Development of PON
PON is a broadband access technology based on an optical distribution network (ODN). It uses a P2MP topology, has independent upstream and downstream transmission wavelengths, and transmits data in time division multiplexing mode. The ODN between the OLTs and ONUs utilizes only optical fibers, and is passive throughout, highly environment-adaptive, and easy to expand and upgrade. Over the past decade, PON has been put into large-scale deployment thanks to its advantages over copper technology in terms of fiber, P2MP and passive nature.
The development of PON is largely promoted by the standard organizations ITU-T/FSAN and IEEE. GPON and EPON are massively deployed today to provide users with up to 100 Mbps access bandwidth. Newer 10G PON technologies (10G GPON and 10G EPON) have also been commercially deployed on a large scale, delivering a  bandwidth of up to 1 Gbps to support scaled deployment of 4K/8K videos and the introduction of VR/AR in the early stage.

50G PON: The Next-Gen PON Technology After 10G PON

Video has become a basic service carried by broadband networks. The application of PON technology is extending from home broadband to enterprise domains such as telemedicine, smart manufacturing, and factory or mine communications. These developments pose higher requirements on network metrics, including bandwidth, latency, packet loss, jitters, QoS, and user experience. For example, VR service requires more than 1 Gbps bandwidth and an RTT of 5 ms while telemedicine demands an end-to-end latency of less than 50 ms and jitter less than 200 μs.
To meet service requirements in the era after 10G PON, the IEEE and ITU-T/FSAN started to research the post-10G PON technology after finalizing the 10G PON standard. The next-generation PON will mainly develop in two directions: improving the single-wavelength rate or increasing the total rate through multi-wavelength multiplexing. The industry consensus is that the bandwidth of the next-generation optical access network will be increased to 50 Gbps. How to simply and efficiently achieve that bandwidth upgrade has become a hot topic in PON research.
The IEEE was the first organization to start formulating the next-generation PON standard, which supports 25 Gbps downstream and 10 Gbps/25 Gbps upstream over a single fiber and is compatible with 10G EPON. To deliver 50 Gbps rates, it uses wavelength stacking and channel bonding technology to provide two 25 Gbps channels. The ITU-T/FSAN, after considering the needs of home, enterprise and mobile backhaul/fronthaul, established next-generation PON requirements with a focus on 50G PON providing a single-wavelength rate of 50 Gbps. In 2018, the ITU-T/FSAN started the standardization of single-wavelength 50G PON referred to as G. higher speed PON (G.hsp). The standard is expected to be released by the end of 2021. 
50G PON is able to provide over 4-fold increase in bandwidth, better service support, and stronger network security compared to 10G GPON. It also should support the compatibility with the existing ODN.
Both the upstream and downstream wavelengths of 50G PON operate in the O band, making 50G PON unable to coexist with GPON and 10G GPON at the same time. 50G PON uses an LDPC FEC scheme. To better support low-latency applications, 50G PON introduces technologies like dedicated activation wavelength (DAW) and cooperative dynamic bandwidth allocation (CO-DBA). Table 1 shows a comparison of key performance metrics between 50G PON and 10G GPON.

10G GPON and 50G PON: Evolution and Coexistence

During network evolution, using the existing network resources to save upgrade and evolution costs has always been a priority for operators. In order to realize the smooth evolution from 10G GPON to 50G PON and meet the networking requirements of different services, 10G GPON and 50G PON will coexist for a certain period of time. A good coexistence scheme minimizes the equipment footprint, reduces the energy consumption of the optical access equipment, reuses the existing ODN, and reduces the network construction cost of the operator. Using the multi-mode optical transceiver modules in the OLT, such as the Combo PON module that supports both 50G PON and 10G GPON, has been regarded as an effective way so far. 
The coexistence of 10G GPON and 50G PON should meet the following requirements: coexistence of 10G GPON and 50G PON over the same fiber, avoiding or minimizing service interruption on ONUs that are not upgraded, and supporting compatibility with the services of 10G GPON.
After discussions, the ITU-T has determined that 50G PON will not coexist with both GPON and 10G GPON at the same time. To enable coexistence, the evolution to 50G PON can be completed in two steps: first from GPON to 10G GPON and then from 10G GPON to 50G PON. This approach achieves continuous bandwidth upgrades while ensuring that the network evolution is smooth and cost-effective.

Key 50G PON Technologies

Wavelength Selection
Currently, 50G PON can only use a small portion of wavelengths in the O band, which are not enough. The ITU-T has decided on some wavelengths and is still discussing the other wavelengths.
Line Coding
The ITU-T had considered a number of line coding schemes for 50G PON, including PAM-4, duobinary modulation, and NRZ code. It ultimately decided to adopt NRZ code that has the highest signal receiving performance, because the PON system requires a very high optical power budget.
Line Rate
The ITU-T has made clear the rate requirement of 50G PON, and supports a combination of symmetric and asymmetric rates, with one downstream rate and four upstream rates available.
FEC
The increase in the line rate of 50G PON reduces receiver sensitivity. To reuse the considerable ODN resources, receiver performance has to be improved. To lower the specifications requirements for optical components, 50G PON uses the LDPC scheme for FEC.
Common Transmission Convergence  
50G PON implements low latencies mainly through DAW, CO-DBA, and shortening the bandwidth allocation interval.
—DAW: A DAW can be a wavelength newly defined for 50G PON or one deployed for a PON system preceding 50G PON. It can be an independent upstream wavelength or a pair of upstream and downstream wavelengths. The DAW technology avoids opening a quiet window on the upstream wavelength, thereby eliminating the delay caused by the quiet window.
—CO-DBA: The OLT learns about the upstream service transmission requirement of the ONU through the upstream equipment, and then allocates bandwidth to the ONU in advance. This mechanism reduces as much as possible the time for which the service data is cached in the ONU.
—Shortening the bandwidth allocation interval: The interval between bandwidth allocations for the ONU is slashed, thus reducing the service data cache time in the ONU. Each T-CONT supports a maximum of 16 burst frames in a 125 us period.
PHY Components
The PHY components of 50G PON mainly include key optoelectronic devices such as optical transmitter modules, optical receiver modules, laser device drivers (LDDs), burst-mode TIAs, and clock-data recovery (CDR) chips. The OLT can use EML or integrated SOA-EML components as the transmitter module, and APD or integrated SOA-PIN components as the receiver module. The ONU differs from the OLT in that the ONU driver needs to support the burst function and that the ONU receiver does not require the burst-mode CDR feature. Experiments and simulations in the industry show that, by using a 50G EML transmitter and an APD receiver, 50G PON can attain a single-wavelength rate of 50 Gbps.

As the industry works on the standardization of 50G PON, some of its technical requirements and possible development directions need further research. One example is to provide exclusive bandwidth for operators or services. Another example is to address the latency and cost/performance requirements to meet the tight upstream optical power budget of 50G PON. It is estimated that 50G PON will be put into commercial use by about 2025. Before that time, 10G PON will be deployed on a large scale, laying a solid foundation for its smooth evolution to 50G PON.
As an active player in the industry, ZTE has submitted over 300 proposals in the PON field to standard organizations such as ITU-T SG15 Q2/FSAN, and holds key positions (e.g. editor) of many standards related to PON technologies. ZTE is now promoting the standardization of 50G PON and improvement of the relevant industry chain. In addition to making breakthroughs in the technologies for some key 50G PON components, ZTE has submitted more than 30 proposals on 50G PON, and those related to physical layer parameters, low latency, and FEC have been adopted by the standard organizations.