CS计算机代考程序代写 flex scheme ant assembly data structure algorithm 1

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Elements of Physical Layer
!
Source Transmitter
Receiver Destination
DCE DTE
DTE DCE Transmission Medium
•Data Terminating Equipment (DTE)
•Data Circuit Terminating Equipment (DCE)
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Periodic signal
t where T is the period of the signal
• s(t) = Asin(2􏰚ft + 􏰛) !
• Parameters: # Amplitude: A
Frequency: Phase : 􏰛

• A signal s(t) is periodic if and only if s(t+T)=s(t) for all

• Example: sine wave s(t)
f
4
2
Continuous signal vs discrete signal
amplitude
amplitude
Continuous signal
(e.g. spe”ech) => 类比
Analog signal
time
!
Discrete #signal (e.g. binary 1s and 0s)
=> Digita
time
l signal

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Varying the sine wave parameters
Time domain vs frequency domain
!
!
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Frequency components
S1 S2
$S3 = S1 + S2
f 2f 3f
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傅立叶级数
Fourier Series
g(t)
To t
g􏰜t􏰝􏰠 a0 􏰡
n 􏰠1
a cos􏰜n􏰢 t 􏰡􏰛 􏰝 n0n
􏰟 􏰞
2􏰚 􏰢0 􏰠 T0
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Spectrum of Signal (1)
G (f)
f1 f2f
Spectrum of Signal: Range of frequencies it has energy/power
带宽
con# tent
Absolute Bandwidth: Width of its spectrum ( = f2 – f1) %
10
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离散光谱
Discrete Spectrum
G (f)
fo 2fo 3fo 4fo 5fo f
fo =1/To
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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
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通道容量
Channel capacity (without noise)
Nyquist Formula
C = 2Wlog2M
where
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
损害
Channel
Signal Impairments
!
Impact:
‐ d e g r a d e t h e s i g n a l q’u a l i t y f o r a n a l o g s i g n a l s
‐ introduce errors in digital signals (i.e. 0 may be
changed to 1 and vic
Types:
‐ signal a”ttenuation ‐ delaydistortion失真 ‐ noise ”
e‐versa)
衰减

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衰减
Attenuation
振幅 (
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
􏰣 Fact:
􏰣 As a signal propagates along a transmission path there is
传播
loss, attenuation of signal strength.

transmitter
􏰣 Solution: ” 补偿
receiver
􏰣Tocompensatetheattenuation, wecanusedevices inserted at various points to “boost” signal’s strength (amplifiers).

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Signal impairments
Pt
Attenuation is measured in dB
!
Pr
transmitter
receiver
Decibel (1)
• Gains and losses are expressed in decibels (dB)
• Definition: )
= power at destination
N
P
􏰠10log
= number of decibels
dB
10
r t
where: N dB
P
r
P
= power at source
log10 = logarithm base 10 (also noted log)
t
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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
放大
􏰣 Amplifiers: Amplify the received signals (this includes useful signa&l plus noise).
􏰣 Repeaters: Recover the digital information, and retransmit it.
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Amplifiers
amplifier
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􏰣 Can be used with Analog and Digital Communication Systems
“&
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transmitter
receiver
boosts the energy of the signal (also boosts the noise component)
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Decibel (3)
• Useful to determine overall gain or loss in a system. This is done simply by adding or subtracting
amplifier
– gain of the amplifier is 30 dB
– loss of second portion of line is 40 dB
– theoveralllossis23dB(i.e.-13+30-40=-23dB)
station
station
if
#– loss on first portion of line is 13 dB
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Repeaters
􏰣 Can be used with Digital Communication Systems r”epeater
transmitter
receiver
Function of Repeater
#
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Amplifiers
!
X
a X + N1
bX + (b/a) N1 +N2
cX + (c/a) N1 + (c/b) N2 + N3
X + (1/a) N1
X + (1/a) N1 +(1/b) N2
transmitter
receiver
Repeaters
!
X
bX + N2 a X + N1
cX + N3
X
transmitter
receiver
X
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Noise
!
• 4 categories
– therma*l noise – intermodulation noise
– crosstalk
– impulsive noise
Thermal Noise
搅动
• The amount of th!erma+l noise power in a BW of 1 Hz is given by:
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1-27
• Due to the thermal agitation of electrons in a conductor (uniformly distributed across the frequency spectrum)
N0 􏰠kT !
where:
– N0 = noise power density
– k = Boltzmann’ s
– T = temperature (oK)
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ant = 1.3803 x E-23 J/oK
const
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x(t) y(t)
Output
!
Input
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2
互调
• It is produced when different frequencies are passed through’the same non-linear
device (e.g. non-lin
Effect:
Inter-modulation Noise (1)
ear amplifier)
produces signals at frequencies that are the multiple, sum or dif’ference of the
frequencies the orig
inal signal contains.
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3
Inter-modulation Noise: Example
x(t) y(t)
y(t) = x(t) + a {x(t)}2
x(t) = cos(2􏰚f1t) + cos(2􏰚f2t)
f1 f2
f1 f2 2f1 2f2 f1+f2
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y(t) = cos(2􏰚f1t) + cos(2􏰚f2t) + (a/2) + (a/2) {cos(2􏰚􏰤f1t) + cos(2􏰚􏰤f2t)}+
a {cos(2􏰚 􏰦f1+f2 ] t) + cos(2􏰚 􏰦f1-f2 ] t) }
f2-f1
串话
Crosstalk
• Unwanted coupling between signal paths
• Example: more than one conversation can be
heard.
,
!
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Power Spectr”al Density of Additive White Gaussian Noise (AWGN)
No/2
!
f
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浮躁
• Non-continuous no’ise consisting of irregular pulses or noise spikes of short duration and
Impulsive Noise
of relatively high amplitude.
Causes:
• external electromagnetic di
faults in the communication system
振幅
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骚乱
sturbances and

