11899009
| Question | Answer |
| What are the origins of carrier frequency offsets and phase offsets? | The origins are the carrier frequency and phase differences between the transmitting laser and the local oscillator laser at the receiver. |
| Name the different frequency recovery algorithms and explain the basic ideas. |
- Blind Frequency Search (BFS): Sweep through a set of frequency offsets and compare the Mean Square Distance Error for deviations in a test sample.
- Mth Power Algorithm: Take the Mth power of the signal in time domain which removes the modulation format (everything collapses in the constellation diagram to one point). Use the FFT -> the peeks can be used for the estimation of the frequency offset.
- Viterbi-Viterbi Algorithm: Works similar to the Mth power but is used for the phase offset. From this offset the frequency offset can be calculated.
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| Name the different phase recovery algorithms and explain the basic ideas. |
- Viterbi-Viterbi Algorithm: Take the Mth power of the signal in time domain which removes the modulation format (everything collapses in the constellation diagram to one point).
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10 3 (binary/octet-stream)
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| What is the purpose of a PLL and how does it work? |
Phase Locked Loop (PLL): Used for heterodyne/homodyne detection to lock the frequency and phase:
-> Phase detector detects phase
-> Loop filter (low pass)
-> Voltage Controlled Oscilator
IF (intermediate freq) and VCO signals are mixed -> if the mixed signal is zero then the frequency and phase are locked.
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| Which additional equalizer step can further improve the signal quality? |
Adaptive Least Mean Square Equalizer (for example): Revert remaining channel impairments, matched filter, minimize ISI
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| What is the fundamental idea of error correction? What are the drawbacks? | idea: Allow higher BER (less SNR) via error correction. Algorithm can take care of e.g. 1:100 error rate drawbacks: Requires overhead. Of n bits only m bits are data bits and k=n-m bits are parity information. (larger BW required) |
| Give one definition for coding gain. |
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| Classify briefly the different Generations of FEC (Forward Error Correction). |
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