Published June 21, 2026 • 8 min read • Telecom Insights
The 5G Active Antenna Unit (AAU) represents the fusion of radio and antenna into a single, tightly integrated system — the definitive architecture for sub-6 GHz massive MIMO in 5G NR. Unlike traditional deployments where a passive antenna panel connects via coaxial jumpers to a separate RRU, the AAU integrates 64 (or 32) transceiver chains directly behind 192+ antenna elements in a single enclosure, enabling full digital beamforming with three-dimensional spatial control.
Key Takeaway: A 64T64R AAU can generate up to 64 independent beams simultaneously, providing spatial multiplexing that increases cell capacity 3–8× compared to traditional 2T2R or 8T8R systems — the single most impactful capacity enhancement in 5G NR.
AAU Hardware Architecture
A typical C-band (3.4–3.8 GHz) 64T64R AAU contains: 192 dual-polarised patch antenna elements arranged in an 8×12 grid (8 columns × 12 rows, 2 polarisations each), 64 transceiver chains each comprising PA, LNA, TDD switch, and digital step attenuator/phase shifter, digital beamforming processor (FPGA or ASIC) implementing beam weight computation and application, DFE and CFR/DPD, and eCPRI fronthaul interface (25/50 Gbps). The entire unit measures approximately 400×500×200 mm and weighs 25–40 kg — a factor of 3–5× heavier than a comparable passive antenna + RRU combination.
Beamforming: Digital, Analog, and Hybrid
The AAU implements fully digital beamforming — every antenna element has a dedicated transceiver chain, enabling simultaneous formation of multiple independent beams with arbitrary amplitude and phase weighting. This contrasts with analog beamforming (single transceiver with RF phase shifters — common in mmWave) and hybrid beamforming (fewer digital chains than elements). Digital beamforming enables Multi-User MIMO (MU-MIMO) — simultaneously serving up to 16 users on the same time-frequency resources, each with a dedicated beam formed to maximise SINR while nulling interference toward other users.
3D Beam Management and Grid of Beams
5G NR defines a comprehensive beam management framework. The AAU transmits SSB (Synchronization Signal Block) bursts across up to 64 beam directions in a periodic sweeping pattern (5, 10, or 20 ms period). UE measurements of SSB RSRP are reported, enabling the gNB to select the optimal beam pair. CSI-RS based beam refinement then fine-tunes beam direction and width using dedicated reference signals. The AAU supports both grid of beams (GoB) — a fixed set of pre-defined beams — and adaptive beamforming based on real-time channel estimates from SRS (Sounding Reference Signals).
Antenna Array Design and Calibration
The antenna array uses dual-polarised (±45° slant) patch elements with inter-element spacing of 0.5–0.7 λ (approximately 40–55 mm at 3.5 GHz). Tight spacing minimises grating lobes but increases mutual coupling (>−15 dB S21 between adjacent elements) which must be compensated in the beamforming weights. Array calibration — compensating for amplitude and phase mismatches across the 64 chains — is critical: ±1 dB and ±10° accuracy are required to achieve the predicted beam pattern. Integrated calibration couplers and dedicated calibration transceiver paths enable self-calibration in the field.
Thermal and Power Challenges
A 64T64R AAU with 200 W total RF output (3 W per channel) consumes approximately 800–1200 W DC input. With all electronics integrated directly behind the antenna, thermal density (watts per litre) is extreme. Passive cooling using the full rear surface as a heatsink is standard, with some designs incorporating active fan cooling despite outdoor deployment concerns. PA efficiency is paramount — every percentage point improvement saves 10–15 W. The AAU's power consumption and weight have become critical deployment constraints, driving intense R&D into higher-efficiency PAs and lighter-weight materials.
AAU technology continues to advance with 32T32R variants for cost-sensitive deployments, AI-driven beam management for mobility optimisation, and the extension to 6 GHz and 7–8 GHz bands in 5G-Advanced and future 6G systems.
