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NRZ, Duo-Binary or PAM-4, Which One Is Better in 100G Optical Communication?

Number of visits: Date:2018-3-15 09:42


The wave of global broadband speed-up has arrived. 4K / 8K Ultra High Definition Video, Virtual Reality, Smart Home, Internet of Things and other applications will become part of people's daily life and work. More and more countries have already increased or plan to increase the rate of broadband access. At present, more than 50 operators worldwide are offering gigabit broadband services. In South Korea, the United States, Hong Kong, and other regions, operators have already offered 2G and even 10G services for enterprises and home users. In China, the State Council released a national broadband strategy in 2013, which planned to enable the access rate of home users in developed cities to reach 1 GB / s by 2020. In the EU and the United States, governments also have been accelerating the promotion of the national basic bandwidth or providing greater support for broadband development.

Single-wave High Speed: The Next Generation PON Key Technology

With the increasing demand for user terminal bandwidth, the current mainstream access network system architecture - PON (Passive Optical Network) network is also constantly upgrading. Nielsen Norman's co-founder Dr. Jakob Nielsen proposed the famous Nielsen's Law in 1998: user bandwidth will grow at a rate of 50% per year. According to the statistical results from 1983 to 2014, the trend of bandwidth growth fits well with this law. Following this pattern, user bandwidth will reach 1.6G by 2020. If we use common 1:64 spectral ratios of PION network to calculate, the bandwidth of the current mainstream PON system will not be able to meet the needs of users.

At this stage, GPON and EPON are in a large-scale mass deployment, XG-PON1 and 10G-EPON have also begun the small-scale commercial use. The main camp of international standard of PON system includes FSAN leading ITU-T series GPON standard, and IEEE leading 802.3 series EPON standard. Among them, the ITU-T GPON standard series has been developed from GPON, XG-PON1 to NG-PON2. The current NG-PON2 standard system has explicitly adopted the TWDM-PON system architecture with a single-wave rate of 10G and a total system capacity of 40G or 80G by using 4-wave or 8-wave.

Comparing to the faster ITU-T standard, the formulation of IEEE802.3 series of standards lag slightly behind. EPON and 10G-EPON standards were released in 2004 and 2009, and NG-EPON standard corresponding to the NG-PON2 communication capacity is in an early stage. In July 2015, the NG-EPON CFI (call for interest) was formally approved and a Study Group was formed. At the end of September 2015, the NG-EPON standard was released. Currently, NG-EPON targets have defined several schemes, including single-wave 25G uplink & downlink and N × 25G uplink & downlink. The main modulation format technologies include NRZ (not return-to-zero code), Duo-Binary (duo-binary coding) and PAM4 (4 pulse amplitude modulation coding), etc.

At present, although many candidate technologies are still being discussed and compared, the single wave rate of more than 10G has been basically agreed, and the direction of the target evolution is mainly single-wave rate of 25G. In the whole PON system, single-wave 25G PON can be used as mainstream technology for home user access. For government and enterprise users, due to their greater bandwidth demand, it can achieve a higher bandwidth of 2 x 25G or 4 x 25G by the wavelength overlay on the basis of single-wave 25G. The ITU-T GPON standard is expected to launch a project discussion for the single-wave 25G standard in the second quarter of 2016.

From the above analysis, it can be seen that the single wave 25G has become an important node rate for optical access. The industry, including Huawei, has accumulated a lot of multi-wave superposition techniques in the process of the previous 4 × 10G TWDM PON research. Therefore, the realization of the current single-wave 25G technology has become the key to 100G PON.

The Challenges for Single-wave High-speed PON Technology

In the field of optical access, the main demand of operators is that optical fiber networks can also be reused while bandwidth upgrade. Since ODN links involve infrastructure construction, which is difficult and costly, its construction cost accounts for most of the PON network deployment. Therefore, operators have a strong appeal for not changing the ODN link in the next generation of PON network upgrade. Currently, ODN links generally need to support a minimum of 20Km optical fiber and 1: 32 optical splitters. Therefore, the main challenges of single-wave high-speed PON will be focused on dispersion, power budget and rate selection.

