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408 Fiber Optic Communications

DSP

I r d
ADC 1 1
Chan.
r 2 d
Fiber IQ Complex S/P FFT Phase Est. 2
link receiver data Est. &
r Equal. d
Q N 3
ADC
Local
Osc.

Figure 9.18 Block diagram of an optical OFDM receiver. S/P = serial to parallel, ADC = analog-to-digital converter,
DSP = digital signal processing.

block would be the actual data in each subcarrier. However, because of the noise and fiber propagation effects,
the signal is distorted. To undo these distortions, phase and channel estimation and equalization are carried
out in the digital domain. More details can be found in Ref. [38].

9.4.4 Optical OFDM Experiments

Transmission of a 1-Tb/s OFDM superchannel over 8000 km of SSMF link at a spectral efficiency of
3.1 bit/s/Hz has been demonstrated experimentally [39]. Each channel of a WDM system can use OFDM
as modulation format. Since OFDM has a compact spectrum, the guard band between WDM channels can
be reduced. The experimental demonstration of an 8-channel dense WDM (DWDM) transmission with a
spectral efficiency of 7 bit/s/Hz using a 65.1-Gb/s coherent PDM–OFDM signal in each channel with 8-GHz
channel spacing utilizing 32-QAM data on each subcarrier of an OFDM over a 240-km SSMF link has been
carried out [40].
Using a coded modulation scheme for coherent OFDM, a 231.5-Gb/s OFDM signal at a record spectral
efficiency of 11.15 bit/s/Hz over an 800-km ultra-large-area fiber link has been demonstrated experimentally
[41]. The spectral efficiency in this experiment is close to the Shannon limit. However, the signal reception in
most of the experimental reports is processed off-line. This is because it is hard to implement DFT and IDFT
in real time at higher data rates. Multi-band real-time coherent OFDM reception at a data rate of 110 Gb/s
based on a field programmable gate array (FPGA) with an individual sub-band of 3.33 Gb/s over a 600-km
fiber-optic link has been demonstrated [42].

Example 9.4

An optical OFDM system has 128 subcarriers with a frequency spacing of 78.125 MHz between subcarriers.
2
The dispersion of the transmission fiber is −22 ps /km. The guard interval is 1.28 ns. Calculate the maximum
reach up to which the carrier orthogonality is preserved.
Solution:
The carrier orthogonality will not be preserved if
| |(2NΔfL) < T . (9.90)
2
g