1
Common Public Radio Interface
Tapas Jana
RWTH-Aachen University
Abstract—This paper provides a basic overview on a new architecture reduces costs for service providers because only
interface specification named, CPRI (Common Public Radio the remote radio heads containing the RE need to be situated
Interface). CPRI is a joint industry initiative to define a publicly in environmentally challenging locations. The base stations
available specification that standardizes the protocol interface
between the radio equipment control (REC) and the radio equip- can be centrally located in more accessible and affordable
ment (RE) in wireless base stations. This allows interoperability locations where footprint, climate, and availability of power
of equipment from different vendors, and preserves the software are more easily managed. Figure 1 shows block diagram of
investment made by wireless service providers. In this seminar CPRI network interface. Main motivation and scope of the
paper, we present scope, benefits, design specification and goal CPRI are briefly discussed below [2].
of CPRI.
Index Terms—CPRI, Radio Equipment Control, Radio Equip-
ment.
A. Motivation of CPRI:
CPRI Simplified Radio Base station architecture dividing
into two distinct parts radio (RE) and control part (REC)
I. I NTRODUCTION by specifying one new unique interface between them. This
T he Common Public Radio Interface (CPRI) initiative was
formed in June of 2003 by five companies [2] (Ericsson,
Huawei, NEC, Nokia Siemens Networks and Alcatel-Lucent)
allows Flexible and efficient product differentiation for radio
base stations and independent technology evolution for RE
and REC. Network operator gets more flexibility to work in
and defined a new standardized interface for the interconnect different domains for control, management and maintenance
point between the radio equipment (RE) and radio equipment procedures in network system. It gives global standardization
controller (REC) within the base station to provide network for equipment and software vendors and network operators to
operator more deployment flexibility. In conventional wireless incorporate technology evolution.
network systems base-stations are located adjacent to the
antenna in a small room at the base of the antenna tower. B. Scope of CPRI Specification:
Now it is a challenging task to find a suitable place for setting
CPRI is a publicly available specification free for all compa-
up a base-station room and the antenna tower adjacently in
nies to use. Scope of this specification is limited on hardware
congested urban area and due to problem regarding structural
dependent layers (L1 and L2). This ensures independent tech-
reinforcement of rooftops, workspace for maintenance and
nology evolution (on both sides of the interface), with a limited
repairing, and the availability of both primary and backup
need for hardware adaptation. It includes necessary items for
power sources.
transport, connectivity and control using User Plane data,
Control and Management Plane transport mechanisms and
synchronization procedures. CPRI specification is restricted to
the link interface only, which is basically a point to point
interface. CPRI can be used for all relevant 3G standard.
In Section 2 system architecture of CPRI interface
is described, design goal and hardware implementation are
discussed. Section 3 describes shortly on effects and benefits
of CPRI and section 4 gives a brief conclusion.
II. S YSTEM A RCHITECTURE
In wireless network systems (UMTS, GSM, WiMAX) the
radio base station is composed of two subsystems a) Radio
Equipment Control (REP) and b) Radio Equipment (RE).
Fig. 1: CPRI Interface These two units are physically separated by high speed optical
fiber link using CPRI. RE consists of transmitting and receiv-
ing antenna and circuitry and provides connectivity through
CPRI allows the use of a distributed architecture where Air interface. REC consists of control unit, signal multiplexing
base stations, containing the REC, are connected to remote and modulation unit, database and storage unit etc. It provides
radio heads (transmitting antenna) via lossless fibre optic access to the radio network controller, helps in multiple access
link. Interface data are exchanged through this link. This and traffic control. One or more REs can be connected with
, 2
one REC via CPRI link. In this section some basic overview
on system architecture is presented. The CPRI interface is
graphically presented in the figure 2. Some basic properties
of the CPRI interface are mentioned below.
A. Interface:
• Support both electrical and optical interface.
• A digitized serial interface enabling single-hop and multi-
hop topologies.
• Three different information flows (User Plane data, Con-
trol and Management Plane data, and Synchronization
Plane data) are multiplexed over the interface.
• Multi-mode or single-mode optical fiber for the uplink or
downlink, using the same Optical connectors.
