ZTE Communications

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50 km
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75 km
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Received Optical Power (dBm)
▲Figure 9. System performance for different access distances.
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b2b, 50 GHz
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25 km, 10 GHz
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25 km, 50 GHz
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Received Optical Power (dBm)
FEC: forward error coding
▲Figure 10. System performance for different wavelength intervals.
0 GHz; that is, when the wavelengths are the same, the
upstream signal has BER greater than 10 -3 and cannot be
recovered using FEC.
4.2 Colorless OFDM-PON with the Same Lightwave Source
To realize a colorless upstream link, a blank downstream
carrier for the whole ONU is used. This downstream carrier
can be used as optical carrier for the upstream signal. In this
experiment, the ONU uses a reflective optical modulator to
add the upstream signal to the blank carrier. The blank carrier
is highly coherent, which eliminates the optical beating
interference (OBI) noise at the OLT. The distance between the
ODN and ONU is different, and Fig. 11 shows the system
performance for these different distances. When the distance
is almost the same for each ONU, the system performs well,
and the receive sensitivity at BER = 10 -3 is about 11.8 dBm.
When the distance is different, performance deteriorates
significantly mainly because the blank optical carriers for the
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FEC Limit
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S pecial Topic
The Key Technology in Optical OFDM-PON
Xiangjun Xin
ONUs experience a different link environment, and the phase
noise reduces coherency.
5 Experiment Setup and Performance of
40 Gb/s OFDM-PON
OFDM-PON has been widely proposed as one of
candidates for next-generation 40 Gb/s optical access
networks. In this paper, we adopt a two-carrier scheme to
realize the 40 Gb/s OFDM-PON, and the experiment setup is
shown in Fig. 12(b). Two distributed feedback lasers with
60 GHz frequency space are the light sources, and a 20 Gb/s
16-QAM OFDM downstream signal is carried on each
wavelength, so the total transmission speed is 40 Gb/s. This
scheme makes full use of the large wavelength resources in
PON to achieve 100 GHz within the 40 Gb/s OFDM access
network. Compared with NEC Corporation’s proposed
scheme [14],[22], which uses polarization and RF multiplexing
(Fig. 12a), the proposed scheme simplifies the network
structure, and reduces costs associated with OLTs and
ONUs. Especially at the ONU, the proposed scheme avoids
the need for a complicated MIMO algorithm. It also avoids the
need for many polarization components and local radio
frequency, and this is the essence of colorless access. In the
scheme proposed by NEC Corporation, the signal needs to
be loaded to a different local radio frequency. Strictly
speaking, this is also a kind of colorless scheme. The
upstream scheme still uses double-wavelengths lasers and
existing 10G PON for 20 Gb/s upstream transmission. This is
because achieving a colorless OFDM signal is difficult. If
OFDM and TDM is used in this way, timing is still a problem.
Fig. 13 shows the performance of a downstream 40 Gb/s
optical OFDM access signal. The received optical power is
about -14 dBm when BER = 10 -2.6 , but the received signal
can be recovered with the help of enhanced forward error
coding (EFEC). Because the launch power of the system is
about 3 dBm, the OFDM-PON can support 25 km access and
16 users, which means it is a class-B optical access network.
Fig. 13 shows the constellations of the signal before and after
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Different Distances for each ONU
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Received Optical Power (dBm)
ONU: optical network unit
▲Figure 11. System performance for same laser source.
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March 2012 Vol.10 No.1 ZTE COMMUNICATIONS 43
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