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How Does CP Length Affect the Sensing Range for OFDM-ISAC?

Xiaoli Xu, Zhiwen Zhou, Yong Zeng

TL;DR

This work challenges the conventional CP-length bound in OFDM-ISAC by showing that random OFDM data act as a probabilistic mask that attenuates ISI/ICI during radar processing, allowing sensing beyond the CP-limited range. It derives closed-form SINR expressions for range estimation with arbitrary CP and multipath delay, demonstrating that the maximum sensing distance can greatly exceed the ISI-free CP limit while preserving communication efficiency. To further boost long-range sensing, it introduces a sliding window detection method that iteratively detects and cancels short-range echoes before shifting the detection window, effectively compensating power loss without additional CP overhead. Numerical results validate the analytic SINR model and show substantial gains in maximum sensing range, providing practical guidance for CP-length design in OFDM-ISAC systems and enabling robust sensing at much larger distances than previously considered feasible.

Abstract

Orthogonal frequency division multiplexing (OFDM), which has been the dominating waveform for contemporary wireless communications, is also regarded as a competitive candidate for future integrated sensing and communication (ISAC) systems. Existing works on OFDM-ISAC usually assume that the maximum sensing range should be limited by the cyclic prefix (CP) length since inter-symbol interference (ISI) and inter-carrier interference (ICI) should be avoided. However, in this paper, we provide rigorous analysis to reveal that the random data embedded in OFDM-ISAC signal can actually act as a free ``mask" for ISI, which makes ISI/ICI random and hence greatly attenuated after radar signal processing. The derived signal-to-interference-plus-noise ratio (SINR) in the range profile demonstrates that the maximum sensing range of OFDM-ISAC can greatly exceed the ISI-free distance that is limited by the CP length, which is validated by simulation results. To further mitigate power degradation for long-range targets, a novel sliding window sensing method is proposed, which iteratively detects and cancels short-range targets before shifting the detection window. The shifted detection window can effectively compensate the power degradation due to insufficient CP length for long-range targets. Such results provide valuable guidance for the CP length design in OFDM-ISAC systems.

How Does CP Length Affect the Sensing Range for OFDM-ISAC?

TL;DR

This work challenges the conventional CP-length bound in OFDM-ISAC by showing that random OFDM data act as a probabilistic mask that attenuates ISI/ICI during radar processing, allowing sensing beyond the CP-limited range. It derives closed-form SINR expressions for range estimation with arbitrary CP and multipath delay, demonstrating that the maximum sensing distance can greatly exceed the ISI-free CP limit while preserving communication efficiency. To further boost long-range sensing, it introduces a sliding window detection method that iteratively detects and cancels short-range echoes before shifting the detection window, effectively compensating power loss without additional CP overhead. Numerical results validate the analytic SINR model and show substantial gains in maximum sensing range, providing practical guidance for CP-length design in OFDM-ISAC systems and enabling robust sensing at much larger distances than previously considered feasible.

Abstract

Orthogonal frequency division multiplexing (OFDM), which has been the dominating waveform for contemporary wireless communications, is also regarded as a competitive candidate for future integrated sensing and communication (ISAC) systems. Existing works on OFDM-ISAC usually assume that the maximum sensing range should be limited by the cyclic prefix (CP) length since inter-symbol interference (ISI) and inter-carrier interference (ICI) should be avoided. However, in this paper, we provide rigorous analysis to reveal that the random data embedded in OFDM-ISAC signal can actually act as a free ``mask" for ISI, which makes ISI/ICI random and hence greatly attenuated after radar signal processing. The derived signal-to-interference-plus-noise ratio (SINR) in the range profile demonstrates that the maximum sensing range of OFDM-ISAC can greatly exceed the ISI-free distance that is limited by the CP length, which is validated by simulation results. To further mitigate power degradation for long-range targets, a novel sliding window sensing method is proposed, which iteratively detects and cancels short-range targets before shifting the detection window. The shifted detection window can effectively compensate the power degradation due to insufficient CP length for long-range targets. Such results provide valuable guidance for the CP length design in OFDM-ISAC systems.

Paper Structure

This paper contains 19 sections, 1 theorem, 46 equations, 16 figures, 1 table, 1 algorithm.

Table of Contents

  1. Introduction
  2. System Model
  3. OFDM Signal Model
  4. Signal Processing at ISAC Receiver
  5. OFDM Radar Sensing Range
  6. Interference Beyond CP Protection
  7. ISI and ICI Analysis
  8. Interference from All Targets
  9. OFDM Radar Sensing Range
  10. SINR for Sensing of Single Target
  11. SINR for Sensing of Multiple Targets
  12. Sliding Window Sensing Detection for CP-limited OFDM-ISAC
  13. Detection and Elimination of Short-ranged Targets
  14. Detection of Long-ranged Targets
  15. Complexity Analysis
  16. ...and 4 more sections

Key Result

Lemma 1

If $x_n\sim\mathcal{CN}(0,\sigma^2),n=0,...,N-1$ and $X_k=\frac{1}{N}\sum_{n=0}^{N-1}x_ne^{j\frac{2\pi}{N}nk}$, then $X_k\sim\mathcal{CN}(0,\frac{\sigma^2}{N})$.

Figures (16)

  • Figure 1: OFDM-ISAC system model.
  • Figure 2: An illustration of CP and signal transmission time.
  • Figure 3: An illustration of signal detection at communication and sensing receivers.
  • Figure 4: The sensing procedures for target range estimation.
  • Figure 5: An illustration of ISI and ICI when $\tau_l>T_{cp}$. The red portion shows the included part of previous symbol and the yellow portion shows the missing part in current symbol.
  • ...and 11 more figures

Theorems & Definitions (4)

  • Remark 1
  • Lemma 1
  • proof
  • Example 1