• SFP/SFP+ transceiver module is used in CPRI interface Fig. 3: Protocol Overview[2]
[5].
D. CPRI Frame Structure:
All CPRI line bit rates have been chosen as same as basic
UMTS chip rate of 3.84 Mbit/s [2]. The length of a basic frame
is 1 Tc = 1/fc = 1/3.84 MHz = 260.416667ns. A basic frame
consists of 16 words with index W=015. 1st word of each
frame is a control word. The length T of the word depends
on the CPRI line bit rate. Word length (T) is given by (Line
Rate in Mbps)/76.8 [9]. Each bit within a word is addressed
with the index B, where B=0 is the LSB and B=T-1 is the
Fig. 2: CPRI System and Interface Definition[2]
MSB. Each BYTE within a word is addressed with the index
Y, where B=0 is LSB of Y=0, B=7 is MSB of Y=0, B=8
is LSB of Y=1, etc. The other 15 words are used to carry
user plane data (IQ data block). Figure 4 shows a basic frame
B. Protocol Layers: structure for 188.8 Mbit/s CPRI line bit rate.
CPRI defines protocol layers for physical layer (L1) and
data link layer (L2). L1 defines data transfer by Electri-
cal/Optical medium, time division multiplexing of the different
data flows and control Low level signalling. Function of L2
is Medium access control, Flow control Data protection of
the control and management information flow. Basic protocol
overview for CPRI is given in figure 3.
C. CPRI Data Plane:
The following data flows are discerned [2]:
Control Plane: Control data flow used for call processing.
Management Plane: This data is management information for
the operation, administration and maintenance of the CPRI
link and the nodes.
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 nodes. The central clock for
frequency generation in the RE shall be synchronized to the bit
clock of one slave port. With 8B/10B line coding the bit clock Fig. 4: Basic frame structure for 1228.8 Mbit/s CPRI line bit
rate of the interface shall be a multiple of 38.4MHz. Any RE rate [2]
receives on its slave port a clock traceable to the main REC
clock. It reuses the same clock at its master port and forward
to slave port of next RE.
Common Public Radio Interface
Tapas Jana
RWTH-Aachen University
Abstract—This paper provides a basic overview on a new architecture reduces costs for service providers because only
interface specification named, CPRI (Common Public Radio the remote radio heads containing the RE need to be situated
Interface). CPRI is a joint industry initiative to define a publicly in environmentally challenging locations. The base stations
available specification that standardizes the protocol interface
between the radio equipment control (REC) and the radio equip- can be centrally located in more accessible and affordable
ment (RE) in wireless base stations. This allows interoperability locations where footprint, climate, and availability of power
of equipment from different vendors, and preserves the software are more easily managed. Figure 1 shows block diagram of
investment made by wireless service providers. In this seminar CPRI network interface. Main motivation and scope of the
paper, we present scope, benefits, design specification and goal CPRI are briefly discussed below [2].
of CPRI.
Index Terms—CPRI, Radio Equipment Control, Radio Equip-
ment.
A. Motivation of CPRI:
CPRI Simplified Radio Base station architecture dividing
into two distinct parts radio (RE) and control part (REC)
I. I NTRODUCTION by specifying one new unique interface between them. This
T he Common Public Radio Interface (CPRI) initiative was
formed in June of 2003 by five companies [2] (Ericsson,
Huawei, NEC, Nokia Siemens Networks and Alcatel-Lucent)
allows Flexible and efficient product differentiation for radio
base stations and independent technology evolution for RE
and REC. Network operator gets more flexibility to work in
and defined a new standardized interface for the interconnect different domains for control, management and maintenance
point between the radio equipment (RE) and radio equipment procedures in network system. It gives global standardization
controller (REC) within the base station to provide network for equipment and software vendors and network operators to
operator more deployment flexibility. In conventional wireless incorporate technology evolution.
network systems base-stations are located adjacent to the
antenna in a small room at the base of the antenna tower. B. Scope of CPRI Specification:
Now it is a challenging task to find a suitable place for setting
CPRI is a publicly available specification free for all compa-
up a base-station room and the antenna tower adjacently in
nies to use. Scope of this specification is limited on hardware
congested urban area and due to problem regarding structural
dependent layers (L1 and L2). This ensures independent tech-
reinforcement of rooftops, workspace for maintenance and
nology evolution (on both sides of the interface), with a limited
repairing, and the availability of both primary and backup
need for hardware adaptation. It includes necessary items for
power sources.
transport, connectivity and control using User Plane data,
Control and Management Plane transport mechanisms and
synchronization procedures. CPRI specification is restricted to
the link interface only, which is basically a point to point
interface. CPRI can be used for all relevant 3G standard.
