5G Base Station

Overview

A 5G base station, formally a gNodeB, is the radio access node that links subscriber handsets to the mobile core network. This unit follows the distributed-RAN split: a roof- or tower-mounted Active Antenna Unit (AAU) handles all radio-frequency work, while an indoor Baseband Unit (BBU) performs digital baseband processing. The two are joined by the Optical Fronthaul Cable, a single-mode fiber run carrying eCPRI-framed traffic at 25 Gbit/s. Site infrastructure comprises a -48 V Power System with rectifier and battery backup, a GPS / Sync Module for timing, a galvanized Tower Mounting Kit, and a Grounding Kit for lightning protection.

The AAU integrates a 64-element dual-polarized array with one transmit/receive chain per element, enabling digital beamforming. Each T/R Module contains a GaN power amplifier, a low-noise amplifier and a phase shifter, and the array radiates through an RF-transparent radome. Steering weights are computed by the Beamforming ASIC Board.

How it works

Downlink data arrives at the BBU from the core network. Baseband boards perform layer-1 PHY processing, channel coding, modulation and the inverse FFT that builds the OFDM waveform, then send per-antenna I/Q samples across the fronthaul to the AAU. There the digital samples are upconverted, amplified by the T/R modules and emitted by the array. Because each element is independently phased, the station forms narrow beams aimed at individual users, raising signal strength and letting many users share the same spectrum through spatial multiplexing (MU-MIMO).

Precise timing is essential for TDD operation, where transmit and receive share one frequency and alternate in time. The GNSS module disciplines an oven-controlled oscillator to hold phase alignment within roughly a microsecond, so neighboring cells switch between transmit and receive in lockstep and avoid mutual interference. The uplink reverses the chain: weak signals are boosted by the LNAs, digitized, beam-combined and returned to the BBU for decoding.