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Bidirectional Integrated Sensing and Communication: Full-Duplex or Half-Duplex?

Zhaolin Wang, Xidong Mu, Yuanwei Liu

TL;DR

This work proposes a bidirectional integrated sensing and communication (ISAC) framework with both full-duplex and half-duplex operation in narrowband and wideband regimes. It develops SCA-based, Karush-Kuhn-Tucker (KKT)–optimal beamforming solutions to characterize the tradeoff between communication rate and sensing accuracy, using the CRB as the sensing metric and DAM for wideband frequency-selective channels. The key finding is that full-duplex does not universally outperform half-duplex; the relative gains depend on LOS conditions and sensing-prior versus communication-prior regimes, and wideband systems necessitate a dedicated sensing signal to meet sensing requirements. The results advocate a hybrid, context-aware duplex strategy and provide tractable optimization frameworks for joint S&C design in both narrowband and wideband ISAC systems.

Abstract

A bidirectional integrated sensing and communication (ISAC) system is proposed, in which a pair of transceivers carry out two-way communication and mutual sensing. Both full-duplex and half-duplex operations in narrowband and wideband systems are conceived for the bidirectional ISAC. 1) For the narrowband system, the conventional full-duplex and half-duplex operations are redesigned to take into account sensing echo signals. Then, the transmit beamforming design of both transceivers is proposed for addressing the sensing and communication (S&C) tradeoff. A one-layer iterative algorithm relying on successive convex approximation (SCA) is proposed to obtain Karush-Kuhn-Tucker (KKT) optimal solutions. 2) For the wideband system, the new full-duplex and half-duplex operations are proposed for the bidirectional ISAC. In particular, the frequency-selective fading channel is tackled by delay pre-compensation and path-based beamforming. By redesigning the proposed SCA-based algorithm, the KKT optimal solutions for path-based beamforming for characterizing the S&C tradeoff are obtained. Finally, the numerical results show that: i) For both bandwidth scenarios, the existence of the interference introduced by sensing results in full-duplex may not always outperform half-duplex, especially in the sensing-prior regime or when the communication channel is line-of-sight-dominated; and ii) For both duplex operations, it is sufficient to reuse communication signals for sensing in the narrowband system, while an additional dedicated sensing signal is required in the wideband system.

Bidirectional Integrated Sensing and Communication: Full-Duplex or Half-Duplex?

TL;DR

This work proposes a bidirectional integrated sensing and communication (ISAC) framework with both full-duplex and half-duplex operation in narrowband and wideband regimes. It develops SCA-based, Karush-Kuhn-Tucker (KKT)–optimal beamforming solutions to characterize the tradeoff between communication rate and sensing accuracy, using the CRB as the sensing metric and DAM for wideband frequency-selective channels. The key finding is that full-duplex does not universally outperform half-duplex; the relative gains depend on LOS conditions and sensing-prior versus communication-prior regimes, and wideband systems necessitate a dedicated sensing signal to meet sensing requirements. The results advocate a hybrid, context-aware duplex strategy and provide tractable optimization frameworks for joint S&C design in both narrowband and wideband ISAC systems.

Abstract

A bidirectional integrated sensing and communication (ISAC) system is proposed, in which a pair of transceivers carry out two-way communication and mutual sensing. Both full-duplex and half-duplex operations in narrowband and wideband systems are conceived for the bidirectional ISAC. 1) For the narrowband system, the conventional full-duplex and half-duplex operations are redesigned to take into account sensing echo signals. Then, the transmit beamforming design of both transceivers is proposed for addressing the sensing and communication (S&C) tradeoff. A one-layer iterative algorithm relying on successive convex approximation (SCA) is proposed to obtain Karush-Kuhn-Tucker (KKT) optimal solutions. 2) For the wideband system, the new full-duplex and half-duplex operations are proposed for the bidirectional ISAC. In particular, the frequency-selective fading channel is tackled by delay pre-compensation and path-based beamforming. By redesigning the proposed SCA-based algorithm, the KKT optimal solutions for path-based beamforming for characterizing the S&C tradeoff are obtained. Finally, the numerical results show that: i) For both bandwidth scenarios, the existence of the interference introduced by sensing results in full-duplex may not always outperform half-duplex, especially in the sensing-prior regime or when the communication channel is line-of-sight-dominated; and ii) For both duplex operations, it is sufficient to reuse communication signals for sensing in the narrowband system, while an additional dedicated sensing signal is required in the wideband system.
Paper Structure (37 sections, 1 theorem, 61 equations, 10 figures, 1 table, 2 algorithms)

This paper contains 37 sections, 1 theorem, 61 equations, 10 figures, 1 table, 2 algorithms.

Table of Contents

  1. Introduction
  2. Prior Works
  3. Studies on Full-duplex Communication
  4. Studies on ISAC
  5. Motivations and Contributions
  6. Organization and Notations
  7. Narrowband Bidirectional ISAC Systems
  8. Transmission and Reception Schemes
  9. Performance Metrics
  10. Achievable Rate of Communication
  11. Cramér-Rao Bound of Sensing
  12. Joint Beamforming Design for S&C Tradeoff
  13. Problem Formulation
  14. SCA-based Algorithm for Solving Problem \ref{['problem:narrow']}
  15. Convergence, Optimality, and Complexity Analysis
  16. ...and 22 more sections

Key Result

Proposition 1

Denote $\{ \mathbf{w}_{k,i}^{[\infty]}, \mathbf{Q}_{k,i}^{[\infty]} \}$ as the converged solution obtained by Algorithm alg:SCA_narrow as $t \rightarrow \infty$. Then, $\{ \mathbf{w}_{k,i}^{[\infty]}, \mathbf{Q}_{k,i}^{[\infty]} \}$ is a KKT point of problem problem:narrow.

Figures (10)

  • Figure 1: Illustration of the bidirectional ISAC system.
  • Figure 2: Half-/full-duplex protocols for narrowband system (the view from transceiver $A$).
  • Figure 3: Half-/full-duplex protocols for wideband system (the view from transceiver $A$).
  • Figure 4: Convergence behavior of Algorithm \ref{['alg:SCA_narrow']} when $w = 0.5$.
  • Figure 5: S&C Tradeoff region achieved by full-duplex and half-duplex operations in the narrowband system.
  • ...and 5 more figures

Theorems & Definitions (4)

  • Remark 1
  • Remark 2
  • Proposition 1
  • proof