REVIEW 2 major objections 3 minor 23 references
Optimal User and Target Scheduling, User-Target Pairing, and Low-Resolution Phase-Only Beamforming for ISAC Systems
T0 review · 2 major / 3 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read Joint scheduling, pairing, and low-resolution beamforming in ISAC exactly reduces to a MILP.
desk verdict The joint ISAC formulation is new, but the claimed exact MILP reformulation uses mis-scaled big-M constants and is a strict restriction of the original problem. 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 load-bearing device is the reformulation chain itself, and its main components are: binary phase-selection vectors $x_{u,n}$ that pick one of the $L$ constant-modulus phases per antenna element; lifted matrices $Y_{u,n,m} = x_{u,n}x_{u,m}^{T}$, whose integrality is guaranteed by total unimodularity after relaxing entries to $[0,1]$; big-M constants $D_t = (P_{\mathrm{tx}}/K)\mathrm{Tr}(G_t)$ and $B_u = P_{\mathrm{tx}}\mathrm{Tr}(\tilde{H}_u)+1$ that switch off sensing and SINR constraints when scheduling or pairing indicators are zero; and auxiliary binary products $\pi_{t,q}$ that linearize $\lambda_t\lambda_q$. Together these variables and inequalities eliminate every nonconvexity and nonlinear coupling, leaving a linear objective and linear constraints.
What would settle it
Solve the original problem P and the reformulated problem Q on a minimal instance with a single user, a single weak target, and $N=1$ antenna. If the optimal $\tau$ in P can exceed $D_t = (P_{\mathrm{tx}}/K)\mathrm{Tr}(G_t)$ while Q's constraint J1 with $\lambda_t=0$, $\rho_{u,t}=0$ forces $\tau \le 2D_t$, the claimed solution-space preservation fails. Similarly, with two targets where $D_q > D_t$, check whether an optimal solution that illuminates target q while pairing the user with target t is excluded by K5; if so, Q is not equivalent to P.
Extended reading notes
Core claim
The core discovery is a chain of eight equivalent reformulations (Propositions 1-8) that map the nonconvex MINLP P onto the MILP Q. Discrete-phase beamforming is encoded with binary phase-selection vectors; SINR constraints are linearized by a big-M bound and a rank-one lifting that is exact; products of binary variables are replaced by auxiliary matrices whose integrality follows from total unimodularity; and sensing power-gain and cross-interference constraints are rewritten with big-M constants derived from a trace inequality. At each step the original solution space is preserved, so an optimal solution of Q is claimed to be optimal for P as well. Solving Q by branch-and-cut yields the globally optimal schedules, pairings, and low-resolution beamformers.
Load-bearing premise
The reformulation is exact only if the big-M constants $D_t$ are large enough to deactivate the sensing constraints when a target is unscheduled or a pair is inactive; if a desired sensing level $\tau$ exceeds $D_t$ for a weak target, or the needed bound for a cross-interference term is $D_q$ rather than $D_t$, the MILP becomes a strict restriction of the original problem.
Editorial extensions
If this is right
- A practitioner can now compute provably optimal schedules, pairings, and low-resolution beams for ISAC systems of moderate size, replacing heuristic stage-wise designs.
- The advantage of joint optimization is scenario-dependent: the best heuristic baseline changes between correlated and uncorrelated user channels, while the optimal design is consistently best.
- Higher phase resolution improves attainable sensing accuracy, as shown by the Cramér-Rao bound on angle-of-arrival estimation, and the optimal design exploits this automatically.
- The linear structure of the reformulated problem lets branch-and-cut solvers reach the global optimum in a small fraction of the exhaustive-search complexity.
Reading between the lines
- The exactness claim implicitly requires the big-M constants to be upper bounds in all inactive cases; a natural stress test is to verify on random instances whether Q's optimum ever violates a constraint of P.
- Because the reformulation is exact, the MILP optimum can serve as a certificate against which any heuristic can be measured, allowing one to quantify the suboptimality gap of stage-wise designs in a given scenario.
- The lifting-and-total-unimodularity technique is transferable to other discrete-phase ISAC resource-allocation problems, such as many-to-one pairing or multiple users per target, which the paper notes as possible extensions.
- The Cramér-Rao analysis suggests that phase resolution, scheduling, and pairing interact; an extension could treat phase resolution as an optimizable variable rather than a fixed parameter.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies joint user/target scheduling, user-target pairing, and low-resolution phase-only beamforming for an ISAC downlink. The problem is formulated as a nonconvex mixed-integer nonlinear program (MINLP), and the authors propose an exact mixed-integer linear program (MILP) reformulation through a chain of eight equivalence propositions. The reformulation is intended to preserve the original solution space and thus yield a globally optimal solution to the MINLP. The paper also provides simulations comparing the proposed optimal design against heuristic scheduling/pairing baselines.
