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Log (BER) -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -4.0 -23 ■ ■ : 25 km ●: 50 km ▲: 75 km -22 -21 Received Optical Power (dBm) ▲Figure 9. System performance for different access distances. Log (BER) -1 -2 -3 -4 -5 -6 ▲ ■ : b2b, 0 GHz ◆: b2b, 10 GHz ▲: b2b, 50 GHz □: 25 km, 0 GHz ○: 25 km, 10 GHz △: 25 km, 50 GHz -30 □ ◆ ▲ △ □ ■ ○ ◆ ▲ △ -28 ○ □ △ ◆ ▲ ○ □ ■ ○ ◆ △ ▲ -26 -24 -22 -20 -18 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 □ ● ■ ▲ -20 □■ ■ ▲● -19 □ ■ □ ○ ○ ◆ ◆ ○ ◆ ○ △ ▲ △ ▲ △ ▲△ -16 ■□ ■ ●▲ -18 ■ FEC Limit -14 -12 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 Log (BER) -1 -2 -3 -4 -5 -16 ■ -14 ● ■ : Same Distance for Each ONU ●: Different Distances for each ONU -12 -10 -8 Received Optical Power (dBm) ONU: optical network unit ▲Figure 11. System performance for same laser source. ■ ● ■ ■ March 2012 Vol.10 No.1 <strong>ZTE</strong> COMMUNICATIONS 43 ● ■ -6 ● -4
S pecial Topic The Key Technology in Optical OFDM-PON Xiangjun Xin OLT-Downstream Transmitter Downstream CW Laser ECL λ1 Downstream CW Laser DL OFDM Transmitter 1 DL OFDM Transmitter 2 a IM ▶ 50 G Interleaver fc=12.5 GHz Pol-X Pol-Y Data Data Data Data ADC: analog-digital convertor ECL: external cavity laser MIMO: multiple-input multiple-output ONU: optical network unit SSMF: standard single-mode fiber ▲Figure 12. a) Scheme proposed by NEC Corporation, and b) two-carrier scheme proposed in this paper. Log (BER) PC IM PC IM Data Training ▲Figure 13. Downstream performance of 40 Gb/s OFDM access signal. 44 -2.0 -2.2 -2.4 -2.6 -2.8 -3.0 2fc X Y 2fc a ECL IM PC IM ECL DL OFDM Transmitter 1 a IM PC IM DL OFDM Transmitter 2 OLT-Downstream Transmitter -24 ■ : B2B ●: After 25 km Transmission -22 -20 2fc b c PBC X Y b Coupler b fc Pol-X Pol-Y … … … … Data d e Training 25 G Interleaver Training 2fc Training Data Data fc … … … … X Y t t (a) 20 km SSMF fc Splitter 20 km SSMF Splitter X FBG 25 km transmission. Pol-X’ PBS Pol-Y’ fc 2fc fc PD PD PD ONU-Downstream Receiver A B A B A B a b c -18 -16 -14 -12 Received Optical Power (dBm) -10 -8 (b) c fc Y DL OFDM Receiver 2 (MIMO) PolDeMux Receiver 6 Conclusion In this paper, OFDM-PON is introduced and optical OFDM signal performance analyzed. Through experimentation, system performance is determined for different parameters. OFDM-PON is tolerant to linewidths of the lightwave source, and the system is influenced by the OSNR and mapping style. A two-carrier 40 Gb/s OFDM-PON was also proposed, and 25 km SMF was successfully transmitted. ADC (DPO72004) ONU-Downstream Receiver DL OFDM Receiver 1 DL OFDM Receiver 1 a b c d e ■ <strong>ZTE</strong> COMMUNICATIONS ● ■ B2B After 25 km Transmission ■ ● ■ March 2012 Vol.10 No.1 ■ ● ● ■ PD ADC (DPO72004) DL OFDM Receiver 2 X fc 2fc fc Demodulated Signal References [1] N. U. Kim, H.-S. Lim, M. Kang,“Fair Bandwidth Allocation Using Effective Multicast Traffic Share in TDM-PONs,”IEEE J. Lightwave Technol,. vol. 26, no. 7, pp. 756-767, 2008. [2] J. H. Moon, K. M. Choi, and C. H. Lee,“Overlay of Broadcasting Signal in a WDM-PON,”in Proc. Opt. Fiber Commun. Conf. (OFC ’06), Anaheim, CA, 2006, Y ➡To P. 53
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