The “Broadband China” strategy defines the broadband network as “the strategic public infrastructure during the economic and social development of China in the new era” at the state level for the first time. As the key part of the broadband network, the broadband access network has some noticeable features such as large investment, long construction period, and high network complexity. With the rapid development of new services such as cloud computing, high-definition video, and Virtual Reality (VR), the user bandwidth grows 10 times larger every five to seven years. The current access network technologies need to be continuously upgraded to meet larger bandwidth and higher technical requirements. Point-to-multipoint PON technologies are the mainstream broadband access technologies, and have already evolved through EPON and GPON to 10G-PON. The current global broadband access market already enters the gigabit era. During the coming years, 10G Fiber-to-the-Home will become an inevitable trend during the construction of broadband access networks. With the accelerated development of 4K video and 5G technologies, 10G-PON technologies are difficult to meet the future bandwidth requirements in the access to the Customer Premises Network (CPN), mobile fronthaul and backhaul. The PON technologies supporting 25 Gbps, 100 Gbps, and higher rates are gradually becoming a hot research topic in the industry.
Development of the Next-Generation PON Standard
The evolution of PON technologies following 10G-PON technologies can be implemented in one of the following ways: increase of the single-wavelength rate (the baud rate is increased from 10 Gbps to 25 Gbps or 40 Gbps) and multi-wavelength overlapping (the rate per wavelength is 10 Gbps or 25 Gbps, and the rate reaches 40 Gps, 80 Gps, or 100 Gbps after multi-wavelength overlapping.
The FSAN Alliance started the R&D of NGPON2 in 2011, and finished the work in 2015. The Alliance selected TWDM-PON as the key technology solution (4/8 wavelength overlapping). The 10G TDM mode is used for each wavelength, and the point-to-point WDM overlay technology can be used in mobile backhaul and for business customers. The key requirements for NGPON2 are 40 Gbps downlink and 40 Gps or 10 Gbps uplink, aiming to achieve a transmission distance of 20 km and 1:64 splitting. The ITU is also concerned with the research process of single-wavelength 25G PON. The 25G-PON standard is expected to be formulated in the near future.
IEEE began the NG-EPON research in 2013, and already set up IEEE ICCOM to analyze the market demands and technical solutions for NG-EPON. IEEE also published the NG-EPON Technical Whitepaper in March 2015. It began the formulation of the 100G-EPON standard (named after IEEE 802.3ca) in July 2015. The 100G-EPON standard is expected to be released in 2019. This standard defines three types of MAC-layer rates: 25 Gbps, 50 Gbps, and 100 Gbps. As far as 25 Gbps is concerned, it involves asymmetric (10/25 Gbps) and symmetric (25/25 Gbps) modes.
Analysis of the 25G/100G-PON Modulation Technology
Access network technologies are continuously upgraded, a large scale of networks are involved, and the investment cost is high. Therefore, high performance and low costs are always the key factors determining the evolution of access network technologies. Optical components account for a large share of the total cost, and they need to be specially taken into account during the upgrade of access network technologies. Currently, the industry chain of optical components (E/GPON and 10G-PON) is already mature, and the 25-Gps or 40-Gbps optical components following 10G-PON have technology-intensive and high-cost features. The industry chain is yet to be developed. For the traditional OOK modulation-based PON technologies, large dispersion and receiving sensitivity degradation problems occur after 10G-PON technologies. Therefore, dispersion compensation and the equalizing algorithm are used in design to improve performance. The bandwidth of high-speed optical components is the key factor to guarantee performance and restrict costs. If low-bandwidth optical components are used to transmit high-speed signals, advanced modulation technologies (such as duobinary and PAM-4) are required. However, this also increases the complexity in the implementation of circuits (for example, the use of high-speed AD/DA and DSP components).
For the next-generation PON technologies following 10G-PON, single-wavelength 25G-PON is more researched, and the implementation mode can be NRZ modulation, duobinary modulation, or PAM-4 modulation.