Significance. If correct, the exact MILP reformulation would be a valuable contribution: it would allow global optimality for a practically relevant ISAC resource-allocation problem and provide a benchmark for heuristic designs. The paper is clearly written and the reformulation machinery (big-M linearizations, total unimodularity arguments) is standard. However, the central claim of exactness is load-bearing, and it is false as stated. The simulation results are therefore not evidence for the optimality of the proposed method.
major comments (2)
- [Proposition 7, constraint J1] The big-M constant D_t = (Ptx/K)Tr(G_t) is an upper bound on Tr(G_t W_u), but the linearized constraint J1 also needs a bound on the auxiliary variable τ. In the inactive case λ_t=0 (and hence ρ_{u,t}=0 by C6), J1 reduces to Tr(G_t W_u) ≥ τ − 2D_t. Since τ can be much larger than D_t when a strong target is scheduled while a weak target t is unscheduled, this inequality is not automatically satisfied and cuts feasible solutions. Concrete counterexample: N=1, K=1, Ptx=10, U=1, T=2, J=1, α1=1, α2=0.1, with the single user paired with target 1. The original problem P achieves τ=10, but J1 for the unscheduled target 2 gives 1 ≥ τ − 2, so τ ≤ 3 in Q. Thus Q is a strict restriction of P, and the statement that an optimal solution to Q is also optimal to P is false.
- [Proposition 8, constraint K5] The same mis-scaled big-M constant appears in K5: Tr(G_q W_u) ≤ ξ_th + (2 − π_{t,q} − ρ_{u,t}) D_t. The left-hand side is bounded by D_q = (Ptx/K)Tr(G_q), not by D_t. When D_q > D_t, the constraint for an unpaired or unscheduled strong target q can become active, excluding beams that are perfectly feasible in the original problem P. For example, if λ_q=1 but ρ_{u,t}=0 or π_{t,q}=0, the right-hand side may be smaller than the maximum possible value of Tr(G_q W_u), so the reformulation further restricts the feasible set. This confirms that the error is not isolated to J1 but affects the full reformulation.
minor comments (3)
- [Abstract and Section I] There is a typo: 'commmunications' appears in the abstract and again in the first line of Section I.
- [Fig. 5 and Scenario IV] The paper reports specific Cramér-Rao bound values for different phase resolutions but does not describe how these bounds are computed for the given beampatterns; a brief derivation or reference would help reproducibility.
- [Appendix, proof of Proposition 6] The proof invokes total unimodularity to justify relaxing H3 to I3, but the argument that the row/column sums force the outer product can be made directly; the current wording is slightly misleading.
Circularity Check
No significant circularity: the MILP reformulation is a self-contained transformation chain, and the only self-citation is a non-load-bearing reference to a standard big-M technique.
full rationale
The paper's central claim is that the nonconvex MINLP P is equivalently recast as the MILP Q via Propositions 1 through 8. Each proposition transforms a constraint using binary encodings, matrix lifting, or big-M linearization, and none of these steps defines the target quantity (e.g., the optimal value tau or the scheduling/pairing decisions) in terms of the quantity it is supposed to predict. There is no data fitting, no calibrated parameter that is later called a prediction, and no benchmark result that is used to infer the model parameters. The only self-citation is reference [10], which is cited in the proofs of Propositions 2, 7, and 8 as a source for the big-M method. That citation is not load-bearing: the big-M technique is a standard textbook transformation, the paper states the transformation explicitly, and the equivalence claimed in the propositions does not depend on the correctness or content of the authors' prior work. The potential flaw noted by the skeptic, namely that the big-M constant D_t in Proposition 7 may not dominate tau when a target is unscheduled, is a mathematical correctness issue in the claimed equivalence; it is not a circular argument in which the output reduces to the input by construction. Accordingly, no circular step is identified.
