What is CPRI (Common Public Radio Interface)?
CPRI (Common Public Radio Interface) is a specification for wireless communication networks that defines the key criteria for interfacing the transport, connection and control of communications between baseband units (BBUs) and remote devices. radio unit (RRUs), which are also called remote radio heads (RRHs).
CPRI defines key internal interfaces (digitized serial) between radio equipment (RE) and local/remote radio equipment controllers (RECs) of radio base stations. It provides a standardized framework to facilitate the development of equipment for mobile telecommunications networks. As an open specification, CPRI benefits any entity in the wireless industry.
Radio equipment is the collective term for RRUs or RRHs, while RECs are also known as baseband units. As a specification for a serial interface between BBUs and RRUs, CPRI provides for a high-speed connection between these parts of a radio base station.
CPRI consists of Layers 1 and 2 of the OSI model, with Layer 1 supporting electrical and optical interfaces at base stations and Layer 2 supporting increased flexibility and scalability. In addition to defining the interface of radio base stations, the first version of the specification (CPRI v1.0) also defines objects for transport, connection and control, including user plane data, control of transport mechanisms in plane and synchronization methods.
The CPRI specification was originally developed in 2004 by a consortium of five original equipment manufacturers: Nokia, Huawei Technologies, Ericsson, NEC, and Nortel. Nortel left the consortium in 2009.
CPRI system architecture
The basic CPRI system architecture consists of a radio base station divided into a radio part and a control part with a CPRI link defining the interface between them. REC and RE comprise the control and management module, sync module, and user plane — all at Layer 2 of the OSI stack.
The sync module and its L1 inband protocol are defined in the CPRI specification. In particular, the UMTS protocol defined by CPRI v1 and v2, WiMax by CPRI v3, LTE by CPRI v4, etc.
At Layer 1 of the OSI stack, the CPRI specification addresses how time division multiplexing (TDM) occurs. This is important because the specification is based on TDM of digitized I/Q bits on multiple antennas and carriers in the form of in-phase and quadrature samples. As such, CPRI is bandwidth inefficient although efficiency can be improved by compressing CPRI, with little degradation of overall CPRI performance. Since CPRI supports both electrical and optical interfaces, both types of transmission can occur (also at Layer 1).
Important features of CPRI
CPRI specifications support L1 inband protocol, High-Level Data Link Control and Ethernet for configuration and management data. Other data types supported by CPRI include synchronization for frame and time alignments. The CPRI specification is not a standard, but it covers 3street Generation Partnership Project (3GPP) and 3GPP2 (3G and Long-Term Evolution or LTE) frameworks.
An important feature of CPRI is its support for separation between the base frequency band and the radio frequency band. In addition, it supports electrical and optical interfaces, as well as many topologies, including point-to-point, star, ring and daisy chain. CPRI refers to the communication link, the fronthaul network that interfaces between radio transceivers and base stations.
CPRI has proven its usefulness for fronthaul communications between RRHs and BBUs through multiple generations of wireless networks. CPRI benefits include:
- It is a flexible and scalable specification for wireless radio base stations.
- Base station manufacturers may use a common protocol.
- It provides a broad portfolio of radio base stations that can be easily adapted to many deployment scenarios.
- The details are free and publicly available.
- The public can contribute ideas and suggestions to improve the details.
- It provides efficient and flexible data interfaces for many wireless standards, including the Global System for Mobile Communications, Wideband Code Division Multiple Access, and LTE.
What is eCPRI?
In 2017, the CPRI specification eCPRI 1.0 became available for download. In 2019 eCPRI v2.0 was released. This specification provides intelligent CPRI mapping to minimize bandwidth requirements and allows new and legacy equipment to coexist on the same Ethernet-based fronthaul network. In addition, the updated specification further expands the flexibility of eCPRI for fronthaul transport between eCPRI nodes and CPRI nodes.
The reason for the development of eCPRI is that high rates cannot be made cost-effective with the CPRI serial interface, even with CPRI compression of at least 3-to-1. For example, a typical 5G use case will use MIMO (multiple input, multiple output) technology to transfer large amounts of data simultaneously and require extremely high CPRI data rates in the tune of hundreds of Gbps.
This disadvantage of CPRI led to the development of a new functional split option called eCPRI. eCPRI provides a way to divide and transfer some functions of the BBU to the RRUs to reduce the burden on the fiber between the BBU and the RRUs.
The updated specification is expected to provide higher network throughput, better efficiency, and better connectivity with less fiber. It also provides a good balance between the low latency and high reliability requirements of 5G. Also, since eCPRI is an open interface, carriers can work together to create a more connected and faster 5G network.
The consortium that developed CPRI and eCPRI continues to work toward refining both specifications. As it develops, eCPRI is expected to become a viable option for all networks with dedicated fronthaul fiber connections, including 5G applications.
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