Beyond 10G PON:Evolution of Next Generation Pon Technology

China’s fixed broadband access rate is growing exponentially and is expected to gradually increase to 1G in the next few years.At the same time, as 5G has launched large-scale commercialization worldwide, Enhanced mobile broadband (eMBB) is also available to provide users with the ability to access gigabit, the fixed and mobile dual gigabit era has come. In this context, the three major domestic operators have successively entered the 10G PON scale deployment stage. Meanwhile,At the same time, the next-generation PON, namely 10G+PON technology evolution, is also on the agenda.

In order to achieve gigabit access on a large scale, not only EPON\GPON but also 10G EPON\ xG-PON are inadequate, so a higher-speed PON technology is required. When discussing the choice of next-generation PON technology, there are 25G, 50G, 100G and other options in terms of speed. The difference between 25G and existing 10G is too small, while 100G is too difficult, and 50G is moderate, which is basically consistent with the previous 4-fold increase (2.5g to 10G). 50G PON is suitable as the evolution technology of domestic 10G PON.

Background knowledge: Why did China choose 50G PON as the evolution technology of 10G PON?

The current scale deployment of 10G PON includes two technologies, 10G GPON and 10G EPON, which belong to the two major standard systems of ITU-T and IEEE respectively. The Full Service Access Network (FSAN) once defined NG-PON2 as the evolution technology of 10G GPON, and started the standardization of NG-PON2 in 2011, and completed the standard formulation in 2015.In the early stage of the formulation of NG-PON2, new scenarios were considered as the traction, and available technologies other than TDM-PON could be selected.And the final network model is determined to be a composite network structure based on TWDM-PON (TDM+WDM). ITU-T has also successively released G.989.1-G.989.3 in 2013-2015, which defines Multi-wavelength superimposed TWDM+PtP PON technical specification. TWDM PON integrates a large number of XG(S)-PON technology, uses a wavelength tunable laser on the ONU side, which can work at any wavelength, and achieves coexistence with GPON and XG(S)-PON technology through WDM technology. However, the single-wavelength rate increase in the NG-PON2 network model is not obvious, and it is difficult and costly to implement the tunable transmitter and tunable receiver in the burst mode at the technical level, which brings the overall network construction The cost is greatly increased, so domestic operators are not actively choosing this technology.With the large-scale application of 25G/50G speed Ethernet, the maturity of the high-speed industry chain makes the N*10G system architecture face obsolete.

IEEE launched the 25Gb/s, 50Gb/s, 100Gb/s EPON standard in December 2015 after 10G EPON, with a single-wavelength rate of 25Gb/s, using 2 wavelengths or 4 wavelengths to achieve 50G or 100G capacity.After two years of discussion, due to the difficulty of 100G technology implementation, IEEE 802.3ca finally eliminated the 100G project goal and only retained the single-wavelength 25G EPON and the 50G EPON based on the superposition of two wavelengths. However, because the capacities of 25G PON and 10G PON are too close, the bandwidth improvement from 10G to 25G is too small, and the cost of 2*25G based on two-wavelength stacking is not as cost-effective as single-wave 50G in the long run.

Considering the long-term network demand, network construction cost and other factors, 50G TDM PON (single-channel 50G PON) is selected as the next-generation PON technology, and deployment towards 2025 has become an industry consensus.Internationally, NORTH American MSO operators choose to support THE IEEE P802.3CA standard of 25G PON, based on the network that has recently rapidly deployed 25G PON and can be upgraded to 50G gradually through wavelength stacking.Since the 21st century, FTTH and broadband have experienced nearly 20 years of development and have become the world trend.Through this selection of 50G PON technology as the evolution of the 10G PON technology to explore, will historically guide the trend of the international PON access network, and further promote the integration of PON technology.

10G+ PON: System architecture and technical requirements

The 10G+ PON system architecture is shown in the figure below.

The physical layer of the 10G+ PON network (high-speed PON network) can support the following set of uplink and downlink rates:

50Gbit/s downlink, 10Gbit/s uplink

50Gbit/s downlink, 25Gbit/s uplink

50Gbit/s downlink, 50Gbit/s uplink

At the same time, the high-speed PON network must meet the corresponding OPL grade, namely 28/29dB and 31/32dB ODN network attenuation requirements;The optical splitting ratio must reach at least 1:64, and the maximum fiber distance must reach at least 20km; the equipment must support the ITU-G.652 fiber, which has been widely used in the existing network, should also be compatible with G.657 fiber. In terms of system-level requirements, high-speed PON networks should support power-saving modes in addition to verification and identification of ONUs (including GPON/XG(S)-PON), data encryption, DBA, and eye/protection mechanisms.The equipment should also support power saving mode, which greatly reduces Operator’s operating costs.In terms of coexistence and integration with existing networks, in order to maximize the investment utilization of deployed PON equipment, high-speed PON networks must support the same type of ODN network shared with XG-PON/XGS-PON/10G EPON, and Ensure the seamless and smooth upgrade capability of XG-PON/XGS-PON/10G EPON users

