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CPRI protocol stack | RRH protocol stack

This article section on CPRI basics covers following sub topics:
Remote Radio Head   CPRI protocol stack  CPRI frame structure 

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.

CPRI protocol stack,RRH protocol

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
Downlink Uplink Downlink Uplink
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)
Measurements Measurements


RRH (Remote Radio Head) Basics
CPRI protocol stack or RRH Protocol stack
CPRI or RRH frame structure

RF and Wireless Terminologies

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