Direct Detection vs Coherent Detection for 100G Metro Data Center Interconnect (DCI)
As massive amounts of data are managed, distributed, and exchanged across the globe nowadays, data center networks are expanding in number and capacity, which drives the need for data center interconnect (DCI). While industries are moving quickly with the development of 100G, the choice between these two modulation formats—direct detection and coherent detection, has become the key question metro DCI infrastructure designers should face when building today’s 100G DCI networks. This article will focus on these two detection methods and make a comparison between them.
Coherent detection is an optical signal detection technique that is useful both in fiber-optic communication systems and in optical measurement and sensors. It uses a local oscillator at the receiver, enabling 100Gbit/s transport on a single wavelength for long-haul DWDM networks. With coherent detection modulation, an optical receiver can track the phase of an optical transmitter so as to extract any phase and frequency information carried by a transmitted signal.
Though coherent detection is a concept derived from wireless communication systems, it has been widely used in optical fiber communications, consequently bringing great benefits to long-haul optical networks. For 100G metro DCI, coherent detection enhances performance and spectral efficiency, but the increased cost and power consumption of coherent solutions are troubling problems.
Direct detection can only detect the amplitudes of the signals when receiving the receive terminal. Different from coherent detection, which can extract amplitude, frequency, and phase information from an optical carrier, direct detection systems can only modulate optical waves via intensity modulation.
Direct detection is optimized for short reaches and point-to-point connections in metro networks. By leveraging PAM4, direct detection can transport 100G direct detect signals up to 80km while complying with requirements for optical signal-to-noise ratio. With the benefit of small size, simplified structure, and low consumption, intensity modulation direct detection (IM/DD) communication system becomes the ideal option for short-distance optical communication. Besides, technological advances, such as PAM4, have led to direct detection as a compelling choice for metro DCI networks.
Direct Detection vs Coherent Detection: Which One Is Better?
Now that both direct detection and coherent detection can be used for 100G data center interconnect, which is the more appropriate choice? To make it clear, the relevant considerations are discussed below:
Compared to direct detection, coherent detection can extract amplitude, frequency, and phase information from an optical carrier, and therefore can achieve much higher fiber capacity in the same bandwidth.
Coherent detection uses digital signal processor (DSP) which can compensate for the large chromatic and polarization mode dispersion without the need for optical amplifiers and optical dispersion compensators. However, direct detection requires hardware resources or a smart optical line system for the compensation of mode dispersion. Therefore, coherent detection is much simpler than direct detection.
Coherent detection networks require application-specific integrated circuit and digital signal processors that direct detection networks do not need. This means that direct detection eliminates the significant costs of 100G transceivers, making it more attractive in terms of cost compared to coherent detection.
The need for additional optical components as well as DSP in the coherent detect DCI results in not only higher cost but also higher power consumption. On the contrary, direct detection removes the power-hungry DSP, thus consuming less power than coherent detection systems.
As technology development accelerates, direct detection can transport 100G signals with a maximum distance of up to 80km. While coherent detection can be deployed all over the world and the transmission distance can be thousands of kilometers.
There is no doubt that coherent detection is in great demand in metro DCI applications, but we can not deny that direct detection is rapidly emerging as an economically viable method in metro DCI and 100G direct detection is challenging the dominance of coherent detection for metro DCI applications. If you are faced with the dilemma of choosing between them, optimize and balance these considerations mentioned above.