Every generation of mobile network brings about new technology and infrastructure improvements to support faster data rates, greater bandwidth, and improved efficiency and coverage. To keep up with these advancements, base station architectures have steadily evolved over time. The introduction of fourth generation cellular (4G) required major changes in the traditional analog radio architecture, such as the migration of all analog circuitry to the remote radio unit (RRU).
This migration included the elimination of the coaxial connection between the baseband unit (BBU) and RRU, a common source of signal degradation due to insertion loss, reflections and other imperfections encountered along the signal path. Now the BBU and RRU can be separated by up to 20 km of high-performance, high-throughput fiber-optic cabling. The Common Public Radio Interface (CPRI) is the specification that defines the communication protocol running over the dedicated fiber link.
Overview of CPRI Specification
CPRI is a high-speed serial communication protocol for transferring digitized radio data and control information between the BBU (also called the Radio Equipment Control or REC) and RRU (also called the Radio Equipment or RE). In the Downlink (DL) path, the REC is responsible for signal conditioning and conversion of data into in-phase and quadrature (IQ) data, prior to sending it to the RE via the CPRI link. The RE converts the digitized IQ data back to analog RF signals and transmits it over the air via the antenna. Conversely, a similar process occurs in the opposite, or Uplink (UL), path.
Functionalities of the REC and RE for the Downlink path (source: IEEE)
Key Features of CPRI
Because CPRI was developed as an open specification by multiple industry parties – including Ericsson, Huawei, NEC, and Nokia – one of its strengths (and sometimes complications) is that the “standard” supports a multitude of deployment, equipment and vendor specific configurations. The protocol lays out a basic architecture for communication between the REC and RE, while still allowing flexibility for manufacturers to customize their equipment and for carriers to customize their own infrastructure to their specific operating needs.
Below are some of the main requirements and functionalities that the CPRI specification defines:
· The interface must be able to support multiple radio standards, including 3GPP UTRA FDD, WiMAX, 3GPP E-UTRA, and 3GPP GSM/EDGE.
· A wide range of channel bandwidthsare supported. For example, the transmission bandwidth of CDMA is 1.25 MHz, while the transmission bandwidth of LTE can be as much as 20 MHz.
· CPRI allows REs to support one or multiple antennae (sectors). For example, a typical topology may use 1 to 4 carriers x 2 antennas x 3 sectors per RE.
· While the most basic configuration consists of a point-to-point connection between one REC and one RE, the specification outlines numerous network topologiesincluding star, chain, tree, ring, and multi-hop connections.
Star topology using CPRI links (source: CPRI.info)
eCPRI: The Next Evolution of CPRI
With 5G networks on the horizon, the density of REs will significantly increase to meet the need for faster data rates, lower latency, and widespread coverage. Although the RE needs to reside near or at the antennae, the REC can be consolidated into common areas to reduce deployment and maintenance costs. Running dedicated optical fiber links to each RE is neither practical nor economical. Therefore, eCPRI seeks to leverage existing network infrastructure, such as Ethernet, for the fronthaul transport network. This new eCPRI fronthaul would allow for a truly distributed cloud-based radio access network (C-RAN) architecture to be realized where, for instance, a site contains five to six centralized RECs that each talk to dozens of remote REs over Ethernet.
eCPRI system architecture and interface definition (source: CPRI.info)
Interested in learning more? Read the full white paper provides detailed information about the CPRI protocol stack, frame structure, IQ data mapping, and networking differences between CPRI and eCPRI.