{"id":"e23079de-2bbe-45b7-8763-fcd4d9988db8","arxiv_id":"2505.12166","paper_version":1,"verdict":"REJECT","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"high","formal_verification":"none","parameter_count":4,"one_line_summary":"A pilot-only sliding-window detector is claimed to estimate bistatic OFDM target range and velocity beyond the cyclic-prefix duration, matching Cramer-Rao bounds in high SNR.","lead":"This paper proposes a sliding-window receiver for bistatic OFDM sensing that uses only pilot symbols to estimate target range and velocity beyond the cyclic-prefix limit. If correct, it could extend integrated sensing and communication to longer ranges without redesigning the waveform, but the published equations and evaluation method leave important gaps.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Equation (24) omits the coarse delay offset from the sliding-window search, so the manuscript as written cannot recover absolute range beyond the CP limit.","rationale":"The reader's formal weakest_assumption concerned conditional validity of the ECRBs, but the reader's rationale also flagged the coarse-delay omission in Eq. (24). My independent stress-test identifies the estimator equation itself as the most load-bearing issue: even a perfect CRB comparison would not help if Eq. (24) does not compute the absolute range. This is a correctness risk, not merely a validation subtlety, because the printed equations omit the coarse CP-block offset and appear to swap the periodogram axes. The sliding-window approach is plausible and the problem is relevant, so I am not claiming the underlying method is impossible; I am claiming the manuscript as written does not establish the central claim. Since the reader's verdict is already REJECT and this concern reinforces that rejection, no adjustment to the verdict is needed.","tokens_in":9501,"tokens_out":17365,"duration_ms":180448,"concrete_test":"Implement Eqs. (18)-(24) literally for the noiseless Scenario I with the Tab. I parameters at a known delay tau_NLOS = 2.3 Tcp, and compare the Eq. (24) output with c*tau_NLOS. If the estimate is off by a multiple of c*Tcp or another coarse-block multiple, or if the implementation requires adding a bk or h term absent from Eq. (24), the central beyond-CP claim is not supported as written.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central estimator is not reproducible from the printed equations. Sec. III-C selects a coarse window and states that the delay lies in [bkTs, bkTs+Tcp), but the final range estimate in Eq. (24), bRbis = ep c/(np Δf Np), depends only on the interpolated periodogram peak and has no dependence on bk or the hypothesis index h. For delays longer than Tcp, the fine periodogram phase is periodic in the pilot subcarrier spacing, so the peak alone is ambiguous by an integer multiple of the unambiguous delay interval; the coarse offset must be added to recover the absolute bi-static range. The numerical setup in Sec. IV deliberately places targets in CP block 7 or later, so the RMSE-vs-ECRB curves cannot follow from the estimator as stated. In addition, Eqs. (24) and (27) appear to use the wrong periodogram axes: range should be read from the q/time axis and velocity from the p/frequency axis, yet Eq. (24) uses ep and Eq. (27) uses eq. The sliding-window idea may be repairable, but as written the text does not contain a working range estimator for the claimed beyond-CP regime.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript proposes a sliding-window, pilot-only receiver for bistatic OFDM ISAC that aims to estimate target range and velocity when the echo delay exceeds the cyclic prefix duration. The receiver divides the maximum sensing range into CP-length blocks, computes least-squares channel estimates on pilot subcarriers under each block hypothesis, and selects a refined window with the largest periodogram peak. Range and velocity are then read from the interpolated periodogram, and the results are compared with expected Cramér-Rao bounds (ECRBs) taken from the authors' prior work, for both LOS-blocked and LOS-present scenarios.","tokens_in":9698,"tokens_out":8266,"duration_ms":92833,"significance":"If the claims were fully established, the pilot-only sliding-window approach would be a useful contribution: bistatic sensing receivers generally lack access to the full transmitted modulation symbols, and several prior beyond-CP studies assume full symbol knowledge or monostatic