The widespread adoption of advanced mobile applications and their migration to lower-end, high-volume phones is driving base station manufacturers to develop higher bandwidth base station hardware at lower price points for the wireless infrastructure market.
More wireless customers are checking e-mail, obtaining real-time driving directions and road status, surfing the Internet, or watching video on mobile handsets. As customers become dependent on these capabilities, the network service providers will be continuously driven to improve their network performance and provide the services at a very competitive cost.
This is the driving force behind the development in 2003 of the Common Public Radio Interface (CPRI), a publicly available specification that defines the key internal interface between the Baseband Radio Equipment Control (REC) and Radio Equipment (RE) in a wireless base station.
The CPRI protocol provides the opportunity for more-flexible network architectures, faster time-to-market, lower-cost components and the ability to upgrade base station modules rather than the entire system.
This article will discuss the advantages of CPRI implementations, the range of system architectures supported, the scope of the CPRI specification and the basic technical details of CPRI.
CPRI definition and history
The CPRI specification is the result of an industry cooperation between five of the leading base station hardware suppliers: Ericsson AB, Huawei Technologies Co. Ltd., NEC Corporation, Nortel Networks and Siemens AG.
The goal was to define a simple and flexible interconnect for the key internal interface between the baseband processing subsystem in the REC and the radio or radio frequency (RF) processing function in the RE that could be used for all third-generation (3G) standards and that simplified the overall base station architecture.
The CPRI protocol is based on the basic 3G Universal Mobile Telecommunications System (UMTS) 10ms radio frame, and has evolved from the original specification to include provisions for support of WCDMA-, CDMA 2000-, TD-SCDMA- and WiMAX-based systems.
CPRI is an open standard and is easily accessible for general use. To encourage adoption, CPRI is made available under a non-assertion principle that the developing companies will not assert any essential patents against any party using or developing products or systems that include fully compliant CPRI interfaces.
The CPRI specification defines the user plane data, control plane transport mechanisms and the means for synchronization between the REC and RE.
Click here for Figure 1.
Figure 1. CPRI System and Interface Definition: The CPRI link is the connection between the Baseband Radio Equipment Control (REC) connected to the network and the Radio Equipment (RE) connected to the air interface. The CPRI specification defines OSI Layer 1 and 2 functionality, and includes User, Control & Management, and Synchronization flows.
The specification includes Layers 1 and 2 of the Open Systems Interconnection (OSI) Stack and is perfect in applications that utilize electrical or optical interface technology.
CPRI was developed to complement the Third-Generation Partnership Project (3GPP) Specification Release 5, which defines the globally applicable technical specifications for 3G networks, including Universal Terrestrial Radio Access (UTRA) in Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes.
Since the original specification release in 2003, there have been two additional specification levels defined to cover expanding market demands. CPRI 1.0, currently at revision 1.4, was the original specification that outlined the basic parameters, formats and bandwidth levels of the interface.
CPRI 2.0, currently at revision 2.1, includes support for additional network configurations beyond the original star topology. CPRI 3.0, the most recent specification version, which was introduced in 2006, has added provisions for WiMAX support and one additional higher speed line rate.
