The Common Public Radio Interface, or CPRI, is a widely used protocol to transmit digitized RF data over optical fiber between radios and baseband equipment. As a serial stream without the possibility of packet aggregation, CPRI is bandwidth inefficient, which leads to higher transport costs. The IEEE 1914.3 working group has developed a new Radio over Ethernet (RoE) standard to address this.
Currently, there is a big push to packet fronthaul in order to reduce bandwidth and associated transport costs, but this poses a challenge to the large installed base of CPRI radios which must still be served.
According to the GSMA, 4G connections will still account for 85% of global connections by 2025. Thus, a method is needed to packetize the constant bit rate CPRI streams to take advantage of bandwidth efficiencies and utilize converged transport with other Ethernet traffic (e.g., enhanced CPRI, or eCPRI, flows).
Interworking and bandwidth efficiency
Radio over Ethernet (RoE) defines several methods to packetize CPRI streams using Ethernet frames. Serial CPRI streams are mapped onto Ethernet frames for transport over a packet fronthaul network and demapped back to CPRI on the other end. This packetization of 4G/5G CPRI flows allows coexistence with 5G eCPRI flows over a shared Ethernet fronthaul transport network using time-sensitive network (TSN) switches (figure 1).
Different RoE modes have been developed to maximize either multi-vendor interworking or bandwidth efficiency. These include structure-agnostic, line-coding-aware, and structure-aware mapping modes (figure 2).
The structure-agnostic tunneling mode is RAN agnostic, as no visibility into the CPRI frame is required. In this mode, all CPRI information, including the line coding, is encapsulated into one RoE flow and transported transparently.
In the line-coding-aware mode, the mapper understands the CPRI framing, thus enabling removal of the line coding and saving on bandwidth compared to tunneling mode. However, this mapping may not be vendor agnostic since vendor-specific information could be embedded in the line coding.
The RoE structure-aware mode uses knowledge of the CPRI frame structure to remove some of the unused CPRI frame information. These optimizations can lead to further transport efficiencies, although this mode is more operationally complex to implement and provision because visibility into the semi-proprietary structure of the CPRI protocol is needed.
The actual bandwidth savings attainable using structure-aware mode depends on the RAN configuration. Several parameters, such as the CPRI rate, carrier spectral bandwidth (MHz), and number of transmit/receive antennas, will impact the antenna carrier (AxC) occupancy level. Lower occupancy levels lead to higher bandwidth savings, as traffic streams with low fill rates have more empty containers than can be discarded.
In addition to the bandwidth savings, having AxC-level visibility brings additional benefits for features that require processing of CPRI content:
- CPRI muxing – multiple low rate CPRI radio interfaces can be aggregated to a higher rate at the baseband unit leading to savings on the number of ports needed
- CPRI switching – AxC level switching can be used to connect multiple radio units with multiple baseband resources enabling BBU pooling gains
- Low-PHY conversion – allows conversion of eCPRI flows towards CPRI radios performing frequency-domain to time-domain conversion. This is useful for dynamic spectral sharing where operators can use the same spectrum bands for different radio access technologies (e.g., 4G and 5G).
For RoE to work properly, a jitter buffer is used to compensate for differential delays. In addition, time/phase and frequency synchronization must be tightly controlled.
To ensure proper RAN performance, the nodes participating in the RoE mapping and demapping functions must be in the same time domain with aligned timebases (figure 3). This ensures that the presentation time embedded within the RoE packet aligns with the RAN and when it expects the CPRI frames.
By employing a mix of the standardized RoE mapping modes, operators can optimize their packet fronthaul transport networks for maximum interoperability or bandwidth efficiency. These modes allow existing CPRI radios and baseband units to be used, thus saving on cost by leveraging existing investments while paving the way to new Ethernet-based fronthaul protocols.
— Hector Menendez is product marketing manager, IP/optical networks at Nokia. He develops and markets service provider solutions on topics including mobile fronthaul, mobile backhaul, secure optical transport, and Ethernet/wavelength services. Menendez has over 30 years of telecommunications experience and has held a variety of positions including project analyst, events management, market development, and solutions/product marketing at AT&T, Alcatel-Lucent, and Nokia.