Common Public Radio Interface
The Common Public Radio Interface (abbreviated CPRI ) defines an interface between the radio equipment control (English Radio Equipment Control, REC) and radio equipment (English radio equipment, RE) of a mobile - base station . Usually the REC is the baseband signal processing unit of the base station, the RE an antenna module.
CPRI is an industrial cooperation between Ericsson , Huawei Technologies , NEC Corporation , Nortel Networks , Alcatel-Lucent and Nokia Solutions and Networks, who founded the CPRI initiative in 2003. The CPRI interface supports both electrical and optical transmission of I / Q samples . The optical transmission allows the radio equipment control and the radio equipment to be set up spatially separated from one another. The specification of the CPRI interface is freely available online.
System architecture
In principle, RECs and REs can have any number of ports to other RECs or REs. However, these are always individual point-to-point connections. A connection consists of a master and a slave port , which mainly differ in the following points:
- The slave can recover the master's reference clock.
- The slave adapts to the link speeds of the master according to its possibilities.
- Usually the slave port is part of a RE and the master part of a REC
CPRI defines two network layers, the datalink layer and the physical layer . The data link layer defines the data access control and the flow control, while the physical layer defines the optical and electrical transmission properties and contains a time division multiplex method for transmitting different data channels. There are essentially three different data channels: a user data channel for I / Q samples, a control data channel and a synchronization channel .
Link speeds
The current CPRI specification V 4.2 (status: September 2, 2011) supports the following link speeds:
- 614.4 Mbit / s
- 1228.8 Mbit / s ( 2 × 614.4 Mbit / s)
- 2457.6 Mbit / s ( 4 × 614.4 Mbit / s)
- 3072.0 Mbit / s ( 5 × 614.4 Mbit / s)
- 4915.2 Mbit / s ( 8 × 614.4 Mbit / s)
- 6144.0 Mbit / s (10 × 614.4 Mbit / s)
- 9830.4 Mbit / s (16 × 614.4 Mbit / s)
Frame structure
The duration of a CPRI frame is 10 ms. This corresponds to the duration of an LTE radio frame. The CPRI frame is in turn divided:
- A CPRIFrame is divided into 150 hyperframes
- A hyperframe is divided into 256 BasicFrames
- A BasicFrames is divided into 16 words
- A word is 8 to 128 bits wide. The different word lengths depend on the speed of the connection. (see also link speeds ).
The first word of each BasicFrame (i.e. 1/16 of all data) is a control word. These control words contain control and synchronization data, all other words contain user data.
Advantages of CPRI over proprietary interfaces
Before there were standardized interfaces between the antenna module and the baseband signal processing unit, proprietary LVDS interfaces were mostly used. On the one hand, these require agreements between manufacturers, since these interfaces have not been made public, and on the other hand, no optical transmission is possible with LVDS. The advantages of optical transmission are complex, as it ensures that REC and RE can be up to a few kilometers apart. For example, it is possible to mount the RE, which is only a small part of the base station, in close proximity to the antennas, while traditional base stations are so large that they can only be mounted at the base of an antenna mast and not at the top of the mast. This means that the power losses that occur in the electrical cables between the amplifier (which is located in the RE) and the antennas can be neglected, since the distance between the amplifier and the antenna can be kept very small. So far, the loss has usually been 3 dB to 10 dB. This means that 50% - 90% of the power is lost before the signal reaches the antenna. Another advantage of the spatial separation is that one baseband signal processing unit can be used for many antenna modules, whereby computing resources can be distributed as required. This is of particular interest because the trend towards smaller radio cells can be seen in modern mobile radio systems.
There are also other competing standards that describe an interface between baseband signal processing and the antenna module. In addition to CPRI, the OBSAI standard is the most widespread.
Web links
- CPRI specification on cpri.info (English)
Individual evidence
- ↑ Does the wireless industry really need all these digital if standards? (PDF; 392 kB) (No longer available online.) In: IEEE Radio Communications Magazine. 2005, archived from the original on September 24, 2015 ; accessed on September 3, 2011 (English). Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ a b c CPRI specification V4.2. In: cpri.info. 2010, accessed on September 3, 2011 .
- ^ Enabling Distributed Base Station Architectures with CPRI, Gerry Leavey, Burnaby, BC, February 1, 2006
