Common Public Radio Interface (CPRI) Protocol
The Common Public Radio Interface (CPRI) protocol consists of a specification that enables flexible and efficient product differentiation for radio base stations and independent technology evolution for RE and REC. In other word, the goal of CPRI is to allow base stations manufacturers to share a common protocol and more easily adapt platforms from one customer to the other. Besides that, CPRI focuses on simplified radio base station architecture by dividing the radio base station into a radio and a control part, by specifying one new interface. The groups cooperating on defining CPRI specifications are Ericsson AB, Huawei Technologies Co. Ltd, NEC Corporation, Nortel Networks Ltd, Alcatel Lucent and Nokia Siemens Networks GmbH & Co. KG.
Principal Advantages of CPRI
The CPRI specification enables flexible and efficient product differentiation for radio base stations and independent technology evolution for RE and REC. In addition, CPRI has little overhead compared to the one in OBSAI, which makes it more advantageous to implement. Another significant advantage using CPRI is the bit error rate for the dataplane is 10-12, which is less strict than the OBSAI RP3 requirement of 10-15, to achieve a cost efficient solution on layer 1.
CPRI Protocol Overview
CPRI supports transmission of all IQ data between REC and RE in both directions compliant to 3GPP UTRA FDD, Release 5 (June 2004) radio standard in logical scope. CPRI supports a continuous range of distances between master and slave port. The minimum required range for the cable length is:
- Lower limit – 0 m
- Upper limit – > 10 km
For the transfer of user plane, control and management information (C&M) as well as synchronization information between REC and RE, CPRI defines 2 layer protocols:
- The user plane information is sent in the form of IQ data. These IQ data coming from different antenna carriers are multiplexed by a time division multiplexing scheme onto an electrical or optical transmission line.
- C&M data are sent either as inband protocol (for time critical signaling data) or by layer 3 protocols (not defined by CPRI) that reside on top of appropriate layer 2 protocols. Two additional layer 2 protocols for C&M data, which are High level Data Link Control (HDLC) and Ethernet, are also supported by CPRI. These control and management data are time multiplexed with the IQ data.
- Any type of vendor specific information is also transferred with the additional time slots.
The information flows supported by the interface are:
|IQ data||user plane information of in-phase and quadrature modulation data (digital baseband signals)|
|Synchronization||data used for frame and time alignment|
|L1 Inband Protocol||information related to the link and is directly transported by the physical layer for system start-up|
|C&M data||control and management information exchanged between the control and management entities within the REC and RE|
|Protocol Extensions||information reserved for future protocol extensions, may be used for more complex interconnection topologies or other radio standards|
|Vendor Specific Information||reserved for vendor specific information|
CPRI Basic Frame Structure
A CPRI basic frame structure consists of 16 words indexed by W = 0…15 where the first word W = 0 is used for a control word. The word size on the CPRI depends on the link rate (which is also the line bit rate), where five different rates are defined in order to achieve the required flexibility and cost efficiency. It is compulsory that each REC and RE support at least one of the line bit rates listed below:
|Link Rate (CPRI line bit rate)||Word Size in byte(s)|
|1x (614.4 Mbps)||1|
|2x (1228.8 Mbps)||2|
|4x (2457.6 Mbps)||4|
Notice that all CPRI line bit rates have been chosen in a way that the basic UMTS chip rate of 3.84 Mbps can be recovered in a cost-efficient way from the line bit rate taking into account the 8b/10b coding. For example, the 1228.8 Mbps correspond to an encoder rate of 122.88 MHz for the 8b/10b encoder and a subsequent frequency division by a factor of 32 provides the basic UMTS chip rate. The length of a basic frame is 1 Tchip = 1/3.84 MHz = 260.416667 ns. There are 256 basic frames in a hyper frame (HFN = 66.67 µs) and an UMTS radio frame consists of 150 hyper frames (10 ms). Below are CPRI basic frame structures:
|B||bit index (0 to 7)|
|W||word index (0 to 15)|
|Y||byte index within a word (0 to 3)|
|X||basic frame number (0 to 255)|
|Z||hyper frame number (0 to 149)|
|Z.X.Y||control words definition|
Basic frame structure for 614.4Mbps CPRI line rate
Basic frame structure for 1228.8 Mbps CPRI line rate
Basic frame structure for 2457.6 Mbps CPRI line rate
Three topologies are supported by AIF module in CPRI mode:
Star chain: multiple point-to-point links between a REC and several Res
Daisy chain: connection in series of REC and Res
U Daisy chain: redundant connection of Daisy chain