Hello everyone！
Today I am happy to share with you three concepts in WDM knowledge: OSNR, BER, and Q Value.
OSNR
The optical signal-to-noise ratio (OSNR) is used to quantify the degree of optical noise interference on optical signals. It is the ratio of service signal power to noise power within a valid bandwidth.
OSNR(dB)= 10lg(Pi/Ni)+10lg(Bm/Br)
Pi: signal power on the I channel
Bm: equivalent noise bandwidth
Ni: noise power introduced within the Bm
Br: reference optical bandwidth, which is usually 0.1 nm
BER
Bit error rate (BER) is the ultimate indicator for measuring transmission quality. Due to the impact of factors such as noise, non-linear effects, and dispersion (PMD/CD), the waveforms of optical signals coupled into fibers will be distorted when the optical signals arrive at the end of fiber links. Bit errors are therefore present when the receiver converts the optical signals into electrical signals.
OSNR and BER
OSNR indirectly reflects BER and can provide a warning of potential BER deterioration.
In an experimental environment where factors such as loss, dispersion, and non-linear effects are excluded, if the OSNR is less than the specified threshold, the pre-FEC BER will be excessively large and uncorrectable bit errors will be generated. The OSNR threshold in this case is called B2B OSNR tolerance.
Q Value
Q Value:
I1 and σ1 are the mean value and variance output by Gaussian pulse 1.
I0 and σ0 are the mean value and variance output by Gaussian pulse 0.
Mapping between Q value and BER:
Q(dB):
The Q value and BER have a one-to-one mapping. The Q value can represent the system tolerance in dB, just like dBm substitutes for mW to represent optical power. A smaller pre-FEC BER indicates a larger Q value and better link performance.
That's all. Thank you!
