Page 332 - Fiber Optic Communications Fund

Transmission System Design 313

P = 1.99 × 10 −5 mW. (7.81)
1r
The mean of bit ‘1’ is
√
I = 2R P P , (7.82)
1r LO
1
= 2.46 × 10 −2 mA. (7.83)
The noise variance of ‘1’ may be calculated as before,
2
= 2qB R(P + P )+ 4k TB ∕R (7.84)
1 e 1r LO B e L
2
= 2.22 × 10 −11 A . (7.85)
For bit ‘0’,

I =−I 1 (7.86)
0
=−2.46 × 10 −2 mA,
= . (7.87)
1
0
Therefore, the Q-factor is
I 1
Q = = 5.23. (7.88)

1
The approximate Q-factor for PSK is given by Eq. (7.62),
√
2P 1r
Q PSK = (7.89)
hfB
e
= 5.38. (7.90)

7.3 Dispersion-Induced Limitations

Consider a simple fiber-optic system consisting of an OOK transmitter, a receiver, and a fiber, as shown in
Fig. 7.11. Fig. 7.12 shows the input and output bit patterns. As can be seen, a pulse corresponding to bit ‘1’
broadens and occupies the adjacent bit slot corresponding to bit ‘0’. This is known as inter-symbol interference
(ISI), and it leads to performance degradation. To estimate the maximum achievable transmission distance
with negligible ISI for the given bit rate, let us consider a single Gaussian pulse launched to the fiber,
( )
t 2
u(t, 0)= A exp − . (7.91)
in 2
2T
0

Figure 7.11 A simple fiber-optic system.