Carriers face ever-increasing needs for bandwidth and capacity in their metro, regional, and long-haul optical networks due to the demands of high-speed data services, Internet video services, data centers, and higher-bandwidth residential broadband connections. Until recently, most DWDM systems supported up to 88 channels with 10-Gbps data rates per channel. To provide additional network capacity, improved spectral efficiency, and lower cost per bit, the optical transport industry has been developing 100G technologies for the last 3–4 years.
A limited number of vendors introduced 100G transponders and muxponders, based on single-carrier dual-polarization quadrature phase-shift keying (DP-QPSK) modulation and coherent detection, in 2011. Carriers have started to deploy these 100G units for capacity-constrained routes and to support 100-Gigabit Ethernet private line services, a trend that will continue to grow over the next few years. One of the key benefits of 100G transponders and muxponders is the ability to expand existing WDM network capacity by 10X, eliminating the need for costly overbuild networks.
The optical industry now is shifting focus and R&D activities to enable even greater capacity. Some possible options include:
- Increasing optical channel rates
- Increasing the number of WDM channels
- Adding parallel systems over additional fiber pairs
- Combinations of the approaches above.
Each option has its own set of tradeoffs, which are being studied and evaluated. For example, increasing channel rates from 100G to 400G also incurs additional optical signal-to-noise ratio (OSNR) requirements, which can limit the overall optical reach of a signal, requiring additional regeneration nodes on long-haul routes. Adding parallel WDM systems over separate fiber pairs to increase capacity offers the benefit of using currently available technology and WDM platforms, but requires significant additional investment, as well as using additional fiber resources.
Carriers are likely to adopt many, if not all, of these approaches in one form or another. In the near term, capacity is being increased by using additional fiber pairs, as well as migrating to 100G interfaces. Future systems will use even higher speed, 400G optical interfaces.
400G – Capacity versus reach
With the introduction of 100G, the industry shifted from very simple modulation techniques (on/off keying) that transported a single bit of data, to much more advanced phase modulation techniques (DP-QPSK) capable of encoding and sending multiple bits at once. Along with coherent receivers, these more advanced modulation techniques enable much higher data rates and improved compensation for optical impairments such as chromatic dispersion, polarization mode dispersion, and optical loss.
The tradeoff with these advanced modulation techniques is they require higher OSNR. OSNR translates directly into the optical distances that can be achieved prior to a regeneration node. In other words, the more sophisticated and powerful the modulation, the shorter the optical reach. This tradeoff between modulation technique, channel size, and OSNR requirements is at the heart of current 400G research efforts.
Researchers are evaluating a number of advanced modulation schemes and channel sizes for use at 400G, as shown in Figure 1. In general, the higher order modulation techniques, such as 16QAM and 64QAM, encode more bits per symbol and can be squeezed into smaller channel sizes, but with the previously mentioned tradeoff of much higher OSNR requirements.
|Figure 1. Advancements in optical interfaces, 1980–2015.|
As vendors and the optical industry evaluate these different 400G modulation, channel size, and OSNR options, it will be critical to adopt a single, standardized approach. The industry achieved such a consensus at 100G for long-haul applications, working through the Optical Internetworking Forum (OIF). A similar approach to 400G OIF standardization will be needed to ensure a healthy, robust, component supply chain with wide choices and competitive pricing.
Spectral efficiency and subcarriers
While the OIF has not yet started such a standardization process, a number of vendors have active 400G research and development efforts underway. One likely candidate for 400G modulation will be DP-16QAM using two subcarriers to continue the progress that has been made in improving spectral efficiency.
Spectral efficiency is one measure of how efficient an optical interface or modulation scheme is at using the available fiber, and is measured in the number of bits transmitted per second per Hz of optical spectrum (bits/s/Hz). Existing 10G wavelengths use simple OOK for modulation and easily fit within the 50-GHz channel grid spacing, as shown in Figure 2. However, at 10G much of the 50-GHz channel is unused, resulting in relatively low spectral efficiency of only 0.2 bits/s/Hz. With 100G modulation techniques, 10X the data rate is transmitted in the same 50-GHz channel spacing, resulting in 2 bits/s/Hz spectral efficiency.
|Figure 2. Capacity versus OSNR advancement modulation.|
As mentioned before, efficient transmission of 400G will require the optimum combination of modulation format, channel size, and OSNR requirements. DP-16QAM with two subcarriers looks very promising in this context. Using subcarriers offers a number of key advantages. Subcarriers enable very high data rates to be divided and transported over any number of closely spaced, or slightly overlapping, subcarrier channels. The lower data rates on each subcarrier enable implementations that fit within existing component-level silicon technologies, one example being the high-speed analog-to-digital converters (ADCs) used in the coherent receivers. In addition, subcarrier channels can be spaced on existing 50-GHz grid channels to provide compatibility with existing WDM networks, or future flexible-grid spaced WDM systems.
DP-16QAM modulation using two subcarriers with a total of 87.5 GHz channel spacing is shown in Figure 3. The spectral efficiency of this approach is approximately 4.6 bits/s/Hz.
|Figure 3. 10G and 100G spectral efficiency.|
With 100G development efforts largely complete, the optical transport industry is evaluating modulation techniques, channel size, and OSNR requirements for 400G, with the goal of a single, industry-standard approach, working through the OIF. Although still early, one leading candidate is DP-16QAM using two subcarriers.
Randy Eisenach is a WDM product marketing manager at Fujitsu Network Communications, Inc.