The slides give a basic introduction on why should one model a communication system. And the basic idea of power penalty.

1. SYSTEM MODEL AND POWER
PENALTY
Prepared By:
Pratyusha
SSN College of Engineering

2. FIBER OPTIC COMMUNICATION
SYSTEM

3. SYSTEM MODEL

4.  Transmitter: set of DFB lasers, one for each
wavelength.
 Optical multiplexer: signals at different
wavelengths combined into single fiber.
 Optical power amplifier: increase the
transmission power.
 In-line amplifier: signal amplified after long
distance.
 Optical preamplifier: At receiving end, signal
amplified before passing through a de-
multiplexer.

5.  Quality of transmission is measured using:
 BER
 Power Budget margin
 Usually the required bit error rates are of the
order of 10−9
to 10−15
.

6. POWER PENALTY
 For an ideal system, the SNR calculation &
BER calculation assumes that:
 Optical energy of each bit is impulse
response h(t).
 Zero energy sent during bit ‘0’.
 No random variation in amplitude and
arrival time of bit.

7.  Definitions as in book:
 increase in signal power required to maintain
same bit error rate.
 reduction in SNR ratio as quantified by the
value of 𝛾.

8. BER FOR IDEAL SYSTEM
𝐵𝐸𝑅 = 𝑄
𝑅 𝑃1 − 𝑃0
𝜎1 + 𝜎0
where
o 𝑃1-optical power received during 1 bit.
o 𝑃0- the power received during a 0 bit.
o R-responsivity of the photodetector.
o 𝜎1-noise standard deviation for 1 bit.
o 𝜎0- noise standard deviation for 0bit.
Assume noise is Gaussian,

9. PENALTY FOR IMPAIRMENT SYSTEM
𝑃𝑃 = −10𝑙𝑜𝑔
𝑅 𝑃′1 − 𝑃′0
𝜎′1 + 𝜎′0
𝑅 𝑃1 − 𝑃0
𝜎1 + 𝜎0
Where
o 𝑃′1 and 𝑃′0 are received powers
o 𝜎′1 and 𝜎′0 are noise standard
deviations

10.  In case of an optimized receiver, the Penalty is
presented in 2 cases, they are:
 CASE 1: For unamplified direct detection pin
receivers:
𝜎1=𝜎0 = 𝜎𝑡
𝑃𝑃 = −10𝑙𝑜𝑔
𝑃′
1−𝑃′
0
𝑃1−𝑃0
• CASE 2: For amplified APD receivers
• 𝜎1 ∝ 𝑃1 and 𝑃0 ≪ 𝑃1
• 𝑃𝑃 = −5𝑙𝑜𝑔
𝑃′
1
𝑃1

11. REFERENCES
o Optical Networks: A Practical Perspective, 3rd
Edition 0003 Edition (Galen H Sasaki Kumar N
Sivarajan Rajiv Ramaswami)