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Delay distortion
传播
失真
􏰣 The speed of propagation of a sinusoidal signal along a transmission line varies with the frequency
-!
􏰣 The effect of delay distortion tends to increase with the size of the signal bandwidth (=> it increases with an increase in transmission ra”te)
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正弦曲线
􏰣 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 pulses become distorted, spread in time and can spill over to neighboring pulses, making their detection difficult
)!
符号间干扰 翻译
􏰣 (Intersymbol Interference) => incorrect interpretation of the
received signal
大气 吸收
Atmospheric absorption
􏰣 Strength of signal falls ‘off because the atmosphere absorbs some of its energy
衰减
􏰣 Attenuation and delay are greater at higher frequencies, causing distortion
1.5 1 0.5
Signal 1
Signal 2
1.5
Signal 1
Signal 2
Sum
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Sum 1
0.5
-0.5 -1 -1.5
-0.5 -1 -1.5
00 0 5 10 15
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 interfe)res 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)
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Intersymbol Interference (2)
Impact of ISI on received signal of binary communication system
!
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Illustration of knife-edge diffraction
!
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Causes of Impairments
􏰣 Scattering – occurs when incoming signal hits an “‘!
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􏰣Reflection – occurs when signal encounters a surface
that is large relative to the wavelength of the signal.
衍射
坚不可摧
􏰣 Diffraction – occurs at the edge of an impenetrable
wave.
散射
body that is large compared to wavelength of radio
object whose size is in the order of the wavelength of the signal or less.

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Multipath Propagation
障碍
􏰣 Obstacles reflect signals so that multiple copies of a signal may arrive at the receiver at different times, and therefore with different phases.
.%
破坏性的
􏰣 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.
厘米
􏰣 Also produces ISI.
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ISI due to Multipath
!
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Communication System
Transmission Transmitter Medium Receiver
Impairments
Channel
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Solution: Use of Equalizers
Transmitter
Transmission Medium
Impairments
Transmission Medium
Impairments
Receiver
Equalizer
均衡器
Channel
Transmitter
Equalizer
Receiver
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Channel
!

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Adaptive Equalization
补偿
􏰣 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 transmitted signal
the frequency components of
!’
􏰣 It is commonly adaptive in the sense that the channel response
is periodically estimated and the equalizer adapts accordingly
战斗
􏰣 It is useful to combat intersymbol interference
复杂的
􏰣 It involves sophisticated digital signal processing algorithms
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Channel capacity without noise
􏰣Nyquist Form/ula ‐ C = 2Wlog2M
where
‐ 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 f”ormula ‐ C = W log2(1+S/N)
!
where
0 ‐ W = bandwidth in hertz
‐ S/N = signal‐to‐noise ratio
‐ C = maximum theoretical capacity in bits per second
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类比
• Digital technology
– VLSI product became “low cost”
Digital vs. Analog Transmission (1)

• Data integrity
– Example: the use o”f repeaters guarantees the integrity of the data being transmitted
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Digital vs. Analog Transmission (2)
利用率
• Capacity utilization
– links can be shared (multiplexed)effectively
!””
多路复用
• Security and privacy
– encryption techniques can be applied easily to digital data
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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
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!
n = numberofbitsperchange
!
where
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Transmission media
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Transmission media
引导的
Guided Transmission media
􏰣 twist1ed pair 同轴电缆
􏰣 coaxial Cable 光纤
􏰣 optical Fiber
Wireless transmission 􏰣 micro1wave
􏰣 radio 红外线
􏰣 infrared and millimeter waves
􏰣 light‐wave transmission
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双绞线
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Twisted pair (1) Conductor
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导体
• Widely used in the
W-ire telephone network 电介质
dielectric
Foil shield braid shield
jacked
箔盾
编织屏蔽顶
• Used for either analog and digital transmission
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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 (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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Coaxial cable (1)
copper core in
braided outer
基带同轴电缆
铜芯
• Baseband coaxial cable • one channel
编织外导体
宽频 (
• Broadband coaxial cable
commonly used for analog transmission
conductor
protective plastic covering
TV, CD-quality audio.
• multiple
channels, e.g. analog dual cable system
模拟电视
• two types:

single cab”le system
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绝缘
commonly used for digital
sulating material
transmission

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Optical Fibers (cont’d)
Jacket
Core
Cladding
• Idea: refraction principle
“#
light at less than critical angle is
absorbed in jacket
Angle of incidence
Angle of reflection
被困
• Utilization: light trapped by total internal reflection when…
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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
覆层
the jacket.
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Fiber optics(3)
􏰣 Attenuation very low
2􏰣 High noise immunity
􏰣 Error rate: 10‐15
􏰣 ~100 km of fiber => ~2 Gbps
􏰣 Unfamiliar technology: high skills required
􏰣 Lightweight
􏰣 Expensive
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Fiber optics (2)
Three components: 􏰣 light1source (LED or
􏰣 transmission medium (fiber)
􏰣 dete
Two major t1ypes of fiber
􏰣 multimode fiber (largely used)
􏰣 single mode fiber (expensive but can be used for long distances)
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光电二极管
ctor (photodiode)
激光二极管
laser diode)

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Optical Fiber (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,…
􏰣 Applications (cont’d) 订户
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Optical Fiber (cont’d)
􏰣 Applications ⻓途
􏰣 Long‐Haul trunks
􏰣 Rural‐exchange trunks +!
􏰣 25 to 100 miles
􏰣 less than 5,000 voice channels
􏰣 average length 900 miles
􏰣 20,000 to 60,000 voice channels 大都市
􏰣 Metropolitan networks
􏰣 average length 7.8 miles
􏰣 100,000 voice channels 农村交流干线
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Optical Fiber (cont’d)
波分复用
􏰣 Wavelength Division Multiplexing (WDM)
+􏰣 Send more than one wavelengths through the same fiber
􏰣 Allows re‐use of fiber
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Spectrum Allocation
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有价值 商品
Spectrum is valuable commodity; needs to be shared

