Transmission Impairments Surasak Sanguanpong nguan@ku.ac.th - - PDF document

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Transmission Impairments

Surasak Sanguanpong nguan@ku.ac.th http://www.cpe.ku.ac.th/~nguan

Last updated: 25 November 2004

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Type of Impairments

Attenuation Attenuation Distortion Distortion Noise Noise Impairment Impairment

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Attenuation

Transmitter Receiver Amplifier

P1 P2 P4 P3

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Decibel

1

2 3

Amplifier

• Decibel is a base 10

logarithmic measure

• The gain and attenuation of

a system are defined by the ratio of the output power

• ver the input power

Power ratio Decibel Value 1 0 2 3 4 6 8 9 10 10 100 20 1000 30 Power ratio Decibel Value 1 0 2 3 4 6 8 9 10 10 100 20 1000 30

Typical Decibel Values

Attenuation 10 log10 (P2/P1) dB Gain 10 log10 (P3/P2) dB P = Power

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Distortion

Original signal Received signal

Components are in phase Transmitter Receiver Components are out of phase

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Effect of noise

Signal Noise Signal+Noise

0 1 1 1 1 0 0 0 0 1

Data Received Sampling times

Bit error

0 1 0 1 1 0 0 1 0 1

Original data

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Noise types

Atmospheric Noise

Lightning : static discharge of clouds Solar noise : sun’s ionized gases Cosmic noise : distant stars radiate high frequency signal

Gaussian Noise

Thermal noise : generated by random motion of free electrons

Crosstalk

NEXT FEXT

Impulse Noise : sudden bursts of irregularly pulses

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Noise

Effect

distorted a transmitted signal attenuated a transmitted signal

signal-to-noise ratio to quantify noise S = average signal power N = noise power

SNRdB = 10 log10 S N

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Bit Error Rate

The BER (Bit Error Rate) is the probability of a

single bit being corrupted in a define time interval

BER of 10-5 means on average 1 bit in 10-5 will be

corrupted

A BER of 10-5 over voice-graded line is typical. BERs of less than 10-6 over digital communication is

common.

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Crosstalk

NEXT (near-end crosstalk)

interference in a wire at the transmitting end of a signal

sent on a different wire

FEXT (far-end crosstalk)

interference in a wire at the receiving end of a signal

sent on a different wire NEXT FEXT

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Nyquist formula

M Max data rate (C) 2 6000 bps 4 12000 bps 8 18000 bps 16 24000 bps M Max data rate (C) 2 6000 bps 4 12000 bps 8 18000 bps 16 24000 bps W = bandwidth in Hz M = number of discrete signal

Theoretical capacity for Noiseless transmission channel Channel capacity calculation for voice bandwidth (3000 Hz) Example: A noiseless 3KHz channel cannot transmit binary (two-level) signal at a rate exceeding 6000 bps

C = 2W log2 M

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Shannon Capacity

C = W log2 (1+ S N )

W = bandwidth in Hz S = average signal power in watts N = random noise power in watts

Bandwidth = 3300-300 Hz = 3000 Hz Typical SNR = 35 dB, thus S/N=3162 C = 3000xlog2 (1+3162) = 34881 bps

A theorem that describes a model fot determining the maximum data rate of a noisy, analog communications channel.

Sample Calculation for a telephone line

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Performance

Transmitter Receiver

0101110101010111110 010110101111111

Throughput = Number of bits passing through in a second

Transmitter Receiver

0101110 0101110

distance=D t1 t2 propagation delay = t2-t1 = D/propagation speed

Throughput Propagation delay & propagation speed