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Optical Networks - _4_4 Demodulation_50

Optical Networks - _4_4 Demodulation_50 - 256 Modulation...

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256 Modulation and Demodulation Photodetector Front-end amplifier Receive filter Decision circuit Clock/timing recovery Sampler Figure 4.5 Block diagram showing the various functions involved in a receiver. turn impose additional limits on channel capacity. Recent work to quantify the spec- tral efficiency, taking into account mostly cross-phase modulation [Sta99, MS00], shows that the achievable efficiencies are of the order of 3–5 b/s/Hz. Other nonlinear- ities such as four-wave mixing and Raman scattering may place further limitations. At the same time, we are seeing techniques to reduce the effects of these nonlinearities. Another way to increase the channel capacity is by reducing the noise level in the system. The noise figure in today’s amplifiers is limited primarily by random spontaneous emission, and these are already close to theoretically achievable limits. Advances in quantum mechanics [Gla00] may ultimately succeed in reducing these noise limits. 4.4 Demodulation The modulated signals are transmitted over the optical fiber where they undergo attenuation and dispersion, have noise added to them from optical amplifiers, and sustain a variety of other impairments that we will discuss in Chapter 5. At the receiver, the transmitted data must be recovered with an acceptable bit error rate (BER). The required BER for high-speed optical communication systems today is in the range of 10 9 to 10 15 , with a typical value of 10 12 . A BER of 10 12 corresponds to one allowed bit error for every terabit of data transmitted, on average. Recovering the transmitted data involves a number of steps, which we will discuss in this section. Our focus will be on the demodulation of OOK signals. Figure 4.5 shows the block diagram of a receiver. The optical signal is first converted to an electrical current by a photodetector. This electrical current is quite weak and thus we use a front-end amplifier to amplify it. The photodetector and front-end amplifier were discussed in Sections 3.6.1 and 3.6.2, respectively. The amplified electrical current is then filtered to minimize the noise outside the bandwidth occupied by the signal. This filter is also designed to suitably shape the pulses so that the bit error rate is minimized. This filter may also incorporate
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4.4 Demodulation 257 (a) (b) Vertical opening Horizontal opening Bit boundaries Figure 4.6 Eye diagram. (a) A typical received waveform along with the bit boundaries. (b) The received waveform of (a), wrapped around itself, on the bit boundaries to generate an eye diagram. For clarity, the waveform has been magnified by a factor of 2 relative to (a). additional functionality, such as minimizing the intersymbol interference due to pulse spreading. If the filter performs this function, it is termed an equalizer. The name denotes that the filter equalizes, or cancels, the distortion suffered by the signal.
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