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System Model and Power Penalty
- 1. SYSTEM MODEL ANDPOWER PENALTY Prepared By: Pratyusha SSN College of Engineering
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- 4. Transmitter: setof DFB lasers, one for each wavelength. Optical multiplexer: signals at different wavelengths combined into single fiber. Optical power amplifier: increase the transmission power. In-line amplifier: signal amplified after long distance. Optical preamplifier: At receiving end, signal amplified before passing through a de- multiplexer.
- 5. Quality oftransmission is measured using: BER Power Budget margin Usually the required bit error rates are of the order of 10−9 to 10−15 .
- 6. POWER PENALTY Foran ideal system, the SNR calculation & BER calculation assumes that: Optical energy of each bit is impulse response h(t). Zero energy sent during bit ‘0’. No random variation in amplitude and arrival time of bit.
- 7. Definitions asin book: increase in signal power required to maintain same bit error rate. reduction in SNR ratio as quantified by the value of 𝛾.
- 8. BER FOR IDEALSYSTEM 𝐵𝐸𝑅 = 𝑄 𝑅 𝑃1 − 𝑃0 𝜎1 + 𝜎0 where o 𝑃1-optical power received during 1 bit. o 𝑃0- the power received during a 0 bit. o R-responsivity of the photodetector. o 𝜎1-noise standard deviation for 1 bit. o 𝜎0- noise standard deviation for 0bit. Assume noise is Gaussian,
- 9. PENALTY FOR IMPAIRMENTSYSTEM 𝑃𝑃 = −10𝑙𝑜𝑔 𝑅 𝑃′1 − 𝑃′0 𝜎′1 + 𝜎′0 𝑅 𝑃1 − 𝑃0 𝜎1 + 𝜎0 Where o 𝑃′1 and 𝑃′0 are received powers o 𝜎′1 and 𝜎′0 are noise standard deviations
- 10. In caseof an optimized receiver, the Penalty is presented in 2 cases, they are: CASE 1: For unamplified direct detection pin receivers: 𝜎1=𝜎0 = 𝜎𝑡 𝑃𝑃 = −10𝑙𝑜𝑔 𝑃′ 1−𝑃′ 0 𝑃1−𝑃0 • CASE 2: For amplified APD receivers • 𝜎1 ∝ 𝑃1 and 𝑃0 ≪ 𝑃1 • 𝑃𝑃 = −5𝑙𝑜𝑔 𝑃′ 1 𝑃1
- 11. REFERENCES o Optical Networks:A Practical Perspective, 3rd Edition 0003 Edition (Galen H Sasaki Kumar N Sivarajan Rajiv Ramaswami)
Editor's Notes
- #3 the fiber optics transmission involves transmission of signals in the form of light from one point to the other,consists of transmitting and receiving circuitry, a light source and detector devices like the ones shown in the figure. the input data, in the form of electrical signals, is given to the transmitter circuitry, it converts them into light signal with the help of a light source. Basically 2 components are used as light sources: laser diodes & LED’s. The light beam from the source is carried to the rx, thro’ optic fibre. The Receiver circuit consists of a photo detector. The purpose of photo detectors is to convert the light signal back to an electrical signal. Two types :PN photo diode and avalanche photo diode. These components are sufficient for transmitting the sgl in a noiseless and dispersion free medium. But what if noise come? Will this system be efficient enough? Hence Accessories like connectors, switches, couplers, multiplexing devices, amplifiers and splices are used.
- #4 So an example of such a system which is modeled using mux & amp is wdm system. Wdm is wavelength division multiplexing wherein different channels r assigned different freqz & they are multiplexed. Pre amp usually installed at the receiver end of the DWDM network to amplify the optical signal to the required level to ensure that it can be detected by the receiver. high gain to ensure that the optical signal is detectable. Power amp is installed in the transmitting end of the fiber optic network, which can amplifier the amplify the optical signal launched into the fiber link. It is usually used in DWDM network where the multiplexer attenuates the signal channels. A booster amplifier usually provides low gain and high output power. The gain provided by the pre-amplifier and booster amplifier might not be enough due to the optical loss caused by long haul transmission. In-line amplifier is installed in the fiber optic link every 80-100km . It has moderate gain and has similar output power to those of booster amplifier.
- #6 The BER depends on the amount of noise as well as other impairments that are present in the system. a bit error rate is defined as the rate at which errors occur in a transmission system. This can be directly translated into the number of errors that occur in a string of a stated number of bits.
- #7 The physical layer design must take into account the effect of number of systems impairments, this leads to system degradation.
- #8 In practice however, the bits are distorted due to various reasons and the optical signal is degraded in addition to the system noise. Such violation demands increase in received signal power to ensure given error probability. To compensate for the system degradation, the signal power has to be increased to achieve the same SNR or BER performance as that of an ideal system. This increase in power is called the Power Penalty.
- #11 Polarization plays an important role in system impairments, where signals interfere with each other. So system must be designed, to accommodate the worst case-identical polarizations. One way of careful budgeting for the power penalties is to determine the ideal value of γ . For BER =10^-12,we need In practice, power penalties must be added to ideal value of γ . Required value of γ after adding all allocations is 31 dB.