5G performance specifications of high-speed data throughput, very low latency and high reliability can only be met with extensive fiber optic cable connectivity between all network elements from the remote radio units (RRU) at every cell site to the network core.
Fiber in wireless networks is not new. As wireless moved from circuit-switched to packet technology, the industry adopted fiber initially in backhaul networks for reliable, high capacity IP/Ethernet transport over longer distances between cell sites and to the evolved packet core (EPC). Fiber for backhaul performed better than either licensed point-to-point microwave systems or high-maintenance copper cables.
With 4G LTE mobile data volumes skyrocketing, fiber was the logical choice for high-bandwidth applications in the access portion of the network. Splitting the radios into their functional components – the baseband unit (BBU) and RRU – improved RF data handling capability. Until then, radios installed on the ground connected to antennas on the tower via coaxial cables. Coax both introduced RF losses and added to the tower loading.
By contrast, RRUs are installed next to the antennas on the tower and communicate with the BBU on the ground via lightweight fiber cable over common public radio interface (CPRI) signaling. Companion copper cables supply power to the RRUs. The BBU-RRU fiber connection is called “fronthaul.”
A BBU-EPC backhaul connection was still required. Separating BBUs and RRUs brought about the notion that BBUs could be co-located in a central site with a common backhaul to the EPC, saving transport and power costs. This is called centralized radio access network or C-RAN. With 5G, the BBU itself now is split into two functional components – a centralized unit (CU) and distributed unit (DU) – connected via fiber in a “mid-haul” configuration. The idea is that the CUs and DUs can be open standards-based, software-programmable elements.
Called Open RAN (O-RAN) or virtualized RAN (V-RAN), this concept enables wireless service providers to deliver bandwidth-on-demand with less expensive hardware and at lower operating cost than traditional networks. O-RAN/V-RAN afford service providers software-enabled, automated, scalable networks that yield significant operating efficiencies, capex and opex savings while delivering very responsive service. A CU located in a central office or at a network hub connects to the 5G Core via a high-capacity fiber optical transport system such as dense wave division multiplexing (DWDM) with reconfigurable optical add-drop multiplexers (ROADM).
Expect to see DUs in small form factors deployed in various outside plant (OSP) cabinets connecting to CUs on one side and RRUs at macrocells and small cells on the other. Such fiber connections could be implemented via a passive optic network (PON) scheme similar to fiber-to-the home (FTTH). With small form factor, easily deployable optical devices, wireless carriers can aggregate multiple DU-CU connections into high-capacity wavelengths for backhaul traffic to the EPC.
Service providers can build out their wireless infrastructure with fiber several ways – lease dark fiber and install their own optical terminals, lease optical capacity on a wholesale basis from third-party fiber network operators, or install and operate their own proprietary fiber optic networks. Each carrier takes a different approach depending on the markets they serve, their network density and available fiber facilities.
Verizon Wireless (VZW) believes owning its own fiber is a competitive advantage. In 2017, the company signed a three-year $1.05 billion purchase agreement with Corning to supply critical fiber cable and related hardware for VZW’s nationwide wireless network. Corning committed to deliver 12.4 million fiber-miles each year from 2018 through 2020. This purchase agreement augments VZW’s strategic acquisitions of the WOW! network to connect VZW macrocells and small cells in Chicago, and XO Communications that operates metro fiber rings in 45 cities. Moreover, VZW relies heavily on Crown Castle for high volume, fiber-connected small cell deployments.
Fiber in wireless networks has big implications for the cost of building and operating next generation infrastructure and the carriers’ service delivery capabilities. In many ways, we’re just beginning.
By John Celentano, Inside Towers Business Editor