Carving Up the 5G RAN

The migration to 5G is critical for carriers who are also moving towards cloud RANs and wanting, where possible, to move to open solutions. 5G will bring higher bandwidths, lower latencies, new radio technologies and a new wireless core. RAN disaggregation enabled by cloud RANs and open interfaces gives carriers the opportunity to benefit from a wider ecosystem but may also create new challenges as they migrate to 5G. The decisions made by carriers, and their suppliers, on cloud RAN architectures will have big implications on the connection bandwidths needed in the 5G RAN and the systems required.

In my previous two blogs in this series I discussed virtualized/cloud RAN architecture development, reviewed the roll-out of 5G and the benefits, and highlighted the key challenges for service providers. In this blog I will look further into the 5G RAN architecture development and the options for carriers and their suppliers.

Centralized RAN architectures have a baseband unit (BBU) that handles all baseband processing and is connected to the mobile core over a backhaul network, and to multiple remote radio heads (RRH) through CPRI (Common Public Radio Interface) links. In a 5G cloud RAN architecture the BBU functionality is split into two functional units; a centralized unit (CU) and a distributed unit (DU). The physical location of these units depends on the specific architecture and geographical locations available. In a centralized 5G cloud RAN the DU is hosted in an Edge cloud data center or central office and the CU can be collocated with the DU or hosted in a regional cloud data center. In a distributed 5G cloud RAN architecture the DU is collocated with the radio unit.

The exact functionality supported in the CU, DU and RU has been in discussion within 3GPP and the industry in general for some time. The diagram below shows the key BBU and RU functions between the backhaul to the wireless core and the radio heads. The key elements are the Radio Resource Control (RRC), Packet Data Convergence Protocol (PDCP), Medium Access Control (MAC) and Physical Layer (PHY). The diagram also shows the main options for splitting the functions between the CU and DU (High Layer Split) and the DU and RU (Low Layer Split).

The critical question is where to make these splits between CU, DU and RU. Where all the systems and software in a cloud RAN architecture are supplied by a single vendor then the splits can be dictated by the vendor. This approach does not support open architectures and therefore makes it difficult for carriers to source systems and software from multiple suppliers. The industry has coalesced around split option 2 for the high layer split as shown in the diagram. This is now the standard 3GPP F1 mid-haul interface between the CU and DU. There is less agreement on the low layer split between DU and RU although many deployments will use split option 7-2, a variant of split option 7.

The split options can have a significant impact on network bandwidth within the RAN and the complexity of the systems. If the DU is collocated with the RU (split option 2), making this a more expensive system, then the bandwidths for the mid-haul network from the CU are relatively low, below 10 Gbit/s for a typical sub-6GHz macro cell deployment. If the DU is located away from the RU (split option 7), making this a relatively inexpensive system, then the front haul bandwidth will be significantly higher, up to 100 Gbit/s.  For mmWave small cell deployments, the aggregate bandwidths can be even higher due to the massive spectrum available.

The CPRI interface is widely used in LTE RAN front haul deployments and will support bandwidths up to 25 Gbit/s. The eCPRI specification removes many of the restrictions of CPRI and has been developed to enable 5G and RAN disaggregation. eCPRI, supports different RAN functional splits and enables flexible radio data transmission via a packet based front haul network such as Ethernet or IP. eCPRI allows the use of standard Ethernet interfaces and switches that support synchronisation. This enables flexible front haul networks that support bandwidths of 25, 50, 100 Gbit/s or more.

Several leading carriers are issuing RFIs calling for RAN disaggregation and open solutions. The challenge for the industry is meeting these requirements while also delivering the performance and flexibility required for 5G services. In my last blog in this series I will look at the latest open RAN developments and priorities