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The New Direction of Optical Communication Modulation: PAM4

Number of visits: Date:2018-3-30 10:20

Abstract:

With the arrival of big data&cloud computing and the growth of traffic, a new and more complex modulation method is urgently needed. At present, PAM4(4 Pulse Amplitude Modulation) is the most popular mode of modulation, likely to drive the industry to a higher-rate optical communication.


When the big data, cloud computing and Internet of Things are developed toward scaled applications, we also follow the development of technology into the era of Internet plus 2.0. Artificial intelligence and intelligent interconnection have become the keyword of the new era. It is shown by research data that by 2020, there will have been 50 billions of interconnected intelligent devices by 2020. On average, 1.5G data volume will be generated via PC, mobile phones and wearable devices per person per day. The intelligent interconnection of all things has caused a huge change in data ecology, and more and more countries plan to increase the broadband rate to cope with the surge of pressure. Currently, more than 50 operators are providing Gigabit broadband services. In Korea, the United States, Honk Kong etc, operators have started 2G or even 10G services for enterprise/home users; in China, the State Council issued the national broadband strategy in 2013, planning to make the access rate of home users in developed city reach 1Gb/s; in the European union and the United States, the governments are also accelerating the upgrading of national basic bandwidth, or providing greater support for broadband development.


Although many candidate technologies are still in the discussion and comparison, and the single-wave rate exceeding the 10G basically comes to agreement. Simutaneously, the evolution direction is toward single-wave rate 25G. In the whole PON system, single-wave 25G PON can be used as the mainstream technology for home user access. For government/business users, on account of the large demand on bandwidth, it can achieve 2&TImes by wavelength superposition on the basis of single wave 25G, 25G or 4&TImes or higher bandwidth. In the field of optical access, the main demand of the operators is to reuse the existing optical fiber network while the bandwidth is upgraded. ODN (Optical Distribution Network) link gets involved in the construction of infrastructures, which is difficult and high-cost. The cost of construction accounts for majority in the deployment of the whole PON network. Therefore, when the next-generation PON network is upgraded, strong appeal for ODN links wihtout change are made. Currently, ODN links generally need to support at least 20km fiber and 1: 32 optical splitter. Therefore, the main challenges of single-wave and high-speed PON will focus on dispersion, power budgeting and rate selection.

The Challenge in Dispersion

When the single-wave rate is up to or more than 25G, the dispersion tolerance of the NRZ modulation format is unable to meet the requirements of 20km fiber transmission. There are two ways to solve this problem. One is to adopt O band with zero dispersion (fiber zero dispersion region), but the band has been occupied by EPON and GPON, hard to use under the conditions of multi-generation PON network for coexistence; another one is to utilize EDC(Electrical Dispersion Compensation)method; the introduction about the modulation format of high dispersion tolerance or power equalization algorithm is a more feasible approach.

The Challenge in Power Budgeting

In the PON system, due to the high requirements for power budget requirements, APD is mainly applied as the optical receiver. The receiving sensitivity of APD is obviously related to the signal rate. When the signal rate is increased from 10Gb/s to 25Gb/s, the receiver's sensitivity will decrease by 4dB. If there is no compensation measure, the link power budget of the system will decrease. At present, 25G APD chip technology and ROSA packaging technology are not yet mature; only a few suppliers declare that they have the technology, but it is expensive. Low-cost optical transceiver in the 25G PON system will be a problem in the industry.

The Challenge in Rate Selection

After the single wave surpasses 10G rate, it will be with problems such as dispersion and insufficient power budget deficit. Meanwhile, higher rate has more influences on system, and the power buget of system is tighter. Compared with single-wave 10G, single-wave 25G can use Duo-binary, PAM4 and NRZ+DSP to solve the above problems. These schemes belong to multi-order modulation. The demands on 100G connection for the data center have been clear, but the approach to make the 100G application a mainstream is not like that. The early-implemented 100G optical transceiver uses the NRZ signal modulation scheme, proved to be feasible. However, same as the transition stage of all mainstream technologies, system designers need clear industry standards to solve lingeringly ambiguious problems among competing methods. This standard should be the agreed standard for designers to reaching agreement faithfully. The single wavelength (lambda) PAM-4 modulation scheme has become the standard.

