Evolution of the Fronthaul Interface with the Advent of 5G

The fronthaul interface enables the communication and synchronization of the Baseband Unit (BBU) with the Radio Unit (RU). To date, multiple protocols have been developed and implemented in order to establish the communication over this link, such as Common Public Radio Interface (CPRI), Open Base Station Architecture Initiative (OBSAI), enhanced CPRI (eCPRI), Next Generation Fronthaul Interface (NGFI), and Open Radio Access Network (O-RAN) fronthaul protocol. Many of these have been designed to be open (e.g., CPRI) but became vendor-specific to achieve better performance, optimization, and efficiency.

With 5G, the fronthaul interface must support higher data rates due to higher bandwidth of the 5G spectrum and new antenna technologies, such as Active Antenna Systems (AAS) where a radio can have an antenna array of 64 Transmit and 64 Receive (64T64R). Therefore, in order to provide enhanced fronthaul link connectivity and leverage the functional splits of the 5G RAN, as defined by the 3GPP Release 15, the CPRI forum and the O-RAN Alliance have designed the eCPRI and O-RAN fronthaul protocol, respectively. These protocols are more efficient packet-based fronthaul systems that can reduce the required bandwidth of the link connection compared with CPRI.  

CPRI is a serial TDM-based fronthaul protocol that has been largely adopted for the deployment of 4G networks. According to the CPRI line rate definition, CPRI can deliver link data rates in the range between 614.4 Mbps and 24.3 Gbps. With CPRI, the capacity of the link scales with the carrier bandwidth and the number of radio branches or MIMO configuration with a constant bit rate independent of the user traffic. The table below provides some examples of the link data rates required by two different carrier bandwidths and different MIMO antenna configurations. Notice that CPRI might not have the potential to cope with the requirements of 5G especially when the MIMO configuration and carrier bandwidth are increased. CPRI has traditionally relied on dark fiber for mobile backhaul, especially when considering higher order MIMO configurations, carrier aggregation, and high bandwidths.

By contrast, eCPRI is a packet-based fronthaul transport network like IP or Ethernet with the following characteristics:

• The link data rates are associated to the carrier bandwidth, the split of the protocol stack, and the actual user traffic without a constant bit rate.
• With eCPRI, the radio branches or MIMO antenna configuration are no longer a deterministic factor for the capacity of the link data rates of the fronthaul. This technology change can be achieved since the 5G NR functional splits and eCPRI splits allow the movement of some of the baseband functionalities of the physical layer to the radio, reducing the bandwidth requirements of the link.
• eCPRI can deliver physical line rates from 10 Gbps to 100 Gbps with delays <100 µs.
• With eCPRI, the traffic can be carried over Ethernet along with other traffic in the same switched network simultaneously.

These advanced features make eCPRI a suitable fronthaul protocol to support the high-speed data rates and latency required for the deployment of 5G networks. With eCPRI (according to Comba Telecom), a 5G network with 100 MHz carrier bandwidth and 8T8R MIMO configuration will require 12 Gbps transmission bandwidth compared with 49.1 Gbps in CPRI fronthaul protocol. These reduced requirements in the transmission bandwidth will enable 5G to realize massive MIMO deployments, which ultimately will result in enhanced spectral efficiency and better user data throughputs. 

However, despite all the benefits, eCPRI is a proprietary protocol, which reduces the number of vendors available in the market. This scenario is gradually changing in the 5G era since the O-RAN and Telecom Infra Project (TIP) are actively working on the disaggregation and open standard solutions based on Ethernet protocols that bring to the mobile network infrastructure innovation and a wider ecosystem.

eCPRI recommends different splits over the protocol stack known as D, ID, IID, IU, and E. These recommended splits overlap with the 3GPP 5G NR functional splits. For example, the split ID corresponds to the split 7.3 in the 3GPP standard. Similarly, splits IID and IU correspond to the split 7.2 in the Downlink (DL) and Uplink (UL) transmission, respectively. Similarly, the recommended splits of the O-RAN fronthaul overlap with the functional splits of 5G 3GPP standard, mainly splits 7.1 and 7.2. Thus, both O-RAN and eCPRI are packet-based fronthaul protocols that can support split 7.2 in the protocol stack to reduce the bandwidth requirements of the interface connection. O-RAN fronthaul protocol can support eCRPI and CPRI, making it suitable for the deployment of 4G and 5G networks.

O-RAN, however, is not just opening the fronthaul interfaces of the RAN. It is also enabling the development and deployment of software-defined networks that can offer huge levels of intelligence, automation, and flexibility to MNOs, which will ultimately result in reduced OPEX and great customer satisfaction.

To date, different MNOs are carrying out trials on the real-life networks leveraging O-RAN. For instance, Turkcell completed an O-RAN call supporting the O-RAN Alliance preferred split 7.2. Rakuten, a full-compliant O-RAN MNO, has unveiled a massive MIMO remote radio head with 32T32R configuration in partnership with NEC and Intel that is expected to support 100 MHz carrier bandwidth. Other operators such as Vodafone in the United Kingdom have deployed O-RAN network solutions in rural mid-Wales.

ABI Research expects that the O-RAN fronthaul solution will be largely adopted by different MNOs because it does not only provide the fronthaul requirements of 5G. O-RAN also gives MNOs access to a wider vendor ecosystem, allowing multiple radio vendors to enter the closed market and providing greater flexibility to deploy all possible frequency bands. Many of the early adopters are deploying O-RAN in rural areas where there is more flexibility to address technical issues. However, ABI Research expects that after experiencing success in different trials with MNOs, O-RAN deployments will gradually move from rural areas to highly congested areas, in outdoor and indoor scenarios, and be used to address enterprise use cases.