11840527
Description
Flashcards by Kaja Jentner, updated more than 1 year ago
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Created by Kaja Jentner
over 6 years ago
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| Question | Answer |
| What defines a baseband signal? | spectrum symmetric around zero (signal before mixing it onto a carrier) a(t) is the signal |
| Passband signal | spectrum located around carrier frequency (signal after mixing) a(t) = signal to be mixed |
| What's the difference between the spectrum of a real-valued and a complex-valued passband signal? | real-valued: symmetric around carrier complex-valued: out of phase a(t) = signal to be mixed |
| which information does the carrier of a signal with modulation index m<1 carry? | carrier never carriers information (script p.64 equation 2.76 and 2.77) |
| what are the reasons to choose a particular modulation format? | - price - spectral efficiency - behavior under transmission - receiver sensitivity and signal quality - recovery of the data clock might be easier with one than with another format |
| RZ vs. NRZ | NRZ: + broader pulses in time -> narrow in spectrum + narrow spectrum, simple scheme - 50% energy in carrier tone (which comprises no information) - high risk of inter-symbol interference, since energy is spread evenly over a bit slot RZ: + avoid fiber burning caused by long 1-1-sequence + avoid symbol interference + avoid losing clock information -narrow puls in time -> broad in frequency |
| describe the following schemes | CSRZ: carrier-surpressed RZ like RZ but with additional phase modulation, all even bits positive amplitude, then all odd bits have negative amplitude -> pi phase shift separates even and odd bits Duobinary: "0" low signal level "1" high signal level, without sign change if there is an even number of “0s” since last “1” bit, with sign change for odd quantity of “0s” since last “1”. Alternate Mark inverison: "0" low signal level "1" high signal level, with every subsequent mark alternating phase by 180 degrees. VSB: "0" low signal level "1" high signal level, with progressive 90 degree phase shift added onto every bit slot |
| pros and cons of PSK | + intensity is constant for all bits -> no non-linear effects (they are caused by intensity change) - very expensive because you need a coherent detector (conventional detector only measures intensity and not phase) - difficult for optical frequencies because of extremely short wavelengths. phase changes fast -> stability issues |
| Why use gray coding for multi-level signals? | two adjacent levels always only differ by 1 bit. if a symbol is detected incorrectly, the error will only differ by one bit |
| optimum constellation diagram | energy per bit as small as possible while the minimum distance between symbols as large as possible |
| what is a constellation diagram? | representation of the complex amplitudes of the symbols. -> it shows the amplitude and phase of all levels in a scheme in a complex diagram |
| two most common modulators in optical communication | 1) electro absorption modulator (EAM) 2) Lithium-Niobate-Modulator (LiNbO3-Modulator), also known as Mach-Zender Interferometer (MZI) |
| difference between EAM and laser diode? | EAM works in reverse bias and a laser diode is forward biased |
| principle of MZI | |
| two typical operation modes of MZI | as can be seen from transfer function, amplitude and phase modulation are possible. In the push-pull mode we have phi1 = -phi2 and there is only amplitude modulation. In the push-push mod we have phi1=phi2 and there is only phase modulation. |
| name two operation points of a MZI | push-pull mode is most common. two interesting points are the null point and the quadrature point phi1 = - phi2 -> phi1+phi2=0 -> 1) phi1 - phi2 = pi, null point 2) phi1 - phi2 = pi/2, quadrature point |
| name the device needed to modulate the amplitude and phase of a carrier | IQ-Mixer |
| advantage of a software-defined transmitter | very flexible, meaning any electrical signal can be converted into an arbitrary analog electrical signal that is passed on to an IQ-modulator. (however they might be too bulky and expensive in practice) |
| why is integrated parallel scheme for QAM modulation outperforming other schemes (tandem, cascaded)? | it requires the least electrical bandwidth and is most dispersion tolerant, because of the phase transitions. and also it is less sensible to interference between symbols |
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