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Abstract—Paper presents theoretical and practical aspects of coherent data transmission in optical DWDM transmission systems. Recent advancements in commercial systems are presented and future guideline outlined. Authors also present real-life challenges and problems that can be encountered during implementing coherent transmission technology in current generation systems. Measurements aspects and problems are discussed. Didactical aspects is proposed.
Index Terms—DWDM systems, coherent transmission, optical transport systems
I. INTRODUCTION
ECENT advancements in electronic and optical equipment enabled to develop new fully coherent optical data transmission schemes. It can be utilized in DWDM transmission systems. New transmission schemes and signal coding allowed to achieve and standarize 40 Gbit/s, 100 Gbit/s data rates on single optical wavelength. 400 Gbit/s and 1Tbit/s data rates are currently under development. TELCO operators are currently heavily investing in these technologies as it allows to reduce operational costs and meet increasing demands for packet transmission devices, particularly from increasing number of mobile devices.
Authors were actively testing coherent transmission equipment and some general conclusion were made. As this technology matures it will allow to develop efficient design and maintenance techniques, it will require new general view at optical network and especially optical fibers. Networks with old and of questionable quality fibers cannot accommodate this new fast data transmission rates.
Along with the new DWDM transmission equipment new measurement equipment needs to follow. New types of devices need to be introduced to on-field testing methods. Many of them were previously only laboratory type equipment.
Technicians also need to properly trained and aware of various potential transmission problems.
Manuscript received November 12, 2012.
Piotr Rydlichowski is with the Institute of Bioorganic Chemistry of Polish Academy of Sciences, Poznań Supercomputing and Networking Center, ul.
Noskowskiego 12/14, 61-704 Poznań, Poland (e-mail:
prydlich@man.poznan.pl).
Piotr Turowicz is with the Institute of Bioorganic Chemisty of Polish Academy of Sciences, Poznań Supercomputing and Networking Center, ul.
Noskowskiego 12/14, 61-704 Poznań, Poland (e-mail:
piotrek@man.poznan.pl).
II. THEORETICAL ASPECTS OF COHERENT OPTICAL
TRANSMISSION
Optical Coherent transmission schemes share much in common with techniques used in RF and microwave systems.
In previous ones data is transmitted using two orthogonal carriers. Each carrier is specifically coded, transmitted and received. Receiver uses optimal receiver concept.
In optical systems data is transmitted on two independently polarized optical carriers. Each of them is also uniquely coded.
Advancements in electronics and photonics allowed to precisely modulate, monitor and receive dual polarized optical signals.
Similarly to RF and microwave signals various coding schemes has been presented for systems with optical transmission.
The main module in optical transponder is the transmit- receive module. Each modulation scheme is designed in such a way that specific transmission capabilities are met. In commercial modules economic aspects also need to be examined and compromises has to be found. Emerging 40G and 100+G solutions’ propositions are compromise between assumptions mentioned above. New signal coding is built using several basic modulation methods. In this paper focus will be made on 40G and 100G signal coding solutions as they are commercially available and will probably stay for long time, especially 100G technology.
Looking at the basic signal characteristics four values can be used to produce a modulation:
• Amplitude on/off keying (OOK) - Non-return-to-zero (NRZ) - Return-to-zero (RZ)
- Carrier-suppressed-RZ (CS-RZ) - Single-sideband RZ (SSB-RZ)
• Phase-shift keying (PSK)
• Frequency-shift keying (FSK)
• Polarization-shift keying (PoSK)
Modulation format has to match specific optical system transmission characteristics. It needs also to ensure limited linear and non-linear impairments. Given modulation format with a narrow optical spectrum may suite DWDM transmission with narrow channel spacing and consequently tolerate more chromatic distortion. Additionally, specific signals with
Coherent Data Transmission in Optical DWDM Systems
Piotr Rydlichowski, Piotr Turowicz
R
2012
constant optical power may be less susceptible to non-linear effects such as SPM and XPM.
A. 10G Technology
10G is today the most common used technology. It met various requirements for WAN and LAN transmission systems.
10G transponders are available in wide range of types and functionalities. Many of these support multiplexing and demultiplexing of low-speed local ports (e.g. ranging from 100Mbps to 2,5 Gbps). Protocols like STM-64, OC-192, 10GbE WAN and LAN, 10G-FC, on both sides – local and network are supported. On line side OTU2 framing is used.
