OCF: Q10

Dominik Werner
Flashcards by Dominik Werner, updated more than 1 year ago
Dominik Werner
Created by Dominik Werner over 2 years ago
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Digital Signal Processing
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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.
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).
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.
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
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.
Classify briefly the different Generations of FEC (Forward Error Correction).
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