Achievements and Prospects of Domestic DWDM Communication Systems
The history of development, achievements and current trends in improving technological solutions to increase the transmission speed, energy and economic efficiency of the domestic fiber-optic communication networks are considered.
V. N. Treshchikov 1, M. A. Gorbashova 1, M. O. Zhulidova 1, V. A. Konyshev 1, A. V. Leonov 1, O. E. Naniy 1, 2,
D. D. Starykh 1, R. R. Ubaidullaev 1, I. I. Shikhaliev 1
T8, Moscow, Russia
Moscow State University Moscow, Russia
The history of development, achievements and current trends in improving technological solutions to increase the transmission speed, energy and economic efficiency of the domestic fiber-optic communication networks are considered.
Keywords: DWDM, fiber optic communication network, coherent detection, spectral efficiency, modulation format, symbol rate, spectrum shaping, spectrum management, FlexGrid, Faraday effect, OSNR margin, required OSNR, BER, error rate, EDFA, ROPA, flatness, extra-long lines, data centers
Статья получена: 20.05.2022
Статья принята: 04.06.2022
1. Development of DWDM communication systems.
The spectral multiplexing technology (WDM, wavelength division multiplexing) allows to multiply the total capacity per fiber due to the use of several carriers. The wavelength division multiplexing concept is the simultaneous transmission of several independent signals over one fiber at different optical wavelengths. At each wavelength, a separate optical data transmission channel is arranged with its own transmitter and receiver . The number of channels can range from two (in the simplest systems) to hundreds or more (when using an extended spectral range).
Initially, the fiber-optic communication systems used the amplitude modulation of laser radiation (on-off keying, OOK) was used to encode information, and direct detection (DD) was applied for reception. The channel rates in the commercial direct modulation systems reached 10 Gbps (STM‑64) in 1995 and 40 Gbps (STM‑256) in 2002. However, the development of systems with direct modulation was brought to stop at this point, since the bandwidth of the amplitude-modulated signal approached the bandwidth of the available spectral bandwidth in the ITU-T frequency grid.
The crucial point in the development of fiber-optic communication systems was the invention of systems with the coherent detection and digital signal processing in the 2000s. The coherent detection concept is that the signal received from the line is mixed with the reference laser radiation at a close frequency. The difference signal is digitized using a high-speed analog-to-digital converter (ADC), after which it is analyzed and processed on a specialized coherent digital signal processor (Coherent DSP). This makes it possible to simultaneously detect the signal amplitude and phase for each polarization.
The transition to the coherent detection opened up the opportunity to use the multilevel amplitude-phase modulation formats and a relevant increase in spectral efficiency.
Spectral efficiency (SE) is the number of bits that can be transmitted using this modulation format in one second in a 1 Hz spectral band with a dimension of bps / Hz. Usually, when comparing various communication systems, the net bit rate (excluding FEC) is considered. The spectral efficiency can be calculated both for a single channel and for the entire system. In the up-to-date systems, where the spectral channels are located close to each other, the spectral efficiency of the system and the spectral efficiency of an individual channel are the same . For example, if a DWDM system with a capacity of 8 Tbps has a spectral bandwidth of 4 THz (80 channels of 50 GHz each), then its spectral efficiency is calculated as 8 Tbps / 4 THz = 2 bps / Hz. Similarly, it is possible to calculate the spectral efficiency for a single channel: SE = 100 Gb / s / 50 GHz = 2 bps / Hz.
To improve spectral efficiency, it is necessary to increase the data rate for the same spectral bandwidth. To do this, the multilevel modulation formats are used. However, at the same time, the inevitable price for increasing the speed is the decreased quality of the transmitted signal and, as a result, the decreased transmission reach (Fig. 1).
During the second decade of the 21st century, the coherent systems have gone through several stages of development: from the first systems with a transmission rate of 40 Gbit / s over a single wavelength (symbol rate: 20 Gbaud, modulation format: DP-BPSK) to the most modern systems with a transmission rate of 600 Gbit / s over a single wavelength (symbol rate: 56 Gbaud, modulation format: DP‑64QAM), see Fig. 2. Thus, the data transmission rate over one carrier is increased by 15 times .
