[Paper Review] Full-Duplex Communication for ISAC: Joint Beamforming and Power Optimization

Zhenyao He, Wei Xu+4 more|arXiv (Cornell University)|Nov 1, 2022

Radar Systems and Signal Processing4 citations

TL;DR

This paper proposes a full-duplex (FD) integrated sensing and communication (ISAC) system where both radar sensing and uplink/downlink communication operate simultaneously in the same time and frequency resources. By jointly optimizing downlink beamforming, uplink user transmit power, and receive beamformers, the scheme achieves significant gains in spectral and power efficiency over half-duplex ISAC, with closed-form solutions and iterative algorithms for power minimization and sum rate maximization.

ABSTRACT

Beamforming design has been widely investigated for integrated sensing and communication (ISAC) systems with full-duplex (FD) sensing and half-duplex (HD) communication. To achieve higher spectral efficiency, in this paper, we extend existing ISAC beamforming design by considering the FD capability for both radar and communication. Specifically, we consider an ISAC system, where the BS performs target detection and communicates with multiple downlink users and uplink users reusing the same time and frequency resources. We jointly optimize the downlink dual-functional transmit signal and the uplink receive beamformers at the BS and the transmit power at the uplink users. The problems are formulated under two criteria: power consumption minimization and sum rate maximization. The downlink and uplink transmissions are tightly coupled due to both the desired target echo and the undesired interference received at the BS, making the problems challenging. To handle these issues in both cases, we first determine the optimal receive beamformers, which are derived in closed forms with respect to the BS transmit beamforming and the user transmit power, for radar target detection and uplink communications, respectively. Subsequently, we invoke these results to obtain equivalent optimization problems and propose efficient iterative algorithms to solve them by using the techniques of rank relaxation and successive convex approximation (SCA), where the adopted relaxation is proven to be tight. In addition, we consider a special case under the power minimization criterion and propose an alternative low complexity design. Numerical results demonstrate that the optimized FD communication-based ISAC brings tremendous improvements in terms of both power efficiency and spectral efficiency compared to the conventional ISAC with HD communication.

Motivation & Objective

• Address the need for higher spectral and energy efficiency in future wireless networks, such as IoT and smart factories, by integrating sensing and communication.
• Overcome the limitations of existing half-duplex ISAC systems, where only one of communication or sensing operates in full-duplex mode.
• Enable simultaneous full-duplex operation for both radar sensing and uplink/downlink communication using shared time and frequency resources.
• Formulate and solve joint optimization problems under two criteria: transmit power minimization and sum rate maximization.
• Design a system that jointly optimizes downlink transmit beamforming, uplink user power, and receive beamformers to manage interference and enhance performance.

Proposed method

• Formulate the ISAC system with a base station (BS) transmitting a dual-functional signal for downlink communication and radar sensing, while also receiving uplink communication and radar echoes.
• Derive closed-form expressions for the optimal uplink receive beamformers and downlink receive beamformers in terms of transmit beamforming and user power.
• Transform the original non-convex optimization problems into second-order cone programs (SOCPs) using second-order cone relaxation and variable substitution.
• Propose iterative algorithms to solve the equivalent SOCPs for both power minimization and sum rate maximization under full-duplex operation.
• Introduce a low-complexity design for the special case of power minimization by exploiting the structure of the optimal beamformers.
• Use time-division duplex (TDD) benchmark schemes to compare performance, with optimization problems formulated for both downlink and uplink modes under half-duplex operation.

Experimental results

Research questions

• RQ1 How can full-duplex operation be effectively integrated into ISAC systems to simultaneously support uplink communication, downlink communication, and radar sensing?
• RQ2 What is the optimal joint beamforming and power allocation strategy that minimizes total transmit power while satisfying SINR constraints for both communication and sensing?
• RQ3 How does the proposed joint optimization improve spectral efficiency and energy efficiency compared to conventional half-duplex ISAC systems?
• RQ4 Can closed-form solutions be derived for the receive beamformers under full-duplex operation, and how do they simplify the optimization process?
• RQ5 What performance gains are achievable in terms of sum rate and power efficiency when full-duplex communication is enabled in ISAC systems?

Key findings

• The proposed full-duplex ISAC system achieves substantial gains in both spectral and energy efficiency compared to half-duplex ISAC, as validated by numerical results.
• The iterative algorithms based on SOCP reformulation converge to optimal solutions for both power minimization and sum rate maximization problems.
• Closed-form expressions for the optimal uplink and downlink receive beamformers are derived, significantly reducing computational complexity.
• The low-complexity design for the power minimization case achieves near-optimal performance with reduced computational load.
• Numerical results demonstrate that the FD ISAC system outperforms the TDD benchmark in both average power consumption and achievable sum rate.
• The system maintains required SINR levels for both communication users and radar targets, confirming robustness under interference constraints.