# Coherent Orthogonal Optical Frequency Division Multiplexing (CO-OFDM)

### Overview

Coherent Orthogonal Optical Frequency Division Multiplexing (CO-OFDM) is a type of WDM technology that uses the mathematical principle of orthogonality to send many secure signals in a small waveband. This makes the information received by a CO-OFDM system have no overlap between signals, i.e., no adjacent signal interference. Since CO-OFDM can transmit many different signals at the speed of light, CO-OFDM systems operate well in both long-distance and high throughput systems such as data centers, telecommunication backhaul, cloud services, high performance computing, and other dense wavelength division multiplexing (DWDM) systems.

### Fundamentals

In signal analysis theory, when two signals are orthogonal to each other, their inner product in the time domain leads to a signal that has zero intensity, preventing intersignal interference. In layman’s terms, this means that when many signals are sent through a single transmission medium like a fiber, if all signals are orthogonal to each other, there will be no signal cross-talk since any interaction between any two signals will cancel down to zero. This simplifies receivers significantly since there will be no need to consider any adjacent signal interference.

To be a fully coherent orthogonal optical system, the source in the transmitter must be coherent and optical. Since CO-OFDM is a DWDM-based system, the source(s) will be in the 1525 – 1610nm waveband. To be a coherent source, a laser or laser diode must be used. DWDM systems most commonly operate in fibers. Current technologies have signal separations of 0.2nm (25GHz) in this waveband, leading to approximately 425 possible signals in a single fiber. If each signal sends 500 Mbps of information, then a single fiber is capable of operating at 850 Gbps. There are hundreds of fibers operating at any given time, so system total throughput for a CO-OFDM system could reach Tbps very easily.