What is a hybrid beamforming architecture and why is it used in 5G millimeter wave systems?
5G Hybrid Beamforming
5G NR at mmWave frequencies (FR2: 24-52 GHz) requires high antenna gain to overcome the severe path loss at these frequencies. A 256-element array at 28 GHz provides approximately 50 dBm EIRP with 30 dBi antenna gain. Full digital beamforming of 256 elements would require 256 ADCs running at multi-GS/s, consuming thousands of watts. This is impractical for commercial base stations.
The hybrid architecture reduces the digital complexity by a factor of N/K (where K is the number of digital chains). For 256 elements with 16 digital chains: the digital complexity is reduced by 16×. Each digital chain controls a sub-array of 16 elements through analog phase shifters. The analog phase shifters steer each sub-array beam, and the digital processor combines the sub-array outputs to form multiple beams, apply multi-user precoding, and perform spatial multiplexing.
The hybrid architecture supports various beam management operations required by 5G NR: beam sweeping (analog beam scanning during initial access), beam tracking (following user movement), beam refinement (narrowing the beam after initial acquisition), and multi-user MIMO (serving multiple users with separate beams simultaneously).
Frequently Asked Questions
How many digital chains are typical?
5G mmWave base stations: 8-16 digital chains (for serving 8-16 simultaneous users). 5G mmWave handsets: 1-4 digital chains (limited by power and space). The number of digital chains determines the maximum number of simultaneously served users and the rank of the MIMO channel.
What about codebook-based beamforming?
5G NR uses predefined codebooks of beam directions that the base station sweeps during beam management. Each codebook entry corresponds to a specific set of analog phase-shifter settings. The UE reports the best beam index, and the base station configures the analog beamformer accordingly. This simplifies the beam management compared to fully adaptive beamforming.
Does hybrid beamforming work for uplink?
Yes. Uplink hybrid beamforming at the base station receives signals from multiple UEs using different analog beams (one per sub-array) and combines them digitally. The base station's digital processor separates the UE signals using multi-user detection. The handset typically uses a single analog beam for uplink transmission.