operation. The paper identifies a real gap and the core mechanism is intuitively plausible. However, as written, the central validation rests on an incomplete range estimator and on a circular conditioning on correct hypothesis identification, so the significance is not yet demonstrated.","major_comments":[{"comment":"The range estimator bRbis = ep c/(np Δf Np) does not include the coarse delay offset bkTs selected by the sliding-window search. The pilot-subcarrier periodogram is periodic in delay with a period determined by np Δf, so the interpolated peak ep identifies only the residual delay within one unambiguous interval. For delays spanning several CP blocks, as in Sec. IV where the bistatic range exceeds 2000 m and corresponds to at least the 7th CP block, the absolute range cannot be recovered from Eq. (24) alone. The printed estimator therefore cannot reproduce the RMSE curves in Figs. 3–5.","section":"Sec. III-C, Eq. (24)"},{"comment":"The threshold computation uses the true hypothesis ell_0 to define the complementary window Wc_0 and the maximum statistic b_ell. This is circular and not implementable, because at the detection stage the receiver does not know ell_0. As a result, the false-alarm probability and the subsequent detection/estimation results are conditioned on information that is not available to the algorithm.","section":"Sec. III-D, Eqs. (28)–(29)"},{"comment":"The statement that the CRBs derived in [5] under the condition tau_NLOS <= Tcp remain exact bounds when conditioned on correctly identifying the true hypothesis is asserted without proof. The windowed observation model involves a discrete coarse-delay selection and a residual-delay estimation problem, so the Fisher information and the resulting bound are not obviously the same as in the CP-limited model. Comparing conditional RMSE to these ECRBs is not a valid benchmark unless the conditional Cramér-Rao bound is derived. The sentence 'This is confirmed by the simulation results' is circular.","section":"Sec. IV, CRB comparison"},{"comment":"The estimated LOS gain b_alpha_LOS = e^{-j2πfc b_tau_LOS} λ/(π b_tau_LOS c) does not match Eq. (14), where alpha_LOS = e^{-j2πfc tau_LOS} λ/(4π c tau_LOS). The factor-of-4 discrepancy in the denominator would directly affect the LOS cancellation step and hence the Scenario II range and velocity results shown in Fig. 5.","section":"Sec. III-E, LOS gain estimate"}],"minor_comments":[{"comment":"The denominator contains the typo '||p - p||' and should read '||p - p_rx||'.","section":"Eq. (9)"},{"comment":"The mapping of the periodogram axes p and q to the interpolated indices ep and eq, and then to range and velocity, is confusing and should be stated explicitly. In particular, clarify that peak positions on the subcarrier axis are used for range and on the symbol-time axis for velocity.","section":"Eqs. (24) and (27)"},{"comment":"The quantity Np is not defined in the manuscript. If it denotes the FFT size Nper, that should be stated; if it denotes the number of pilot subcarriers, its value and role in the estimator should be given.","section":"Eq. (24)"},{"comment":"The stopping rule 'let h be the smallest index such that eta_h >= kappa' is not clearly connected to the refined window selection. Explain how h, the window size W, and the search range W are related, and why W=2 is sufficient for the numerical setup.","section":"Sec. III-C"},{"comment":"The DFT expression uses the phase e^{-j2π(k-Ncp)n/Nsc} with k running from 0; the indexing should be checked and the relation between the CP-removal offset and the delay decomposition in Eq. (15) should be clarified.","section":"Eq. (19)"},{"comment":"The FFT sizes Nper and Mper are larger than Nsc and Msym, respectively. State explicitly whether zero-padding is used in the 2D periodogram and how the periodogram resolution is affected.","section":"Table I"}],"recommendation":"major_revision","confidential_remarks":"The issues are substantial: the printed range estimator is incomplete, the threshold design is circular, and the CRB comparison is conditioned on the algorithm's own success. I recommend a major revision rather than outright rejection because the sliding-window idea is plausible and the equations can in principle be repaired. The authors should be asked to provide a complete estimator with the coarse delay term, a non-circular threshold procedure, and either a valid conditional CRB derivation or a different benchmark. If these cannot be supplied, the paper should not be accepted."