3 ms·
I find that interesting, and am curious: > instead you use a randomly generated symbol/bit sequence which fits into the memory of the DAC. How do you guarante
by jpmattia 4y ago
I find that interesting, and am curious:
> instead you use a randomly generated symbol/bit sequence which fits into the memory of the DAC.
How do you guarantee coverage of the entire spectrum? As I mentioned above, PRBS(N) has every bit sequence possible for N bits, which would expose any drop outs or resonances.
> so one wants to measure down to error rates of 10e-9 unlike coherent systems.
Back in the day, for OC-768 (40 Gbps/43 Gbps with FEC) equipment was measured to 10e-12. Has that relaxed? [IIRC, to gain 95% confidence that BER is 10e-9, you had to measure 10e10 bits. Similarly, for 95% confidence of BER 10e-12 you had to measure 10e13 bits. It's been a while though.]
- cycomanic 4y agoVery late reply so you might not see it. Modern systems use coherent (phase&litude and polarisation, typically QAM formats), together with relatively high overhead (~20%) FEC. The consequence is that for demonstration experiments you do offline processing, which significantly constrains how many bits you can measure (good experiments measure a few million per measurement point). So what people do is measure to the FEC threshold (10e-2) and assume the FEC works (usually a valid assumption). Sometimes people implement the FEC and show error free operation to 10e-6 or so. Also we use Generalized Mutual Information as a more accurate measure of the information content (it essentially measures how much information can be transmitted given a bitwise coding FEC). In other experiments people show FEC implementations running on banks of FPGAs to show that we actually get down to BER 10e-12, but these take weeks on large number of high end FPGAs.