Looking at the transponder functionalities, two different types of transponders can be found: fully transparent without FEC correction and second supporting OTH and OTU-compatible framing (on the line side) with various FEC modes. DWDM systems vendors use both the bit error correction that is defined in FEC standard and proprietary solution (EFEC, GFEC).
Looking at the signal coding the most popular and efficient way is NRZ (Non-Return to Zero). It is simple light intensity modulation.
Below, the list of the most important parameters’ values of 10G transponders that are commonly supported by vendors is presented:
- laser power is less then 4dBm and it can’t be higher because of nonlinear effects. Typical is value between 0 and 3,5 dBm
- typical receiver input range is between -22 to -10 dBm and generally higher receiver sensitivity means lower acceptable OSNR
- OSNR lever typical about 12dB but some vendors producing cards match better with 20dB OSNR lever - CD tolerance: typical is r700 ps/nm more advanced
receivers -1000…+3400 ps/nm.
- Used format coding NRZ determine the PMD tolerance at the level of 10 ps PMD (30 ps DGD) within 1 dB penalty - maximal haul is dependent on count of regeneration points
(approx. 7) hence haul of over 2000 km
- state and tunable cards are produced for 100, 50 and 25 GHz ITU grid
- no onboard dispersion compensation – only line compensation is available (DCMs, Bragg gratings and circulators)
Cost of 10G technology is relatively low and is still decreasing. It will position this technology in end-client markets where previously it was too expensive. Increase in this client-side traffic will allow TELCO operator to move into 40G/100G technology in the backbone networks. Technology upgrade will be cheaper than hire of more dark fibers.
However, it needs to be noted that not all of currently used optical fibers (especially older ones) will sustain 40G/100G transmission. CD/PMD transmission parameters are the deciding factor.
B. 40G Technology
40G transmission is regarded as technology transition from 10G to 100G. Standardization of 40G and 100G technologies was conducted simultaneously. 40G equipment is nowdays cost attractive and is ideal solution for operators that do not have technical and financial capabilities to move to 100G technology.
At the local side, 40G transponders may support multiplexing of various range of protocols e.g. STM-64, OC-192, 10GbE WAN and LAN. In similar to 10G technology 40G transponders from a functional point of view can be divided into two different types: fully transparent without FEC correction and supporting OTH and OTU-compatible framing (on the line side) with a EFEC-3 modes.
Different modulation scheme for 40G guarantees transmission reach and PMD tolerance in comparison to 10G.
RZ-DQPSK (Return-to-Zero Differential Quadrature Phase Shift Keying) modulation format was chosen as the most convenient and suitable for 40G transmission. Transmitter is shown in Fig. 1. This type of modulation proved to be robust and currently is also used for Ultra Long Haul submarine DWDM systems. This kind of modulation format is characterized by superior filtering tolerance. Experiments have shown that after crossing through elements with filters i.e.
OADMs and ROADMs the signal still has good shape and is resistant to many impairments like optical noise, CD and PMD, optical nonlinearity. Practical implementations have shown that this format can support 40G WDM transmission over existing networks. In case of PMD tolerance and OSNR performance it is possible to establish longer transmission spans with fewer regeneration sites and increased number of ROADM nodes per network. RZ-DQPSK modulation allowed the spectral efficiency to increase and also to improve PMD and CD tolerance.
However, more sophisticated transmitter design is required.
Transmitter is equipped with two phase modulator and one intensity modulator. Moreover pulse train has similar RZ format shape and its optical spectrum is relatively narrow. It is applicable in 100GHz ITU grid. Using RZ-DQPSK compared to NRZ gives 3dB RX sensitivity gain.
Fig. 1 Transmitter scheme and RZ-DQPSK modulation diagrams.
The following parameters characterize examplary RZ-DQPSK modulation transponders:
- Tunable cards with 50GHz channel spacing - OTU-3 frame compliant
- On board TDC (Tunable Dispersion Compensator) with dispersion tuning range from -360 to 700 ps/nm - Transmit output power from -1dBm to +4dBm - Receiver input power range from -18dBm to +5dBm - PMD tolerance: 8ps DGD for 1 dB OSNR penalty
with PMDC
As it was previously stated fiber PMD is a determining factor for 40G design. PMD mainly originates from the irregularity of the fiber due to imperfections in the manufacturing process, and transient responses to the environment such as temperature changes, tensions, pressure, vibrations, etc. these changes occur both in time and different points along the fiber.
