Ch10Sec4_1

Ch10Sec4_1 - THE FIBER FORUM Fiber Optic Communications...

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Dr. Joseph C. Palais 10.4 1 THE FIBER FORUM Fiber Optic Communications Dr. JOSEPH C. PALAIS PRESENTED BY
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Dr. Joseph C. Palais 10.4 2 Section 10.4 Digital Modulation Formats
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Dr. Joseph C. Palais 10.4 3 Analog vs Digital Transmission For analog signals which must be converted to digital, digital modulation requires greater bandwidth. Ex. Voice Transmission Analog voice bandwidth = 4 kHz Digitized voice data rate = 64 kbps The required digital signal bandwidth (RZ) is: f 3-dB (electrical) = 64 kHz
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Dr. Joseph C. Palais 10.4 4 Ex. Video Transmission Analog TV bandwidth = 6 MHz Digitized TV bandwidth = (6 X 2) 8 = 96 Mbps The required digital signal bandwidth (RZ) is: f 3-dB (electrical) = 96 MHz Digital compression lowers this bandwidth considerably. Compression uses redundancy in the signal to reduce the amount of data that needs to be transmitted.
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Dr. Joseph C. Palais 10.4 5 Why use digital for fiber links : 1. Nonlinearities in LEDs and LDs degrade analog waveforms but do not degrade single channel digital signals. 2. Digital transmission allows redundancy and error checking to improve fidelity. 3. Some sources of information are digital (e.g., computers).
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Dr. Joseph C. Palais 10.4 6 4. Digital regenerators allow limitless transmission distances over fibers. Regenerators amplify and reshape the digital data stream. Analog amplifiers can only amplify the signals but cannot restore the signal waveshape. There is no such thing as an analog regenerator.
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Dr. Joseph C. Palais 10.4 7 10.4.1 Pulse-Code Modulation (PCM) Also called on-off keying (OOK) Data Rate = R = 1/T Data 1 0 1 1 1 0 1 NRZ t T t RZ T/2
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Dr. Joseph C. Palais 10.4 8 NRZ Code: The received current has a large dc component which varies with the data. This causes changes in the operating point of the receiver’s amplifiers. This changes the amplifier gain. This leads to detection
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Ch10Sec4_1 - THE FIBER FORUM Fiber Optic Communications...

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