2022-01-09
Elements of Physical Layer
Source Transmitter
DTE DCE Transmission Medium
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•Data Terminating Equipment (DTE)
•Data Circuit Terminating Equipment (DCE)
Receiver Destination
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Continuous signal vs discrete signal
Continuous signal (e.g. speech) => Analog signal
Discrete signal (e.g. binary 1s and 0s) => Digital signal
Periodic signal
• A signal s(t) is periodic if and only if s(t+T)=s(t) for all t where T is the period of the signal
• Example: sine wave s(t)
• s(t) = Asin(2ft + ) • Parameters:
Amplitude: A
Frequency: Phase :
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Varying the sine wave parameters
Time domain vs frequency domain
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Frequency components
S3 =S1 +S2
Fourier Series
gta0 ancosn0tn n 1 2
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Discrete Spectrum
fo 2fo 3fo 4fo 5fo f
Spectrum of Signal (1)
Spectrum of Signal: Range of frequencies it has energy/power content
Absolute Bandwidth: Width of its spectrum ( = f2 – f1)
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Spectrum of Signal (2)
Band‐limited Signal: A signal with finite Absolute Bandwidth
All band‐limited signals expand from ‐ infinity to + infinity in time domain ‐‐> No real signal can be band‐limited
Effective Bandwidth: Area of spectrum where MOST of the signal bandwidth is contained
Channel capacity (without noise)
Nyquist Formula
C = 2Wlog2M
W = bandwidth in hertz
M = number of discrete signal levels
C = theoretical maximum capacity in bits per second
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Elements of Physical Layer
Transmission
Source Transmitter Medium Receiver Destination
Impairments
Signal Impairments
‐ degrade the signal quality for analog signals
‐ introduce errors in digital signals (i.e. 0 may be changed to 1 and vice‐versa)
‐ signal attenuation ‐ delaydistortion
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Attenuation
The amplitude of signal decreases with distance over any transmission medium
repeaters and amplifiers are used to restore the signal to its original level
Attenuation is an increasing function of frequency
Signal impairments
As a signal propagates along a transmission path there is
loss, attenuation of signal strength.
Solution:
Tocompensatetheattenuation, wecanusedevices inserted at various points to “boost” signal’s strength (amplifiers).
transmitter
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Signal impairments
Pr Attenuation is measured in dB
transmitter
Decibel (1)
• Gains and losses are expressed in decibels (dB)
• Definition:
NdB 10log10 Prt
= number of decibels = power at destination = power at source
log10 = logarithm base 10 (also noted log)
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Decibel (2)
• Important: the decibel is a measure of relative, not absolute difference.
• For example:
– alossfrom1000Wto500Wisalossof3dB.
– a loss from 10 mW to 5 mW is also a loss of 3 dB.
• In other words, a loss of 3 dB halves the strength; similarly, a gain of 3 dB doubles the strength.
Amplifiers and Repeaters
Amplifiers: Amplify the received signals (this includes useful signal plus noise).
Repeaters: Recover the digital information, and retransmit it.
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Amplifiers
Can be used with Analog and Digital Communication Systems
boosts the energy of the signal (also boosts the noise component)
transmitter
Decibel (3)
• Useful to determine overall gain or loss in a system. This is done simply by adding or subtracting
– loss on first portion of line is 13 dB
– gain of the amplifier is 30 dB
– loss of second portion of line is 40 dB
– theoveralllossis23dB(i.e.-13+30-40=-23dB)
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Can be used with Digital Communication Systems repeater
transmitter
Function of Repeater
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Amplifiers
bX + (b/a) N1 +N2
cX + (c/a) N1 + (c/b) N2 + N3
transmitter
X + (1/a) N1
X + (1/a) N1 +(1/b) N2
bX + N2 a X + N1
transmitter
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• 4 categories
– thermal noise
– intermodulation noise – crosstalk
– impulsive noise
Thermal Noise
• Due to the thermal agitation of electrons in a conductor (uniformly distributed across the frequency spectrum)
• The amount of thermal noise power in a BW of 1 Hz is given by:
– N0 = noise power density
– k = Boltzmann’ s constant = 1.3803 x E-23 J/oK – T = temperature (oK)
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Inter-modulation Noise (1)
• It is produced when different frequencies are passed through the same non-linear device (e.g. non-linear amplifier)
produces signals at frequencies that are the multiple, sum or difference of the frequencies the original signal contains.
