L4 - NOISE: Random disturbances, usually additive Thermal...

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Unformatted text preview: NOISE: Random disturbances, usually additive Thermal noise Burst-type noises Impulse noise Atmospheric noise THERMAL NOISE Due to random electron motion Proportional to absolute temperature Proportional to bandwidth Spectral density concept: S(f) measures how power of a random noise or signal is distributed in frequency. Thermal noise has a constant spectral density over all frequencies used in communication. M aximum data rate of a channel Crudely, if we send the ideal 2W (2H in text notation) samples/second and can distinguish V = 2k levels, we could send 2Hk = 2H*log2V bits per second. This leaves unanswered how many levels V we could hope to distinguish. This is mainly limited by random noise fluctuations. Shannon developed an equation for the maximum data rate, as a function of: noise signal received power bandwidth. Assume pure thermal noise. The noise power in a band of H herz is N0H. S is the signal power. S/N = S/N0H is called the signal-to-noise power ratio. S/N = S/N0H Shannon proved it is not possible to send data reliably at a rate R > C, but it can be sent reliably at any R < C. In practice, it is difficult to send reliably at if R is very close to C. Often S/N is expressed in decibels (abbreviated dB) (S/N)dB = 10 log10 (S/N) If S/N is very large and bandwidth fixed, C is approximately proportional to (S/N)dB. ATTENUATION is bad because: 1. Reduced S reduces S/N, reduces capacity C. 2. Attenuation is usually greater at the higher frequencies - distortion, pulse spread. 3. Amplification to reduce attenuation contributes to nonlinearities, noise. Amplification after a signal is too weak won't help because noise is equally amplified. In relaying a digital signal, it is better to regenerate it than just amplify. Reason: Regeneration - make binary decisions before signal gets too weak; a clean noise-free signal is sent out. Amplification without decision until final receiver - noises accumulate. G UIDED T RANSMISSION MEDIA Twisted pair Coaxial cable Optical fiber Guided media attenuation - dB/kilometer Thus the dB attenuation is additive - measured in dB per kilometer. Attenuation reduces S/N with cable length, which reduces capacity. But there is another factor that reduces capacity further: Higher frequencies generally attenuate more than low frequencies, which reduces the usable bandwidth. T wisted Pair - a pair of wires twisted together to reduce electromagnetic coupling. Common situation: four twisted pairs in a plastic sheath. Often pre wired for 4 telephone connections. Category 3 - fewer twists per centimeter Category 5 - more twists per centimeter Shielded twisted pair (STP) - metallic sheathing around the pair High rates - even 100 Megabits/second possible for short distances, but high attenuation at high frequencies greatly reduces capacity with length. Coaxial cable Insert Figure 2-4 Coaxial cable supplies much higher data rates over long distance than twisted pair. Signaling at frequencies up to 500 mhz is feasible up to about a kilometer distance. Applications Television distribution Long distance telephone Short computer system links Local area networks Optical Fiber Show Figure 2-7 Glass core is very thin. 50 microns, or in single mode fiber, 8-10 microns. Text says current practical limit is 10 Gb/second, primary limited - need for electrical/optical/electrical conversion. 100 Gb/sec. has been achieved in Lab, and > 50,000 Gb/second could be achieved. Instead of frequency, light signals are described in terms of wavelengths, ë. ë = c/f, where c is the speed of light, 3*108 meters/second. Wavelength bands for optical fiber are centered at 0.85, 1.3, and 1.55 microns. In these bands, attenuation is only about 0.2 dB per kilometer. 1 micron = 10-6 meters. Thus, at 1.3 microns, f = 3*108/(1.3*10-6) = 2.31*1014 herz! A deviation of 1 gigaherz = 109 hz from this center is less than 1/100,000 of the center frequency. The bands are about 25,000-35,000 gigaherz wide. Can send thousands of kilometers, small distortion The 0.85 micron band has more attenuation, but the materials and devices are cheaper. Optical signaling is generally by onoff pulses. Optical interfaces can be passive or active. Passive: Passive Star broadcast: Light coming into junction splits about equally. With n receivers, each received power is 1/n of original. Active: ...
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This note was uploaded on 07/16/2011 for the course EE 362 taught by Professor Unknow during the Spring '11 term at Penn College.

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L4 - NOISE: Random disturbances, usually additive Thermal...

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