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1. Basic Concepts of CPRI
The Common Public Radio Interface (CPRI) Alliance is an industrial cooperative organization dedicated to the formulation of the main interface specifications between the Radio Equipment Control Center ( REC for short ) and the radio equipment ( RE for short) inside the wireless base station. As shown below:
The companies that initiated the establishment of the CPRI organization include: Ericsson, Huawei, NEC, Nortel Networks and Siemens.
Purpose :
Make the CPRI specification flexibly and effectively divide the wireless base station products, and independently develop the wireless equipment control center (referred to as REC) and wireless equipment (referred to as RE) technology.
Scope :
The specification includes the necessary elements for transmission, connectivity and control. Specifically, user platform data and control and management platform transport mechanisms and synchronization platform mechanisms. Emphasis is placed on hardware dependent layers (physical and data link layers). This ensures independent technological development (at both ends of the interface) with limited hardware adaptation needs. Furthermore, product division, management, and features by function are unlimited. With a clear understanding of the physical layer (Layer 1) and the data link layer (Layer 2), the scope of the CPRI specification is again limited to just the basic point-to-point link interface. Such links possess all the features necessary to simply and fully use any given REC/RE topology network, including direct interconnection between multiple radios. Error correction techniques are not addressed in the CPRI specification, however, all features required to support error correction, especially system architectures that provide error correction physical interactions (eg, rings), are specified.
The specification system is as follows:
- A digitally continuous internal radio base station interface between the REC and the RE and between the two REs .
- Three different information streams (user platform data, control and management platform data, synchronization platform data) are multiplexed through the interface.
- 3. The specification includes the physical layer (Layer 1) and the data link layer (Layer 2):
- (1) Physical layer (Layer 1): Supports electrical interface (used by traditional base stations) and optical interface (used between base station equipment and remote wireless devices).
- (2) The data link layer (Layer 2) supports mobility and scalability.
2. Related terms
subsystems _ _
The wireless base station consists of two basic subsystems (the wireless equipment control center REC and the wireless equipment RE). .
node ( node )
When referring to either REC and RE , the REC and RE subsystems are also referred to as nodes. A radio base station system may include two or more nodes (one REC and one or several REs ).
Protocol layers
It is divided into physical layer (layer 1 ) and data link layer (layer 2 ).
Physical layer (Layer 1 ) :
- Electrical interface characteristics
- Optical interface characteristics
- Time division multiplexing between different data streams
- low level signal
Data link layer ( Layer 2 ) :
- media access control
- flow control
- Data Protection of C&M Information Flow
Protocol data planes
The following data streams are recognized:
Control plane : The flow of control data for the calling process.
Management platform : management information for CPRI link and RE operation, management and maintenance.
User Platform : The data of the IQ data pattern transmitted between the base station and the mobile device .
Synchronization Platform : A data stream of synchronization and timing information transmitted between REC and RE .
The control plane and management plane are mapped to the control and management plane business access point SAP- CM which will be described below .
User plane data
User platform data is transmitted in IQ data mode. Multiple IQ data streams share a CPRI link. Each IQ data stream corresponds to data for one antenna carrier, called AxC .
Antenna - carrier
One antenna carrier carries a large amount of digital baseband (IQ ) user platform necessary data, which is received or transmitted over a UTRA-FDD carrier on a separate branch of the antenna.
Antenna Carrier Container ( AxC Container )
Contains IQ samples for the duration of one UMTS chip by the antenna carrier .
Service Access Points (Service Access Points ) SAP
The 2-layer service access points of various protocol data platforms are mainly used for performance evaluation, which are divided into control and management platform service access point SAP - CM , synchronization platform service access point SAP - S , user data service access point SAP - IQ . Each link principal component defines a service access point.
link _ _
Used to represent a bidirectional interface between two ports of a directly connected node (between REC and RE or between two REs ), with separate transmission lines in each direction. A working link includes a master port, a bidirectional cable, and a slave port.
The working link defined by this specification is the master / slave link . Master / Master links and Slave / Slave links are not allowed .
Passive Link _
A passive link does not support any control management channel, that is, it only carries IQ data and synchronization information. It can be used for capacity expansion or error correction purposes.
hop _ _
All links between directly connected nodes are called hops. A hop can be between REC and RE or between two REs .
Multi - hop connection
Consists of a set of continuous hops from REC to RE or between REs .
Logical connection _
A logical connection defines the connection between a SAP belonging to a certain REC port (such as SAPCM ) and a SAP belonging to a certain RE port ( such as SAP CM ), and establishes one or more hops between the REC and one of its REs Connection. Classified by C&M data, user data, and synchronization data.
Master port and Slave Port
Two ports with different functions and roles connected by a link, namely master port and slave port. Continuing the definition inherent in CPRI v1 , the master port is in the REC and the slave port is in the RE . The division of master / slave roles is precisely based on the following set of interface flows:
- Sync function
- C&M Channel Negotiation
- restart instruction
- startup sequence
Such a definition allows the reuse of key features of the CPRI v1 specification within the scope of CPRI v2 , such as: each link must have one terminal as the master port and one terminal as the slave port.
The port of the REC is always the master port , and the RE has at least one slave port and can arbitrarily designate other ports as master port or slave port.
Under normal conditions, a link always has one master port and one slave port. It is not normal for two master ports or two slave ports to be connected together and, therefore, outside the scope of this specification.
downlink _ _
The logical connection from the REC to the RE and the link direction from the master port to the slave port is called downstream.
Up ( uplink )
The logical connection from RE to REC and the direction of the link from the slave port to the master port is called upstream.
The following figure is a schematic diagram of each basic definition.
