Optical Link Design Flashcards
When designing an optical link, I usually have a BER requirement needs to be satisfied.
What following constraints need to be considered:
- Power budget
- OSNR
- Other impairments
IPAD Q: Draw a typical optical link
X
What is the objective of the power budget for an amplified system?
The received power is >= receiver sensitivity
Give the equation for received power
Precv=Plaunch-L1+G1-L2+G2…-Ln+Gn - M> receiver_sensitivity
What happens for long distances in terms of amplifiers
For long distances, multiple optical amplifiers are used in cascade
Name the three types of noise in an optical system and describe them
- Amplified Spontaneous Emission; optical amplifiers -> amplifies signal = noise
- Thermal noise; random thermal-driven motion of electrons at the receiver -> sums up to signal carrying information
- Shot noise; photons carrying the information arrive at the detector with random arrival times
What is OSNR and describe its relation with BER
- Optical signal-to-noise-ratio is the relation bet. signal power and noise power @ receiver
Reminder: BER is a measure of the accuracy of data transmission in a digital communication system -> we want a low BER
inc. OSNR LOWER BER -> easier to determine whether a received bit has a 0 or 1 value
Which error correction codes are typically used in optical communications
Turbo codes
What is the Forward Error Correction threshold
The maximum BER a code can correct
Explain what the noise figure is
A measure of the noise introduced by the amplifier; measured in dB
Explain how changing the spacing affects the OSNR
Amplifiers typically spaced 80km apart
Reducing spacing -> inc. OSNR
In a chain of amplifiers, each additional amplifier amplifies both incoming signal and incoming noise, plus it will add its own noise.
Explain what happens of the OSNR
After each amplifier the OSNR decreases and this is permanent, it cannot be recovered.
Explain the concept of margins
Margin = safety distance (SNR) you decide to keep, not being sure where you stand w.r.t the edge
Optical communication margin -> distance to the FEC threshold
Explain the concept of signal regeneration
Before the OSNR drops below the target BER, we need to regenerate the signal:
- terminate signal
- calculate BER
- re-modulate the signal for next fibre span
- for new span, calculate new OSNR as for previous span
What do we know about regenerating signals at multiple points?
Regenerating the signal at multiple points, the total BER will be the sum of the BER of the individual spans.