11899012
| Question | Answer |
| Discuss the relations of SNR, EVM, Q and RIN. |
If we assume data aided reception we have that:
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| How are the SNR and the OSNR defined? |
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| How is the SNR related to the Shannon limit? |
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| Name the different processing steps which need to be done in an optical digital coherent receiver. | 1. Chromatic Dispersion Compensation 2. Deskew & Orthonormalization 3. Timing Recovery 4. Polarization Division De-Multiplexing 5. Carrier and Phase Recovery 6. Equalizing 7. Demapping |
| How can chromatic dispersion be compensated and what is the characteristic of the compensation filter? |
A dispersion compensating fiber (DCF) is one example of a filter which can undo chromatic dispersion. The inverse filter is defined as:
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| Describe the impairments which should be mitigated by de-skewing and orthonormalization. | - Timing might be of because of different optical path lengths in the Rx after the splitting into Ix, Qx, Iy, Qy. - The 90° coherent Rx could have an offset from the ideal 90°. |
| What are the origins of timing errors? | Tx and Rx have different clocks which aren't synchronized. Rx typically samples at a slightly higher rate to prohibit data loss (aliasing). |
| Name different timing recovery algorithms and explain the basic ideas. | -Early-Late Gate: 3 samples. If s0=s2 then s1 is at ideal position -Mueller Müller: weight s[n] and s[n-1] with the expectation and use their difference to estimate the correction. -Square: Square the signal and look at the spectrum. Peeks should indicate Fourier coefficients from which the the timing can be estimated. |
| Which effect should be equalized by polarization division de-multiplexing? How can this effect be mathematically described? |
Rotation, Phase-Retardation:
estimate: theta and phi for PDM
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| Describe the different techniques which are used for polarization division de-multiplexing. How do their cost functions look like? |
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