|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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|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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