Modulation Methods in Optical Communication

NRZ(Non-Return-to-Zero)

In the process of optical path, it is necessary to deal with the problems in the aspect of digital signal representation and signal synchronization on the both of receiving port and transmitting port. For transmission digital signals, the most simple and commonly-used method is to represent two binary numbers with different voltage levels; namely, digital signals consist of rectangular pulses. According to the way of digital encoding, it can be divided into unipolar code and bipolar code. Unipolar code uses positive or negative voltage to represent data; bipolar code is binary code, "1" mean inversion, and "0" mean to maintain zero level. According to the signal to return to zero or not to, it can be divided into RZ and NRZ code. The signal in the intermediate of zero code'element returns to the 0 level, such as "1" as a positive level, "0" as a negative level; after each data representation is completed, it will return to zero level; while NRZ code does not return to zero level, such as the "1" as a high level, "0" as a low level.

1. Unipolar NRZ Code

No voltage (that is, no current) is used to represent "0", and a constant positive voltage is used to represent "1". The intermediate point of each code element's time is the sampling time, and the Tdd-offset is half amplitude level(0.5). That is to say, the value of the received signal is between 0.5 and 1 or between 0 and 0.5, which is regarded as "1" or "0". The number of binary symbols per second is called "code speed".

2. Bipolar NRZ Code

Both "1" code and "0" code have current, but the "1" code is the positive current, the "0" code being the negative current. The amplitude of positive and negative current is equal, so it is called as the bipolar code. The Tdd-offset at this moment is zero level, and zero decision device or positive/negative decision device is applied on the receving interface. The value of received signal above/blow the zero level is positive/negative, it will be judged as "1" code/"0" code.


The above two codes emit or do not emit electric current (unipolar) in the whole time of a symbol, and transmit positive or negative current (bipolar). Each code occupy the width of the entire code, so these two codes belong to the fully-wide code, also called as the NRZ (Non Return Zero)code.

Ideal NRZ Transmission Eye Diagram(Top)and NRZ Transmitter(Bottom)

Ideal NRZ Transmission Eye Diagram(Top)and NRZ Transmitter(Bottom)

Doubinary

According to certain rules, duobinary transforms the original binary logic signal "1" and "0" into logical signals "+1", "-1" and "0", so that the bandwidth of the signal is reduced to half of the original one. The adoption of duobinary modulation can reduce the bandwidth taken by signal, improve the utilization rate of frequency, and increase transmission distance of the optical signal in the optical fiber. The process of modulation and demodulation is divided into precoding, encoding and decoding. Due to the utilization of precoding, only one module 2 arithmetic device is required to decode on the receiving interface. In the process of modulation, there will be three modulation signals: "0", "+1" and "-1". "+1" and "-1" correspond to the same logic code.


Currently, most optical communication systems use NRZ modulation. NRZ is more suitable for long-distance transmission lines, on account that the dispersion of a single mode fiber can be compensated by adding a negative dispersion fiber. However, if the compensating single-mode fiber is not added, other modulation methods need to be considered. Duobinary is the second modulation method used in industry because it is not sensitive to dispersion and there is much less complexity increased compared with PAM4. Duobinary uses a bandwidth less than R/2Hz to transmit the R bps baud rate. According to Nyquist's theorem, if transmission has no Rbps baud rate data of intersymbol interference (ISI), the minimum bandwidth requirement is R/2 Hz, which means duobinary modulation should use Intersymbol Interference. This Intercode Interference is added by an intentional calculation, so it can be removed at the time of reception.

Doubinary Encoder

Doubinary Encoder

 

In the condition of dispersion, the BER of duobinary is much less than that of NRZ. It is shown by the experimental results that duobinary can enhance the transmission distance of NRZ without adding compensation optical fiber. The core of duobinary's coding principle is to use the characteristics of the transmission channel to add controllable Intercode interference. Before it is applied in optical communication systems, duobinary has been used for a long time in other systems, such as disk systems.

