Digital and Hybrid Precoding and RF Chain Selection Designs for Energy Efficient Multi-User MIMO-OFDM ISAC Systems

Using multiple-input multiple-output (MIMO) with orthogonal frequency division multiplexing (OFDM) for integrated sensing and communication (ISAC) has attracted considerable attention in recent years. While most existing works focus on improving MIMO-OFDM ISAC performance, the impact of transmit power and radio-frequency (RF) circuit power consumption on energy efficiency (EE) remains relatively underexplored. To address this gap, this paper investigates joint precoding and RF chain selection for multi-user MIMO-OFDM ISAC systems, and develops energy-efficient designs for both fully digital and hybrid precoding architectures through the joint optimization of precoding and RF-chain activation. Specifically, we first formulate a novel EE maximization problem subject to sensing performance constraints. Then, efficient optimization algorithms are proposed for both architectures, together with analyses of their computational complexity and convergence behavior. Building on the proposed approaches, spectral efficiency-power consumption tradeoff designs are also provided. Simulation results demonstrate that, compared with existing schemes, the proposed approaches achieve significant improvements in the EE-sensing tradeoff for ISAC systems.

💡 Research Summary

The paper addresses the largely unexplored problem of energy‑efficient design for multi‑user, multi‑target MIMO‑OFDM integrated sensing and communication (ISAC) systems. While most prior work on ISAC focuses on improving communication throughput or sensing accuracy, the authors recognize that the total power consumption of a wideband MIMO‑OFDM transmitter is dominated not only by the radiated transmit power but also by the RF‑chain circuitry, which can be substantial when many antennas are employed. Consequently, they formulate a novel energy‑efficiency (EE) maximization problem that jointly optimizes the precoding matrices across all OFDM sub‑carriers and the on/off status of each RF chain, subject to a sensing‑performance constraint (e.g., a bound on the Cramér‑Rao‑type metric).

The system model comprises a base station equipped with (N_t) transmit antennas, (N_{\text{sen}}) radar‑receive antennas, and serving (N_{\text{UE}}) communication users while simultaneously illuminating (N_{\text{tar}}) point targets. The communication signal follows the standard MIMO‑OFDM formulation, yielding a per‑user achievable rate expression (2). The radar signal is processed by angle‑domain beamforming, division‑FFT based target removal, and IFFT/FFT to generate a range‑Doppler (RD) map; detection is performed via cell‑averaging CFAR. The sensing constraint is expressed in terms of the detection probability or CRB on the target parameters.

Total power consumption is modeled as
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