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Coherent optical OFDM: What is it good for?

Coherent optical OFDM: What is it good for?

By Vivian (Xi) Chen | Posted: 11 March 2014 5:05:13 PM
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OFDM, short for orthogonal frequency-division multiplexing, is a method of encoding data on multiple carrier frequencies. And the coherent optical OFDM (CO-OFDM) is OFDM data that is being modulated to light frequency and being detected in coherent manner. Though OFDM has been standardized in wireless communication (e.g. IEEE 802.11) for a long time, the CO-OFDM was proposed around 2008 and is a relatively new concept for optical fiber transmission.
Such a novel modulation format combines two powerful techniques in optical communications, coherent detection and OFDM. This modulation format holds the promise of delivering high electrical and optical spectral efficiency, receiver sensitivity, and optical dispersion resilience. As such, CO-OFDM has emerged as one of the attractive candidates for the 100 Gb/s and 1 Tb/s Ethernet transport.

High spectral efficiency

Unlike the conventional single carrier modulation who has a broad and leaky spectrum (e.g. raised cosine shape with a certain excess factor), the OFDM spectrum is a rectangular one. Almost all the energy is confined inside rectangular spectrum and little is left outside. This means the occupied spectrum is much narrower compare with conventional single carrier modulation. In other words, high spectral efficiency can be achieved.

Flexible to add/drop

CO-OFDM offers good flexibility for optical signal add and drop. The orthogonality and the rectangular spectrum bring much convenience, and the flexible add/drop is one of them. Signals can be combined with narrow guard band (one or two GHz would be sufficient), and each of them is able to be dropped efficiently with proper filters.
 
CO-OFDM does have drawbacks. The OFDM signals have high peak-to-average power ratio (PAPR) and usually require better RF components (e.g. amplifiers) at signal transmitter side. CO-OFDM is also susceptible to laser phase noise, since the symbol is relatively long and is a frequency domain signal.
 
More information and technical discussion on optical OFDM can be found in the following sessions:

  • Tuesday 11 March, Room 125, 14:00-16:00
    Tu2G OFDM I
  • Tuesday 11 March, Room 125, 16:30-18:30
    Tu3G OFDM II

Posted: 11 March 2014 by Vivian (Xi) Chen | with 0 comments

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