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Spectrum

Allocation
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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.
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Narrow Wideband#Broadband
10 kbps
Fixed Wireless Access
Wireless LAN
Direct to Home Satellite
100 kbps 1 Mbps 10 Mbps 100 Mbps
,
1Gbps
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MSAT
2.5 G
3G
2G
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The m”ost valuable segment
Not Enough Bandwidth Poor Radio Coverage
2 GHz
Frequency
VHF 300 MHz UHF 3 GHz SHF 30 GHz EHF
Cellular/PCS/3G WLAN Fixed Wireless
Satellite
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Licensed & Unlicensed Bands
Licensed
800-900 MHz 1800-1900 GHz 2 GHz
5 GHz
28 GHz
Cellular/PCS 3G/4G WiMax
Unlicensed
915 MHz 2.4 GHz
5 GHz
60 GHz ??
Wireless LAN ITS
IMS Applications
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Licensed
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
%
900 MHz General Application
60 GHz ??? New
Unlicensed
2.4 GHz IMS (802.11b/g/n, Bluetooth)
5 GHz UNII (802.11a), DSRC (802.11p)
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Employed Frequencies
􏰣 Wireless channel’s behaviour is dependent on the frequency band of the signal.
-3
􏰣 Frequency band of operation depends on the 天线
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 9400 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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无线电
Radio transmission
􏰣 Radio waves
􏰣 easy to generate, can trave’l long distances, penetrate buildings at 全向的
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”
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穿透

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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”
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1
Wireless transmission patterns
􏰣 directional ‐
􏰣 the transmitting antenna’puts out a focused electromagnetic 光束
beam
地面微波
example: terrestrial microwave, satellite
全向的
􏰣 omnidirectional
􏰣 the transmitted signal spreads out in all directions 􏰣 example: broadcast radio
!
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天线

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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.
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2
Microwave transmission
􏰣 Microwave transmission is widely used
􏰣 long‐distance communication, cellular phones, TV distribution,

