5G is accelerating toward maturity and widescale deployment. Compared with 4G, 5G networks are distinctly different. It is forecasted that the 5G base stations installed in China will be two to three times as many as 4G ones. The Centralized Radio Access Network (C-RAN) mode where the Distributed Units (DUs) for many Active Antenna Units (AAUs) are placed at a centralized location significantly increases the fronthaul distance between DUs and AAUs. If all DU-AAU connections are through fiber, the amount of fiber required will rise by 10-fold. That entails heavy civil works and enormous investments.

25Gbps WDM-PON is Ideal for 5G Fronthaul

WDM-PON is a passive optical networking technology that can be used to address the fiber deployment challenges. A WDM-PON design can be used to separate optical-network units (ONUs) into several virtual point-to-point connections over the same physical infrastructure, a feature that enables efficient use of fiber compared to point-to-point direct fiber connection and offers lower latency than TDM-based technologies. A notable benefit of this technology is high bandwidth, low latency, and fiber savings. 5G fronthaul based on 25Gbps WDM-PON technology has the following technical advantages:

  • Support for CPRI and eCPRI standards as well as 4G/5G hybrid networking.
  • 25Gbps high bandwidth per wavelength, which can smoothly evolve to 50Gbps in the future.
  • Up to 20 pairs of wavelengths on a single trunk fiber.
  • Colorless ONU technology allows flexible wavelength allocation and wavelength routing.
  • In the future, a colorless Small Form-Factor Pluggable (SFP) ONU can be directly inserted into the AAU for easy installation.
  • The Arrayed Waveguide Grating (AWG) incurs a power loss of about 5.5 dBm, which is lower than that of the optical splitter.

5G+FTTH Converged Gigaband Access Solution

WDM-PON is a key innovation that enables 5G+FTTH converged gigaband access. Compared with direct fiber connections between DU and AAU, the WDM-PON based fronthaul mode saves trunk fibers by more than 90 percent (shown as Figure 1). Another advantage of WDM- PON is that wavelengths can be flexibly allocated and resources can be remotely, centrally managed.

WDM PON enriches and perfects 5G fronthaul technology, giving operators more options by allowing for 5G+FTTH converged gigaband access in dense urban areas.

Based on the above principles, ZTE and China Telecom jointly launched the 5G+FTTH Converged Gigaband Access solution. The solution has unique advantages in trunk fiber, equipment space and power savings. Specifically, it:

  • Cuts 95 percent trunk fiber by allowing up to 20 AAUs to share the same trunk fiber.
  • Saves 10 percent power and shrinks space through OLT reuse.
  • Reduces overall investments by 50 percent.

In addition to the high-density WDM-PON cards, the OLT also innovatively provides TDM-like channels to ensure a processing latency of less than 7µs in the OLT. If the distance between OLT and ONU is 5 kilometers, a transmission latency of 25µs will ensue over the fiber. Consequently, the total end-to-end latency is less than 32μs, which is 68 percent lower than the 5G URLLC requirements. The TDM-like channel handles traffic sent from 5G AAUs in real time without the queuing, buffering, forwarding, routing and searching processes. The resulting low-latency forwarding meets the stringent latency requirements of 5G fronthaul for URLLC applications.

Industry’s First WDM-PON for 5G Fronthaul Validation

In December 2018, ZTE and the Technology Innovation Department and Optical Access Research Department of China Telecom jointly completed the industry’s first validation of Nx25Gbps WDM-PON for 5G fronthaul on the live network of China Telecom Suzhou Branch. The validation demonstrated that 25Gbps WDM-PON could carry 5G fronthaul services stably and transparently, with the data rate and end-to-end latency equal to those in a point-to-point direct fiber connection.

Achievements in WDM-PON Standards and Technologies

ZTE and China Telecom collaborate to actively participate in the standardization of WDM-PON. The collaboration has produced numerous achievements including:

  • Editor of “ITU-T G.Sup66 : 5G wireless fronthaul requirements in a passive optical network context” in October 2018.
  • Submitted five proposals to the ITU-T, applied for 23 patents and released two papers.

3) Co-led the formulation of the “Nx25Gbps WDM-PON for 5G Mobile Fronthaul” series standards of China Communications Standards Association (CCSA).

4) Formulated the industry’s first enterprise standard on Nx25Gbps WDM-PON management channels for China Telecom.

5) Proposed the concept of SFP ONUs, which has been accepted by CCSA and is being incorporated into its standards.

Besides the achievements in WDM-PON standards, ZTE has also made breakthroughs in key WDM-PON technologies, including:

1) Developed 25Gbps WDM-PON optical modules with low cost, low power consumption, and high transmission power.

2) Developed the technology of ultra-low latency forwarding of CPRI/eCPRI traffic based on cell switching.

Summary and Prospects

25Gbps WDM-PON is ideal for 5G fronthaul. It is a key innovation to enable 5G+FTTH converged gigaband access in an economical way. ZTE is currently extending WDM-PON based fronthaul from outdoor AAUs to a 5G indoor distribution system. When indoor 5G fronthaul is combined with the Passive Optical LAN (POL), 5G+FTTO (Fiber To The Office) converged dual-gigabit rates can be achieved in industrial parks.