Different sections of the fiber cable could and usually have different PMD. For currently used fibers, typical PMD value is 0,1ps/km0,5 . Older fibers have ca. 1ps/km0,5 In the newest fibers PMD coefficient is 0.05 ps/km0,5. Influence of PMD on transmission range is given in tab. 1
Tab. 1. Comparison system range with different type of PMD coefficient.
PMD tolerance
Fiber with 0.05 ps/km0,5
Fiber with
0.1 ps/km0,5
Fiber with
0.2 ps/km0,5
Fiber with
1 ps/km0,5
3 3600 km 900 km 225 km 9 km
8 >4000 km
>4000
km 1600 km 64 km
12 >4000 km
>4000
km 3600 km 144 km The 40G transmission is four-times more sensitivity to noise.
In case CD this tolerance increases 16 times.
C. 100G Technology
Currently 100G interfaces are coming to the market and promise to be next 10G technology in the backbone systems.
The commercial systems emerged in year 2011. IEEE standardized the following types of interfaces:
- 10x10,2Gb/s SDM with the maximal haul of 10m over copper cable
- 10x10,2Gb/s 850nm SDM with the maximal haul of 100m and based on MMF Ribbon
- 4x25,8Gb/s 1,3Pm LWDM with the maximal haul of 10km and based on SMF
- 4x25,8Gb/s 1,3Pm LWDM with the maximal haul of 40km and based on SMF
-
The main problem and challenge for the vendors of the optical transmission systems was the construction of the line side
interfaces. Nowadays the vendors present the following solutions:
- 4*28Gb/s (OFDM) transmission for the short haul of
<200km for SMF
- 1*112Gb/s transmission using M-ary (ASK-)PSK modulation for Metro Ethernet solutions <600km for SMF
- 1*112Gb/s serial transmission taking advantage of Coherent PolMux-QPSK modulation for the Backbone solutions <2000km fot SMF
- 1*112Gb/s serial transmission taking advantage of DP-QPSK modulation for the Backbone solutions
<2000km fot SMF
The first 4x28Gb/s OFDM solution is designed for short-haul transmission. It is operating on the four independent wavelengths with the speed of 28Gbps. For DWDM systems this approach is less effective as it has four-times wider spectrum comparing to serial transmission. This disadvantage is eliminated in serial transmission of 1x112Gbps using M-ary (ASK-) PSK modulation, it could be used in Metro Ethernet.
DQPSK modulation has been chosen for the 100G serial transmission. The advantage of this solution is 50Gbaud/s symbol rate and hence lower requirements for electronic.
Transponder that utilizes such technique can operate with 100GHz ITU grid.
Figure 2. DQPSK modulator.
Another solution for 100+G serial transmission is DP-DQPSK modulation (POL MUX DQPSK). This scheme is combined Polarization Division Multiplexing and QPSK Quadri-Phase Shift Keying. It decreases the baud-rate by factor of 4. Each symbol codes 4 bits. At 100G this scheme suffers less impact than 40G from nonlinear effects. CD tolerance increases because of doubled symbol duration. Electronic components are not critical, it operates at lower speed of 25 Gbps. Common 100GHz ITU grid can be used due to narrower optical spectrum. Thus 100G transmission could be implemented in existing networks. In many cases it is impossible to exchange optical cables and build infrastructure with new fibers both from technical and economical point of view.
Two kinds of approaches in the 100+ Gbps Serial Transmission are shown on figure 3.
Figure 3. Block diagram of TX – RX modules of 100+ Gbps serial transmission.
Another long haul 100G transmission is based on coherent, single-carrier, dual-polarization QPSK with a digital intradyne receiver and NRZ pulse shaping (coherent NRZ-DPQPSK).
This modulation scheme yields high CD and PMD tolerance using digital-receive filter. It achieves high spectral efficiency in the range of 2(bit/s)/Hz, which allows it to fit within a 50GHz WDM grid. Drawbacks include complexity and the associated cost of the digital filter that must be able to process four A/D-converted bit streams of 28Gbit/s in real time (the capacity to process ~1.1Tbit/s). Figure 4 shows a block diagram of a DP-QPSK system.
Figure 4. Coherent intradyne DP-QPSK transmission system.
LPF denotes a low-pass filter, PC denotes a passive optical polarization controller, and PBS denotes a polarization beam splitter [1].
This diagram shows the location of additional optional pulse carvers, should RZ-DP-QPSK be the goal. Incoherent DP- DQPSK requires ultra-fast, reliable polarization controllers.