Assumptions & free parameters
assumptions (7)
- standard math Trace inequality Tr(AB) ≤ Tr(A)Tr(B) for PSD matrices (Lemma 1)
- standard math Cyclic property of trace Tr(ABCD)=Tr(BCDA) (Lemma 2)
- standard math Linearization of a product of binary variables via c≤a, c≤b, c≥a+b−1, c∈[0,1] (Lemma 3)
- standard math Total unimodularity of the multiple-choice and transportation constraint matrix
- standard math Validity of the big-M method for linearizing products of binary and continuous variables
- domain assumption Monostatic co-located radar with identical AoD/AoA and point-target model
- domain assumption Rician fading channel and 3GPP UMa path-loss model
Cite this review
Pith. "Pith review of Optimal User and Target Scheduling, User-Target Pairing, and Low-Resolution Phase-Only Beamforming for ISAC Systems." pith.science (2026). https://pith.science/paper/BLUQJZ4N
@misc{pith2026250111593,
author = {Pith},
title = {Pith review of: Optimal User and Target Scheduling, User-Target Pairing, and Low-Resolution Phase-Only Beamforming for ISAC Systems},
year = {2026},
howpublished = {\url{https://pith.science/paper/BLUQJZ4N}},
note = {Machine review of arXiv:2501.11593}
}
read the original abstract
We investigate the joint user and target scheduling, user-target pairing, and low-resolution phase-only beamforming design for integrated sensing and communications (ISAC). Scheduling determines which users and targets are served, while pairing specifies which users and targets are grouped into pairs. Additionally, the beamformers are designed using few-bit constant-modulus phase shifts. This resource allocation problem is a nonconvex mixed-integer nonlinear program (MINLP) and challenging to solve. To address it, we propose an exact mixed-integer linear program (MILP) reformulation, which leads to a globally optimal solution. Our results demonstrate the superiority of an optimal joint design compared to heuristic stage-wise approaches, which are highly sensitive to scenario characteristics.
Figures
Reference graph
Works this paper leans on
-
[1]
Joint radar and communication design: Applications, state-of-the-art, and the road ahead,
F. Liu, C. Masouros, A. P. Petropulu, H. Griffiths, and L. Hanzo, “Joint radar and communication design: Applications, state-of-the-art, and the road ahead,” IEEE Trans. Commun., vol. 68, no. 6, pp. 3834–3862, 2020
2020
-
[2]
Waveform design for joint sensing and communications in millimeter- wave and low terahertz bands,
T. Mao, J. Chen, Q. Wang, C. Han, Z. Wang, and G. K. Karagiannidis, “Waveform design for joint sensing and communications in millimeter- wave and low terahertz bands,” IEEE Trans. Commun., vol. 70, no. 10, pp. 7023–7039, 2022
work page 2022
-
[3]
A case for digital beamforming at mmWave,
S. Dutta, C. N. Barati, D. Ramirez, A. Dhananjay, J. F. Buckwalter, and S. Rangan, “A case for digital beamforming at mmWave,” IEEE Trans. Wireless Commun., vol. 19, no. 2, pp. 756–770, 2020
work page 2020
-
[4]
CRB minimization for RIS-aided mmWave integrated sensing and communications,
W. Lyu, S. Yang, Y . Xiu, Y . Li, H. He, C. Yuen, and Z. Zhang, “CRB minimization for RIS-aided mmWave integrated sensing and communications,” IEEE Internet Things J. , vol. 11, no. 10, pp. 18 381– 18 393, 2024
work page 2024
-
[5]
Waveform design for joint radar-communications with low com- plexity analog components,
A. Kaushik, A. Arora, C. Tsinos, C. Masouros, F. Liu, and S. Chatzino- tas, “Waveform design for joint radar-communications with low com- plexity analog components,” in Proc. of IEEE JC&S , 2022, pp. 1–5
work page 2022
-
[6]
Dual-function radar-communication systems with constant-modulus and similarity con- straints,
C. G. Tsinos, A. Arora, S. Chatzinotas, and B. Ottersten, “Dual-function radar-communication systems with constant-modulus and similarity con- straints,” in Proc. of IEEE SAM , 2022, pp. 231–235
work page 2022
-
[7]
Simultaneous multi-beam sweeping for mmWave massive MIMO integrated sensing and communication,
Z. Xiao, S. Chen, and Y . Zeng, “Simultaneous multi-beam sweeping for mmWave massive MIMO integrated sensing and communication,” IEEE Trans. Veh. Technol., vol. 73, no. 6, pp. 8141–8152, 2024
work page 2024
-
[8]
Secure dual-functional radar-communication transmission: Hardware-efficient design,
N. Su, F. Liu, C. Masouros, T. Ratnarajah, and A. Petropulu, “Secure dual-functional radar-communication transmission: Hardware-efficient design,” in Proc. of ACSSC , 2021, pp. 629–633
work page 2021
Show all 23 references
-
[9]
Joint waveform and discrete phase shift design for RIS-assisted integrated sensing and communica- tion system under Cramer-Rao bound constraint,
X. Wang, Z. Fei, J. Huang, and H. Yu, “Joint waveform and discrete phase shift design for RIS-assisted integrated sensing and communica- tion system under Cramer-Rao bound constraint,” IEEE Transactions on Vehicular Technology, vol. 71, no. 1, pp. 1004–1009, 2022
2022
-
[10]
RadiOrchestra: Proactive management of millimeter-wave self-backhauled small cells via joint optimization of beamforming, user association, rate selection, and admission control,
L. F. Abanto-Leon, A. Asadi, A. Garcia-Saavedra, G. H. Sim, and M. Hollick, “RadiOrchestra: Proactive management of millimeter-wave self-backhauled small cells via joint optimization of beamforming, user association, rate selection, and admission control,” IEEE Trans. Wireless...