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Two things to know about arXiv:2505.12166. The problem is real—bistatic OFDM ISAC with a pilot-only receiver is exactly the kind of setup you'd want for a practical 6G sensing node, and extending the usable range beyond the CP limit matters. And the paper makes a genuine adaptation: it takes the sliding-window principle from monostatic [17], applies it to a bistatic pilot-only receiver, and adds an explicit LOS cancellation stage. That combination is new.\n\nThat said, the central estimator as printed does not work. Equation (24) gives the range as ep c/(np Δf Np), with no dependence on the coarse delay block bk selected by the sliding window. For delays that span multiple CP blocks, the fine periodogram peak is periodic in the pilot subcarrier spacing; without adding the block offset you cannot recover the absolute bistatic range. The axes are also swapped: range should come from the delay axis q, but Eq. (24) uses the Doppler index ep; Eq. (27) uses the delay index eq for velocity. So the text as written does not contain a working estimator for the claimed beyond-CP regime. This is not a nitpick; it means the RMSE curves in Figs. 3–5 cannot be produced from the equations given.\n\nThe weaknesses go a bit further. The threshold computation in Sec. III-D requires knowing the true hypothesis to define the complementary window; that's oracle information, and the numerical results are conditioned on successful detection and correct hypothesis selection. The claim that the CRBs from the authors' own [5] remain valid under this conditioning is asserted without proof. And Scenario II assumes τNLOS − τLOS ≤ Tcp, which is restrictive and not discussed.\n\nCredit where due: the system model is clearly written, the LOS cancellation step is thoughtfully designed, and the paper honestly states when it conditions on successful detection. The figures show the expected qualitative behavior.\n\nThis is a repairable paper. A competent referee should send it back for major revision: fix the range/velocity equations, remove the oracle threshold (or provide an unconditional evaluation), and either prove the CRB claim or drop it. The core idea is plausible and worth pursuing. I would not cite it in current form, but it deserves a serious referee—not a desk reject—and with corrections it could become a solid incremental contribution.","headline":"Plausible sliding-window extension for bistatic pilot-only ISAC, but the printed estimator equations drop the coarse delay and swap axes, so the central claim isn't reproducible.","tokens_in":10262,"tokens_out":5644,"would_cite":false,"duration_ms":52766,"reading_group":"maybe","serious_thinker":"no","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"In a bistatic OFDM ISAC link, a sliding-window receiver that sees only pilot symbols estimates target range and velocity for delays beyond the cyclic prefix, approaching the Cramer-Rao bounds at high SNR.","keywords":["integrated sensing and communication","OFDM","cyclic prefix","bistatic radar","sliding-window detection","pilot symbols","Cramer-Rao bound","range and velocity estimation"],"falsifier":"Fix a target with bistatic delay just above the CP duration, tell the receiver the true CP-block hypothesis $\\ell_0$, and compare the pilot-only LS estimator RMSE to the Cramer-Rao bound from [5]; if the RMSE floor stays above the CRB at high SNR, the conditional-validity premise fails.","tokens_in":1724,"feed_emoji":"📡","tokens_out":2109,"duration_ms":61950,"temperature":0.7,"pith_summary":"This paper tries to show that the sensing range of an OFDM-based bistatic integrated sensing and communication link need not stop at the cyclic prefix (CP), the guard interval that normally caps allowable echo delay. The proposed receiver knows only pilot symbols, not the full data stream, yet it slides an FFT window over CP-length blocks, forms least-squares channel estimates on the pilot grid, and takes periodogram peaks to detect a target and estimate its bistatic range and velocity for delays several CPs long. The paper reports simulations in which this estimator tracks the Cramer-Rao bounds at high SNR for three pilot densities, both when the line-of-sight path is blocked and when it is present. If correct, the result removes the main distance cap in bistatic OFDM sensing without changing the transmitted waveform or requiring full symbol knowledge at the sensing