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Inter-modulation Noise: Example
y(t) = x(t) + a {x(t)}2
x(t) = cos(2f1t) + cos(2f2t)
f1 f2 2f1 2f2 f1+f2
y(t) = cos(2f1t) + cos(2f2t) + (a/2) + (a/2) {cos(2f1t) + cos(2f2t)}+
a {cos(2f1+f2 ] t) + cos(2f1-f2 ] t) }
• Unwanted coupling between signal paths
• Example: more than one conversation can be heard.
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Impulsive Noise
• Non-continuous noise consisting of irregular pulses or noise spikes of short duration and of relatively high amplitude.
• external electromagnetic disturbances and faults in the communication system
Power Spectral Density of Additive White Gaussian Noise (AWGN)
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Delay distortion
The speed of propagation of a sinusoidal signal along a transmission line varies with the frequency
For a signal composed of more than one frequencies, the signal frequency components arrive at the receiver with different delays from each other
This distorts the signal
The effect of delay distortion tends to increase with the size of the signal bandwidth (=> it increases with an increase in transmission rate)
the pulses become distorted, spread in time and can spill over to neighboring pulses, making their detection difficult
(Intersymbol Interference) => incorrect interpretation of the received signal
Atmospheric absorption
Strength of signal falls off because the atmosphere absorbs some of its energy
Attenuation and delay are greater at higher frequencies, causing distortion
-0.5 -1 -1.5
-0.5 -1 -1.5
00 0 5 10 15
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Intersymbol Interference (1)
Intersymbol interference (ISI) occurs when a pulse spreads out in such a way that it interferes with adjacent at the sample instant.
Example: assume polar NRZ line code. The channel outputs are shown as “smeared” (width Tb becomes 2Tb) pulses (spreading due to bandlimited channel)
Intersymbol Interference (2)
Impact of ISI on received signal of binary communication system
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Causes of Impairments
Reflection – occurs when signal encounters a surface that is large relative to the wavelength of the signal.
Diffraction – occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave.
Scattering – occurs when incoming signal hits an object whose size is in the order of the wavelength of the signal or less.
Illustration of knife-edge diffraction
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Multipath Propagation
Obstacles reflect signals so that multiple copies of a signal may arrive at the receiver at different times, and therefore with different phases.
If phases add destructively the signal level declines, and vice versa.
When the transmitter or the receiver moves even short distances (of the order of , which is a few centimeters for the frequencies used) the signal amplitude can vary greatly.
Also produces ISI.
ISI due to Multipath
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Communication System
Transmission Transmitter Medium Receiver
Impairments
Solution: Use of Equalizers
Transmitter
Transmission Medium Channel
Impairments
Transmission Medium Channel
Impairments
Transmitter
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Adaptive Equalization
Used to compensate the distortion introduced by the channel
It basically tries to reverse the unequal response of the channel, both in amplitude and phase, to the frequency components of the transmitted signal
It is commonly adaptive in the sense that the channel response is periodically estimated and the equalizer adapts accordingly
It is useful to combat intersymbol interference
It involves sophisticated digital signal processing algorithms
Channel capacity without noise
Nyquist Formula ‐ C = 2Wlog2M
‐ W = bandwidth in hertz
‐ M = number of discrete signal levels ‐ C = capacity in bits per second
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Channel capacity with noise
Shannon‐Hartley formula ‐ C = W log2(1+S/N)
‐ W = bandwidth in hertz
‐ S/N = signal‐to‐noise ratio
‐ C = maximum theoretical capacity in bits per second
Digital vs. Analog Transmission (1)
• Digital technology
– VLSI product became “low cost”
• Data integrity
– Example: the use of repeaters guarantees the integrity of the data being transmitted
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Digital vs. Analog Transmission (2)
• Capacity utilization
– links can be shared (multiplexed)effectively
• Security and privacy
– encryption techniques can be applied easily to digital data
Bit rate and baud rate
Bit rate:
number of bits transmitted per second
Baud rate:
number of signal changes per second
Relation
bit rate = baud rate * n
n = numberofbitsperchange
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Transmission media
Transmission media
Guided Transmission media
twisted pair coaxial Cable optical Fiber
Wireless transmission
microwave
infrared and millimeter waves
light‐wave transmission
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Twisted pair (1)
Conductor Wire dielectric Foil shield braid shield jacked
• Widely used in the telephone network
• Used for either analog and digital transmission
Twisted pair (2)
Attenuation very strong with frequency
analog: amplifiers every 5‐6 km
digital: repeaters every 2‐3 km Low noise immunity
crosstalk is a problem
poor channel characteristics
Easy to install, repair, .. Low cost
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Coaxial cable (1)
copper core insulating material
braided outer conductor
protective plastic covering
• Baseband coaxial cable commonly used for digital transmission
• one channel
• Broadband coaxial cable commonly used for analog transmission
• multiple channels, e.g. analog TV, CD-quality audio.
