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EIE339 Digital Transmission and Switching Systems

Lecturer: Dr. W.Y.Tam– Office: DE604 – Telephone no.: 2666-6265– email address: enwytam@polyu.edu.hk

Continuous Assessment– Tests 25%– Assignments and quizzes 25%– Practical work 50%

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Textbook & References

Textbook– Haykin, S.S., “Communication Systems,” Wiley, 2001.

References– Stallings, W., “Data and Computer Communication s,” Prentice Hall, 1996.– Shanmugam, K.S., “Digital and Analog Communication Systems,” Wiley, 1979.

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Transmission impairmentsAfter this lecture, you will be able to

– describe different types of impairments during transmission• Attenuation distortion• Delay distortion• noise

Reference:– Section 2.3 Transmission Impairments

Data and computer communications, Prentice Hall.

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IntroductionIntroduction

– The purpose of a communication system is to transmit information-bearing signals or baseband signals through a communication channel separating the transmitter from the receiver.

– The received signal differs from the transmitted signal due to various transmission distortion.

Example: Analog signal: “snow flakes” appear in TVDigital signal: a binary 1 is transformed into binary 0

– The most significant distortion are• Attenuation distortion• Delay distortion• Noise

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Attenuation– Strength of a signal falls off with distance over any transmission medium.

– For guided medium, this strength is generally expressed as

and the attenuation is expressed as

frequency offunction a: )0()( ααxeVxV −=

[ ]

[ ] [ ][ ]{ } [ ] dB/km):(unit log20/)0(/)(log20

log20)0(/)(log20

dB):(unit log20)0(/)(log20unit) (no )0(/)( :nattenuatio

1010

1010

1010

eaxVxVeaxVxV

eVxVeVxV

x

x

−=⇒−=⇒

=⇒

=−

−

α

α

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Attenuation– For unguided medium, attenuation is more complex function of distance and the makeup

of the atmosphere.

Example: Signals transmit from a satellite to the ground station passing through different layers of

atmosphere including the ionosphere.

– received signal must have sufficient strength so that the electronic circuitry in the receiver can detect and interpret the signal

– received signal must maintain a level sufficiently higher than noise to be received• solved by adding amplifiers or repeaters at regular intervals

– attenuation is a function of frequency --- attenuation distortion• solved by equalizing attenuation across a band of freqeuncy

Impacts of attenuation

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Example: Thee attenuation as a function of frequency for typical leased line is shown below.(attenuation is measured relative to the attenuation at 1000Hz, which is the reference

frequency specified in North American)

The solid line shows the attenuation of a typicalleased line.

The dashed line shows theequalized attenuation of a typical leased line.

.

Impacts of Attenuation

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Delay distortion– This distortion is caused by the fact that the velocity of propagation of a signal through a

guided medium varies with frequency.

– Delay distortion is critical for digital data.• Because of delay distortion, some of the signal components of one bit position will

spill over into other bit position, causing intersymbol interference (ISI), which is a major limitation to maximum bit rate.

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Delay distortionExample

– Equalizing techniques can also be used for delay distortion.

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Noise– Any unwanted signals that are inserted somewhere between transmission and reception.

• A major limiting factor in communications system performance.

– Four categories• Thermal noise• Intermodulation noise• Crosstalk• Impulse noise

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Thermal Noise

– Occurs in all transmission media and in all communication equipment arising from random electron motion.

– Every equipment element and the transmission medium contribute thermal noise to a communication system, provided the temperature of that element of medium is above absolute zero.

– It cannot be eliminated (unless lowering the temperature) and therefore places an upper bound on communication systems performance.

– Characterized by a uniform distribution of energy over the frequency spectrum and a normal (Gaussian) distribution of levels.

– Thermal noise in a bandwidth of B Hz is

Kelvinin Temperture : /101.38constant sBoltzman':

watts):(unit 23-

TKJk

kTBNo

×=

=

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Thermal Noise

– ExampleAt room temperature,

For a 10MHz bandwidth,

290Kor 17 CT °=

HzW

No

/104

2901038.121

23

−

−

×=

⋅×=

W

No15

621

104

10104−

−

×=

⋅×=

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Intermodulation Noise

– Intermodulation noise is produced when there is some nonlinearity in the transmitter, receiver, or intervening transmission system

Example, if a signal composes of two sinusoids, , passing through an nonlinear device,

the second term is

The derived signals could interfere with an intended signals

)(teo)(tei)()( 2 teete iio +=

tftftei 11 2cos2cos)( ππ +=

[ ]

[ ] [ ] ( ) ( )tfftfftftftftftftf

tftftei

212121

2122

12

221

2

2cos2cos2/4cos12/4cos12cos2cos22cos2cos

2cos2cos)(

−++++++=++=

+=

ππππππππ

ππ

21212121 ,,2,2,,frequency at sinsodscotainssignaloutput ffffffff −+∴

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Crosstalk

– Crosstalk refers to unwanted coupling between signal paths.

Example: hear another conversation when using the telephone

– Three causes of crosstalk• electrical coupling between transmission media

Example: occur by electrical coupling between nearby unshielded cable such as twisted pair

• Poor control of frequency response (i.e., defective filter or poor filter design)

• Nonlinearity performance in analog (FDM) multiplex systemsExample: coaxial cable lines carrying multiple signals

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Impulse Noise

– Impulse noise is non-continuous

– irregular pulses or noise spikes of short duration of relatively high amplitude.

– Generated from a variety of causes, including external electromagnetic disturbances such as lightning

– minor annoyance for analog data• voice transmission may be corrupted by short clicks with no loss of intelligibility

– primary source of error in digital data communication• sharp spike of energy of 0.01 seconds duration destroy 50 bits of data being

transmitted at 4800 bps

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Impulse Noise

Example: effects o thermal noise and impulse noise