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 cellular base station. REC is usually represents the baseband signal processing unit of the base station, while the RE is an antenna module.

CPRI is an industrial cooperation between the companies Ericsson, Huawei Technologies, NEC Corporation, Nortel Networks, Alcatel Lucent and Nokia Solutions and Networks which have founded the CPRI initiative in 2003. The CPRI interface supports both electrical and optical transmission of the I / Q samples. By enabling optical transmission ensures that the radio equipment control and the radio equipment can be located spatially separated. The specification of the CPRI interface is freely available online.

System Architecture

Basically, RECs and RE can have any number of ports to other RECs or REs. This is always, however, to single point-to - point connections. A connection consists of a master and a slave port, which differ mainly in the following points:

  • The slave can recover the reference clock of the master.
  • The slave adapts according to its capabilities to the link speeds of the master.
  • Typically, the slave port is part of a REs and the master part of RECs

CPRI defines two network layers, the data link layer and the physical layer. The data link layer defines the data access control, and flow control, while the physical layer defines the electrical and optical transmission characteristics, and a time division method for transmitting different data channels includes. Essentially, there are three different data channels: a user data channel of the I / Q samples, a control data channel and a synchronization channel.

Link speeds

The latest CPRI Specification V4.2 (Updated: September 2, 2011) supports the following link speeds:

  • 614.4 Mbit / s
  • 1228.8 Mbit / s ( 2 x 614.4 Mbit / s)
  • 2457.6 Mbit / s ( 4 x 614.4 Mbit / s)
  • 3072.0 Mbit / s ( 5 x 614.4 Mbit / s)
  • 4915.2 Mbit / s ( 8 x 614.4 Mbit / s)
  • 6144.0 Mbit / s ( 10 x 614.4 Mbit / s)
  • 9830.4 Mbit / s ( 16 x 614.4 Mbit / s)

Frame structure

The duration of a CPRI frame is 10 ms. This corresponds to the duration of an LTE radio frames. The CPRI frame is subdivided:

  • A CPRIFrame is divided into 150 Hyper frames
  • A hyperframe is divided into 256 basic frames
  • A basic frame is divided into 16 words
  • A word is 8-128 bits wide. The different word lengths depending on the speed of the connection. ( see link speeds).

The first word of each Basic frame ( ie 1/16 of all the data ) is in each case a control word. These control words contain control and synchronization of data, all the other words contained useful data.

Benefits of CPRI over proprietary interfaces

Before it was standardized interfaces between the antenna module and baseband signal processing unit proprietary LVDS interfaces have been mostly used. These require for a collusion between manufacturers, since these interfaces were not made public, on the other hand no optical transmission is possible with LVDS. The advantages of optical transmission are complex, as this ensures that REC and RE spatially up to a few kilometers of each other can be separated. For example, it is possible that RE, which is only a small part of the base station to be mounted in close proximity to the antennas, while the traditional base stations, are so large that they can only be installed at the foot of an antenna mast and not the top of the mast. This allows the power losses in the electrical cables between the amplifier ( which is located in the RE ) arise be neglected and antennas, since the distance between the amplifier and the antenna can be kept very small. So far, the loss of 3 dB is usually up 10dB. This means that 50 % - 90 % of the power is lost, before the signal reaches the antenna. Another advantage of the spatial separation is that a base-band signal processing unit can be used for many antenna modules, thereby computing resources can be distributed as needed. This is therefore particularly interesting because of the trend to 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 module and antenna. In addition to the CPRI intermateability standard is the most widespread.

Mobile telephony Payload (computing) Open Base Station Architecture Initiative
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