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
– Assignments and quizzes
– Practical work
25%
25%
50%
A.1
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.
A.2
Transmission impairments
After 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.
A.3
Introduction
Introduction
– 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 TV
Digital signal: a binary 1 is transformed into binary 0
– The most significant distortion are
• Attenuation distortion
• Delay distortion
• Noise
A.4
Attenuation
– Strength of a signal falls off with distance over any transmission medium.
– For guided medium, this strength is generally expressed as
V ( x) = V (0)e −αx
α : a function of frequency
and the attenuation is expressed as
attenuation : V ( x) / V (0) = e −αx
(no unit)
⇒ 20 log 10 [V ( x) / V (0)] = 20 log 10 e −αx
(unit : dB)
⇒ 20 log 10 [V ( x) / V (0)] = −20ax[log 10 e]
⇒ {20 log 10 [V ( x) / V (0)]} / x = −20a[log 10 e]
(unit : dB/km)
A.5
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.
Impacts of attenuation
– 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
A.6
Impacts of Attenuation
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 typical
leased line.
The dashed line shows the
equalized attenuation of
a typical leased line.
.
A.7
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.
A.8
Delay distortion
Example
– Equalizing techniques can
also be used for delay distortion.
A.9
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
A.10
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
N o = kTB
(unit : watts)
k : Boltzman' s constant = 1.38 × 10- 23 J / K
T : Temperture in Kelvin
A.11
Thermal Noise
– Example
At room temperature, T = 17°C or 290K
N o = 1.38 × 10 − 23 ⋅ 290
= 4 × 10 − 21W / Hz
For a 10MHz bandwidth,
N o = 4 × 10 − 21 ⋅ 106
= 4 × 10 −15W
A.12
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,
through an nonlinear device,
eo (t ) = ei + ei2 (t )
ei (t ) = cos 2πf1t + cos 2πf1t
ei (t )
, passing
eo (t )
the second term is
ei2 (t ) = [cos 2πf1t + cos 2πf 2t ]
2
= cos2 2πf1t + cos2 2πf 2t + 2 cos 2πf1t cos 2πf 2t
= [1 + cos 4πf1t ] / 2 + [1 + cos 4πf 2t ] / 2 + cos 2π ( f1 + f 2 )t + cos 2π ( f1 − f 2 )t
∴ output signal cotains sinsods at frequency f1 , f 2 ,2 f1 ,2 f 2 , f1 + f 2 , f1 − f 2
The derived signals could interfere with an intended signals
A.13
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 systems
Example: coaxial cable lines carrying multiple signals
A.14
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
A.15
Impulse Noise
Example: effects o thermal noise and impulse noise
A.16