5G interface protocol: from CPRI to ECPRI

In the architecture of early 2G and 3G base stations, the RRU that processes radio frequency signals and the BBU that processes signals are placed indoors. Because the antenna is hung on the tower, and the tower is generally tens to 100 meters high, the feeder must also be tens to 100 meters long. However, signal transmission is lossy, and the longer the feeder, the greater the loss. From the base station to the antenna, more than half of the signal has been lost before it is sent out. Therefore, the base station under the tower must increase the power transmission to make up for this loss. The most notable feature of this architecture is its complexity and high power consumption.

In the 3G era, a new architecture was born, namely distributed sites. The popularization of distributed sites in the 4G era has completely changed the problem. BBU is small and exquisite with low power consumption, while RRU is large and has high power consumption. The high-power RRU is also hung on the tower and placed together with the antenna. In this way, there is no need for a long feeder connection, and the loss is small, and the power consumption is naturally reduced, and natural heat dissipation can be achieved. This is the new distributed site architecture.

Since the RRU and BBU are far apart, there are some problems with connection and data transmission. In 2003, Ericsson, Nortel Networks, Alcatel-Lucent, NEC, and Huawei initiated the definition of the Common Public Radio Interface (CPRI) protocol, which is open to other organizations and manufacturers.

CPRI is a standardized protocol that defines the digital interface between the radio equipment control (REC) and radio equipment (RE) of wireless infrastructure base stations. This enables interoperability of equipment from different vendors and protects the software investment of wireless service providers.

The physical layer data transmitted by the CPRI protocol between the BBU and RRU not only contains the carried data, but also contains a large amount of physical layer information, and distributes this information to each antenna. The amount of data is very huge.

Nine options are defined in the CPRI protocol, with a maximum rate of up to 12 Gbps.

However, in the 5G era, new application scenarios require the emergence of Massive MIMO AAU, which greatly increases the carrier bandwidth and places higher requirements on CPRI. For example, a 100M 64-antenna network requires a rate of up to 172.8Gbps, and this is only in the Sub6G frequency band.

The millimeter wave band has a larger bandwidth, which inevitably requires an upgrade of CPRI, which is eCPRI. The data transmitted on the communication protocol stack will be encoded layer by layer, and the amount of data will increase as it goes to the physical layer. As shown in the figure below, the data processed on the BBU is moved up one layer (BBU processing on High Phy) and the data below is handed over to the RRU for processing (RRU processing on Low Phy). In this way, the amount of data between the BBU and RRU is reduced, which can greatly reduce the fronthaul bandwidth, but the complexity of the RRU will also increase.

Taking the 100M carrier bandwidth plus 64 antennas mentioned above as an example, the CPRI protocol requires an optical port rate of 172.8Gbps, while eCPRI only requires an optical port rate of 24.3Gbps, and the bandwidth is only 14% of the original.

In the future development of 5G, eCPRI will be the mainstream, and we look forward to the emergence of more applications.