Page 432 - Fiber Optic Communications Fund

Channel Multiplexing Techniques 413

9.6 Additional Examples

Example 9.7
In a polarization-multiplexed WDM system based on QAM-16, the number of channels = 24 and the symbol
rate per polarization = 28 GBaud. Ideal Nyquist pulses are used in each channel. Assuming that the channel
spacing Δf is equal to the signal bandwidth f in a channel, calculate (a) the total data rate and (b) the spectral
s
efficiency.
Solution:
(a) The data rate of a channel per polarization, B = 28 GBaud. For QAM-16, we have
s
B = B log 16
2
s
= 112 Gb/s. (9.95)
Here, B is the data rate of a channel per polarization.

Total data rate = B × no. of polarizations × no. of channels
= 112 × 2 × 24 Gb/s
= 5.376 Tb/s. (9.96)

(b) For a Nyquist pulse, the signal bandwidth f = B = 28 GHz. With f =Δf, the total WDM signal band-
s s s
width is
NΔf = 28 × 24 GHz
= 672 GHz, (9.97)

total data rate
spectral efficiency =
total bandwidth
5376
= b/s/Hz
672
= 8 b/s/Hz. (9.98)

Example 9.8

In an optical OFDM system, each subcarrier is modulated by QPSK data. The guard interval is 7% of the
OFDM symbol period and the carrier orthogonality should be preserved over a transmission distance of at
least 5000 km. Find the number of subcarriers required to transmit information at a data rate of about 10 Gb/s.
2
Assume =−22 ps /km.
2
Solution:
Let the symbol rate of a subcarrier be B . For QPSK, we have
s
B = B log 4 = 2B . (9.99)
s 2 s