Integrated Constrained Manifold Optimization for Wideband OFDM-DFRC Hybrid Beamforming

Hybrid beamforming (HBF) offers a cost effective solution for multiple-input multiple-output dual-functional radar-communication (MIMO-DFRC) systems by reducing hardware complexity and power consumption. While narrowband HBF has been widely studied, wideband HBF design, particularly for transceiver-side hybrid architectures, remains rather underexplored. Existing HBF methods for wideband MIMO-DFRC systems typically adopt a sequential optimization (SO) framework based on the consensus alternating direction method of multipliers (CADMM), assuming a fully digital receiver architecture during the initial optimization stage. This assumption restricts the design flexibility and limits the achievable spectral efficiency (SE) on the communication side. To this end, an integrated constrained manifold optimization (ICMO) framework is proposed. The ICMO method embeds the transmit and receive HBF matrices, subject to constant modulus (CM) and transmit power per subcarrier, into an integrated variable and projects them onto an ICM-space for joint optimization. This transforms the original constrained problem into an unconstrained manifold optimization problem. Subsequently, a parallel simplified quasi-Newton (PSQN) method is developed to optimize the integrated variable within the ICMO framework, ultimately yielding the transmit and receive HBF coding matrices. Simulation results under a Pareto behavior analysis show that the proposed method dominates the CADMM baseline across the SSME and SE frontier, achieving up to 40% higher SE at comparable SSME and about 4.4 dB lower sidelobe levels, thereby enabling spectrum efficient and low sidelobe DFRC design.