􏰣 Microwave transmissi”ons can be made easier
etc. directional
􏰣 repeaters are needed (for 100‐m high towers, repeaters can be spaced 80 km apart)
􏰣 Higher signal to noise ratio 穿透
􏰣 They do not penetra”te deep into buildings 衰退
􏰣 Multipath fading effect can occur !
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d􏰠3.57􏰜􏰩h 􏰡 􏰩h 􏰝 12
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Terrestrial Microwave
􏰣 Physical Description 抛物面
5!
78
􏰣 parabolic dish
􏰣 ~ 10 feet in diameter 视线
􏰣 line‐of‐sight transmission
􏰣 maximum distance between 2 antennas (in km):
where
􏰣 h1 = height of antenna one 􏰣 h2 = height of antenna two 􏰣 K = 4/3 (adjustment factor)
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Terrestrial Microwave (cont’d)
􏰣 Applications
􏰣 long‐haul communications
􏰣 TV and voice communications
􏰣 transmission in small regions (radius < 10 km) 1-78 3 79 2021-01-18 Terrestrial Microwave (cont’d) 􏰣 Transmission Characteristics 􏰣 attenuation is the major source of loss " 􏰪4􏰚d􏰭2 L􏰠10log􏰬􏰫 􏰧 􏰯􏰮 dB 􏰣 where d is the distance and 􏰧 is the wavelength, expressed in the same units. IMPORTANT: loss varies as the square of the distance ! 1-79 80 4 Satellite Microwave 􏰣 Physical Description 􏰣 a satellite is a microwave' 中继 transmitters/receivers. 饶轨道的 􏰣 a single orbiting satellite will operate on a number of frequency bands, called transponder channels. 􏰣 Geostationary ‐ a sate4llite is required to remain stationary with respect to its position over the earth. This match occurs at ~36,000 km relay station used to link 2 or more ground‐based microwave 􏰣 Low Earth Orbit (LEO), Medium Earth Orbit (MEO) systems (satellite phones) 1-80 81 " Satellite Microwave (cont’d) 􏰣 Applications: 􏰣 television distribution (broadcast) 􏰣 long‐distance telephone 􏰣 private business networks (VSAT/USAT networks) 2021-01-18 Satellite Microwave (cont’d) receives on one frequency (uplink) transmits on another frequency (downlink) 1-81 82 1-82 5 Satellite Microwave (cont’d) 􏰣 Transmission characteristics 􏰣 4 G H z / 6 G H*z , 1 2 G H z / 1 4 G H z , 1 5 G H z / 1 7 G H z 2021-01-18 􏰣 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) 83 1-83 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 1-84 84 6 85 2021-01-18 Free Space Lightwave transmission #􏰣 An application: connect two LANs in two buildings via lasers • high bandwidth, very low cost and easy to install • Characteristics 2 • laser beams cannot penetrate rain or thick fog • laser beams work well on sunny days, but .... 1-85 86 7 Digital Data/Digital Signals 􏰣 Modulation rate is t4he 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 发出 takes for the transmitter to emit a bit. For a data date R, the bit duration, tB, is 1/R. 1-86 87 2021-01-18 Data Rate vs. Modulation Rate 􏰣 data rate: "R = 1/tB wheretB isthebitduration example: In the case of code Manchester, the maximum modulation rate, Dmax, is 2R 1-87 Data Encoding 1-88 88 8 89 " Square Pulse 90 1-90 2021-01-18 9 基带 􏰣 Encoding onto a digital signal located at baseband ! Data Encoding - Baseband Transmission x(t) g(t) digital x(t) g(t) or analog digital Encoder Decoder t 1-89 91 2021-01-18 Digital Data/Digital Signals 􏰣 Definition 􏰣 Bipolar signalling: one logic state is represented by a positive voltage level an the other by a negative voltage level v(t) t 1-92 92 10 Digital Data/Digital Signals 􏰣 Unipolar signal: all the signal elements have the same algebraic sign, all positive or all negative v(t) 􏰣 Definition 单极 " ! t 1-91 93 2021-01-18 Digital Signal Encoding Schemes 􏰣 Five evaluation factors: 1) signal spectrum & 􏰣 lack of high‐frequency components means that less bandwidth is required for transmission 2) clocking 􏰣 every bit being received needs to be identified 1-93 Digital Signal Encoding Schemes 3) error detection 􏰣 useful to be able to detect errors at the physical level 4) signal interference and noise immunity 5) cost and complexit"y ! 1-94 94 11 95 2021-01-18 Some encoding techniques NRZ # NRZI Bipolar-AMI Pseudoternary Manchester Differential Manchester 1-95 Digital Signal Encoding Schemes 􏰣 3 main te(chniques: 􏰣 Nonreturn to Zero (NRZ) 􏰣 Multilevel Binary 􏰣 Biphase 1-96 96 12 97 2021-01-18 Digital Signal Encoding Schemes 􏰣 Non‐return to zero (NRZ) 􏰣 maintains a constant value for the duration of a bit )! time. example 1: NRZ‐L (nonreturn‐to‐zero‐level) 􏰣 during a bit interval there is no transition 􏰣 two different levels for the two binary digits 􏰣 binary 0 ‐ negative voltage 􏰣 binary 1 ‐ positive voltage 1-97 Some encoding techniques NRZ NRZI Bipolar-AMI Pseudoternary Manchester Differential Manchester 1-98 98 13 99 2021-01-18 Digital Signal Encoding Schemes 􏰣 Non‐return to zero (NRZ) 􏰣 example 2: NRZI (non‐return to zero, invert on ones) ! 􏰣 the data is encoded as the presence or absence of a signal transition 6 at the beginning of the bit time. 􏰣 this type is called differential encoding (the signal is decoded by comparing the polarity of adjacent signal elements) 1-99 Some encoding techniques NRZ NRZI Bipolar-AMI Pseudoternary Manchester Differential Manchester 1-100 100 14 101 2021-01-18 Digital Signal Encoding Schemes 􏰣 Multilevel Binary 􏰣 uses more than 2 signal levels example 1: b i p o l a r A M I ‐ t h e b i n a r y 1 )p u l s! e s a l t e r n a t e i n p o l a r i t y example 2: Pseudo‐ternary ‐ the binar " y0 pulses alternate in polarity 1-102 102 15 Digital Signal Encoding Schemes 􏰣 Non‐return to zero (NRZ) 􏰣 Advantage: " 􏰣 make efficient use of bandwidth 􏰣 Drawback: " 直流电 同步化 􏰣 presence of a dc component and lack of synchronization (used in digital magnetic recording) " 1-101 103 2021-01-18 Some encoding techniques NRZ NRZI Bipolar-AMI Pseudoternary Manchester Differential Manchester 1-103 Digital Signal Encoding Schemes 􏰣 Multilevel Binary 􏰣 adva)ntages: 1) since signal alternate in voltage, there is no net DC component 2) pulse alternation property provides a simple means of error detection 􏰣 draw1back: 1) loss of synchronization bipolar AMI ‐ if a long string of 0’s occurs pseudoternary ‐ if a long string of 1’s occurs 1-104 104 16 105 2021-01-18 Digital Signal Encoding Schemes 双相 􏰣 Biphase: 􏰣 there is a transition at(the middle of each bit period Example 1: Manchester ‐ the mid‐bit transition serves as a clocking mechanism and also for transporting data. 􏰣 Biphase: example 2: Differential Mancheste%r ‐ mid‐bit transition provides clocking binary 0 ‐ transition at the beginning of a bit binary 1 ‐ no transition at the beginning of a bit 1-105 Some encoding techniques NRZ NRZI Bipolar-AMI Pseudoternary Manchester Differential Manchester 1-106 106 17 107 2021-01-18 Digital Signal Encoding Schemes Normalized Signal Transition Rate of Various Digital Signal " Minimum 0 (all o's or 1's) 0 (all 0's) 0 (all 0's) 0 (all 1's) 1.0 (1010...) 1.0 (all 1's) Encoding Schemes NRZ-L NRZI Binary-AMI Pseudoternary Manchester Differential Manchester 101010... Maximum 1.0 1.0 0.5 1.0 (all 1's) 1.0 1.0 1.0 1.0 1.0 2.0 (all o's or 1's) 1.5 2.0 (all 0's) 1-108 108 18 Digital Signal Encoding Scheme 􏰣 Biphase 􏰣 ad#vantages: a) synchronization ‐ predictable transition permit to the receiver to resynchronize. b) absence of expected transition can be used to detect errors c) no DC component 􏰣 drawback a) higher modulation rate than NRZ => higher BW
调制
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Evaluation
􏰣 NRZ
􏰣 lack of synchronization ca/pability; widely used for digital magnetic
recording but not for signal transmission 􏰣 Multilevel binary
􏰣 long string of 0s (Bipolar‐AMI) and 1s (pseudoternary) cause synchronization problems (scrambling techniques are used to address this deficiency);
!7′
􏰣 Biphase
􏰣 it is easy to detect isolated errors; it is not as efficient as NRZ (three signal levels are used instead of 2 levels used in NRZ)
􏰣 no synchronization problems; good error detection; more bandwidth is needed (as many as two transitions per bit time)
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Data Encoding
􏰣 Modulation onto an analog signal
S(f)
m(t) digital or analog
fc
s(t) analog
m(t)
Modulator
Demodulator
fc
f
1-110
110 & 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(2􏰚ft + 􏰛) • Parameters:
Amplitude: A
Frequency: Phase : 􏰛
111
f
1-111
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Modulation Techniques
􏰣Amplitude modulation
-8
􏰣 S(t)=[1+naX(t)]cos2􏰚fct, where cos2􏰚fct is the carrier and x(t) is the input signal, na is the modulation index (ratio of the amplitude of x(t) to the carrier)
􏰣 simplest form of modulation 􏰣 Angle modulation
􏰣 􏰣
􏰣
S(t)=Acos[2􏰚fct + 􏰛(t)]
phase modulation : 􏰛(t)=npm(t) where np is the phase modulation
index
frequency modulation: d􏰛(t)/dt=n modulation index
f
m(t) where n
f is the frequency
1-112
Modulation of a sine-wave carrier by a sine-wave signal
!
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Modulation of digital signal
Amplitude-shift keying
Frequency-shift keying
Phase-shift keying
1-114
Multileve”l modulation
Quadrature phase-shift keying Quadrature amplitude modulation (R=2*D) (R=4*D)
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Multiplexing
1-116
Spectrum Allocation
Spectrum is valuable commodity; needs to be shared
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多路复用
Multiplexing
• Objective: share a link between ‘several users (i.e., telephone companies transmit many conversations over a single physical
trunk)
• Two basic techniques: FDM (Frequency Division Multiplexing) and TDM (Time Division Multiplexing)
1-118
Frequency-d”ivision multiplexing
1-119
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m(t) digital or analog
FDM
s(t) analog
fc S(f)
fc !
m(t)
Modulator
Demodulator
f􏰡􏰰f f c
1-120
Frequency Division Multiple Access FDMA
bandwidth BW
guard band
!
f
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121
6