Dispersion Puzzle: Due to the simple structure and low cost of NRZ, the modulation format is adopted for EPON, 10G-EPON, GPON, XG-PON1 and NG-PON2 at the single-wave rate of 10G and below. At this time, dispersion is not the main problem of PON network. However, when the single-wave rate reaches or exceeds 25G, the dispersion tolerance of the NRZ modulation format cannot meet the requirement of transmitting 20Km. There are two ways to solve this problem. One is the use of zero-dispersion O-waveband (fiber zero-dispersion area), but this waveband has been occupied by EPON and GPON, which is difficult to adopt in multi-generation PON networks co-existence scenario. Second is to use electric dispersion compensation method, in which the introduction of high dispersion tolerance modulation format or equalization algorithm is more feasible approach.

Power Budget Is Tight: PON network is a point-to-multipoint system architecture. Since the optical splitter in the ODN link introduces a large extra insertion loss, the power budget becomes a bigger challenge for the PON network. In general, it can be realized by increasing the transmitting optical power and increasing the receiving sensitivity. At present, the mainstream detectors are PIN (photodiode) and APD (avalanche photodiode). In PON systems, APDs are primarily light-receiving devices due to the high power budget requirements. The receiving sensitivity of APD has a clear relationship with the signal rate. When the signal rate is increased from 10Gb / s to 25Gb / s, the receiving sensitivity of the receiver will drop by 4dB. If there is no compensation, the system link power budget will be brought decline. The current 25G APD chip technology and ROSA packaging technology is not yet mature. Only a few suppliers announced that they have possessed of the technology and it is expensive. The low-cost 25G PON optical transceiver devices will be the problem that the industry has to be faced with.

Rate Selection: When the single-wave rate exceeds 10G, interference such as chromatic dispersion and insufficient power budget will be encountered. Moreover, the rate is higher, the impact of dispersion on the system is bigger and the system power budget will be tighter. Relative to single-wave 10G, single-wave 25G can use Duo-binary, PAM4 and NRZ + DSP and other programs to solve these problems. These programs are multi-stage modulation, encoding and decoding is relatively simple, and the device requirements are not high. For single-wave 40G, however, the cost of single-wave data increases is at the expense of more sophisticated high-order modulation or more complex DSP algorithms with tighter power budgets. Theoretical analysis and simulation show that the single-wave 40G mode can hardly meet the current power budget level requirements of 10G-EPON. And correspondingly, the current industry 25G circuit technology has matured, such as 25G laser driver, 25G transimpedance amplifier and 25G data clock recovery circuit and so on.

3 Kinds of Single-wave High-speed Solution Analysis

Single-wave 25G NRZ Solution

Due to the simplicity of the NRZ modulation format, this modulation format is used in EPON, 10G-EPON, GPON, XG-PON1 and NG-PON2 systems. Under the single-wave 25G rate, if the O-band transmission is adopted, the dispersion tolerance of the NRZ format optical signal can meet the requirement of transmitting 20Km. However, if the C or L band (optical positive dispersion region) is adopted, the single-wave 25G NRZ scheme will not meet the conventional 20Km transmission requirement of PON systems because the dispersion tolerance is not sufficient. In this situation, dispersion compensation, either optically or electrically, is required, including 25G electro-absorption modulated laser on the transmit side and 25G APD receiver on the receive side. Although the structure of the PON optical module under the scheme is simple, the cost of the 25G optical device is relatively high, and it is the maximum disadvantage of the solution without enough dispersion tolerance. The remedy for the drawback is to compensate the dispersion by using the DSP algorithm on the receiving side. If the algorithm is properly optimized, 10G optical devices can even replace 25G optical devices on the receive side, and signal distortion caused by insufficient device bandwidth can also be compensated by algorithms.