10G+ PON: Available modulation technology

Generally speaking, different modulation technologies have different advantages and disadvantages, and different modulation technologies can meet different needs in different scenarios. The coexistence requirements between different generations of PON networks, line rate and power budget requirements will have a significant impact on the choice of modulation technology.NRZ coding is relatively simple and has high receiving sensitivity, but it requires the highest optical device bandwidth and the lowest dispersion tolerance. Dual binary coding and PAM4 code can reduce the bandwidth requirements of optical devices and have higher dispersion tolerance, but the receiver sensitivity is reduced and the device complexity is increased. OFDM has the lowest requirements on the bandwidth of optical devices, but it requires high optical device linearity, complex electronic device process requirements, and a significant reduction in receiver sensitivity.Between EDB and ODB, in general, ODB requires a more complex modulator on the transmitter side, but the receiver is relatively simplified (like NRZ receiver). For 50G PON, NRZ scheme has simple structure and high sensitivity, but dispersion compensation is needed at the receiving end. The 50G EDB scheme uses 25G receiver and 25G transmitting optical device on the ONU side, which is relatively low in cost. However, the 50G receiving scheme requires EDB three-level decoding, which will introduce additional ONU cost. At the same time, the EDB receiving sensitivity is low, thus aggravating the challenge of power budget. In order to improve the performance of multilevel decoding, equalization technology is generally needed.The main advantages of the ODB scheme are high downlink receiving sensitivity and simple downlink reception. But the transmitting side is more complicated and a phase modulator is introduced.

In PAM-4 scheme, the baud rate is halved, and the bandwidth of photoelectric devices is reduced, but higher requirements for device linearity are put forward, and the PAM-4 transceiver chip will bring cost and power consumption issues. At the same time, PAM-4 compared with the previous schemes, the sensitivity is lower. In the DMT solution, 10G receiver can be used at the receiving end, and the anti-dispersion performance is good, but the encoding and decoding requires DSP, which brings higher cost issues and lower receiving sensitivity. For 50G PON, as the rate increases, it is necessary to face the following technical challenges: higher-speed optical and electrical devices requirements, higher transmitted optical power, high-sensitivity high-speed receivers,and overcoming the dispersion problem caused by high speed transmission. In the DMT solution, 10G receiver can be used at the receiving end, and the anti-dispersion performance is good, but the encoding and decoding requires DSP, which brings higher cost issues and lower receiving sensitivity. For 50G PON, as the rate increases, it is necessary to face the following technical challenges: higher-speed optical and electrical devices requirements, higher transmitted optical power, high-sensitivity high-speed receivers, and overcoming the challenges brought by high-speed transmission Dispersion problem. In short, the maturity of 50G PON technology is inseparable from the maturity of the high-speed optical device industry chain and technological progress.

10G+ PON: Available optical amplifier technology

In a multi-wavelength system architecture, it is also necessary to overcome the additional insertion loss caused by multiplexing/demultiplexing devices. In order to achieve the same power budget and benefit the traditional ODN, an optical amplifier may be required in a system based on 10Gb/s+ line rate. Optical amplifier technologies that can be used in systems above 10Gb/s: EDFA and SOA.

EDFA, or erbium-doped fiber amplifier, uses erbium-doped fiber as a gain medium to amplify optical signals, and is currently widely used in DWDM optical fiber transmission systems.The disadvantage of the EDFA amplifier is its larger size and higher cost. It will be more economical to use it in a multi-wavelength system. Multiple channels can share the same amplifier and share the cost. But so far, EDFA has only been developed in the C-band (approximately 1525nm-1565nm) and L-band (approximately 1570nm-1610nm). If the wavelength is in the S-band (1490nm region) or O-band (1300nm region),  thulium-doped or praseodymium-doped fiber amplifiers are required.

SOA, or semiconductor optical amplifier, uses semiconductors as gain media amplifiers. Compared with EDFA, SOA can only provide medium gain and has a higher noise figure, but SOA is small in size and electrogenic pump is much cheaper than EDFA. In addition, SOA can also be integrated with semiconductor lasers, modulators, detectors, etc., and can also be used for each semiconductor laser operating band, such as O-band, S-band, C-band, L-band, etc.

On the transmitter side, SOA can be used to increase the transmission power. The most direct method is to integrate SOA on the transmitter. If we want to increase the  downlink power budget, then integrating SOA in the OLT transmitter will be the most suitable solution, because it will only increase the cost of a small part of the OLT transmitter side and can maintain the same OLT port density. SOA can also be used as a preamplifier on the receiver side. Due to the relatively high noise figure of SOA, a large amount of ASE noise will be introduced after the signal passes through the SOA preamplifier. After SOA, narrow band filters can be used to improve the signal-to-noise ratio.

10G+ PON: Selectable wavelength allocation

Wavelength allocation is one of the most important parts of all PON standards, because it has a significant impact on cost and coexistence of traditional PON networks. For the next-generation high-speed PON with a single wavelength of 10Gb/s or more, the wavelength allocation is more complicated. There may be many possible migration paths from traditional PON to 50G PON, such as GPON to 50G PON, XG-PON to 50G PON or GPON/XGS-PON to 50G PON, etc. At present, the industry generally believes that 25G and 50G TDM PON are two possible high-speed PON exceeding 10G.