. Moore, D. Ramage, S. Hampson, and B. A. y Arcas. Communication-efficient learning of deep networks from decentralized data. In Artificial Intelligence and Statistics , pages 1273--1282. PMLR, 2017
2017
-
[23]
J. G. Proakis and M. Salehi. Digital communications , volume 4. McGraw-hill New York, 2001
2001
-
[24]
S. M. Shah, L. Su, and V. K. N. Lau. Robust federated learning over noisy fading channels. IEEE Internet of Things Journal , 10(9):7993--8013, 2022
2022
-
[25]
S. U. Stich. Local SGD converges fast and communicates little. arXiv preprint arXiv:1805.09767 , 2018
2018 arXiv
-
[26]
Tegin and T
B. Tegin and T. M. Duman. Blind federated learning at the wireless edge with low-resolution ADC and DAC . IEEE Transactions on Wireless Communications , 20(12):7786--7798, 2021
2021
-
[27]
Upadhyay and A
A. Upadhyay and A. Hashemi. Noisy communication of information in federated learning: An improved convergence analysis. In 2023 57th Asilomar Conference on Signals, Systems, and Computers , pages 666--669. IEEE, 2023
2023
-
[28]
R. A. Wannamaker, S. P. Lipshitz, J. Vanderkooy, and J. N. Wright. A theory of nonsubtractive dither. IEEE Transactions on Signal Processing , 48(2):499--516, 2002
2002
-
[29]
Wei and C
X. Wei and C. Shen. Federated learning over noisy channels: Convergence analysis and design examples. IEEE Transactions on Cognitive Communications and Networking , 8(2):1253--1268, 2022
2022
-
[30]
B. Xiao, X. Yu, W. Ni, X. Wang, and H. V. Poor. Over-the-air federated learning: Status quo, open challenges, and future directions. Fundamental Research , 2024
2024
-
[31]
Y. Youn, Z. Hu, J. Ziani, and J. Abernethy. Randomized quantization is all you need for differential privacy in federated learning. arXiv preprint arXiv:2306.11913 , 2023
2023 arXiv
-
[32]
K. Yang, T. Jiang, Y. Shi, and Z. Ding. Federated learning via over-the-air computation. IEEE Transactions on Wireless Communications , 19(3):2022--2035, 2020
2022
-
[33]
H. Yang, P. Qiu, J. Liu, and A. Yener. Over-the-air federated learning with joint adaptive computation and power control. In 2022 IEEE International Symposium on Information Theory (ISIT) , pages 1259--1264. IEEE, 2022
2022
-
[34]
J. Yao, W. Xu, Z. Yang, X. You, M. Bennis, and H. V. Poor. Wireless federated learning over resource-constrained networks: Digital versus analog transmissions. IEEE Transactions on Wireless Communications , 23(10):14020--14036, 2024
2024
-
[35]
Zhang, J
Y. Zhang, J. C Duchi, and M. J Wainwright. Communication-efficient algorithms for statistical optimization. The Journal of Machine Learning Research , 14(1):3321--3363, 2013
2013
-
[36]
Zhang, M
N. Zhang, M. Tao, J. Wang, and S. Shao. Coded over-the-air computation for model aggregation in federated learning. IEEE Communications Letters , 27(1):160--164, 2022
2022
Reviewed August 15, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.