Lecture9 - System Design Considerations; Digital Systems;...

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1 System Design Considerations; Digital Systems; Analogue Systems; Multiplexing Techniques; Examples of Applications EE4035 Optical Communications Semester A 2010-11 Lecture 9
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2 Intended Learning Outcomes (ILOs) Describe, in the system level, the characteristics of various key components in an optical communication system. Describe the functions of a regenerative repeater. Calculate the repeater spacing and the power budget for simple systems. Describe various digital and analogue systems. Describe various multiplexing techniques. Evaluate system performance in the presence of optical amplifiers. Describe various applications of optical communication systems.
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3 Basic system requirements: 1. Transmission type: digital or analogue 2. Acceptable system fidelity: BER (Bit-Error-Rate) for digital systems SNR (Signal-to-Noise Ratio) for analogue systems 3. Required transmission bandwidth 4. Acceptable spacing between the terminal equipment or intermediate repeaters 5. Cost 6. Reliability Major component choices: 1. Optical fibres: multimode or single-mode; size, refractive-index profile, attenuation, dispersion, mode coupling, strength, cabling, jointing, etc. 2. Optical sources: laser or LED, optical power, rise and fall time (speed), stability, etc. 3. Optical detectors: PIN or APD, responsivity, response time, active diameter, bias voltage, dark current, etc. 4. Optical amplifiers: SLAs or EDFAs. 5. Transmitters and receivers (electronics) System Design Considerations The operating wavelength is largely dictated by the overall requirements for the system performance, the availability of suitable reliable components, and the cost.
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4 Advantages of digital systems: 1. Better SNR at the receiving end (20 – 30 dB better) 2. Less problems with source nonlinearity and temperature dependence 3. Compatible with digital computing techniques Disadvantages of digital systems: 1. Much wider bandwidth 2. Expensive terminal equipment (e.g. A-D and D-A converters) Digital Systems Versus Analogue Systems The vast majority of optical fibre communication systems are designed to convey digital information (e.g., analogue speech encoded as PCM). However, analogue optical fibre transmission can be used advantageously in situation where the optical fibre link is part of a larger analogue network. There are many applications such as direct cable television and common antenna television (CATV), where analogue optical fibre systems may be utilized.
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5 Sampling rate > Nyquist rate (twice the max. signal frequency) e.g., 3 dB telephone bandwidth ~ 3.4 kHz, sampling rate = 8 kHz Digital Systems – Pulse Code Modulation (PCM) 0 1 2 3 4 5 6 7 57 6 3 1 0 2 101 111110 011 001 000 010 Code number Binaryrepresentation PCM signal (NRZ) Code levels Analog signal Sampling signal PAM t t t
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6 Digital Systems – Pulse Code Modulation (PCM) Baseband PCM transmission system Time-division multiplexing of 3 PCM channels onto a single transmission link Decoder Low pass filter Sampler Quantizer Encoder Quantized PAM Quantizer PCM PCM PAM Receive terminal Transmit terminal Transmission link Analog
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This note was uploaded on 04/17/2011 for the course EE 4035 taught by Professor Prof.chiang during the Spring '10 term at City University of Hong Kong.

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Lecture9 - System Design Considerations; Digital Systems;...

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