Moreover, the modulation format complication inevitably leads to a drop in reach. The more discrete states of the optical signal are used, the higher the symbolic coding gain, the lower the signal quality and transmission range (see Fig. 3).
The transmission reach L is no less important for the telecoms operators than the transmission rate over a single wavelength. The reach in the long-haul systems means the transmission reach in a multi-span line with a cascade of amplifiers and without signal regeneration. The greater this reach, the less often it is necessary to install the transceivers on the long-haul line; therefore, the solution cost is also lower.
The up-to-date coherent systems support the possible selection of a modulation format, so that the operator can set the optimal balance between the range and data rate for his tasks.
Further, it is possible to determine the main indicator of a fiber-optic communication line, the objective of which is to transmit the maximum data rate over the optical fiber at the maximum distance with the minimum use of the spectral range. Thus, to compare the communication systems, it is convenient to use the specific capacity parameter, namely the product of the spectral efficiency and the transmission reach (SE ∙ L) .
The spectral efficiency SE of communication equipment is usually known or easily calculated. The main difficulty in comparing the specific capacity of various applied and promising technologies is the transmission reach L calculation that can be provided by a particular communication system. The maximum specific capacity is achieved in the 100 Gbit / s systems with DP-QPSK - modulation.
The increase in the channel transmission rate is achieved not only due to the use of multi-level formats. The symbol transmission rate supported by the electronic component base continues to be grown. In 2020, the world’s leading manufacturers announced the active development of symbol rates in the range of 64–100 Gbaud at the component base level that makes it possible to transmit the data flows up to 800 Gbps over a single wavelength.
The miniaturization of optical components and accessories is also continued, including through the active use of integrated photonics.
The fiber optic aggregation / access system market for a new generation of wireless networks (5G) is actively developing. The next-generation wireless networks will require much denser base station location than for 4G networks (the number of base stations is estimated to be increased by hundreds of times), the higher fiber optic line velocity factors to each base station (10 Gbps or more), significantly lower signal delays (for example, for control of the unmanned vehicles in the smart city systems). All these factors develop the specific requirements for DWDM / OTN equipment that is planned to be used at the level of the 5G system transmission network.
DWDM is actively breaking into the new markets, primarily the market of communication systems for data processing centers (DPC or DCI). In contrast to the telecommunications industry, where the DWDM systems are conventionally used mainly for the long-haul data transmission over long distances, the DWDM technology in the data center market is used for the high-speed transmission over short distances (Fig. 4). This imposes new requirements on the equipment not available in the field of telecommunications, and leads to the emergence of a new class of solutions. The data center solutions are distinguished by the high port density, high data transfer rate per slot / unit, high compactness, including availability of a wide range of compact hybrid units that save the rack space .
In Russia, the leading manufacturer of fiber-optic communication systems based on the DWDM technology is T8. The company employs more than 350 people, including four doctors of sciences and more than twenty Ph. Ds. The core of the company’s team are graduates from the leading universities, such as the Moscow Institute of Physics and Technology, Moscow State University, Bauman Moscow State Technical University, National Research Nuclear University MEPhI, Moscow Technical University of Communications and Informatics, Saint-Petersburg State University of Telecommunications, etc.
The company has been developing the high-speed DWDM systems since 2008. In 2012, T8 demonstrated a 100 Gbps coherent transponder developed in house, becoming one of the top five companies in the world who had this technology at that time. In 2018, the company introduced 200 Gb / s commercial equipment. By 2020, a range of DWDM devices with the support of channel rates of 2.5, 10, 40, 100 Gb / s, 200 Gb / s (one optical carrier) has been developed and commercialized; the development of 600 Gb / s devices (over two wavelengths in the 100 GHz frequency grid) and 800 Gbit / s devices (one wavelength) has begun. The performance of T8’s DWDM systems is comparable to the world-class commercial long-haul DWDM systems.
