While the CU/DU split adds a lot of extra flexibility in how services are deployed, there is still an area of cost that needs to be optimized is the RU. Today, the interface between the BBU and RU in 4G LTE is proprietary to mobile equipment vendors. It is based on the Common Public Radio Interface (CPRI ) interface, but this is not an open interface today as there are dependencies in the implementation of BBUs and RRHs that require both to be sourced from the same vendor. In addition, it is a costly bottleneck as it is based on transport of digital radio signals directly over a point-to-point optical fiber. CPRI is adequate when RAN architectures are based on macro-cells alone, but becomes a major cost when a point-to-point fiber connection needs to be made between multiple micro-cell RUs to BBUs installed 20 km away. As the amount of data to be transmitted increases, the cost of the interface also increases. This is because the CPRI interface requires a constant bit rate no matter the load and there is therefore no possibility for statistical multiplexing.
In 2017/18 an update to this interface called enhanced CPRI (eCPRI ) was introduced. The eCPRI interface uses Ethernet as the L2 interface, which allows existing solutions for control, management and synchronization to be used. Ethernet allows packet-based switching and statistical multiplexing of several RU connections onto a single backhaul fiber. This vastly improves the cost of deploying micro-cells.
Let’s discuss CPRI & eCPRI in detail below:
CPRI: Common Public Radio Interface
CPRI Transport The CPRI is an industry forum defining a publicly available specification for the interface between a radio equipment control (REC) and a radio equipment (RE) in wireless networks. CPRI specifies a digitized serial interface between a base station referred to as REC in CPRI terminology and an RRH or RE. The specifications cover the userplane, the control-plane transport mechanisms, as well as the synchronization schemes. The specification supports both electrical and optical interfaces as well as point-to-point, star, ring, daisy-chain topologies.
The CPRI interface provides a physical connection for I/Q samples transport as well as radio unit management, control signaling, and synchronization such as clock frequency and timing synchronization.
Antenna Carrier: CPRI transports I/Q samples to/from a particular antenna port and RF carrier. This is called an antenna-carrier (AxC) and is the amount of digital baseband (I/Q) user-plane data necessary for either reception or transmission of only one carrier at one independent antenna element.
Antenna Carrier group: Itis an aggregation of multiple Antenna Carrier streams with the same sample rate, the same sample width, and the same destination.
Antenna Carrier Container: Itconsists of a number of Antenna Carriers and is a part of a basic CPRI frame.
Data is organized into basic frames of 16 words. The first word of each basic frame is the control word. Each word can be 8, 16, or 32 bits, depending on the width of the I/Q samples. The width of the word depends on the CPRI line rate. For example, in an LTE system, if I =16 bits and Q = 16 bits, then one Antenna carrier is 32 bits.
Each 256 basic frames make up a hyperframe and 150 hyperframes are needed to transport an LTE 10 ms frame. Data in a basic frame is encoded with 8B/10B encoding, that is, 8 bits of data are encoded in 10 bits. The extra bits are used to detect link failures. Some of the CPRI rates support 64B/66B encoding scheme and this extension is used to detect sync header impairments and link failures.
One of the CPRI technical requirements defines the clock frequency accuracy of RRH as 0.002 ppm. This requirement states that the maximum impact of jitter from the CPRI fronthaul on the frequency accuracy of RRH should be less than 0.002 ppm.
eCPRI Transport The concepts of CPRI-over-Ethernet and replacing the TDM-like CPRI format with Ethernet messaging both hold the promise of reducing the bandwidth requirements of CPRI transport and making fronthaul affordable and available to all mobile operators.
eCPRI, introduces improved transport efficiency to match the speed and bandwidth requirements of 5G fronthaul networks.The main advantages of the eCPRI protocol include support of functional split option 7, flexible bandwidth scaling according to user-plane traffic, and the use of mainstream transport technologies, which makes it possible carrying eCPRI and other traffic simultaneously in the same switched network.
As shown in figure below, in eCPRI, the radio base station is divided into two building blocks: eCPRI radio equipment control (eREC) and eCPRI radio equipment (eRE), which are physically separated and are connected via a transport network. The eREC implements part of the physical layer functions and higher layer functions of the air interface, whereas the eRE contains the remaining part of the physical layer functions and the analog RF functions. Userplane data, control and management, and synchronization signals (i.e., synchronization data used for frame and timing alignment) are packetized, multiplexed, and transferred over the transport network which connects eREC(s) and eRE(s). The eCPRI does not rely on specific transport network and data-link-layer protocols, thus any type of network can be used for eCPRI.
eCPRI Protocol planes: This method typically involves the transmission and reception of eCPRI messages.
- C&M Plane: Control and management data flow for the operation, administration and maintenance of nodes.
- User Plane: Data flows covered by user plane as:
- Data flow to be transferred from radio base station to user equipment and vice versa.
- Other eCPRI flows not covered by others flows and protocol planes.
- Real time control data.
- Synchronization Plane: As eCPRI is being used in Open RAN architecture and nodes are from different OEM’s So Synch plane has been introduced to maintain the data flow for synchronization timing information between nodes.
The main difference between eCPRI and CPRI can be summarized by looking at their respective characteristics
• It is intrinsically a point-to-point interface.
• There is a master port and a slave port connected directly by optical/electrical cable (s) as a hop.
• Networking functions are application layer functions and not supported by the CPRI interface itself. .
• Supported logical connections include point-to-point (one REC to one RE) and point-to-multipoint (one REC to several REs).
• Redundancy, QoS, security, etc. are REC/RE functions.
• An eCPRI network consists of eCPRI nodes (eRECs and eREs), transport network, as well as other network elements including grand master for timing and EMS/NMS for management.
• There is no longer a master port/slave port classification at physical level
• The eCPRI layer is above the transport networking layer.
• The eCPRI layer does not depend on a specific transport network layer (TNL) topology.
• The transport network may include local network and local switches provided by the eREC/eRE vendors.
• Supported logical connections include point-to-point (one eREC to one eRE), point-to-multipoint (one eREC to several eREs), multipoint-to-multipoint (eRECs to eREs, eRECs to eRECs, eREs to eREs).
• Redundancy, QoS, security, etc. are mainly transport network functions; eCPRI nodes need to implement proper TNL protocols to support these capabilities.