Page 407 - Fiber Optic Communications Fund

388 Fiber Optic Communications

of 10 −9 if the signal is (a) PSK, (b) OOK. Assume that quantum efficiency is 0.9 and rectangular
NRZ pulses are used to transmit the data. Ignore thermal noise.
(Ans: (a) −4.55 dBm; (b) −1.55 dBm.)
8.4 Explain the difference between synchronous and asynchronous detection.

8.5 Show that the BER of DPSK with a heterodyne receiver and asynchronous detection is
( het )
1 −
P = exp . (8.389)
b
2 2
8.6 Write a computer program to estimate the BER of the following modulation schemes with a hetero-
dyne receiver and asynchronous detection: (a) OOK, (b) FSK, (c) DPSK.

8.7 N and N are the mean number of signal photons and noise photons at the receiver of the unrepeatered
n
s
fiber-optic system with a preamplifier. The spontaneous noise factor and gain of the preamplifier are
n and G, respectively and N = n (G − 1). Show that the error probabilities of FSK and DPSK are
sp
sp
n
given by
( )
−N s
P FSK = exp , (8.390)
b
2N n
( )
−N s
DPSK
P = exp , (8.391)
b N
n
respectively. Ignore shot noise and thermal noise.
8.8 Rectangular NRZ pulses are used in a direct detection 25-Gb/s DPSK system operating at 1540 nm.
The average optical power at the receiver is 0 dBm. The fiber-optic link consists of 20 spans of identi-
cal fibers followed by amplifiers which exactly compensate the loss of fibers preceding. Each amplifier
(1) −3
introduces ASE whose PSD is ASE . It is desirable that the BER <= 2.1 × 10 . Find the upper limit
(1)
on .
ASE
(Ans: 3.654 × 10 −16 W/Hz.)
References
[1] B.P. Lathi, Modern Digital and Analog Communication Systems, 3rd edn. Oxford University Press, New York, 1998.
[2] S. Haykin, Communication Systems, 4th edn. John Wiley & Sons, New York, 2001.
[3] S. Betti, G. De Marchis, and E. Iannone, Coherent Optical Communication Systems. John Wiley & Sons, New York,
1995.
[4] K.P. Ho, Phase-modulated Optical Communication Systems. Springer-Verlag, Berlin, 2005.
[5] G.P. Agrawal, Fiber-optic Communication Systems, 4th edn. John Wiley, & Sons, Hoboken, NJ, 2010.
[6] J.G. Proakis, Digital Communications, 4th edn. McGraw-Hill, New York, 2001, chapter 2.
[7] P.A. Humblet and M. Azizoglu, J. Lightwave Technol.,vol. 9, p. 1576, 1991.
[8] D. Marcuse, J. Lightwave Technol.,vol. 8, p. 1816 1990.
[9] D. Marcuse, J. Lightwave Technol.,vol. 9, p. 505 1991.