RRU Remote Radio Unit: RF-to-Digital Conversion, Power Efficiency & Tower-Mount Design
Published June 21, 2026 • 8 min read • Telecom Insights
The Remote Radio Unit (RRU) — also called Remote Radio Head (RRH) — is the tower-mounted active RF subsystem that sits physically close to the antenna, converting between digital baseband (CPRI/eCPRI) and RF signals. Deployed in their millions across global macro cell sites, RRUs have evolved from simple RF-to-optical converters into sophisticated software-defined radios with integrated Crest Factor Reduction (CFR), Digital Pre-Distortion (DPD), and passive intermodulation (PIM) cancellation.
Key Takeaway: Moving the RRU from the base station shelter to the tower top eliminates 2–4 dB of feeder cable loss — translating to 30–50% improvement in cell-edge coverage and equivalent reduction in PA output power requirements.
RRU System Architecture
A modern 4T4R or 8T8R RRU comprises: CPRI/eCPRI interface (up to 25 Gbps per fiber), digital front-end (DFE) implementing CFR, DPD, and digital up/down-conversion, transceiver (integrated AFE with JESD204B/C interface to DFE), PA module (2–8 channels of GaN Doherty PAs delivering 20–60 W each), LNA and TDD switch, duplex filter (cavity or ceramic), and power supply (AC-DC or DC-DC). The entire assembly is housed in a weatherproof (IP65/IP67) cast aluminum enclosure with passive convective cooling fins.
GaN PA Technology for RRU Efficiency
RRU power efficiency is dominated by the PA — typically consuming 60–70% of total unit power. GaN-on-SiC HEMT technology has become the universal choice for macro RRU PAs, offering PAE of 50–55% in Doherty configuration at 8 dB back-off. A typical 8T8R RRU with 40 W per channel (320 W total RF output) requires approximately 700 W DC input — any improvement in PA efficiency directly reduces OPEX (electricity cost) and simplifies thermal design. Envelope tracking — while standard in handsets — sees limited adoption in RRUs due to the bandwidth-power product challenge at 100+ MHz instantaneous bandwidth.
Digital Front-End: CFR and DPD
The DFE's Crest Factor Reduction (CFR) algorithm limits the peak-to-average power ratio (PAPR) of the OFDM signal — typically from 10–12 dB to 7–8 dB — through peak windowing or pulse cancellation, enabling the PA to operate closer to saturation with acceptable EVM degradation (<1%). Digital Pre-Distortion (DPD) — using a generalised memory polynomial model — linearises the PA across the full 100–200 MHz instantaneous bandwidth, achieving ACLR improvement of 15–20 dB. The DPD adaptation engine runs continuously, tracking PA characteristic changes due to temperature, aging, and VSWR variation.
Passive Cooling and Thermal Design
With 500–1000 W of heat dissipation, the RRU relies entirely on passive natural convection cooling — no fans (unreliable in outdoor environments). The cast aluminum enclosure features tall, closely-spaced fins oriented vertically to maximise chimney-effect airflow. Heat pipes or vapor chambers embedded in the baseplate spread heat from the PA and power supply hot spots to the full fin area. Thermal design must maintain PA junction temperature below 150°C at 55°C ambient with solar loading (1120 W/m² per GR-487).
CPRI and eCPRI Fronthaul
The RRU connects to the BBU via CPRI (Common Public Radio Interface) or the newer eCPRI (enhanced CPRI) standard. CPRI carries digitised IQ samples at constant bit rate (e.g., 9.8 Gbps for 20 MHz LTE 4T4R). eCPRI — specified by the same industry cooperation — supports packet-based transport over Ethernet, enabling statistical multiplexing and lower fronthaul bandwidth through functional decomposition (e.g., Split 7.2 where only frequency-domain IQ symbols are transported). eCPRI line rates of 25 Gbps and 50 Gbps are typical for 5G NR.
The RRU continues to evolve toward higher integration, with ongoing development of integrated antenna-RRU assemblies (Active Antenna Units), massive MIMO RRUs with 64 transceivers, and O-RAN compliant units with standardised management interfaces (NETCONF/YANG).
