Chapter 20: Hybrid Beamforming

Learning Objectives

• Explain why fully digital beamforming is infeasible at mmWave/sub-THz and how the hybrid architecture trades hardware for performance
• Derive the constant-modulus factorization $\mathbf{W} = \mathbf{F}_{\text{RF}} \mathbf{F}_{\text{BB}}$ and state the fundamental bound on hybrid spectral efficiency
• Compare fully-connected and subarray architectures in terms of phase-shifter count, insertion loss, and beamforming flexibility
• Execute the OMP-based spatially sparse precoding algorithm and prove its optimality under sufficient RF chains
• Derive the performance penalty of $b$-bit phase quantization and select a resolution for a target spectral efficiency
• Understand lens-based beamforming (Butler matrix, Rotman lens, beamspace MIMO) and the CommIT array-fed multibeam architecture