Complete Duobinary Modulation and Demodulation System

Complete Duobinary Modulation and Demodulation System

Duobinary Receiving Interface

Duobinary Receiving Interface

Duobinary Receiving Interface

 

PAM4

As mentioned above, more complex modulation methods are paid more attention by the industry due to the growth of traffic. PAM4 is the most popular modulation method at present. The 53G PAM4 modulation can reach the rate of 100G in single mode fiber.


The PAM4 modulation mode uses 4 different signal levels to carry out the signal transmission, each symbol period can represent 2 bits of logic information (0, 1, 2, 3). Due to that each symbol cycle of PAM4 signal can transmit 2bit information, to achieve the same signal transmission capability, the symbol rate of PAM4 signal only needs half of NRZ signal. After that, the loss caused by transmission channel is greatly reduced. In fact, in the 802.3bj standard issued by IEEE Association in 2014 for 100G backplane, PAM4 simultaneously defined two signal transmission modes: 4 groups of 25.78G baud rate NRZ signal, or 4 groups of 13.6G baud rate PAM4 signal. However, with the development of chip technology, PCB board and connector technology, NRZ technology with 25G baud rate quickly realizes commercial application. While PAM4 has not been truly applied in 100G Ethernet technology due to the problems in the maturity and cost. In the formulation process of the new-generation 200G/400G interface standards, the general demand is that the data rate on each pair of differential lines needs to be improved to over 50Gbps. If NRZ technology is still applied, because the cycle of each symbol is less than 20ps, the requirement for the time margin of the transceiver chip and the transmission link will be more severe. Therefore, the adoption of PAM4 technology is almost an inevitable trend.

The Composition of PHY in Optical Link

The core component is the optical link PHY to achieve optical communication system of 100G/400G. Currently, there have been some 25Gx4 and 50Gx2 multiplex optical link PHY in the stage of application. But for the cost, power consumption and line management, single-wavelength 100G optical link PHY is undoubtedly the best option.


As mentioned above, PAM4 has already been the most advantageous modulation mode in single-wavelength optical link PHY. In the rate of 100G, NRZ will not be widely utilized by the market because the dispersion will be inferior to PAM4 in the transmission distance. While in the optical link PHY, the performance of power FET is crucial, on account that the power required by PAM4 is greatly increased. MACOM's FinFET has a qualitative leap in efficiency compared with the competitor's flat technology FET. The leakage current of FinFET is greatly reduced because of the use of multi-grid manufacturing technology. It is very suitable for the single-wavelength optical link PHY of 100G.


On account that grid adopts the new manufacturing tech, leaked current get well-controlled at the place far away from source polar and leaked polar, compared with FET of traditional flat tech. It is the key to enhance the effeciency of FinFET.

The First Product with 100G Single-wavelength Optical Link Interface: MATP 10025

MACOM has recently exhibited the first industry PHY module with 100G single-wavelength PAM-4 tech in 2017 CIOE, which is mixed with 16nm FinFET 100Gb/s PAM4 DSP and TIA of MACOM.

Conclusion and Prospect

PAM-4 is advocated by enterprises such as AppliedMicro, Cisco,etc., and is applied by IEEE. It has been proven that PAM-4 is the most cost-effective and the most effective factor to drive 100G and 400G in data center so far. For the 100G transceiver, the number of lasers is reduced to one and the need for the optical multiplexing is also eliminated via single-wavelength PAM-4 technology. Only four optical components are needed for the 400G implementation. MACOM has achieved the integration of two combined and complementary product by the merger and acquisition of AppliedMicro and the cooperation with other partners. Therefore, it can provide mature solutions, including designing PAM-4 DSP, linear driver and linear TIA for each λ module to achieve 100G communication. MACOM has the core advantages in optical network technology to help to speed up the deployment of 100G transceivers suitable for datacenter applications, and is with great significance for the industry to clearly implement 400G applications.

TypeInfo: Blog Articles

Keywords for the information:Doubinary  PAM4  NRZ