receiver.","feed_headline":"Bistatic OFDM sensing sees past the cyclic-prefix limit","feed_subtitle":"A pilot-only sliding-window receiver estimates range and velocity for long delays and tracks Cramer-Rao bounds in simulation.","key_machinery":"The load-bearing mechanism is the sliding-window delay decomposition: $\\tau_{\\text{NLOS}} = \\ell T_{cp} + \\epsilon$ with $0 \\le \\epsilon < T_{cp}$, where each hypothesis $H_\\ell$ corresponds to a different CP-length block. For each hypothesis the receiver shifts its FFT window by $\\ell N_{cp}$ samples, which turns the long delay into a residual delay within the CP, so pilot-based LS channel estimates remain ISI-free. The decision metric is the peak of a 2D periodogram computed from the pilot-grid channel estimates, followed by a finer sliding window over sample indices and quadratic interpolation for sub-bin accuracy.","core_discovery":"The central claim is that in a bistatic OFDM ISAC system, the sensing receiver can detect a target and estimate its bistatic range and projected velocity even when the NLOS echo delay exceeds the CP duration, using only the pilot symbols embedded in the time-frequency grid. The method decomposes the delay into CP-length blocks plus a residual, and for each block hypothesis computes LS channel estimates on the pilot subcarriers after shifting the FFT window. The decision metric is the peak of the resulting two-dimensional periodogram; after a coarse block-level search and a finer sample-level window, quadratic interpolation yields the range and velocity estimates. Numerical results show RMSE closely approaching the expected Cramer-Rao bounds in the high-SNR regime, for both the LOS-blocked and LOS-present scenarios, indicating the CP is no longer the limiting factor for sensing range.","pith_inferences":["A natural test is to separate detection loss from estimation loss: compute an unconditional Cramer-Rao bound that accounts for the search over the hypothesis $\\ell$, rather than conditioning on a correct hypothesis choice; this would show how much of the gap between RMSE and ECRB comes from the sliding-window decision itself.","The same sliding-window idea could be applied to interference suppression in communication receivers, where a long-delay signal from a second transmitter could be treated as a target-like component and cancelled using only pilot positions.","Combining the sliding window with coherent compensation of the residual delay, along the lines of the coherent-compensation approach cited in the paper, may improve SINR beyond what the current method achieves, since the current method discards energy outside the selected window.","In a multi-static ISAC network, several transmitter-receiver pairs could share the same pilot grid and fuse their periodogram peaks to localize targets without exchanging full data symbols, an extension the paper explicitly leaves for future work."],"forward_implications":["A bistatic OFDM sensing link can extend its ISI-free range from the CP-limited value $c T_{cp}$ (300 m in the paper's example) up to a chosen maximum range $R_{max}$ (3000 m), a tenfold extension, without changing the OFDM frame structure.","Because only pilots are needed, the sensing receiver does not require full modulation-symbol knowledge or time-domain reconstruction, so the approach works with standard OFDM communication waveforms.","Sparse pilot patterns with overhead 0.125 perform nearly as well as denser patterns with overhead 0.5, suggesting sensing can piggyback on existing pilot grids without major throughput loss.","Velocity estimation is sensitive to angle-of-arrival error: range estimation stays accurate under AoA mismatch, but velocity degrades, so practical deployments need AoA estimation at the sensing receiver.","When a LOS path is present, the method detects and subtracts it, but reliable NLOS estimation requires the NLOS echo to arrive within one CP after the LOS path; outside that delay difference the cancellation argument is not made."],"supporting_citations":[{"why":"Supplies the Cramer-Rao bound expressions for range and velocity estimation that the paper uses as its theoretical benchmark.","marker":"[5]"},{"why":"Proposes a sliding-window detection approach for sensing beyond the CP limit, which this paper extends to the bistatic, pilot-only setting and contrasts with its own method.","marker":"[17]"},{"why":"Provides the quadratic interpolation method used to refine the periodogram peak location for