What advantages do active antennas provide?

Eliminating the feeder cable saves 3 to 5 dB of loss, directly improving both transmit EIRP and receive sensitivity. Per-element amplitude and phase control enables advanced beamforming: beam steering (directing energy toward specific users), null steering (avoiding interference), and spatial multiplexing (serving multiple users simultaneously on the same frequency). Energy efficiency improves 30 to 50 percent because smaller PAs operating at lower power per element replace a single high-power PA with lossy cable. System complexity is reduced by eliminating separate RRU, jumper cables, and tower-mounted amplifiers. Digital beamforming in massive MIMO provides up to 16 spatial layers, multiplying cell capacity.

What beamforming architectures exist?

Analog beamforming: RF phase shifters adjust element phases in the analog domain. One data stream, one beam. Simple but limited to single-beam operation. Used in mmWave with many elements. Digital beamforming: each element has its own DAC/ADC and up/down converter. Full flexibility for multiple simultaneous beams and spatial multiplexing. Maximum degrees of freedom but highest cost and power (used in sub-6 GHz mMIMO, typically 64T64R). Hybrid beamforming: combines analog sub-arrays with digital baseband processing. Each digital stream feeds an analog beamformer controlling a sub-array of elements. Balances flexibility and cost. Used in 5G mmWave (typically 2 to 4 digital streams, each driving a 16 to 32 element analog array).

What are typical 5G AAU specifications?

Sub-6 GHz massive MIMO (band n78, 3.5 GHz): 64T64R, 192 antenna elements (cross-pol dual-pol patch array), 200 W total TX power, 25 dBi array gain, 320 MHz instantaneous bandwidth, 25 kg weight, supports up to 16 spatial layers DL and 8 UL. Power consumption approximately 1000 to 1500 W. mmWave AAU (band n258, 26 GHz): 512 to 1024 elements, 2 to 4 digital streams with 128 to 256 element analog sub-arrays, hybrid beamforming, 35 to 40 dBi array gain, 400 to 800 MHz bandwidth, beam scanning plus or minus 60 degrees, 10 to 15 kg. Vendors: Ericsson AIR 6488, Nokia AirScale mMIMO, Samsung 5G Radio.

5G Infrastructure

Active Antenna

/ak-tiv an-ten-uh/ — AAS / AAU

Integrates RF electronics (PA, LNA, phase shifters, DAC/ADC) directly with antenna elements. Eliminates feeder cable loss (3-5 dB saved). 5G mMIMO: 64T64R, 192+ elements, digital beamforming, up to 16 spatial layers. Benefits: improved EIRP/sensitivity, per-element control, spatial multiplexing, 30-50% energy savings. Architectures: analog (single beam), digital (full flexibility), hybrid (balanced).

5G: 64T64R mMIMO

Elements: 192-1024

Gain: 25-40 dBi

Understanding Active Antennas

The transition from passive to active antennas is the most significant architectural change in base station design since the move from indoor BTS to remote radio heads. In a passive system, the radio unit sits at the base of the tower and connects to the antenna via long coaxial cables that lose 3-5 dB of the precious transmit power and add noise to the receive path. The active antenna eliminates these cables entirely by co-locating the RF electronics with the antenna elements.

This integration enables massive MIMO, where 64 or more independent transmit/receive chains drive hundreds of antenna elements. Each element can be individually controlled in amplitude and phase, creating highly directive beams that can be steered electronically to individual users. The result is a 5-10x increase in spectral efficiency compared to traditional macro cells with passive antennas.

Active Antenna Performance

Array gain:
G_array = G_element + 10log(N)
192 elements, 5 dBi each:
G = 5 + 10log(192) = 28 dBi

Cable loss elimination:
Passive: P_ant = P_PA − L_cable
Active: P_ant = P_PA (no cable)
Typical saving: 3-5 dB

Capacity gain (mMIMO):
C = min(N_TX,N_RX) × log₂(1+SNR)
64T64R, 16 layers: 16× throughput

Beamforming EIRP:
EIRP = P_total + G_array + G_BF
200W + 28dBi = 81 dBm EIRP peak

Active Antenna Architecture Comparison

Architecture	Streams	Beams	Cost	Application
Analog BF	1	1 steered	Low	mmWave simple
Digital BF	N_elements	Multiple	High	Sub-6 mMIMO
Hybrid BF	2-4	2-4 + sub-beams	Medium	5G mmWave
Passive + RRH	2-4	Fixed sectors	Low	LTE macro
mMIMO 64T64R	64	16 layers	Highest	5G NR sub-6

Common Questions

Frequently Asked Questions

Advantages?

Eliminates 3-5 dB cable loss. Per-element control enables beam steering, null steering, spatial multiplexing (16 layers). 30-50% energy savings (distributed small PAs vs. single large PA + lossy cable). Replaces separate RRU + jumpers + TMA. Digital BF provides maximum flexibility for capacity optimization.

Beamforming architectures?

Analog: RF phase shifters, 1 stream/beam. Simple, low cost. mmWave with many elements. Digital: per-element DAC/ADC, full flexibility, multiple beams/layers. 64T64R sub-6 mMIMO. Hybrid: analog sub-arrays + digital baseband. 2-4 digital streams, each driving 16-32 element sub-array. 5G mmWave standard.

5G AAU specs?

Sub-6 (3.5 GHz): 64T64R, 192 elements, 200W TX, 25 dBi, 320 MHz BW, 16 DL layers, ~1200W DC. mmWave (26 GHz): 512-1024 elements, hybrid BF, 35-40 dBi, 800 MHz BW, ±60° scan, 10-15 kg. Vendors: Ericsson AIR 6488, Nokia AirScale, Samsung 5G Radio.

Related Terms

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5G Systems

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