2. Volga platform
T8 has developed and is producing a full-fledged multi-service Volga DWDM platform that has received the status of the Russian-made telecommunications equipment and is listed in the Unified Register of Russian Radioelectronic Products. This platform is accompanied with the complete range of equipment for the fiber optic communication systems based on the DWDM technology. The equipment includes the units of transponders and aggregating transponders (aggregators) with various transmission rates and various combinations of the customer and line interfaces, units of optical path equipment (amplifiers, multiplexers, etc.), chassis and control units, auxiliary units, as well as the network management and monitoring systems of the EMS / NMS class (Fig. 5).
In addition to the equipment for standard telecommunications applications, the systems are being produced for the data center communication infrastructure (DCI). The range of equipment adapted for the use on 5G networks is also being developed.
The family of the Volga multiservice platform for the high-speed DWDM networks supports the channel rate of up to 800 Gbps, and up to 28 Tbps in one pair of fibers. The system provides the bandwidth capacity of up to 28 Tbps in the C-band over a pair of fibers. Data transmission in each spectral channel is performed at the rates of 800, 600, 400, 200, 100, 40, 10 Gbit / s and less. The network reconfiguration and its expansion can be performed without traffic interruption. The high signal quality at the transponders allows to enter the channels on existing lines built earlier for the low-speed channels.
The chassis allows to quickly increase the network capacity by adding the required units to the free slots. The platform chassis range includes the dimensions from 1U to 10U. The chassis is available with the redundant control system and power supply units (PSU) of two types: DC 36–72 V or AC 220 V. The chassis is installed in the standard 19 / 21» racks.
3. High-speed coherent
long-distance DWDM communication systems
The main physical parameters of DWDM systems include the capacity, spectral efficiency, performance and specific capacity, as well as the required optical signal-to-noise ratio (OSNR) of the transponder in the back-to-back configuration.
Capacity (C) is the product of the number of channels and the maximum supported rate in the channel. For example, the capacity of a DWDM system that is capable of transmitting 80 channels of 100 Gbps each is 8 Tbps.
It is possible to increase the DWDM system capacity both by increasing the rate in a single channel and by increasing the number of channels in the entire system.
The communication system performance is the product of the capacity C and the maximum transmission reach L and is measured in bit ∙ km / s:
PE = C · L. (1)
Using the definition of spectral efficiency expressed as the ratio of data rate to the used spectral range Δν, the formula (1) can be shown as follows:
PE = SE · Δν · L. (2)
The maximum reach of a multi-span communication path depends on many specifications, such as the span length, attenuation in the fiber, the noise factor of the amplifiers, non-linearity factor (that expresses the influence of non-linear effects on the signal propagation), the input powers in each span, the number and type of transmitted channels, the frequency plan used, and guard band, the required operating margin for OSNR, the transponder sensitivity threshold OSNR_T. The line input power values can be selected in different ways depending on the line optimization method used (BER minimization, OSNR margin maximization, etc.). The optimization methods for the systems with coherent signal detection differ from those used in the systems with non-coherent detection. To compare various technologies, it is necessary to recalculate all experimental or calculated reaches based on a single set of input parameters (line characteristics).
Figure 6 shows three methods to increase the regenerative length, and Figure 7 shows three methods to increase the total transmission rate over a pair of fibers.
Increase in the data rate in the optical communication networks due to the use of multi-level formats reduces the non-regeneration transmission reach and the communication system performance. This poses the following question: how will the increase in the channel data transfer rate due to the increased symbol rate affect the data transmission reach and performance? We will compare the low speed (narrowband with 35 GHz channel bandwidth) and high speed (broadband with 70 GHz channel bandwidth) channel configurations (see Figure 8). In both cases the total input power of the span will be equal. Since there are 2 times fewer channels in a broadband configuration, therefore, the channel power of broadband channels should be 2 times higher. As is known, the ASE noise power spectral density is primarily determined by the total power arriving at the EDFA. Therefore, at a first approximation, the ASE noise power spectral density will remain the same in both cases.
