Active Antenna Unit (AAU)
Understanding the Active Antenna Unit (AAU)
If you look at an old 4G cell tower, you will see a massive grey metal box on the ground (the Radio) and a white plastic box at the top (the Antenna). They are connected by 100 feet of thick black coaxial cable. In 5G, that design was completely destroyed and replaced by the Active Antenna Unit (AAU).
| Antenna Type | Gain (dBi) | Beamwidth | Bandwidth |
|---|---|---|---|
| Dipole | 2.1 | 360° (H) | Moderate (~10%) |
| Patch | 5-8 | 60-90° | Narrow (2-5%) |
| Horn | 10-25 | 10-60° | Wide (>50%) |
| Parabolic | 25-45 | 1-10° | Wide |
The Death of the Coaxial Cable
In the old 4G design, the radio generated 100 Watts of power on the ground. By the time the radio wave traveled up 100 feet of copper cable to the antenna, 50% of the power was lost to heat (Cable Loss). It was incredibly inefficient.
The AAU fixes this by putting the heavy silicon computer directly inside the antenna box on the roof.
- Instead of thick copper cables, the AAU connects to the ground using a single, microscopic Fiber-Optic Cable (CPRI/eCPRI).
- The fiber shoots pure digital light up the tower. The AAU catches the light, instantly converts it into a massive analog radio wave, and blasts it out the front of the antenna with exactly zero cable loss.
The Massive MIMO Requirement
The AAU is not just about saving power; it is physically mandatory for 5G Massive MIMO.
To bend a radio wave around a building (Beamforming), the antenna must use an array of 64 or 128 microscopic transmitters, all firing in perfect, microsecond-synchronized mathematical harmony. If the computer was on the ground, running 128 separate copper cables up the tower to synchronize the array would be physically impossible. The AAU places the digital phase-shifting supercomputer millimeters away from the metal array, making the complex calculus of Massive MIMO a physical reality.
Frequently Asked Questions
Are AAUs heavier than old antennas?
Massively heavier. A legacy passive antenna is essentially an empty plastic box with some light copper wires inside, weighing roughly 40 pounds. An AAU is packed with heavy solid-state silicon, massive copper heat-sinks, and heavy aluminum armor. A modern 64T64R (64 Transmit / 64 Receive) AAU can easily weigh 120 to 150 pounds, often requiring telecom carriers to structurally reinforce the steel cell tower before they can install it.
How does an AAU handle the heat?
Heat is the AAU's greatest enemy. Because the massive radio computer is trapped inside a sealed box baking in the direct 120°F summer sun, the silicon threatens to melt. The entire back half of the AAU is a massive, highly engineered block of jagged aluminum 'fins'. These fins act as a passive heat-sink, relying entirely on the wind to constantly pull the massive heat away from the silicon without the use of fragile moving fans.
What is the difference between an AAU and an RRU?
An RRU (Remote Radio Unit) was the half-step between 4G and 5G. In an RRU setup, the radio is moved to the top of the tower, but it is still a separate physical box connected to the antenna with short 3-foot jumper cables. An AAU (Active Antenna Unit) completely eliminates the jumper cables, physically fusing the radio and the antenna into one single, inseparable block of hardware.