• two types:
dual cable system
single cable system
Coaxial cable (2)
Attenuation linear with frequency Better noise immunity
Error rate:
baseband: 10‐7 broadband:10‐9
~ 1 Km Baseband Cable =?‐2 Gbps
~100 km Broadband cable => 300‐450 MHz Moderate cost
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Optical Fiber
Physical Description
an optical fiber is a thin (2 to 125 m), flexible medium
capable of conducting an optical ray
an optical fiber cable has a cylindrical shape and consists of three concentric sections: the core, the cladding and the jacket.
Optical Fibers (cont’d)
Jacket Cladding
light at less than critical angle is
absorbed in jacket
Angle of incidence
• Idea: refraction principle
• Utilization: light trapped by total internal reflection when…
Angle of reflection
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Fiber optics (2)
Three components:
light source (LED or laser diode)
transmission medium (fiber)
detector (photodiode)
Two major types of fiber
multimode fiber (largely used)
single mode fiber (expensive but can be used for long distances)
Fiber optics(3)
Attenuation very low
High noise immunity
Error rate: 10‐15
~100 km of fiber => ~2 Gbps
Unfamiliar technology: high skills required
Lightweight
Expensive
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Optical Fiber (cont’d)
Applications
Long‐Haul trunks
average length 900 miles
20,000 to 60,000 voice channels Metropolitan networks
average length 7.8 miles
100,000 voice channels Rural‐exchange trunks
25 to 100 miles
less than 5,000 voice channels
Optical Fiber (cont’d)
Applications (cont’d) Subscriber loop
fibers running from the central exchange to the subscriber
Local Area Networks
networks linking 100’s and even 1000’s of workstations Ethernet, ATM‐LAN,…
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Optical Fiber (cont’d)
Wavelength Division Multiplexing (WDM)
Send more than one wavelengths through the same fiber Allows re‐use of fiber
Spectrum Allocation
Spectrum is valuable commodity; needs to be shared
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Spectrum Allocation
Spectrum Allocation
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Spectrum Allocation
Microwave frequencies: range between 109 Hz (1 GHz) to 1000 GHz. Respective wavelengths: 30cm to 0.03 cm.
Narrow Wideband Broadband
Fixed Wireless Access
Wireless LAN
Direct to Home Satellite
10 kbps 100 kbps 1 Mbps 10 Mbps 100 Mbps 1Gbps
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The most valuable segment
Not Enough Bandwidth
VHF 300 MHz
Cellular/PCS/3G
Poor Radio Coverage
UHF 3 GHz SHF 30 GHz EHF
Fixed Wireless Satellite
Licensed & Unlicensed Bands
800-900 MHz 1800-1900 GHz 2 GHz
Cellular/PCS 3G/4G Wi
915 MHz 2.4 GHz
Wireless LAN ITS
IMS Applications
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TACS/NMT/AMPS/TDMA 400 MHz AMPS/CDMA/TDMA 800 MHz GSM/TACS/NMT 850 MHz Subscriber Radio 1.4 GHz GSM/DECT 1.8 GHz
PCS/PHS/3G 1.9 GHz MMDS 2.5 GHz Point to multi-point 10.5 GHz LMDS 24/26/28/32/40 GHz
Unlicensed
900 MHz General Application
60 GHz ??? New
2.4 GHz IMS (802.11b/g/n, Bluetooth)
5 GHz UNII (802.11a), DSRC (802.11p)
Employed Frequencies
Wireless channel’s behaviour is dependent on the frequency band of the signal.
Frequency band of operation depends on the availability of spectrum, antenna characteristics, propagation behavior, and technological preferences.
Licensed wireless systems operate at 150 MHz, 450 MHz, 800 MHz, 2 GHz, 28 GHz.
