Supersizing the C-Band
Over the past seven years the optical transport industry has made huge strides in increasing the capacity of a single fiber using the C-band. The combination of higher order modulation, flex grid channel spacing, higher symbol rates and super channels has increased the spectral efficiency to deliver 32 Tbit/s per single fiber using the standard C-band or 38.4 Tbit/s in systems supporting the extended C-band. By developing a new optical amplifier and other system enhancements, Huawei has taken this process one stage further to support 48 Tbit/s per single fiber using a Super C-band (Super C). This innovative solution can deliver up to an additional 50% capacity without the need for parallel L-band optics.
Cloud services and massive increases in broadband capacity for businesses and residential users, over both fixed and mobile networks, are driving demand for higher-bandwidth transport networks between users and data centers, and between data centers. There are several key technology approaches to increasing the capacity of a single C-band fiber link including higher symbol rates, more bits per symbol, greater spectral efficiency, and more flexible line rates and channel spacing. Much has already been achieved and many of these approaches are reaching the limits that can be achieved with current technology. Other alternatives to increasing capacity include using additional spectrum by adding L-band optics (C + L), or by adding many more fibers, both of which requires multiple line systems and can be expensive.
The original ITU G.694.1 C-band channel plan supports 25GHz, 50GHz and 100GHz spacing with built-in guard bands on the lower and upper edges. This provides a 4THz spectrum. A typical implementation supports 8 Tbit/s using 50GHz spacing and 80 channels of 100 Gbit/s with QPSK modulations and 32 Gbaud symbol rate. Doubling the data rate to to 200 Gbit/s using 16QAM delivers a simple upgrade, doubling the capacity per fiber to support 16 Tbit/s, although this does reduce the reach. This upgrade can be achieved by replacing existing 100G CFP optical modules with 200G equivalents.
The flexible grid standardized by the ITU in 2012 is now becoming a reality, allowing flexible channel spacing based on multiples of 12.5GHz. The flexible grid architecture also supports multi-carrier super-channels significantly increasing spectral efficiency. 75GHz is expected to be widely used for 400Gbit/s applications and is supported the latest systems. A standard C-band implementation using 16QAM modulation for 400 Gbit/s channels will support 20.8 Tbit/s over a single fiber. A 600Gbit/s implementation using 64QAM modulation and the 4.8THz extended C-band, supported by the flexible grid architecture, will deliver a maximum bandwidth of 38.4 Tbit/s over a single fiber (64 x 75 GHz).
The Huawei Super C-band approach takes advantage of all these developments but also increases the width of the C-band further to 6THz, enabling 120 channels with 50GHz spacing or 80 channels with 75GHz spacing. This development has been enabled through a new optical amplifier design that supports the wider spectrum. A 600Gbit/s implementation using 64QAM modulation will now support a maximum bandwidth of 48 Tbit/s over a single fiber (80 x 75GHz), which is six times of the capacity in a typical 100G system today. This approach is compatible with a future Super (C+L) upgrade that would support 10-12THz of spectrum and even higher capacity.
The industry is continually looking for new developments that increase capacity and reduce cost per bit. The transition to 200 Gbit/s that is happening around the world now and the technologies to achieve 400 Gbit/s and 600 Gbit/s are key developments that are enabling a four-fold increase in the capacity of a single fiber using the C band. The Super C-band development from Huawei will take this one stage further adding an additional 50% to a standard C-band implementation, significantly improving fiber utilization and reducing transmission cost.
This article was sponsored by Huawei.
— Simon Stanley, Analyst at Large, Heavy Reading