2023
-
[11]
Sensing-efficient NOMA-aided integrated sensing and communication: A joint sensing scheduling and beamforming optimization,
C. Dou, N. Huang, Y . Wu, L. Qian, and T. Q. S. Quek, “Sensing-efficient NOMA-aided integrated sensing and communication: A joint sensing scheduling and beamforming optimization,” IEEE Trans. Veh. Technol., vol. 72, no. 10, pp. 13 591–13 603, 2023
2023
-
[12]
Joint transmit and receive beamforming design for integrated sensing and communication,
N. Zhao, Y . Wang, Z. Zhang, Q. Chang, and Y . Shen, “Joint transmit and receive beamforming design for integrated sensing and communication,” IEEE Commun. Lett. , vol. 26, no. 3, pp. 662–666, 2022
2022
-
[13]
Channel sharing aided integrated sensing and communication: An energy-efficient sensing scheduling approach,
C. Dou, N. Huang, Y . Wu, L. Qian, and T. Q. S. Quek, “Channel sharing aided integrated sensing and communication: An energy-efficient sensing scheduling approach,” IEEE Trans. Wireless Commun., vol. 23, no. 5, pp. 4802–4814, 2024
2024
-
[14]
Deep learning-based target-to-user association in integrated sensing and communication systems,
L. Cazzella, M. Mizmizi, D. Tagliaferri, D. Badini, M. Matteucci, and U. Spagnolini, “Deep learning-based target-to-user association in integrated sensing and communication systems,” 2024. [Online]. Available: https://arxiv.org/abs/2401.12801
2024 arXiv
-
[15]
MIMO radar with colocated antennas,
J. Li and P. Stoica, “MIMO radar with colocated antennas,” IEEE Signal Process. Mag., vol. 24, no. 5, pp. 106–114, 2007
2007
-
[16]
Beamforming design for active IRS-aided MIMO integrated sensing and communication systems,
J. Li, G. Zhou, T. Gong, and N. Liu, “Beamforming design for active IRS-aided MIMO integrated sensing and communication systems,” IEEE Wireless Commun. Lett., vol. 12, no. 10, pp. 1786–1790, 2023
2023
-
[17]
Intelligent reflecting surface enabled multi-target sensing,
K. Meng, Q. Wu, R. Schober, and W. Chen, “Intelligent reflecting surface enabled multi-target sensing,” IEEE Transactions on Commu- nications, vol. 70, no. 12, pp. 8313–8330, 2022
2022
-
[18]
Branch-price-and-cut algorithms,
J. Desrosiers and M. Lübbecke, “Branch-price-and-cut algorithms,” in Wiley Encyclopedia of Operations Research and Management Science , 2010
2010
-
[19]
Study on channel model for frequencies from 0.5 to 100 GHz,
3GPP, “Study on channel model for frequencies from 0.5 to 100 GHz,” 3rd Generation Partnership Project (3GPP), Technical Report (TR) 38.901, 2020, version 16.1.0
2020
-
[20]
[Online]
SiBEAM SK62xx-MOD transceiver module , SiBEAM. [Online]. Available: https://fcc.report/FCC-ID/UK2MOD621X/3593226.pdf
-
[21]
Multicast multi- group precoding and user scheduling for frame-based satellite communi- cations,
D. Christopoulos, S. Chatzinotas, and B. Ottersten, “Multicast multi- group precoding and user scheduling for frame-based satellite communi- cations,” IEEE Trans. Wireless Commun., vol. 14, no. 9, pp. 4695–4707, 2015
2015
-
[22]
C. H. Papadimitriou and K. Steiglitz, Combinatorial optimization: Algorithms and complexity . Courier Corporation, 1998
1998
-
[23]
A note on the trace inequality for products of Hermitian matrix power,
Z. Yang and X. Feng, “A note on the trace inequality for products of Hermitian matrix power,” J. Inequal. Pure Appl. Math. , vol. 3, no. 5, pp. 1–12, 2002
2002
Reviewed August 10, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.