Figure 5 shows incoherent DP-DQPSK for comparison.
Figure 5. Incoherent NRZ-DP-DQPSK system. DPC denotes a dynamic polarization controller and R denotes a direct- detection receiver [1].
100GbE modulation techniques offer a wide range of performance and cost points. If a cost differential is sufficiently high at 25% or above, this justifies the use of more than a single, one-size-fits-all modulation scheme. An analysis is required which would take transmission performance, spectral efficiency, cost and latency into account.
In Table 2, modulation schemes and transmission systems discussed are compared. It includes basic 10G NRZ-OOK for reference. The comparison also includes NRZ-DP-16QAM
with a coherent receiver, which has the potential to increase spectral efficiency to 4 (bit/s)/Hz.
Tab. 2 Comparison of modulation schemes[1].
Modulati on
10G NRZ- OOK
NRZ- DP- QPSK
NRZ- DP- DQPSK
NRZ- DC- DP- QPSK
RZ- DPSK- 3ASK
4x28G O- OFDM binary NRZ
4x28G Inv Mx bin NRZ
NRZ- DP- 16QAM
Detection Direct,
no pre- amp
Coh Idyne,
DSP, no pre
Incoh, DLI, DPC, pre-amp
Cog Idyne,
DSP, no pre
Direct, self-coh (DLI), pre-amp
Diect, no pre- amp
Diect, no pre- amp
Coh Idyne, DSP, no
pre WDM
grid (GHz)
50
(25) 50 50 50 100 100 4 x 50
(4 x 25)
25
Spect Eff (b / s) /
Hz 0.2
(0.4) 2 2 2 1 1 0.5
(1) 4
OSNR
(dB) 15 13 15 12 21
21 FEC, 32 FEC
off 21 FEC 29 FEC
off 19
CD +/-
(ps/nm) 600 >1000 600
(TDC) >1000 600 (TDC)
-300 +500
-300 +500 >1000 PMD
(ps) 10 >10 ~5 >10 ~4 ~5 ~5 >10
NL
effects Mod Mod Mod Strong Low Strong Mod Strong
Max reach (km)
2000+ 2000 1500 2000 600 200 km
FEC 600 km
FEC, 1000
Migration to 100G technology is a significant step in capacity and in system complexity. It required significant R&D investments.
High requirements on PMD and CD for the new transponders will require the best parameters of the fibers or shorter distances between amplifier or regeneration elements.
Moreover the quantity of possible regeneration point should be shorten probably to three point only. Presented factors influence on high-requirements of transmission path maintenance and necessity of rebuilding old DWDM installations. Significant investments has to be made in special 100G testing equipment.
Figure 6 shows example of measurement taken for the 100G transceiver line module. Optical Modulation Analyzer was specially developed and is used to test and verify 40G/100G and future solutions. It allows to investigate signal constellation and eye-diagram.
III. CONCLUSION
Paper presented recent advancements in optical data transmission systems. Significant R&D investment was required. Technologies will require new approach to network maintenance. New set of measurement equipment and techniques was developed. Time will tell how these new technologies work in practice in different conditions.
40G/100G technology paved the way for future systems – 400G, 800G and 1T.
Figure 6. Example of measurements for Optical Modulation Analyzer type of equipment.
REFERENCES
[1] ADVA Optical Networking white paper “100GBE – The Future of Ethernet”, August 2009.
[2] EXFO application notes.
[3] IEEE P802.3ba standard.
Piotr Rydlichowski (M’03) finished faculty of Electronics and Telecommunication at Poznan University of Technology. In 2003-2008 PhD student at Poznań University of Technology, chair of Multimedia Telecommunications and Microelectronics. In 2008 joined Institute of Bioorganic Chemistry PAS Poznan Supercomputing and Networking Centre.
Works in the team responsible for the maintenance of Polish NREN - PIONIER network. Area of interest are: electromagnetic wave propagation theory and microwave theory and techniques.
Piotr Turowicz graduated in Electronics and Telecommunication Science at Poznan University of Technology with MSc degree in 1999. Since that year he has been working in Network Department in Poznan Supercomputing and Networking Center. His research interests are in the areas of broadband and optical networks and new generation networks. From the beginning of his work in PSNC he has been responsible for maintenance and development of Poznan Metropolitan Area Network - POZMAN and Polish NREN -PIONIER network.