FDM 3 System
Overview
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􏰣 Crosstalk &
􏰣 guard bands (separating two “adjacent” channels) should be
Frequency‐division multiplexing: problems
carefully chosen
􏰣 a voice signal has an effective bandwidth of 3.1 kHz; a channel of 4 kHz is adequate to avoid crosstalk in analog voice transmissions
􏰣 Intermodulation noise
􏰣 Channels
􏰣 More challenging and expensive RF technology (narrow
filters; large ) 􏰣 Inefficiency
ar effects of amplifiers on a signal in one channel can produce undesirable frequency components in other
􏰣 nonline
123
􏰣 Channels might be allocated to sources that are not using them all the time
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Consider the following samples of 3 users’ data to be multiple accessed
User 1 data
User 2 data User 3 data
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Waveforms of Users 1,2 & 3 after FDM
%
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125
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Input to the amplifier after 3 FDM signals are added
!
1-126
x(t) y(t)
Output
!
Input
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Nonlinear Effects in FDM
􏰣Received signal is sum of multiple carriers.
#􏰣Receiver power amplifiers are operated nonlinearly (near saturation) for maximum efficiency.
􏰣The nonlinearities cause intermodulation (IM) frequencies to appear in the amplifier output.
􏰣IM components can interfere with other channels in the FDMA system.
1-128
128
Inter-modulation Noise
x(t) y(t)
y(t) = x(t) + a {x(t)}2
x(t) = cos(2􏰚f1t) + cos(2􏰚f2t)
f1 f2
f1 f22f1 2f2 f1+f2
1-129
y(t) = cos(2􏰚f1t) + cos(2􏰚f2t)
a + (a/2) {cos(2􏰚􏰤f1t) + cos(2􏰚􏰤f2t) + a {cos(2􏰚 􏰦f1+f2 ] t) + cos(2􏰚 􏰦f1-f2 ] t)
!
f2-f1
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Synchronous Time Division Multiplexing
!
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Time Division Multiple Access
􏰣TDMA systems divide the radio spectrum into time slots.
􏰣Only one user can t&ransmit or receive during one time slot.
􏰣Usually, each user may occupy the channel once during a time frame, where one frame comprises N time slots.
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TDM System ” Overview
1-132
Time‐division multiplexing: problems
􏰣 Fram-e synchronization
􏰣 use an identifiable pattern of bits at the beginning of each frame
􏰣 Pulse stuffing
􏰣 If user does not have data, the assigned slot needs to be staffed with dummy
bit
􏰣 Inefficiency
􏰣 many of the time slots are wasted; slots are allocated to inputs even these input are not sending any data
􏰣 High Pick Transmission power
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12
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Statistical TDM
􏰣 in Synch TDM many slots are wasted
􏰣 Statistical TDM allocates time slots dynamically based
􏰣 line data rate lower than aggregate input line rates -!
on demand
􏰣 multiplexer scans input lines and collects data until frame full
􏰣 may have problems during peak periods 􏰣 must buffer inputs
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Statistical time-division multiplexing
!
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TDMA Systems
􏰣TDMA systems tra&nsmit data in a buffer and burst method.
􏰣The transmission is non‐continuous.
􏰣Unlike FDMA systems which can transmit analog signals, TDMA must transmit data and digital modulation must be used.
1-136
TDMA
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TDMA
Slot 1
Slot 2
Slot 3
Slot N
1 frame

t
Guard time
t
trail bits
sync. bits
info bits
Each slot requires overhead bits. More overhead reduces efficiency.
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User 1,2&3 dat”a before and after TDMA 1-139
139
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TDMA Features
􏰣Only one carrier. N’o intermodulation.
􏰣Number of time slots per frame depends on bandwidth, desired date rate, modulation technique.
􏰣Receiver must syn&chronize to each time slot, thus more synchronization bits are required in TDMA compared to FDMA.
􏰣It is possible to allocate more than one time slot per frame – bandwidth on demand.
140
Spread Spectrum
􏰣 Also known as Code Division Multiple Access (CDMA)
􏰣 Important encoding meth’od for wireless
communications
􏰣 Can be used with analog & digital signal formats
􏰣 Users share both time & frequency domains; their signals overlap, occupying a wide bandwidth
􏰣 The separation is achieved by assigning different codes
to each user.