- NRZ Modulation
Either of the following solutions can be used to implement NRZ modulation-based 25G-PON: One is to use 25 Gbps optical components on both sending and receiving ends. The other is to use 25 Gbps optical components on the sending end for reducing costs and use 10 Gbps optical components on the receiving end (the DSP bandwidth compensation algorithm is used to provide a transmission rate of 25 Gbps). The direct OOK modulation mode is applied to both of the above solutions. The technical key points of the modulation mode are as follows: First, the implementation difficulty of the BCDR of uplink 25G burst-mode electrical chips is large. Second, if the planned wavelength is outside the O band, the dispersion compensation algorithm needs to be added. Third, in order to meet the power budget requirements of the PON network, pre-emphasis is added to the sending end, and the equalizing algorithm is used on the receiving end to enhance sensitivity. The advantages of NRZ modulation are that the system implementation is simple and the mature industry chain of 100G Ethernet and 10G-PON can be reused for key optical components. 25G optical transceivers have high costs, and may be reduced to a certain degree in the future when they are applied to data centers and the FTTx project on a large scale.
- Duobinary Modulation
Duobinary is a binary data encoding method, which converts the logical signal “0” in binary format into “+1” and “–1”, so that the spectral bandwidth of signals can be reduced by half. In optical fiber communications, duobinary has two application modes: three-level amplitude modulation and optical duobinary. In terms of three-level amplitude modulation, duobinary decoding circuits are needed for receivers. Different from the traditional binary IM-DD system, three-level decision may cause the degradation of receiver sensitivity. The advantages of the three-level amplitude modulation solution are that the system implementation is simple and the signal bandwidth of the optical domain can be reduced by half when compared with the NRZ system. The disadvantage is that, different from the NRZ system, the optical power budget of the system is reduced. In the optical duobinary solution, the M-Z modulator and AM-PSK mode are used. The feature is that the receiving end is compatible with the receiver of the traditional binary IM-DD system, which does not cause any sensitivity degradation. The advantage of this solution is that the receiving end does not determine the phase of the received signal and extracts only the signal amplitude. Therefore, only the traditional direct detection components need to be used at the receiving end. The difficulty is that the M-Z modulator is large in size and high in cost.
- PAM-4 Modulation
PAM is a type of high-order modulation technology. Its principle is to map two or more bits into different transmission pulse amplitudes (voltages), thus increasing the bit transmission rate of each symbol. The primary goal of using PAM modulation is to reduce or maintain the bandwidth of transmitted signals when the transmission rate is increased, thus reducing or maintaining the transmission and receiver costs. PAM-4 has four amplitudes, and each amplitude can carry two bits. The dispersion tolerance of PAM-4 modulation can be increased by a factor of 4 relative to NRZ. For PAM-4 modulation, 12.5 Gbps optical components are used to transmit 25 Gbps signals. The cost is that high-speed AD and DA technologies are needed to encode and decode signals at both the sending and receiving ends. A relatively complicated algorithm is needed to implement bandwidth compensation on the receiving end.
Based on the above analysis, two core factors (the bandwidth of optical components and electrical-layer implementation complexity) need to be balanced during the implementation of single-wavelength 25G-PON to determine a solution with a proper price/performance ratio. The NRZ encoding-based 25G-PON has become a hot topic in the formulation of standards and industrial research due to its simple architecture and sophisticated components. After the transmission rate of 25 Gbps is achieved for single wavelengths, the multi-wavelength overlapping and channel binding technologies can be combined to implement 50G-PON (two wavelengths) and 100G-PON (four wavelengths).
25G/100G-PON Wavelength Planning Analysis
For the wavelength selection of the 25G/100G-PON system, the following factors are taken into account: fiber dispersion, fiber loss, compatibility of the existing PON system, optical component costs, and technical implementation complexity.
