New high-speed coherent digital signal processors (DSPs) significantly enhance optical performance while offering improved flexibility and programmability. These photonic engines power the latest generation of high-speed optical transponders, achieving record capacity, spectral efficiency, and optical reach. With these programmable, multi-modulation transponders, carriers can adjust the transponder capacity to the physical characteristics of an optical path. This enables them to maximize network capacity and efficiency on each route.
Previous generations of coherent transponders typically relied on a single line rate (100G) and a single modulation type (polarization-division multiplexed quadrature phase-shift keying, or PDM-QPSK) for all applications. The 100G coherent transponders represented significant technical advancements in high-speed optics and yielded large capacity improvements over 10G wavelengths.
However, these single-modulation transponders were a one-size-fits-all solution. So while the 100G transponders were a good, general-purpose fit for most applications, they demanded compromises on shorter optical paths and very long routes. On shorter optical paths — for example, in metro or regional applications — each optical channel is capable of transporting 200G to 250G of capacity. Therefore, the use of 100G PDM-QPSK transponders on metro/regional routes underutilizes the true capabilities of the optical network. On ultra-long haul routes, 100G PDM-QPSK modulated wavelengths may not be the best fit because they may require costly regeneration nodes.
A new generation of DSPs and transponders eliminates the need for such compromises by supporting programmable, multiple modulation modes. These advanced features enable carriers to optimize capacity, optical reach, and spectral efficiency for each optical route. Such capabilities, collectively known as Super Coherent Technologies, are now ready to reach the field.
Super Coherent Technologies
Super Coherent Technologies vastly improve the flexibility and performance of optical transponders. As shown in Figure 1, Super Coherent Technologies encompass four areas: multiple modulation formats, multiple baud rates, flexible spectrum, and advanced coding.
|Figure 1. Super Coherent Technologies.|
Multi-modulation balances capacity and reach. In optical networking, modulation is used to convert digital ones and zeros into symbols that are transmitted over fiber. In most cases, the modulation formats increase overall capacity by supporting the encoding and transmission of multiple bits per symbol.
Modern coherent optical networks rely on phase modulation, primarily PDM-QPSK, to encode and transmit 100G coherent wavelengths. QPSK modulation encodes two bits per symbol, combined with two different polarization modes (i.e., PDM), resulting in a total of four bits transmitted. QPSK modulation produces the familiar constellation diagram, as shown in Figure 2.
|Figure 2. QPSK (left) and 16QAM modulation constellation diagrams.|
Higher-complexity modulations such as 8QAM and 16QAM increase overall capacity by encoding even more bits per symbol. With PDM-16QAM modulation, for example, it is possible to send 200G of traffic over the same optical channel used to transport 100G PDM-QPSK modulated signals. The higher capacity that comes with these more advanced modulation types also results in shorter optical reach due to the closer spacing of the constellation points. For many metro or regional applications, the optical reach is sufficient and the higher modulation formats provide double the capacity per wavelength.
The tradeoff between capacity and optical reach for different optical modulation types is shown in Figure 3. Simpler modulation methods, such as QPSK, are well suited for long-haul and ultra-long-haul routes. Higher-complexity modulations are ideal for high-capacity routes at metro and regional distances. The new super coherent DSPs support programmable, multiple modulation modes. The multiple modes enable carriers to optimize both the capacity and optical reach of the transponder for each optical path and application.
|Figure 3. Capacity vs. optical reach tradeoff.|
Baud rate flexibility. The capacity of an optical channel is based on its modulation type and baud rate. The modulation type defines how many bits are encoded per symbol, and the baud rate measures how fast the symbols are transmitted. Carriers can increase channel capacity by using more complex modulation formats, which encode more bits per symbol, by using faster baud rates or by combining both techniques.
One limitation of more complex modulations is shorter optical reach, as described previously. An increased baud rate combined with the same modulation type enables longer reach and higher capacity. Baud rate flexibility is a powerful Super Coherent Technology on next-generation DSPs.
The maximum baud rate is limited by the speed of the electronics within the coherent DSP. For current-generation DSPs and 100G coherent transponders, the industry uses DSP baud rates of approximately 32 Gbaud. Total channel capacity is simply the baud rate (32 Gbaud) multiplied by the number of bits per symbol (2 bits/symbol) multiplied by the two polarization modes for PDM-QPSK modulation, resulting in a line rate of approximately 128 Gbps. The 128-Gbps rate is the actual line rate of a nominal 100G signal, including soft-decision forward error correction (SD-FEC) bits and Optical Transport Network (OTN) framing.
Newer, super coherent DSPs support higher baud rates, as well as the ability to program both the baud rate and modulation type to optimize the capacity and optical reach of each optical path. One key application higher baud rates enable is transport of 200G using 8QAM modulation. Carriers have traditionally used PDM-16QAM modulation to transport 200G, which limits its use to shorter distances. Higher baud rates enable 200G to be transported using PDM-8QAM modulation, so carriers get the benefit of both higher capacity and longer optical reach.
