Evolution of towards eCPRI from 3G, 4G and 5G.
User equipment (UE) must establish a connection to the radio access network (RAN) in order to communicate using data/voice (for the network voice is also data). RAN is the gateway to access the core network and it is accessed using radio channels. The two key elements of RAN are the radio unit (RU) and the baseband unit (BBU). This connection between the RU and BBU is commonly known as the fronthaul connection.
Along the evolution journey of wireless technologies from 3G, 4G to 5G, RAN architecture has evolved too as shown below in Figure 1.
In 3G networks each site has its own RU and a dedicated BBU. Hence, It is called as “Distributed RAN” where as in 4G/LTE single RU is connected to a poll of BBUs via CPRI protocol. The BBU is connected to the core network via the backhaul connection. Now what is eCPRI in 5G?
What is eCPRI in 5g?
5G brings progressive network performance enhancements, supporting new emerging use cases such as v2x, massive Machine Type Communication (mMTC), etc. 5G new radio (5G NR) consists of the latest radio access technologies designed specifically for the requirements of 5G. For example, it introduces the use of new frequency bands and numerous architecture design improvements to overcome the shortcomings of LTE (4G). In this article, the focus towards the fronthaul connection improvements in 5G.
What are the requirements in 5G lead to eCPRI?
For the use cases such as extreme mobile bandwidth, and ultra-low latency (URLLC), and Open Radio Access Network (O-RAN) it is required to have communication channel flexibility and higher channel efficiency.
Hence, the flexible fronthaul configurations (link to fronthaul) are a must in 5G but achieving flexible fronthaul configuration was not quite possible with legacy CPRI which is a fixed bitrate serial interface in which the data is multiplexed into the transmission line with time division multiplexing (TDM).
Additionally, since some parts of the CPRI protocol were left vendor specific, different vendors have implemented CPRI in variety of proprietary ways. This resulted in interoperability between equipments from different vendors impossible and it violates O-RAN vision.
In 5G with cloud RAN, BBU functionality is in a cloud based virtual network function (VNF). Thus the connection between RU and BBU needs to be layer 3 capable to route packet data to virtual BBU. This demand for more functionality from CPRI and thus this lead the path to formulation of evolved CPRI or evolved Common Public Radio Interface (eCPRI).
eCPRI and its use cases in 5G and beyond (6G).
As a solution “CPRI cooperation” has defined as a new packet-based fronthaul transmission interface called eCPRI. eCPRI is future-oriented and it provides more flexible fronthaul interface where it optimizes radio performance by analyzing traffic and prioritizing traffic in real time.
CPRI and eCPRI protocols
CPRI protocol
The CPRI protocol is defined in physical layer (layer 1) and data-link layer (layer 2), and it consists of three different data flow segments as follows and as shown in Figure 3 (i):
- User plane: Carries the quantized In-phase and Quadrature (IQ) data.
- Control and Management plane: Carries control signaling between RU and BBU.
- Synchronization plane: Carries timing information to keep the equipment in synchronization (insync).
Since eCPRI protocol is defined (by eCPRI cooperation) with standard Ethernet frames and UDP frames (Figure 3: (ii) ), eCPRI is also a data packet which is in the following Figure 4.
The data structured according to data eCPRI packet defined above (Figure 4) is routed from RU to BBU or vise versa. Instead of requiring a point-to-point physical link, since eCPRI is not defined in the physical layer but works in a higher layer ethernet-switched or IP-routed transport network.
The same data planes of CPRI is included (User plane, sync in signal and control and maintenance) while not requiring physical between RU and BBU functionalities enabling 0-RAN like 5G concepts. eCPRI is the substitute of CPRI with much more flexibility for data rates optimizing available resources, and layer 3 functionality and thus eCPRI data packets can move very flexibly in any conventional ethernet or Internet Protocol (IP) based network. This allows eCPRI packets to flow between any eCPRI capable device within the transport network. Direct connectivity between RU and BBU is not a must. Finally as a summary, we compare eCPRI to CPRI.
CPRI vs eCPRI
| eCPRI | CPRI |
| New packet-based protocol between the radio unit (RU) and the baseband unit (BBU): fronthaul. | Fixed bitrate serial interface with TDM |
| More flexible and dynamic in terms of transport network. i.e., No direct link between RU and BBU is required. | Mainly used as a point-to-point interface: Hence, it requires a direct link between the RU and BBU. |
| More flexible and dynamic in terms of data rates. | Constant bitrate protocol. Several pre-determined fronthaul bitrates for the CPRI transmission line. [614 Mbps – 24 Gbps] |
Additionally, these are some benifits/ advantages eCPRI has over legacy CPRI,
- Free to use blend of providers’ equipment in a functional RAN for network operators. This drives the industry into a more open, free, interoperable direction. Thus, operators are not be tied to buy all the hardware and software from the same vendor for RAN.
- Enhances spectral efficiency by efficient cell coordination and increase energy efficiency since it is possible to optimize and share available resources. Thus reduce the required bandwidth by ten times.
- Instead of requiring a point-to-point physical link, eCPRI instead works in a higher layer ethernet-switched or IP-routed transport network.
- Lowers network operation and maintenance costs enabling technologies like centralized VNF type BBU.