Unlicensed systems at 900 MHz, 2.4 GHz, 5 GHz, [3.1 GHz to 10.3 GHz: UWB], 57 GHz t0 64 GHz, optical frequencies.
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Radio transmission
Radio waves
easy to generate, can travel long distances, penetrate buildings at lower part of spectrum, omnidirectional (all directions from the source)
widely used for indoor and outdoor communication Low noise immunity
interferencefromelectricalequipment multipath interference
Co/adjacent‐channel interference
Attenuation increases with distance “fast”
Wireless transmission patterns
directional ‐
the transmitting antenna puts out a focused electromagnetic
example: terrestrial microwave, satellite omnidirectional
the transmitted signal spreads out in all directions example: broadcast radio
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Multipath Propagation
Obstacles reflect signals so that multiple copies of a signal may arrive at the receiver at different times, and therefore with different phases.
If phases add destructively the signal level declines, and vice versa.
When the transmitter or the receiver moves even short distances (of the order of , which is a few centimeters for the frequencies used) the signal amplitude can vary greatly.
Also produces ISI.
Microwave transmission
Microwave transmission is widely used
long‐distance communication, cellular phones, TV distribution,
Microwave transmissions can be made easier directional
repeaters are needed (for 100‐m high towers, repeaters can be spaced 80 km apart)
Higher signal to noise ratio
They do not penetrate deep into buildings Multipath fading effect can occur
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Terrestrial Microwave
Physical Description
parabolic dish
~ 10 feet in diameter
line‐of‐sight transmission
maximum distance between 2 antennas (in km):
d3.57h h 12
h1 = height of antenna one h2 = height of antenna two K = 4/3 (adjustment factor)
Terrestrial Microwave (cont’d)
Applications
long‐haul communications
TV and voice communications
transmission in small regions (radius < 10 km)
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Terrestrial Microwave (cont’d)
Transmission Characteristics
attenuation is the major source of loss
4d2 L10log dB
where d is the distance and is the wavelength, expressed in the same units.
IMPORTANT:
loss varies as the square of the distance
Satellite Microwave
Physical Description
a satellite is a microwave relay station used to link 2 or more ground‐based microwave transmitters/receivers.
a single orbiting satellite will operate on a number of frequency bands, called transponder channels.
Geostationary ‐ a satellite is required to remain stationary with respect to its position over the earth. This match occurs at ~36,000 km
Low Earth Orbit (LEO), Medium Earth Orbit (MEO) systems (satellite phones)
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Satellite Microwave (cont’d)
receives on one frequency (uplink)
transmits on another frequency (downlink)
Satellite Microwave (cont’d)
Applications:
television distribution (broadcast)
long‐distance telephone
private business networks (VSAT/USAT networks)
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Satellite Microwave (cont’d)
Transmission characteristics
4GHz / 6GHz , 12 GHz / 14 GHz, 15 GHz / 17 GHz
below 1 GHz there is significant noise from natural sources, including galactic solar and atmospheric noise.
20 GHz to 30 GHz
Personal Satellite Communication Systems High‐Definition TV (around 23 GHz)
Infrared, Optical and millimeter waves
They are widely used for short‐range communication
remote control on TV, VCRs, etc.
indoor wireless LANs Characteristics
relatively directional, cheap, easy to build
do not pass‐through solid object (e.g. wall)
mmWave and Visual Optical proposed for use in 5G Access networks
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Free Space Lightwave transmission
An application: connect two LANs in two buildings via lasers
• high bandwidth, very low cost and easy to install
• Characteristics
• laser beams cannot penetrate rain or thick fog
• laser beams work well on sunny days, but ....
Digital Data/Digital Signals
Modulation rate is the rate at which signal level is changed (rate at which signal elements are generated)
Digital signaling rate or just data rate of a signal is the rate, in bits per seconds, that data are transmitted.
Duration of length of a bit is the amount of time it takes for the transmitter to emit a bit. For a data date R, the bit duration, tB, is 1/R.
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Data Rate vs. Modulation Rate
data rate:
wheretB isthebitduration
example: In the case of code Manchester, the maximum modulation rate, Dmax, is
Data Encoding
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Data Encoding - Baseband Transmission
Encoding onto a digital signal located at baseband x(t)
g(t) digital or analog
x(t) g(t) digital
Square Pulse
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Digital Data/Digital Signals
Definition
Unipolar signal: all the signal elements have the
same algebraic sign,
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