A similar statement can be made in relation to the nonlinear noise power spectral density. This statement follows from the reference formula of the nonlinear additive Gaussian noise model. Therefore, the total noise spectral density at the line output will remain constant.
Thus, the spectral power densities of the signal and noise will remain the same in both cases, and hence the spectral signal-to-noise ratio (SNR) will be the same. Moreover, based on the OSNR definition, it follows that for the broadband channels (70 GHz) the total OSNR at the line output will be 2 times higher than for the narrowband ones (35 GHz).
As far as is known, BER is one-to-one functionally related to the SNR value. The specific type of functional dependence is determined by the modulation format (see Table 1). Since the non-linear noise is also determined by the SNR value and not by the OSNR value, the signal quality will be the same when using the optimal channel power (see Fig. 9).
In other words, although doubling the symbol rate increases the optimal channel power (and the optimal OSNR) by 2 times, the maximum transmission reach is not changed.
The result obtained seems paradoxical at first sight only. However, it is easily explained if we consider the relations between SNR and OSNR that described with good accuracy by the approximate formula given below (for the QPSK format, 1 polarization):
It clearly follows from this formula that with a recorded optimal SNR value, an increase in the symbol rate Rs equal to the double electrical bandwidth ((RS = 2 · BWe, Bref = 12,5 GHz), the optimal OSNR value is proportional to the symbol rate Rs and is also increased. However, the achievable signal quality and reach do not depend on the symbol rate (see Fig. 9, 10).
On a practical level, it is necessary to add the adjustments related to the technical limitations of a particular hardware to the required theoretical SNR values.
In T8, the transition from the first generation of equipment with a full rate of 1.2 Tbps (80x200G) to the equipment (40x400G) not only did not reduce the maximum transmission reach, but also increased the reach by 15–65%. Moreover, the main advantage of the new generation of communication systems with an increased channel rate is reduction in the transmitted data cost.
4. Some examples of using the research results in the development, design and improvement of communication equipment
4.1. Nonlinear operating mode of coherent communication systems
The use of modulation formats that are advanced for the industry has led to the need to develop new optimization methods for the fiber-optic communication lines (FOCL). This task has been successfully completed by T8, with the development of a new method for parameter setting of the channeling equipment (line cards) and a mathematical tool that uses these parameters for the FOCL design. The calculation method has been developed for the specifications of coherent fiber-optic communication networks, with due regard to many physical effects that have simultaneous impact on the signal propagation. The proposed algorithms and easy-to-use engineering methods that allow predicting the network performance significantly simplify the design. The research results are published in the articles -.
4.2. Overcoming communication interruption in the case of lightning strikes in the OPGW
The use of optical cables in a ground wire (OPGW) is a cost-efficient technical solution, since it allows to simultaneously implement the functions of a ground wire that protects the power lines from lightning strikes, and the functions of a telecommunications optical cable used to transmit data over the optical fibers.
However, a direct lightning strike in the OPGW can cause the short-term communication interruptions when using the modern coherent high-speed data transmission systems. The studies and computational simulations made it possible to determine the physical mechanism of this phenomenon: a lightning strike generates a strong longitudinal magnetic field in the fiber, the change of which, in turn, leads to the polarization state rotation of the optical signal due to the Faraday effect and to the occurrence of errors on the receiving side -. The up-to-date high-speed coherent transmission systems with the multilevel modulation formats and polarization multiplexing turned out to be the most sensitive to the lightning strikes.
In the case of a lightning strike, the rotation rate of the signal polarization state in the fiber can reach 10 Mrad / s (typically 1–5 Mrad / s).
In the conventional coherent transponders, this leads to the communication interruption. Determination of the interruption reason allowed T8 to develop the 100 / 200 / 400G coherent transponders equipped with the SOP-suppression function that are able to withstand such strong signal disturbances caused by the lightning strikes (see Fig. 11).