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Spread Spectrum
􏰣 Makes jamming and interception harder
􏰣 Initially used for mil”itary communications
􏰣 Two approaches, both in use: 􏰣 Frequency Hopping (FH)
􏰣 Direct Seque #
nce (DS‐SS)
􏰣 Cellular radio (IS‐95, CDMA2000,WCDMA)
􏰣 Wireless LANs (IEEE 802.11 b, g) ”
1-142
Spread Spectrum: Advantages/Disadvantages
􏰣 Resistive to interference, multipath fading #􏰣Easy Encryption
􏰣 Easy traffic multiplexing of discontinuous sources
􏰣Allows “soft” hand‐offs
􏰣 Synchronization imposes a challenge
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General Model of Spread Spectrum System
!
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Pseudorandom Numbers
􏰣 generated by a deterministic algorithm
􏰣 not actually rando”m
􏰣 but if algorithm good, results pass reasonable tests of randomness
􏰣 starting from an initial seed
􏰣 need to know algorithm4and seed to construct the
sequence
􏰣 hence only receiver can decode signal
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Frequency Hopping Example
!
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147
19
Frequency Hopping Spread Spectrum (FHSS)
􏰣 signal is broadcast ov&er seemingly random series of frequencies
􏰣 receiver hops between frequencies in sync with
transmitter
窃听者
􏰣 eavesdroppers hear unintelligible blips
􏰣 jamming on one frequency affects only a few bits
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FHSS (Transmitter)
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Frequency H”opping Spread Spectrum System (Receiver)
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149
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Slow and Fast FHSS
􏰣 commonly use mult#iple FSK (MFSK)
􏰣 have frequency shifted every Tc seconds 􏰣 duration of signal element is Ts seconds 􏰣 S l o w F H S S h a s T c 􏰱’T s
􏰣 Fast FHSS has Tc < Ts 􏰣 FHSS quite resistant to noise or jamming 􏰣 fast FHSS is giving better performance 1-150 Slow MFSK FHSS ! 1-151 151 21 152 2021-01-19 Fast MFSK FHSS ! 1-152 Direct Sequence Spread Spectrum (DSSS) 􏰣 each bit is represented'by multiple bits using a spreading code 􏰣 this spreads signal across a wider frequency band 􏰣 has performance similar to FHSS 1-153 153 22 2021-01-19 Direct Sequence Spread Spectrum System " 1-154 154 Data PN-1 Data spread by PN-1 PN-2 Data despread by PN-2 Data despread by PN-1 1 1001010 1001010 1011100 1101001 1 1-155 ! 155 23 2021-01-19 DSSS Example Using BPSK ! 1-156 156 CDMA Example 157 ! 157 24 158 2021-01-19 CDMA for DSSS ! 1-158 Direct Sequence Spread Spectrum Example ! 1-159 159 25 160 2021-01-19 Approximate Spectrum of DSSS Signal 1-160 Power of DS"‐SS Signals at Tx when square pulses are used noise level t t f f 1-161 161 26 2021-01-19 Do all systems fall into only one of 3 categories?" f t f, t Answer: In practice NO. ! 162 162 Examples of their use in wireless mobile communications systems 􏰣 FDM#/FDMA 􏰣 1st generation: analog cellular, AMPS (each channel was occupying 60 KHz bandwidth) 􏰣 2nd generation North American digital cellular radio; IS‐41 􏰣 2nd generation North American digital cellular radio (improved); IS‐136. 􏰣 Hybrid architecture. TDMA multiplexing 4 users in each 60 KHz AMPS channel. 􏰣 TDM/TDMA 􏰣 2nd and 2.5 generation European cellular radio, GSM/GPRS/EDGE 􏰣 Bluetooth (FH‐SS), ZigBee (DS‐SS), Home RF (FH & DS‐SS) 􏰣 CDMA 􏰣 2nd generation: IS‐95 based North American cellular radio (DS‐SS). 􏰣 3rd generation CDMA2000 and WCDMA (DS‐SS) 􏰣 IEEE 802.11 WLAN (FH & DS‐SS) +! 163 1-163 27 1 2021-01-25 1-1 Outline 􏰣 Synchronization 􏰣 Asynchronous and synchronous transmission 􏰣 Error detection/correction techniques 􏰣 Line Configuration 1-2 2 1 3 2021-01-25 Digital Data/Digital Signals v(t) v(t) t t 1-3 Sender Receiver TB SYNCHRONIZATION Ts sampling at the centre of each bit time 1-4 ! 4 2 5 2021-01-25 Some encoding techniques NRZ NRZI Bipolar-AMI Pseudoternary Manchester Differential Manchester 1-5 6 3 SYNCHRONIZATION • loss of synchronizat'ion – In practice TB and TS are not equal. The result is that the timing of the receiver may slowly 相对于 drift relative to the received signal. 1-6 同步化 7 2021-01-25 SYNCHRONIZATION unpredictable time 11111001 1-7 SYNCHRONIZATION • loss of synchronization – Solution: • data is sent in bit sequences called frames ! • the receiving clock is started at the beginning of each bit sequence ! – Question: ! Since synchronization needs to be kept only for the duration of the frame, what is the length of the frame that will allow us to avoid loss of synchronization? 1-8 8 4 9 2021-01-25 ASYNCHRONOUS TRANSMISSION • Timing or synchronization must only be maintained within e'ach character; the receiver has the opportunity to resynchronize at the beginning of each new character. idle state start bit 11111001 unpredictable time stop bit 1-9 SYNCHRONIZATION • loss of synchronization example: #– a frame consists of 11 bits – assume that the synchronization at the start of the first bit is late at most 10% of TB We must fulfill the following 2 conditions: (10􏰡12)􏰲TS 􏰡0.1􏰲TB 􏰳11􏰲TB and (10􏰡12)􏰲TS 􏰴10􏰲TB These are satisfied if: Ts 􏰵 TB 􏰳 3.8% TB 1-10 10 5 11 2021-01-25 ASYNCHRONOUS TRANSMISSION 􏰣 Timing requirements are modest. Sender and receiver are synchronized at the beginning of every character (8 bits if ASCII) 􏰣 high overhead overhead 􏰠 control_bits total _ bits ! 