Coherent DWDM system: Backbone of 5G transport network

In recent years and decades the speed in which we generate, consume and process information has been growing faster than the increase in information transport capacity. While the transport capacity increases by about 20% per year, the increase in information generation and processing is around 60%. It is expected, that this mismatch will further increase in the next years, driven by the implementation of the next generation of mobile services (5G).

Optical fiber communication networks provide the required capacity and reliability for today’s high-bandwidth internet applications and services and form the backbone of all wired or wireless services. Coherent optical techniques allow to modulate amplitude, phase and polarization of the light for data transport resulting in unprecedented per-channel data rates spectral efficiency, flexibility and robustness. While coherent optical transport is a widespread technique for longhaul terrestrial and transoceanic transmission, it currently also migrates to data center interconnect, access and short reach scenarios.

What is Coherent Transmission?

In simple terms, Coherent Transmission is a system that combines amplitude modulation, phase modulation, and polarization to transmit greater amounts of information through a fiber optic cable than is possible with simple on-off keying. Using digital signal processing at both the transmitter and receiver, coherent optics also offers higher bit-rates, greater degrees of flexibility, simpler photonic line systems, and better optical performance.

Advanced coherent optical technology has a number of key attributes, including:

• High-gain soft-decision Forward Error Correction (FEC).
• Spectral shaping.
• Programmability.
• Strong mitigation to dispersion.

How Coherent Optics Deliver More Data

In standard optical fiber communications, the optical transmission and detection is based on the intensity of the signal. The on-off modulation of the light signal carries the information from the transmitter and the receiver directly detects the signal modulation. This method has worked well for transmission rates up to 10G, but as demands for more speed and capacity over long-haul fiber links has grown, other technologies are required that can send data at rates of 100G or more on a single channel over a single fiber pair.

To achieve higher data rates, more information needs to be encoded in each transmitted symbol. Higher order modulation techniques encode information into the phase, amplitude, and polarization of the light signal rather than just the intensity. Binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), and quadrature amplitude modulation (QAM, 8QAM, or 16QAM) are all known methods for encoding more bits per symbol. These modulation methods are used to encode bits on two polarizations of a single carrier light wave, a technique that is known as “polarization multiplexing.”

Once information has been encoded on a transmitted light signal, the problem remains how to decode that information at the receiver. The ability of the receiver to perform “coherent” detection and extract information from the phase and amplitude of the signal is the source of the term “coherent optics.” Coherent detection uses a laser tuned to a specific frequency as a local oscillator (LO) that “intradynes” with the received signal in an optical coherent mixer. The resulting signal is then passed through photodetectors to a digital signal processor (DSP) that performs analog-to-digital conversion, compensates for chromatic dispersion and polarization mode dispersion in the fiber, and recovers the transmitted bits.

NOKIA’s innovation: Trade of solution between wavelength capacity Vs distance

Maximize the Capacity-Sky limit of DWDM Network

Leveraging sophisticated digital signal processors (DSPs) and advanced photonics, coherent optical technology has revolutionized DWDM transport, enabling wavelength speeds to go from 10 Gb/s in the pre-coherent era to 100 Gb/s and now 800 Gb/s with the latest embedded coherent optical engines, as well as 400 Gb/s with the latest coherent pluggables.

800G capacity per optical channel over longer Distances-Innovation by Infinera

Optimize the use of Optical Fibers and spectral efficiency in DWDM Network

Full duplex coherent solution by CableLabs

Conclusion

Coherent optical communication is a game-changer technology for high-speed data transmissions in long-haul networks and data center interconnects, enabling a widespread upgrade and new deployment of optical transport networks to speeds of 100 Gbps, 200 Gbps, and 400 Gbps per wavelength. Recently, the potential of using coherent optics in access networks incurs a lot of discussions in both industry and academia. Following the continuation of growing bandwidth demands in ultra-high-definition video streaming, cloud computing, immersive gaming, 5G, VR, and remote healthcare, we are pushing really hard on conventional intensity-modulation direct-detection (IMDD) systems and approaching their performance limit.

Thank you!!

Monowar Hossain

Microwave Unit Head (Planning and Operation)

VEON, Bangladesh

Mobile: +8801962424691

E-mail:monowar.hossain@banglalink.net

Originally published at https://www.linkedin.com.