range and velocity estimates.","marker":"[12]"},{"why":"Demonstrates a prior beyond-CP extension using zero-power and non-zero-power reference signals, serving as a baseline for the ISI-free range achievable.","marker":"[14]"},{"why":"Proposes coherent compensation for echo delays beyond the CP, an alternative approach this paper positions against.","marker":"[15]"},{"why":"Introduces a timing-advance mechanism to extend sensing range, another prior technique the paper distinguishes from its own.","marker":"[16]"},{"why":"Supplies the bistatic Doppler-shift relation used to define the projected velocity estimate.","marker":"[18]"},{"why":"Provides the normalization convention used to absorb beamforming gains into the channel gains and noise scaling.","marker":"[7]"}],"fun_headline_variants":["Bistatic OFDM sensing slips past the cyclic-prefix barrier","Pilot sliding window beats CP limit in bistatic OFDM sensing","Extending bistatic OFDM sensing beyond CP with pilot sliding window","Bistatic OFDM sensing: pilots break the cyclic-prefix limit","Bistatic OFDM sensing beyond CP with pilot windows"],"cache_read_input_tokens":12416,"weakest_assumption_plain":"The whole performance comparison rests on assuming that the Cramer-Rao bound formulas from earlier work, derived for delays no longer than the CP, remain exact theoretical bounds when the estimator is first gated by the sliding-window hypothesis test, but the paper does not prove that conditional validity.","fun_headline_variants_meta":{"raw":{"variants":["Bistatic OFDM sensing slips past the cyclic-prefix barrier","Pilot sliding window beats CP limit in bistatic OFDM sensing","Extending bistatic OFDM sensing beyond CP with pilot sliding window","Bistatic OFDM sensing: pilots break the cyclic-prefix limit","Bistatic OFDM sensing beyond CP with pilot windows"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001035,"raw_usage":{"total_tokens":4303,"prompt_tokens":835,"completion_tokens":3468,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":451,"completion_tokens_details":{"reasoning_tokens":3376}},"tokens_in":451,"tokens_out":3468,"duration_ms":25639,"temperature":1.0,"reasoning_tokens":3376,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T20:42:10.582012+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Fix a target with bistatic delay just above the CP duration, tell the receiver the true CP-block hypothesis $\\ell_0$, and compare the pilot-only LS estimator RMSE to the Cramer-Rao bound from [5]; if the RMSE floor stays above the CRB at high SNR, the conditional-validity premise fails.","supporting_citations":[{"cited_title":"Impact of the Pilot Design for OFDM Based Bi-static Integrated Sensing and Communication System","cited_arxiv_id":"2503.20288","evidence_quote":"Supplies the Cramer-Rao bound expressions for range and velocity estimation that the paper uses as its theoretical benchmark."},{"cited_title":"Ofdm radar algorithms in mobile communication net- works,","cited_arxiv_id":null,"evidence_quote":"Provides the quadratic interpolation method used to refine the periodogram peak location for range and velocity estimates."},{"cited_title":"Isi-resistant reference signal design and processing for ofdm integrated communications and long- range radar sensing,","cited_arxiv_id":null,"evidence_quote":"Demonstrates a prior beyond-CP extension using zero-power and non-zero-power reference signals, serving as a baseline for the ISI-free range achievable."},{"cited_title":"Coherent compensation-based sensing for long-range targets in integrated sensing and communication system,","cited_arxiv_id":null,"evidence_quote":"Proposes coherent compensation for echo delays beyond the CP, an alternative approach this paper positions against."},{"cited_title":"Scalable long- distance isac signal design for ofdm systems with theoretical analysis and practical validation,","cited_arxiv_id":null,"evidence_quote":"Introduces a timing-advance mechanism to extend sensing range, another prior technique the paper distinguishes from its own."},{"cited_title":"Bistatic radar,","cited_arxiv_id":null,"evidence_quote":"Supplies the bistatic Doppler-shift relation used to define the projected velocity estimate."},{"cited_title":"Perfor- mance analysis of a bistatic joint sensing and communication system,","cited_arxiv_id":null,"evidence_quote":"Provides the normalization convention used to absorb beamforming gains into the channel gains and noise scaling."}],"review_version":1}