Revisiting energy-efficient hybrid and digital beamforming architectures above 100 GHz

Millimeter-wave spectrum (30 to 300 GHz) is explored in order to provide higher throughput by exploiting the large bandwidth available. At those frequencies, multiple antenna systems are essential to combat severe path loss. Fully digital (FD) architectures, where each antenna connects to its own baseband chain, are considered the most energy-efficient at low frequencies while enabling multi-user multiplexing. However, unlike lower frequencies, channel propagation above 100 GHz is heavily line-of-sight dominated and the operating bandwidth is much larger which poses different constraints on signaling schemes and hardware components. The optimal beamforming architecture configuration is still an open problem above 100 GHz. To address this problem, we compare energy-efficient beamforming architectures for the D-band (110-170 GHz). We estimate the energy efficiency of future systems by following the technology trends in circuit implementation. We show that hybrid fully connected architecture is the most energy-efficient for the 7nm technology node and size-constrained antenna arrays. Hybrid partially connected architecture is the most energyefficient for unconstrained antenna arrays. We show that FD architecture becomes energy-efficient for technology nodes better than 2nm.