1-11 12 6 SYNCHRONOUS TRANSMISSION 􏰣 In this mode, blocks of characters or bits are 前言 )! transmitted. Each block begins with a preamble and ends with a postamble 􏰣 2 types: 􏰣 character‐oriented 􏰣 bit‐oriented 后同步码 1-12 13 2021-01-25 SYNCHRONOUS TRANSMISSION 􏰣 Character‐oriented 􏰣 the frame consists of a sequence of characters ! SYN SYN one or more SYN characters control characters control characters SYN is a unique bit pattern that"signals the receiver the beginning of a block 1-13 data characters SYNCHRONOUS TRANSMISSION 􏰣 character‐oriented ‐ 2 approaches 􏰣 the receiver having detected the beginning of the block # reads the information till it finds the postamble SYN SYN PREAMBLE POSTAMBLE the receiver having received the preamble, looks for extra starts to look for next preamble 1-14 " information regarding the length of the frame SYN SYN LEN 14 7 15 2021-01-25 SYNCHRONOUS TRANSMISSION 􏰣 Bit oriented 􏰣 In this mode, the fram"e is treated as a sequence of bits. Neither data nor control information is interpreted in units of x‐bit characters 􏰣 a special bit pattern indicates the beginning of a frame " 􏰣 the receiver looks for the occurrence of the flag FLAG FLAG control fields ! control fields 1-15 Dealing with presence of errors #• Detect presence of errors (error detection) • Try to correct them (error correction) • If no correction have the mechanism to request retransmission (use of Automatic Repeat Request) 1-16 16 8 17 2021-01-25 Random Errors 􏰣Anerro"roccurswhena&bitisalteredbetween transmission and reception 􏰣 Random, statistically uniformly spread errors 􏰣 occurrence of an error does not increase the probability that other bits, close to the one in error, while be in error 􏰣 white noise is producing such errors 􏰣 For low BER and frames of “reasonable” length, most framers would experience no error or 1 error at most. 􏰣 example: BER = 10^{‐6} and length of frame=[1000bytes*8=] 8,ooo bits 􏰣 probability of receiving a frame correctly = [1‐10^{‐ 6}]^{8,000}>0.992
􏰣 probability of a frame having a single error=8,000*receiving a
[1‐10^{‐6}]^{7,999}* {8,000}=0.007873 correctly = 10^{‐6}*
frame
􏰣 Probability of having more than 1 error<[ 1‐0.992‐ 0.007873=]0.000127 1-17 18 9 Burst Errors 􏰣 Occurrence of an error'having occurred in the sequence, means bits preceding/following the one in error have higher probability than the average bit error probability to be in error 簇 􏰣 impulsive noise 􏰣 “slow” fading/shadowing in wireless (relevance of bit rate to average time/distribution channel attenuation remains below certain level) 􏰣 Error strings (clusters of errored bits closely located in the seq&uence) form 减损 􏰣 Some channel related impairments producing error bursts 􏰣 effect greater at higher data rates 1-18 2021-01-25 1-19 19 10 20 2021-01-26 Error detection and control Objective 􏰣 detect and correct errors that occur in the transmission of frames Types of errors 􏰣 lost frame: a frame that the receiver does not receive (e.g., ! because starting of frame/clock extraction is not achieved due to excessive signal attenuation, increased levels of noise...) 􏰣 damaged frame: a frame that the receiver receives, but some of its bits are in error " ! 1-20 Impact of error location The frame consists of a sequence of characters SYN SYN one or more SYN characters control control characters data characters characters SYN is a unique bit pattern that signals the receiver the beginning of a block 1-21 21 1 22 2021-01-26 Error Detection 1-22 Error Correction/Detection Process ! 1-23 23 2 2021-01-26 How Error Correction & Detection Works 冗余 #􏰣 Adds redundancy to transmitted message 尽管 􏰣 Can deduce original despite some errors 24 􏰣 Example: block error correction code 􏰣 map k bit input onto an n bit codeword 􏰣 each distinctly different 􏰣 if get error assume codeword sent was closest to that received 􏰣 means have reduced effective data rate 􏰣 most of work concerning error correction & detection is making use of Galois field algebra (Boolean algebra ‐ mod_2 arithmetic – is a case of it) 1-24 Code rate & minimum distance Code rate r = # of information bits in a block 􏰠 k # of total bits in a block n The bandwidth expansion is 1 / r = n / k " The energy per channel bit (Ec )is related to energy perinformationbit(Eb)throughEc 􏰠rEb Minimum distance (dmin ): Minimum number of positions in which any 2 codewords differ. " " 1-25 25 3 2021-01-26 26 A simple block code: (7,4) Hamming Code Message 0000 1000 0100 1100 0010 1010 0110 1110 0001 1001 0101 1101 0011 1011 0111 1111 Codeword 000 0000 110 1000 011 0100 101 1100 111 0010 001 1010 100 0110 010 1110 101 0001 011 1001 110 0101 000 1101 010 0011 100 1011 001 0111 111 1111 27 •Error detection w/o error correction? •Error detection with error correction? •Minimum Humming distance? •Error Correction capability? 27 4 Correctable and detectable errors 􏰣A block code can correct at least 􏰨 errors if dmin > = 2 􏰨 +1 􏰣=> if dmin=3, then 􏰨 =1. If there is only one error in the block,
7! d o n o t c o r r e s p o n d t o a c o d e w’o r d .
it can be corrected.
􏰣A block code can detect any error pattern if the received n bits
􏰣If there are 􏰧 errors in the n‐bits codeword, the existence of errors is detected with certainly if 􏰧 < dmin. 􏰣However, even when 􏰧 >=dmin, many of the corrupted blocks can still be detected.
“&
􏰣Out of the 2n possible n‐bit combinations, only 2k code can be generated, thus, there are 2n ‐ 2k= 2k (2(n‐k)‐1) pro
combinations.
􏰣Above statement applies when no error correction is used.
1-26
words
禁止
hibited

