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The Curse of Beam-Squint in ISAC: Causes, Implications, and Mitigation Strategies

Ahmet M. Elbir, Kumar Vijay Mishra, Abdulkadir Celik, Ahmed M. Eltawil

Abstract

Integrated sensing and communications (ISAC) has emerged as a means to efficiently utilize spectrum and thereby save cost and power. At the higher end of the spectrum, ISAC systems operate at wideband using large antenna arrays to meet the stringent demands for high-resolution sensing and enhanced communications capacity. However, the wideband implementation entails beam-squint, that is, deviations in the generated beam directions because of the narrowband assumption in the analog components. This causes significant degradation in the communications capacity, target detection, and parameter estimation. This article presents the design challenges caused by beam-squint and its mitigation in ISAC systems. In this context, we also discuss several ISAC design perspectives including far-/near-field beamforming, channel/direction estimation, sparse array design, and index modulation. There are also several research opportunities in waveform design, beam training, and array processing to adequately address beam-squint in ISAC.

The Curse of Beam-Squint in ISAC: Causes, Implications, and Mitigation Strategies

Abstract

Integrated sensing and communications (ISAC) has emerged as a means to efficiently utilize spectrum and thereby save cost and power. At the higher end of the spectrum, ISAC systems operate at wideband using large antenna arrays to meet the stringent demands for high-resolution sensing and enhanced communications capacity. However, the wideband implementation entails beam-squint, that is, deviations in the generated beam directions because of the narrowband assumption in the analog components. This causes significant degradation in the communications capacity, target detection, and parameter estimation. This article presents the design challenges caused by beam-squint and its mitigation in ISAC systems. In this context, we also discuss several ISAC design perspectives including far-/near-field beamforming, channel/direction estimation, sparse array design, and index modulation. There are also several research opportunities in waveform design, beam training, and array processing to adequately address beam-squint in ISAC.
Paper Structure (23 sections, 5 figures, 1 table)

This paper contains 23 sections, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Beam-Squint Compensation Techniques
  3. Beam Broadening
  4. TTD Networks
  5. SD Phase Shifter Networks
  6. Phase Correction Techniques
  7. Beam-Squint in ISAC
  8. Beamforming
  9. Digital Beamforming
  10. Analog Beamforming
  11. Hybrid Beamforming
  12. Channel Estimation
  13. DoA Estimation
  14. Antenna Selection
  15. Index Modulation
  16. ...and 8 more sections

Figures (5)

  • Figure 1: ISAC beam generation in the presence of beam-squint with various transmitter architectures based on conventional hybrid analog/digital beamformers incorporated with TTD networks, antenna selection, and spatial path IM.
  • Figure 2: Comparison of beam-squint compensation techniques for hybrid beamforming and channel estimation in terms of (left) SE and (right) NMSE when $N=128$, $K=8$, $M=32$ for $300$ GHz carrier frequency and $30$ GHz bandwidth.
  • Figure 3: DoA estimation RMSE vs. SNR for beam-squint and MC calibration when $N=128$ with $300$ GHz carrier frequency over $30$ GHz bandwidth.
  • Figure 4: ISAC antenna selection involving (left) codebook for subarray configurations at different subcarriers, (top right) illustration of inaccurate/accurate antenna selection, and (bottom right) the communications (SE) and sensing (beamforming gain) performance for selecting $8$ out of $N=128$ when $K=8$, $M=32$ for $300$ GHz carrier frequency and $30$ GHz bandwidth.
  • Figure 5: SE performance of IM-empowered ISAC with respect to system bandwidth when $3$ out of $8$ paths are modulated for $\eta = 0.5$, $N=128$, $K=8$, $M=32$, $f_c = 300$ GHz and $\mathrm{SNR} = 0$ dB.