As can be seen from the figure, the use of the polarization state tracking algorithm can significantly improve the transponder stability under the conditions of lightning strikes.
In addition, the results of study of the lightning strike effect on the light polarization led to the development and production of a device for determining the lightning location and power.
4.3. Use of the statistical regularities in the spectra of amplifiers for the communication line optimization
When a multichannel signal propagates along an optical communication line, its spectrum profile is distorted. The spectrum irregularities result in an additional OSNR penalty. The non-uniformity of the EDFA gain spectrum is one of the effects that has a significant impact on the signal spectrum profile. The T8 research department studied the statistical properties of the distribution of values of the accumulated spectrum non-uniformity (Flatness) in a multi-span communication line with EDFA depending on the number of spans. Based on the obtained distributions, the approximate analytical expressions were obtained that allow estimating the signal non-uniformity in the multi-span FOCL and obtaining a conservative estimate of the DWDM signal spectrum non-uniformity (95% of the EDFA sequences will have a non-uniformity better than the estimated one) (Fig. 12). According to the results obtained with a probability of 95%, one T8 amplifier increases the spectrum non-uniformity by no more than 0.72 dB. The exception is the case when a significant number (>40%) of amplifiers with a high gain ratio (~35 dB) is required in the line. In the case of such communication line configuration, with a probability of 95%, one amplifier will increase the spectrum non-uniformity by no more than 0.95 dB.
The empirical rules found make it possible to reduce the cost of line by reducing the number of power spectrum equalization points.
4.4. Extra-long lines
The construction of communication networks with the long spans requires additional research.
In particular, T8 theoretically and experimentally studied the signal degradation mechanisms in the extra-long single-span communication lines with the distributed Raman amplifiers with a channel rate of 100 Gbit / s and coherent detection that limit the maximum length of single-span lines .
The construction options are described and the numerical models for the single-span long-distance communication lines are implemented (Fig. 13) . The optimization methods are proposed for designing the extra-long communication lines. A good coincidence between the network design results and the results of experimental studies is shown.
The distributed Raman and remote erbium amplifiers are the important elements of extra-long single-span communication lines. T8 developed the models of such amplifiers based on the rate equations. For good accuracy of these models, the well-known methods were used to measure the parameters of telecommunication and active erbium fibers, such as the stimulated Raman scattering factors, attenuation factors, absorption and luminescence cross sections, up-conversion coefficient, etc. . The parameter database for various types of telecommunication and active fibers has been developed that makes it possible to design the extra-long lines based on any components.
To assess the accuracy of the developed models, we performed the experimental studies of the integrated amplifier specifications, impact of co-propagating and counter-propagating Raman pumps, as well as the remote erbium amplifiers on the parameters of single-channel and multi-channel single-span communication lines. The results of experimental and computational studies coincide with good accuracy.
According to the results of experimental studies of the Raman and remote erbium amplifiers, it was found that:
An increase in the ambient temperature leads to a decreased efficiency of the remote erbium amplifiers: the gain is decreased, and the noise factor is increase;
The active fibers for EDFA and ROPA have various specifications tailored to the performance of each amplifier;
A fiber for ROPA should have a high pump efficiency to operate at low pump power (Fig. 14.);
The pump efficiency of an erbium amplifier can be increased by decreasing the mode field diameter.
The implemented numerical models of the Raman and remote erbium amplifiers are used in the development of amplifiers, in particular the broadband Raman amplifiers for C-band, hybrid amplifiers , active and passive ROPA units.
The studies performed by the T8 research department made it possible to develop a range of equipment for the single-span communication systems and the optimization method for such lines, used to break several world records in relation to the reach and performance:
10x100 Gbit / s (DP-QPSK, SoftFEC) over a distance of 500 km using two additional pump delivery fibers, using G.652B (Corning ULL) and G.654B (Corning Vascade EX2000) fibers with a total attenuation of 79 dB .
