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294
Transmission System Engineering
Note that on the one hand this penalty represents the decrease in signaltonoise
ratio performance of a system with a nonideal extinction ratio relative to a system
with inﬁnite extinction ratio, assuming the same
average
transmitted power for both
systems. On the other hand, if we assume that the two systems have the same peak
transmit power, that is, the same power for a 1 bit, then the penalty can be calculated
to be
PP
sig

indep
=−
10 log
r
−
1
r
.
Lasers tend to be physically limited by peak transmit power. Most nonlinear effects
also set a limit on the peak transmit power. However, eye safety regulation limits (see
Section 8.7.1), are stated in terms of average power. The formula to be used depends
on which factor actually limits the power for a particular system.
The penalty is higher when the system is limited by signaldependent noise, which
is typically the case in ampliﬁed systems (Section 4.4.5)—see Problem 5.10. This is
due to the increased amount of noise present at the 0 level. Other forms of signal
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This note was uploaded on 01/15/2011 for the course ECE 6543 taught by Professor Boussert during the Spring '09 term at Georgia Institute of Technology.
 Spring '09
 Boussert

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