In Section 2 system architecture of CPRI interface
is described, design goal and hardware implementation are
discussed. Section 3 describes shortly on effects and benefits
of CPRI and section 4 gives a brief conclusion.
II. S YSTEM A RCHITECTURE
In wireless network systems (UMTS, GSM, WiMAX) the
radio base station is composed of two subsystems a) Radio
Equipment Control (REP) and b) Radio Equipment (RE).
Fig. 1: CPRI Interface These two units are physically separated by high speed optical
fiber link using CPRI. RE consists of transmitting and receiv-
ing antenna and circuitry and provides connectivity through
CPRI allows the use of a distributed architecture where Air interface. REC consists of control unit, signal multiplexing
base stations, containing the REC, are connected to remote and modulation unit, database and storage unit etc. It provides
radio heads (transmitting antenna) via lossless fibre optic access to the radio network controller, helps in multiple access
link. Interface data are exchanged through this link. This and traffic control. One or more REs can be connected with
, 2
one REC via CPRI link. In this section some basic overview
on system architecture is presented. The CPRI interface is
graphically presented in the figure 2. Some basic properties
of the CPRI interface are mentioned below.
A. Interface:
• Support both electrical and optical interface.
• A digitized serial interface enabling single-hop and multi-
hop topologies.
• Three different information flows (User Plane data, Con-
trol and Management Plane data, and Synchronization
Plane data) are multiplexed over the interface.
• Multi-mode or single-mode optical fiber for the uplink or
downlink, using the same Optical connectors.
• SFP/SFP+ transceiver module is used in CPRI interface Fig. 3: Protocol Overview[2]
[5].
D. CPRI Frame Structure:
All CPRI line bit rates have been chosen as same as basic
UMTS chip rate of 3.84 Mbit/s [2]. The length of a basic frame
is 1 Tc = 1/fc = 1/3.84 MHz = 260.416667ns. A basic frame
consists of 16 words with index W=015. 1st word of each
frame is a control word. The length T of the word depends
on the CPRI line bit rate. Word length (T) is given by (Line
Rate in Mbps)/76.8 [9]. Each bit within a word is addressed
with the index B, where B=0 is the LSB and B=T-1 is the
Fig. 2: CPRI System and Interface Definition[2]
MSB. Each BYTE within a word is addressed with the index
Y, where B=0 is LSB of Y=0, B=7 is MSB of Y=0, B=8
is LSB of Y=1, etc. The other 15 words are used to carry
user plane data (IQ data block). Figure 4 shows a basic frame
B. Protocol Layers: structure for 188.8 Mbit/s CPRI line bit rate.
CPRI defines protocol layers for physical layer (L1) and
data link layer (L2). L1 defines data transfer by Electri-
cal/Optical medium, time division multiplexing of the different
data flows and control Low level signalling. Function of L2
is Medium access control, Flow control Data protection of
the control and management information flow. Basic protocol
overview for CPRI is given in figure 3.
C. CPRI Data Plane:
The following data flows are discerned [2]:
Control Plane: Control data flow used for call processing.
Management Plane: This data is management information for
the operation, administration and maintenance of the CPRI
link and the nodes.
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 nodes. The central clock for
frequency generation in the RE shall be synchronized to the bit
clock of one slave port. With 8B/10B line coding the bit clock Fig. 4: Basic frame structure for 1228.8 Mbit/s CPRI line bit
rate of the interface shall be a multiple of 38.4MHz. Any RE rate [2]
receives on its slave port a clock traceable to the main REC
clock. It reuses the same clock at its master port and forward
to slave port of next RE.
