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In practice, it is difficult to send reliably
if R is very close to C. Often S/N is expressed in decibels
(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.
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
Guided media attenuation - dB/kilometer Thus the dB attenuation is additive - measured
in dB per kilometer. At...
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- Fall '08