Logical architecture
To understand fronthaul, middlehaul, and backhaul in 5G, first consider the logical architecture of mobile communication:
phone - access network - transport network - core network - transport network - access network - phone
Access network (RAN)
The access network in wireless communications generally refers to the radio access network (RAN), i.e., the base station.
Base station composition
A typical 4G base station usually includes:
- BBU (baseband unit): mainly responsible for signal modulation, channel coding/decoding, baseband modulation/demodulation, and protocol processing, and provides interfaces to higher-layer network elements.
- RRU (remote radio unit): mainly responsible for RF processing. It acts as a bridge between the antenna system and the BBU. On receive, the RRU filters and amplifies the RF signal from the antenna, converts it to an optical or digital form, and transmits it to the BBU. On transmit, the RRU converts signals from the BBU into RF signals and transmits them via the antenna.
- Feeder: connects the RRU and the antenna.
- Antenna system: performs signal transmission and reception, converting energy between guided waves on cables and space waves in the air.
5G RAN functional split: CU, DU, AAU
In 5G, the access network is restructured into three functional entities:
- CU (Centralized Unit): the non-real-time portion of the original BBU is split out and defined as the CU. It handles non-real-time protocols and services.
- DU (Distributed Unit): the remaining BBU functions are redefined as the DU, responsible for physical layer protocol processing and real-time services.
- AAU (Active Antenna Unit): part of the BBU physical layer processing is combined with the RRU and the antenna into the AAU.
Fronthaul, middlehaul, backhaul
These terms denote the connections between different functional entities in the RAN:
- Fronthaul: the connection between the AAU and the DU.
- Middlehaul: the connection between the DU and the CU.
- Backhaul: the connection from the CU upward toward the core network.