25G/100G-PON supports three rates: 25 Gbps, 50 Gbps, and 100 Gbps. The rate of 100 Gbps involves four wavelengths, and each wavelength operates at the rate of 25 Gbps. The available wavelength plan supports the following solutions: all-O band (four pairs of uplink and downlink wavelengths are located on the O band). For O/C/L band 1, the uplink and downlink wavelengths of the first wavelength channel (Lane0) are located on the O band, and other wavelengths are located on C and L bands. For O/C/L band 2, all uplink wavelengths are located on the O band, and all downlink wavelengths are located on C and L bands. The all-O band solution supports NRZ modulation, and complex dispersion compensation processing is not needed. The 100G Ethernet industry chain can be reused, and the Direct Modulated Laser (DML) and Electroabsorption Modulated Laser (EML) are also used. The physical layer is easy to be implemented. The O/C/L band solution allows a large wavelength passband range, and the laser wavelength shift index is reduced. The uncooled laser can be used. The multiplexer/demultiplexer design is easy, and the EDFA amplifier can be used. However, the dispersion of the C or L band is large, and therefore complex modulation and equalization technologies are needed for dispersion compensation. The effect on performance and costs needs to be further studied.
Next-Generation PON Evolution Trend
The evolution of PON technologies can be analyzed from two aspects: technology upgrade and smooth evolution.
In terms of technology upgrade, for the IEEE system, after EPON evolves into 10G-EPON, symmetric 10G-EPON will gradually become a mainstream technology. 10G-EPON may also evolve into single-wavelength 25G-EPON and multi-wavelength 50G/100G-EPON. For single-wavelength 25G-EPON, standard organizations are recently concerned about NRZ. PAM-4 and other advanced modulation technologies may become hot research topics in the future in terms of the single wavelength rate higher than 50 Gbps. For the ITU system, asymmetric XGPON1 will gradually evolve into symmetric XGS-PON. Technology complexity and high costs must be solved if NGPON2 is to be put into commercial use on a large scale. For the next-generation PON technologies following NGPON2, ITU will be gradually integrated with IEEE in physical-layer technologies.
For the compatibility of the next-generation PON technologies, considering the growth of bandwidth requirements, wavelength resources, and cost factors, complying with second-generation PON technologies is a reasonable decision, that is, compatibility between EPON and 10G-EPON, between 10G-EPON and 25G/100G-EPON, between GPON and 10GGPON, and between 10GGPON and NGPON2.
From the perspective of bandwidth requirements, 10G-PON can provide the 100 Mbps to 1 Gbps bandwidth for each user, which meets user bandwidth requirements before 2020. After 2020, 25G-PON, NGPON2, 50G/100G PON can provide users with the 1 Gbps to 10 Gbps bandwidth.
Development of the Next-Generation PON Industry Chain
In terms of the research of 25G/100G-PON following 10G-PON, various uplink and downlink products of the industry chain are recently launched. SiFotonics recently launches silicon low-cost 25G APD components based on the next-generation 100G-PON optical access technology. In April 2016, ZTE and Shanghai University conducted a test and proved that the receiving sensitivity of the components can reach up to –28.5 dB. Avago and Sumitomo are also actively involved in the R&D of 25GEML/DML laser products and have already launched mature products. Optical module manufacturers such as Hisense and Accelink also actively carry out the R&D of 25G/100G-PON optical modules and already launch sample products. In March 2016, multiple scientific research institutes and device vendors such as OFC and ECOC published research articles and launched prototype sample devices. ZTE is also active in the R&D of 25G/100G-PON, and is already one of the sponsors of the 100G-EPON standard and the key contributor of the NG-EPON whitepaper and the 100G-EPON standard. ZTE began the R&D of 25G/100G-PON devices in 2013, and got strong support from the 100G-PON Project sponsored by the Shenzhen Technology Innovation Program and the High-Tech 100G-PON Project sponsored by China’s STCSM.
Currently, 10G-PON is deployed on a large scale, and 25G-EPON/100G-EPON standards are instituted. Single-wavelength 25G becomes a standard and technical research hot topic. 10G-PON meets the recent bandwidth requirements, and 25G/100G-PON will be gradually put into commercial use after 2020. ZTE is actively involved in the R&D of 25G/100G-PON, and gradually promotes the institution of standards and the development of the industry chain.