CPRI protocol stack | RRH protocol stack
As mentioned in the article on CPRI frame structure, CPRI interface lies between REC and REs in RRH system architecture. CPRI interface covers two layers layer-1 (physical layer) and layer-2(data link layer). The figure below depicts CPRI protocol stack mentioning this layers.
The CPRI interface supports following information types:
• IQ Data which is user plane information in the form of in-phase and quadrature phase modulation data (i.e. digital baseband signals).
• Synchronization data used for frame and time alignment.
• L1 Inband Protocol: Signalling information that is related to the link and is directly transported by the physical layer. This information is required, e.g. for system start-up, layer-1 link maintenance and the transfer of time critical information that has a direct time relationship to layer-1 user data.
• C&M data:Control and management information exchanged between the control and management entities within the REC and the RE. This information flow is given to the higher protocol layers.
• Protocol Extensions: This information flow is reserved for future protocol extensions. It may be used to support, e.g., more complex interconnection topologies or other radio standards.
• Vendor Specific Information: This information flow is reserved for vendor specific information.
Layer-1 and Layer-2 functions
In RRH, CPRI protocol stack consists of layer-1(physical layer) and layer-2(data link layer).
RRH protocol stack Layer-1 defines following:
• Electrical characteristics
• Optical characteristics
• Time division multiplexing of the different data flows
• Low level signalling
RRH protocol stack Layer-2 defines following:
• Media access control
• Flow control
• Data protection of the control and management information flow
Following are the functions of various protocol data planes:
The following data flows can be discerned:
Control Plane: Control data flow used for call processing.
Management Plane: Management information for the operation, administration and maintenance of the CPRI link and the RE.
User Plane: Data that has to be transferred from the radio base station to the mobile station and vice versa. These data are transferred in the form of IQ data.
Synchronization: Data flow which transfers synchronization and timing information between REC and RE. The control and management plane will be mapped to a single information flow over the CPRI link.
User plane data:
The user plane data are transported in the form of IQ data. Several IQ data flows will be sent via one physical CPRI link. Here each I/Q data represents per carrier and it is for one antenna. It is referred as antenna carrier.
Antenna-carrier (AxC): One antenna-carrier is the amount of digital baseband (IQ) U-plane data necessary for either reception or transmission of one UTRA-FDD carrier at one independent antenna element.
AxC Container: It contains the IQ samples of one AxC for one UMTS chip duration.
Service Access Points: For all protocol data planes layer 2 services access points are defined that will be used as reference points for performance measurements. These service access points will be denoted as SAPCM, SAPS and SAPIQ.
Downlink: Direction from REC to RE.
Uplink: Direction from RE to REC.
The table below describes the functions of REs and REC entities in a RRH (Remote Radio Head).
|Functions of REC||Functions of RE|
|Radio Base Station Control and Management|
|lub Transport||RRC Channel Filtering|
|lub Frame protocols||D/A conversion||A/D conversion|
|Channel Coding||Channel Decoding||Up Conversion||Down Conversion|
|Interleaving||Deinterleaving||ON/OFF control of each carrier||AGC|
|Spreading||Despreading||Carrier Multiplexing||Carrier De-multiplexing|
|Scrambling||Descrambling||Power amplification and limiting||LNA|
|Adding of physical channels||signal distribution to signal processing units||Antenna supervision|
|Transmit power control of each physical channel||Transmit power control & feedback information detection||RF filtering||RF filtering|
|Frame and slot signal generation(including clock stabilization)|