Page 435 - Fiber Optic Communications Fund

416 Fiber Optic Communications

9.6 Explain the difference between WDM and OFDM systems.

9.7 In a polarization-multiplexed optical OFDM system, there are 256 subcarriers and each subcarrier is
2
modulated by QAM-16 data. OFDM symbol period = 81.92 ns and fiber dispersion =−22 ps /km.
2
The optical OFDM signal needs to be transmitted over a distance of 1000 km. Find (a) the minimum
guard interval to ensure carrier orthogonality and (b) the total data rate.
(Ans: (a) 0.4319 ns, (b) 25 Gb/s.)

9.8 Discuss the significance of the cyclic prefix.

9.9 A polarization-multiplexed OFDM signal is transmitted over a 50-km-long fiber. Total OFDM band-
width = 2.5 GHz, fiber loss = 0.19 dB/km, OFDM symbol period = 204.8 ns. If the total power at the
fiber output is −13 dBm, find the signal power/subcarrier/polarization at the transmitter.
(Ans: 4.362 × 10 −4 mW.)
9.10 In a polarization-multiplexed optical OFDM system, there are 128 subcarriers and each carrier is mod-
ulated by QAM-16 data. It is desired that the guard interval should not exceed 5% of the OFDM symbol
period and the carrier orthogonality should be preserved over a distance of 500 km. Calculate (a) the
2
OFDM symbol period and (b) the spectral efficiency. Assume =−22 ps /km.
2
(Ans: (a) 13.302 ns, (b) 8 b/s/Hz.)
9.11 Write a computer program to simulate the polarization-multiplexed OFDM system with the following
parameters: total data rate = 28 Gb/s, modulation = QPSK, transmission distance = 1000 km, ampli-
2
fier spacing = 100 km, fiber loss = 0.18 dB/km, fiber dispersion =−22 ps /km. The guard interval
2
should not exceed 6% of the OFDM symbol period. Choose the OFDM symbol period such that car-
rier orthogonality is preserved. Assume that Mach–Zehnder modulators operating in the linear regime
are used and ignore fiber nonlinearity and amplifier noise. Plot the OFDM symbol in the time and
frequency domain at the fiber-optic link input and at the receiver after DFT.
9.12 Develop an optical TDM scheme to multiplex four 25-Gb/s data streams into a single 100-Gb/s data
stream. Explain how the TDM signal can be demultiplexed at the receiver.

References

[1] ITU-T-694.1, Spectral grids for WDM applications: DWDM freq-grid. ITU-T website.
[2] H. Toba et al., Electron. Lett., vol. 23, p. 788, 1987.
[3] K. Oda et al., IEEE Photon. Technol. Lett., vol. 1, p. 137, 1989.
[4] B.H. Verbeek et al., J. Lightwave Technol., vol. 6, p. 1011, 1988.
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[6] H. Ishio, J. Minowa, and K. Nosu, J. Lightwave Technol., vol. 2, p. 448, 1984.
[7] G.P. Agrawal, Fiber-Optic Communication System, 4th edn. John Wiley & Sons, Hoboken, NJ, 2010.
[8] M.K. Smit, Electron. Lett., vol. 29, p. 285, 1988.
[9] A.R. Vellekoop and M.K. Smit, J. Lightwave Technol., vol. 9, p. 310, 1991.
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[11] H. Bissessur et al., Electron. Lett., vol. 30, p. 336, 1994.
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