MVR - CH9 ANALOG LINKS.pptx - ANALOG LINKS M.V RAGHUNADH...

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M.V. RAGHUNADH Associate Professor, Dept. of ECE NIT, Warangal – 506004. [email protected] LINKS ANALOG
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Elements of an optical communication system
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most fiber optic systems are implemented digitally Advantageous to transmit original analog signals directly over the fiber without converting them to a digital form. - Example applications:- - Multiplexed Microwave signals - Hybrid fiber/coax subscriber services - Video distribution - Radio over Fiber
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Main parameters are --- CNR, Bandwidth, --- Spur Free Dynamic Range (SFDR) --- Signal distortion from nonlinearities Sub Carrier Modulation (SCM) - Baseband signals are first superimposed on RF subcarriers - RF subcarriers are multiplexed using FDM in electrical domain - Resulting electrical signal is modulated onto optical carrier Ex:- 6 MHz video signals in CATV systems
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RF over Fiber - Transmitting microwave analog signals in the range 0.3 to 300 GHz over a fiber link for distributing broadband microwave signals in broadband wireless networks
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Fig. 9-1: Basic elements and major Noises in an analog link Overview of Analog Fiber Links Transmitter uses LED or Laser diode as source Bias point is set at the midpoint of linear portion Analog signal is sent using modulation scheme
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Direct Intensity Modulation(IM) - optical output is directly modulated by varying the current around the bias point in proportion to the message signal - Information signal is sent directly in baseband Sub Carrier Modulation (SCM) - complex but more efficient method - Superimpose baseband signal onto the electrical subcarrier (by FDM) prior to intensity modulation of the optical source using AM,FM
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Fiber shall have flat amplitude and group delay Response within the passband Single Mode fiber is chosen as the model distortion limited bandwidth is difficult to equalize Optical amplifier enhances link noise as it adds Amplified spontaneous emission noise (ASE) Optical receiver impairments are APD gain noise, and Thermal noise
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Carrier-to-noise ratio (CNR) Analog system uses CNR analysis instead of a SNR It is ratio of rms carrier power to the rms noise power at the input of the RF receiver following the photodetector. Typical CNR values: FSK- BER of 10 -9 translates to a CNR of 36 (15.6 dB) BER of 10 -15 translates to a CNR of 64 (18.0 dB) AM- BER of 10 -9 translates to a CNR of 56 dB FM- BER of 10 -9 translates to a CNR of 15-18 dB
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CARRIER POWER: To find carrier power, first the generated analog signal. the drive current through the optical source is the sum of a fixed bias current and a time-varying sinusoid. If the time-varying analog drive signal is s ( t ), then the output optical power P ( t ) is P ( t ) = P t [1+ ms ( t )] If CNR i is the CNR for an i th noise source, the total CNR is
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Analog Transmission System In case of amplitude modulation the transmitted optical power P(t) is in the form of: where m is modulation index, and s(t) is analog modulation signal.
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  • Spring '18
  • MV Raghunath Sir
  • rin

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