The move to Cloud RAN (Radio Access Network), or increased network virtualization, is a long-term path to improve most aspects of LTE network efficiency. A recent poll of operators showed that opex savings are seen as the biggest benefit of network virtualization.
The original architecture for cellular networks was to have sites with most of the network’s intelligence at the bottom of the tower in the form of an integrated base station radio. This radio was connected via coaxial cable to antennas located at the top of the tower, which radiated the RF energy.
Around 2000, as 3G was introduced, the network architecture began to change with the introduction of the common public radio interface (CPRI) standard. CRPI allowed the base station to be split in two — a base band unit (BBU) located at the bottom of the tower and a remote radio head (RRH) co-located with the antenna at the top.
The elimination of signal degradation by going up the tower improved the efficiency of the system. Cloud RAN takes this separation one step further by moving the baseband processor away from the site and deeper into the core. This architecture will improve network performance through lower costs, greater coordination and more capacity, resulting in higher speeds and improved efficiency.
A network that is completely run in the cloud would have COTS (commercial off-the-shelf) equipment placed centrally in a big data center with simple digital-to-RF converters located at the edge. To accomplish this, you would need dedicated links with very low latency and very high capacity from every cell site to the central core.
Unfortunately, the high speeds and low latency links necessary for true Cloud RAN do not exist ubiquitously, even in countries with large amounts of available fiber connecting current cell sites. In addition, more technology advancements are necessary before general purpose microprocessors are efficient enough to replace some specific purpose chips.
As such, the industry is really at the beginning of a long-term transition to Cloud RAN that will likely occur in stages with greater levels of centralization and virtualization as time goes on.
The three fundamental stages to Cloud RAN are centralization of the current baseband equipment to clusters of sectors, splitting of processing between COTS equipment and specialized chips resulting in additional processing in the RRH and the BBU being made virtual, and total network virtualization.
Each of these stages has costs and benefits. Centralization of current baseband equipment can be done without major equipment changes. Clustering of cell sectors will enable more timely and expedient decision making and reduce operational costs, but it has limited efficiency benefits on the overall network.
In the second phase, most of the call processing that currently takes place on specialized chips can be moved off the BBU and put onto standard server style equipment. At the same time, some radio functions (Layer 1) can move into the RRHs from the BBU, enabling a lower-speed, higher-latency link between the core and the site. This design may allow non-dedicated links to exist, which will reduce cost and decrease installation issues.
The final stage of centralization virtualizes everything at the site except the digital to RF converters in the data center — from signal processing to analytic applications.
I expect phase two will be most common deployment scenario on a wide scale, operating as a sort of pseudo Cloud RAN. The control plane and the data plane from the baseband will be in some way separated, but it will have more latency than true Cloud RAN. Some of the intelligence will be out at the site, like today, and some of the intelligence will be brought into the centralized cloud.
There will also likely be semi-centralized sites that serve clusters of cells where some processing functions will be semi-centrally located. The equipment that remains out at the site will really be there for power management in both pseudo and full Cloud RAN.
It is interesting to note that 5G will be the first wireless technology designed with targets for energy efficiency, and the Cloud RAN network architecture will reflect that need.
Right now, the wireless industry is working out how to move the Layer 2 and Layer 3 radio functions to the cloud servers that sit further back in the core.
Morgan Kurk serves as the senior vice president and Wireless segment leader at CommScope.