Single-wave 25G Duo-Binary Solution

Two symmetric 25G PON systems can be made up of EDB and ODB. In the first form, the uplink and downlink adopt the EDB modulation format. Considering that the cost of the ONU side in the PON system is relatively sensitive, only the EDB modulation of the 25G optical devices can be used on the transmission side of the OLT and the uplink signal of the EDB format can be generated on the transmission side of the ONU by using the 10G optical devices. The distortion of the uplink signal caused by the bandwidth limitation of the device can be compensated by a more complex electric domain algorithm on the cost-insensitive OLT receiving side. In the second form, the ODB modulation format is used on the downlink and the ODB signal is formed on the transmission side of the OLT by phase modulation based on the generated 3 levels using the Mach-Zehnder Modulator (MZM). On the receiving side of the ONU, it only needs to receive the two-level decision similar to NRZ, which can greatly simplify the receiving circuit and reduce the ONU cost. The uplink modulation scheme is consistent with the first form, that is to say, a 10-G optical device is used on the sending side of the ONU to generate a 3-level EDB signal.

Single-wave 25G PAM-4 Solution

PAM-4 modulation, known as 4-level pulse amplitude modulation, modulates every two bit into a single baud when the signal is modulated, so the baud rate for PAM-4 modulation is halved and the frequency efficiency is doubled. The dispersion tolerance of PAM-4 modulation can be up to 4 times higher than NRZ. The 25G PAM-4 modulation uses only 12.5G EML and 12.5G linear drivers on the transmit side and 12.5G APD linear receive optics on the receive side. And because the current mainstream optical devices are 10G, 10G optical devices can also be used instead of 12.5G EML or APD, and then through the electric compensation algorithm for bandwidth compensation. PAM-4 needs to adopt the digital-analog converter to produce 4 levels on the sending side and the receiving side adopts the analog-digital converter to decode 4 levels.

Comparison for 3 Kinds of Solutions

The above three kinds of single-wave high-speed technology solutions have their own advantages and disadvantages. 25G NRZ scheme is simple in structure, but needs to adopt DSP to carry on dispersion compensation at the receiving end, and the cost is higher that adopts 25G optical device at the same time. Symmetrical 25G EDB scheme uses 10G receiver and 10G sending optical device on ONU side, the cost is lower, 25G reception requires 3-level decoding in EDB format, introducing additional ONU costs. For the downlink 25G ODB and uplink 25G EDB solution, its main advantage is the downlink receiver high sensitivity and easy to receive. But the sender is more complex because of the introduction of the phase modulator. Meanwhile, the receiver side also needs to use 25G optical devices. The porter rate of PAM-4 solution halves, the bandwidth of optoelectronic devices will also be reduced. But higher requirements are placed on the linearity of the devices. Moreover, PAM-4 transceivers bring about problems such as cost and power consumption. In addition, PAM-4 solution is also less sensitive comparing to other solutions. However, due to many famous chip companies in the industry are going to introduce PAM-4 codec chips, it is expected to drastically reduce the difficulty and cost of sending and receiving PAM-4 solutions.

In addition, in order to achieve the power budget requirement of the PON network system, each of the above solutions basically needs to use an optical amplifier. The problems of cost, power consumption, and integration of the optical amplifier caused by the above solutions are also need to face and solve for single-wave high-speed PON.

Each generation of PON system evolution and development are inseparable from the cooperation with the industrial chain. The current 25G optoelectronic devices are constantly maturing. Among them, 25G chip has been mature commercial, such as 25G electro-absorption modulation laser driver, 25G Mach Zehnder modulator, 25G data clock recovery and transimpedance amplifier, etc. 25G O-band laser based on PIN receiving optical components have also been mature commercial for many years. The industry is in full swing to develop APG-based 25G receiving optical components. It can be said that 25G optical chip is in rapid development.

With the progress of optical network technology and the continuous development of high-speed photoelectric chips, it is believed that the single-wave rate will continue to be improved and the single-wave high-speed PON system will have a better tomorrow.

TypeInfo: Blog Articles

Keywords for the information:PON  NRZ  Duo-Binary  PAM-4