A Nokia network study compared the percentage of optical routes suitable for 200G 16QAM and 200G 8QAM wavelengths, based on a European network model and a North American network model, as shown in Figure 4. The study found that 16QAM modulation posed significant limitations, especially on North American routes where only 40% of the routes could be supported. It also found that 200G 8QAM was suitable on 100% of the European optical routes and more than 92% of all North American optical routes. Higher baud rates and flexibility to program baud rates are key Super Coherent Technologies enabling more efficient, cost-effective optical networks.
|Figure 4. Applicability of 200G 8QAM on WDM networks.|
Flexible spectrum groups wavelengths closer together. Traditional WDM systems support approximately 90 channels using fixed 50-GHz spaced channels, following the ITU-defined WDM grid pattern. However, grouping these wavelengths slightly closer together can provide up to 120 usable channels – a 30% capacity improvement. Carriers require a few new techniques to group these wavelengths closer together and maintain the same optical performance.
Newer-generation DSPs support a feature called Tx pulse shaping, sometimes referred to as Nyquist filtering. This feature shapes and compresses each wavelength into a slightly smaller bandwidth. With Tx pulse shaping, carriers can transport 100G coherent signals over a 37.5-GHz channel, as opposed to the 50-GHz channel on traditional WDM networks.
Carriers can combine flexible grid channel spacing and Tx pulse shaping techniques to create multi-carrier superchannels. A superchannel is a grouping of two or more carrier wavelengths, as shown in Figure 5.
|Figure 5. Flexible spectrum benefits.|
A superchannel enables the inner wavelengths to be grouped closer together instead of transporting each wavelength in individual 50-GHz channels. The inner subcarrier channels are less exposed to optical network transmission impairments, such as passband narrowing. The combination of Tx pulse shaping and grouping superchannel subcarriers closer together provides improved spectral efficiency — in other words, more bits transmitted for every Hertz of spectrum. Carriers can use the spectrum savings to carry additional channels and capacity.
Advanced coding is the secret sauce. Super Coherent Technologies include advanced coding methods and techniques, many of which are considered highly proprietary by vendors. However, some of the common techniques being developed in the industry include stronger SD-FEC, coded modulation, and symbol encoding/decoding flexibility.
SD-FEC is a powerful error detection and correction algorithm implemented on current-generation DSPs. Each vendor has its own proprietary SD-FEC algorithm. In general, these algorithms provide about 11 dB net coding gain. Some vendors are implementing stronger second-generation SD-FEC algorithms, sometimes referred to as ultra SD-FEC. These new algorithms provide higher net coding gains, resulting in longer optical reach compared to existing SD-FEC implementations.
SD-FEC is applied and processed at the frame layer, within the electrical domain inside coherent DSPs. It is also possible to apply additional error correction information to the optical layer, by way of the modulated symbols themselves. This technique is referred to as coded modulation. The use of coded modulation formats along with the newer ultra SD-FEC algorithms offers substantial performance improvements.
Finally, there are different techniques for encoding and decoding the modulated symbols. These techniques offer different performance levels based on each method. As described previously, coherent optical networks use modulation to encode digital ones and zeros into symbols transmitted on the fiber. The result is a phase modulated constellation pattern, like the sample 100G QPSK modulation shown in Figure 2.
The encoding and decoding of these symbols occur in the transmitter, coherent receiver, and DSP, based on either differential encoding or absolute encoding of the phase. The two encoding methods determine how the coherent receiver tracks and decodes the modulated constellation phases and converts the symbols back into ones and zeros. Each encoding method, differential or absolute, offers advantages depending on the application.
Super coherent DSPs implement robust symbol decoding including advanced polarization tracking, phase tracking, and cycle-slip mitigation algorithms to prevent fiber impairments and network disruptions from causing burst errors in the transmission. Naturally occurring phenomena, such the lightning strike-induced state-of-polarization changes described in "Lightning Affects Coherent Optical Transmission in Aerial Fiber," have been largely resolved by advanced polarization and phase-tracking algorithms. Ideally, super coherent DSPs will offer differential and absolute encoding options, along with a broad range of multi-modulation and baud rate options.
Super Coherent Technologies for a connected world
The latest generation of DSPs includes several new and powerful enhancements, collectively referred to as Super Coherent Technologies. This collection of technologies includes multiple modulation modes, multi-baud rate, Tx pulse shaping, superchannels, and advanced coding techniques.
When combined, Super Coherent Technologies offer a vast improvement in capacity and optical reach compared to current-generation transponders. They also offer the flexibility to program and tune optical transponder capacity and optical reach to each specific optical path. These new super coherent DSPs, and their corresponding optical transponders, improve optical performance, increase network capacity, enhance spectral efficiency, and lower the overall transport cost per bit for optical networks.
Randy Eisenach is responsible for Optical Networks Product Marketing at Nokia. He specializes in optical transport technologies, next generation wavelength routing architectures, and high speed photonics.