EXFO PSO-200 Optical Modulation Analyzer - Manual (Page 95)

Bit Pattern Analysis and the Gearbox
Optical Modulation Analyzer 87
Performing Detailed Bit Pattern Analysis with the Gearbox
Performing Detailed Bit Pattern Analysis with
the Gearbox
To obtain information such as the probability of error for each individual
bit/byte in a pattern, it is essential to get detailed bit pattern analysis
capabilities. The PSO-200 Optical Modulation Analyzer can accurately
recover the time-domain information without any signal processing due to
the high bandwidth achieved with optical sampling. A detailed analysis of
the bit pattern recovered can provide a bit error rate (BER) per polarization
or map the error vector magnitude (EVM) per symbol. To achieve this, the
PSO-200 must receive information on the pattern and sequences used in
the transmitter.
For example, consider the following bytes to transmit: D8 A5 69 FE. We
need to go further down to the bits since the transmitter is bit-based:
We sequence those bits in time by considering the first one to be
transmitted as the farthest one on the left (which is the higher rank bit in
binary convention).
A bit-pattern generator usually sends bits the way we read them, from left
to right. Some may send the bits in reverse order (“0001 1011”), hoping that
the receiver will rebuild the sequence correctly.
No matter how the generator sends bits, the Optical Modulation Analyzer
application needs a way to determine which bit goes to which modulator
input of the transmitter, and which symbol in the constellation represents
the recomposed bit pattern. This is where the Gearbox is required.
D8 A5 69 FE
1101 1000 1010 0101 0110 1001 1111 1110