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. Maximum 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 a theoretical limit to the maximum data rate, as a function of noise, signal received power, and 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 R < C 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. GUIDED TRANSMISSION MEDIA (Transported magnetic media - mentioned in text) Twisted pair Coaxial cable Optical fiber Guided media attenuation - measured in dB per 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. Twisted 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 ...
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This note was uploaded on 09/23/2009 for the course CMPEN 362 taught by Professor Johnmetzner during the Spring '09 term at Penn State.

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

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