Virtualized Radio Access Network (vRAN) Background
Fronthaul using CPRI evolved from the need of mobile operators to reduce their carbon footprint, obtain gains by pooling processing resources, and deploy advanced network features like Carrier Aggregation (CA) and Coordinated Multipoint (CoMP). The disadvantage of this type of Fronthaul is that it needs ‘Ideal Transport’ (Defined by 3GPP as 2.5 µs latency and throughput up to 10 Gbps). Additionally, the CPRI proprietary interface carries demodulated signals from base station antennas, signals that cannot be statistically multiplexed. This results in high capacity demands for the transport networks even for standard LTE deployments.
The introduction of new massive MIMO and new 5G features have led to the realization that CPRI is not scalable for future 4G and 5G deployments. These scalability issues and the proprietary nature of the standard have created a consensus that a rethink of the base station decomposition is needed. This process has been advocated by entities like 3GPP, IEEE1914, and eCPRI. Operators have stepped in to launch their own Fronthaul standardization projects to complement these efforts. The main ones are sponsored by members of x-RAN (Now ORAN) and v-RAN Telecom Infra project. X-RAN is supported by carriers like AT&T, Deutsche Telekom, Telstra, Verizon and SK Telecom. The main objectives of these Fronthaul projects include:
- Cater to multiple deployment scenarios
- Reduce bandwidth utilization compared to proprietary CPRI interface
- Efficiently scale and deploy large number of antennas per base station site
- Support advanced features such as Carrier Aggregation (CA), coordinated multipoint (CoMP), etc
- Evolve to an Ethernet-based solution, independent of proprietary implementations
vRAN Simplifies Transport
On a Virtualized RAN (vRAN) operators can make the decision of where to ‘split’ the base station functionality. A protocol stack resembling the current CPRI Fronthaul will provide more network features (CoMP, CA, etc.) but have more stringent capacity and latency requirements. A ‘split’ with more functionality on the transmitter can still carry some of the network features but will have less stringent Fronthaul requirements. Currently the v-RAN and x-RAN fronthaul projects are concentrating on the physical layer (separating low and upper physical layers as shown in figure 1) looking to provide the maximum functionality, reducing transport capacity and latency requirements and ensuring that this new interface is open.
Figure 1 CRAN vs. vRAN comparison
This relaxation on the Fronthaul requirements created by the new base station functional split gives operators the freedom to select their Fronthaul according to their deployment scenarios. In certain scenarios this can lead to the use of what the 3GPP calls “Non-Ideal Transport” (latency typically between 2-60ms and throughput from 10Mbps up to 10Gbps). The use of “Non-Ideal Transport” allows operators to use cost effective transmission mediums such as copper and microwave. In other scenarios “ideal transport” (high capacity and negligible latency) can also be used providing the same performance that a CPRI interface would provide (but requiring ideal transport).
The vRAN Fronthaul interface can be seen as an Application Programming Interface (API). This API baseband implementation (either virtual or traditional) may be invoked to efficiently transfer structured radio signals from/to the RRU. To accomplish this the interface relies heavily in open Fronthaul interfaces currently being developed like IEEE 1914. This reliance in current Fronthaul standardization efforts and the ability to test and share the results on different operator’s deployment cases will likely attract a diversity of players willing to support the architecture and open interface. As more players participate in this ecosystem it is more likely that it will succeed commercially beyond initial trials.
vRAN Enables Wireless Backhaul
Microwave backhaul is a necessary solution to enable low cost backhaul networks (over 50% of all base stations today are backhauled wirelessly). Millimeter wave and microwave now provide more capacity by using higher modulation schemes (up to 4096QAM), cross polarized antennas, multi-header compression and new multiband solutions where microwave and millimeter wave are being combined on a single link. With these capabilities, 10Gbps backhaul solutions are available on the market today – which is more than adequate capacity for 5G networks.
With the exceedingly high capacity requirements of CRAN/CPRI, wireless backhaul is generally not a viable option, leaving operators with limited backhaul possibilities – which means more cost. vRAN is promising because it enables operators to utilize wireless backhaul solutions which are necessary for building low cost backhaul networks.
What is clear is that if vRAN Fronthaul becomes the next Fronthaul standard, it can help the development of a competitive ecosystem with best of breed solutions in which microwave can be an active player. This is will benefit operators as they can not only reuse their existing infrastructure but also deploy more cost-effective Fronthaul that fits their deployment requirements.