Active Antenna Unit

Description

The Active Antenna Unit (AAU) represents a fundamental architectural shift in radio access network design by integrating active radio frequency (RF) components directly with passive antenna elements into a single physical unit. Unlike traditional base station architectures where remote radio heads (RRHs) and passive antennas are separate components connected via coaxial cables, the AAU combines power amplifiers, low-noise amplifiers, transceivers, and antenna elements in an integrated package. This integration eliminates the need for feeder cables between the radio unit and antenna, significantly reducing signal losses that typically occur in traditional deployments.

At its core, the AAU contains multiple transceiver chains, each consisting of digital-to-analog converters (DACs), analog-to-digital converters (ADCs), power amplifiers (PAs), low-noise amplifiers (LNAs), filters, and antenna elements. These components are arranged in a tightly integrated architecture that enables precise control over each antenna element's phase and amplitude. The AAU connects to the baseband unit (BBU) or distributed unit (DU) via high-speed fiber optic interfaces using protocols like Common Public Radio Interface (CPRI) or enhanced CPRI (eCPRI), which carry digitized baseband signals between the processing unit and the radio unit.

The AAU's architecture enables advanced antenna technologies including beamforming and massive Multiple-Input Multiple-Output (MIMO). By controlling the phase and amplitude of signals at each antenna element, the AAU can steer radio beams electronically without physically moving the antenna. This capability is particularly important for millimeter wave (mmWave) frequencies in 5G, where beamforming is essential to overcome high path loss. The AAU also supports multi-band operation, allowing a single unit to transmit and receive across multiple frequency bands simultaneously, which simplifies site deployment and reduces equipment count.

In operation, the AAU receives digitized baseband signals from the BBU/DU, converts them to analog RF signals, amplifies them, and transmits them through the integrated antenna array. On the receive side, it captures signals through the antenna elements, amplifies them using LNAs, converts them to digital format, and sends them back to the BBU/DU for further processing. The integration of active components with antennas enables more precise calibration and control, resulting in improved signal quality, reduced interference, and better overall network performance compared to traditional separated architectures.

Purpose & Motivation

The AAU was developed to address several limitations of traditional base station architectures where radio units and antennas were separate components. In conventional deployments, the radio unit (often implemented as a Remote Radio Head) connects to passive antennas via coaxial feeder cables, which introduce significant signal losses, especially at higher frequencies. These losses reduce network efficiency, increase power consumption, and limit the practical deployment of advanced antenna technologies. The AAU eliminates these feeder losses by integrating active components directly with the antenna elements, resulting in improved energy efficiency and better signal quality.

Another key motivation for AAU development was the need to support advanced antenna technologies required for 4G LTE-Advanced and 5G networks. Technologies like beamforming and massive MIMO require precise control over individual antenna elements, which is difficult to achieve with separated radio and antenna components due to phase inconsistencies in feeder cables. The integrated design of AAUs enables more accurate calibration and control of antenna elements, making it possible to implement sophisticated beamforming algorithms that dynamically steer radio beams toward users, improving coverage and capacity.

The AAU also addresses practical deployment challenges by reducing site footprint and simplifying installation. Traditional base station sites require separate cabinets for radio equipment, feeder cables running up towers, and large antenna arrays. The AAU consolidates these components into a single unit that can be mounted directly at the antenna location, reducing wind load, tower space requirements, and installation complexity. This integrated approach became increasingly important as networks evolved to support higher frequency bands and more antenna elements, where traditional architectures would become impractical due to cable losses and physical constraints.

Key Features

• Integrated active RF components with antenna elements
• Support for advanced beamforming and massive MIMO
• Elimination of feeder cable losses between radio and antenna
• Multi-band operation capability
• High-speed fiber optic interface to baseband unit
• Support for millimeter wave frequency operation

Evolution Across Releases

Defining Specifications