1x200 Gbit / s (5 bit per symbol modulation format, 56.8 GBaud) over a distance of 520 km using the additional fibers with G.652B and G.654.E fiber with a total attenuation of 84.5 dB .
2x100 Gbit / s (DP-QPSK, SoftFEC) over a distance of 501 km without any additional fibers using G.652B fiber (Corning ULL) with a total attenuation of 80.1 dB .
At the SVIAZ‑2017 trade show (April 25–28, 2017) we demonstrated 1x100 Gbit / s (DP-QPSK, SoftFEC) transmission over a distance of 410 km based on the standard G652.D fiber (Corning SMF‑28) with full attenuation of 77 dB.
5. Prospects and areas for development of the DWDM communication systems in Russia
In the foreseeable future, the development of long- haul FOCL will be performed in the area of increasing the network transmission capacity. In the points where all the main consumers of DWDM equipment (Rostelecom, «Bolshaya Troyka» (Beeline, Megafon, MTS) and TTK) installed coherent equipment along the federal long-haul lines, the number of 100G- and 200G-channels will be gradually increased, the transition from 10G-channels to 100G- channels will take place on the regional networks. Moreover, in the regional traffic aggregation networks the number of 10G-channels will grow, while the number of 1G and 2.5G channels will be rapidly decreased. We can expect a new surge of large-scale DWDM network modernization that will be related to the commissioning of 5G networks. Under these conditions, the terabit-class equipment (supporting the transmission rates of up to 600 / 800 Gb / s per wavelength, up to 1.2 Tb / s per unit) will be in great demand for the telecommunications systems and for communication networks between the data centers and inside them, since such equipment provides the lowest cost per gigabit per second.
To double the total system rate, it will be necessary to develop the high-speed transponders and muxponders up to 800G (1.2T), optical amplifiers and multiplexers for operation in the C ++ range, and expand the product line with additional units.
The main tasks to be solved by the Russian manufacturers are as follows:
Expansion of the range and functionality of passive DWDM devices, primarily the multiplexers / demultiplexers and OADM;
Reduction in the energy intensity of equipment;
Implementation of Flex Grid technology (flexible frequency plan) that is already an important competitive advantage of a number of foreign manufacturers.
Development and implementation of the protection features for DWDM channels, ensuring the uninterrupted FC protocol operation;
Improvement of the network management system (NMS) in order to facilitate its docking with the NMS of other major DWDM equipment suppliers .
The transition to the next generation of coherent communication systems places new demands on the optical components in relation to high performance and reduced footprint.
In the near future, the key trend in the development of coherent optical communication systems will be the increasingly dense packaging of electrical and optical components and the need for higher integration levels in the electrical and photonic circuits.
To meet the new requirements, the photonic components must provide the high symbol rates (> 400 G), linearity to support the high-order modulation formats, low power consumption, and high integration density.
At present, almost the entire component base of the Russian-made high-speed coherent communication systems is produced abroad. Development of the Russian component base of integrated photonics is a critically important task.
The key optical units the domestic production of which is a priority, include the following:
Coherent optical modules and components;
EDFA / RAMAN and other optical components;
Passive optical components.
V. N. Treshchikov, Cand. of Scien.(Phys.&Math.), General Director of T8 company.
M. A. Gorbashova, engineer of T8 company.
M. O. Zhulidova, engineer of T8 company.
V. A. Konyshev, Cand. of Scien.(Phys.&Math.), head of the scientific group of the T8 company.
A. V. Leonov, Doctor of Technical Sciences, Deputy General Director for Technical Development of the T8 company.
O. E. Nanii, Doctor of Scien.(Phys.&Math.), Professor of MSU, Head of the Research Department of the T8 company.
D. D. Starykh, Deputy Head of the Research Department of T8 Company.
R. R. Ubaydullaev, engineer of T8 company.
I. I. Shikhaliev, Cand. of Scien.(Teсh.), firstname.lastname@example.org, head of the group of T8 company.