28
2021-01-26
A simple block code: (7,4) Hamming Code
Message
0000
1000
0100 1100 0010 1010 0110 1110 0001 1001 0101 1101 0011 1011 0111 1111
Codeword
000 0000
110 1000
011 0100 101 1100 111 0010 001 1010 100 0110 010 1110 101 0001 011 1001 110 0101 000 1101 010 0011 100 1011 001 0111 111 1111
•Error detection w/o error correction? 3
•Error detection with error correction? 0
•Minimum Humming distance? 3
•Error Correction capability? 1
1-28
“Popular” Error Detection Techniques
􏰣 Parity Checks
+􏰣 Longitudinal redundancy checks (LRC)
􏰣 Cyclic redundancy checks (CRC)
1-29
29
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30
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Simple Error Detection Scheme
奇偶性
􏰣 Parity check
􏰣 Value of parity bit is such that character has even (even
parity) or odd (odd parity) number of ones
􏰣 Even number of bit errors goes undetected !
1-30
Parity Checks
1-31
31
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Longitudinal Redundancy Checks
1-32
Longitudinal Redundancy Checks
1-33
33
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Longitudinal Redundancy Checks
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Longitudinal Redundancy Checks
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Longitudinal Redundancy Checks
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Longitudinal Redundancy Checks
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Cyclic Redundancy Check
􏰣 Based on cyclic error‐correcting codes
􏰣 For a block&of k bits the transmitter generates n bit sequence
􏰣 n insert redundancy in the codeword
􏰣 Transmitter transmits the k+n bits
􏰣 Receiver uses error detection process to decide if there were errors in the received sequence or otherwise
Cyclic Codes
• “Cyclic code is a block code, where the circular shifts of each codeword gives another codeword that belongs to the code”.
-!
• “They are error-correcting codes having algebraic properties
that are convenient for efficient error correction & detection”.
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Fundamentals of CRC coding
􏰶 CRC codes tr&eat bit strings as representations of polynomials with coefficients of 0 and 1 only (modulo 2 arithmetic)
11001↔1∗𝑋􏰷 􏰸1∗𝑋􏰹 􏰸0∗𝑋􏰺 􏰸0∗𝑋􏰻 􏰸1∗𝑋􏰼 􏰽𝑋􏰷 􏰸𝑋􏰹 􏰸1
􏰶 Polynomial arithmetic is done modulo 2
􏰣 subtraction and addition are similar to EXCLUSIVE OR
􏰣 division is similar to the one in decimal except the subtraction is done modulo 2
􏰣 Make sure you are familiar with mod2 arithmetic/algebra
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2􏰶 The sender and receiver agree upon “a generator polynomial”, G(x), in advance.
CRC: Basic Idea
􏰶 The sender appends a checksum (corresponds to the n redundancy bits) to the end of the (only data) frame, represented by the M(x) polynomial, in a way that the polynomial T(x), representing the {data + checksum bits} frame, is divisible by G(x).
􏰶 Upon receipt of the frame, the receiver (generates and) divides H(x) by G(x) using modulo 2 division.
􏰶 H(x) is the polynomial corresponding to the received sequence.
􏰶 if there is a remainder, there has been transmission error. 43
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How to compute the checksum
􏰣 If n‐1 is the degree of G(x), then append n zero to the low order end of the frame; the resulting frame corresponds to the polynomial X n M (x).
􏰣DivideG(x)into XnM(x) usingmodulo2division.
􏰾􏰿􏱀􏱁􏰾􏱂 􏱃􏱁􏰾􏱂
􏰣D(X): divisor; R(X): remainder
𝑋􏱄𝑀𝑋 􏰽𝐺𝑋􏱅𝐷𝑋􏱆𝑅􏱁𝑋􏱂
11001↔1∗𝑋􏰷 􏰸1∗𝑋􏰹 􏰸0∗𝑋􏰺 􏰸0∗𝑋􏰻 􏰸1∗𝑋􏰼 􏰽 𝑋􏰷 􏰸 𝑋􏰹 􏰸 1=M(X)
11001000􏱄􏱇􏰹1∗𝑋􏱈 􏰸1∗𝑋􏱉 􏰸0∗𝑋􏱊 􏰸0∗𝑋􏰷 􏰸1∗𝑋􏰹 􏰸 0∗𝑋􏰺 􏰸0∗𝑋􏰻 􏰸0∗𝑋􏰼 􏰽𝑋􏱈 􏰸𝑋􏱉 􏰸𝑋􏰹=𝑋􏰹 ∗𝑀􏱁𝑋􏱂
→𝐷 𝑋 ;𝑅􏱁𝑋􏱂
How to compute the checksum
􏰣Subtracttheremainderfrom XnM(x) using modulo 2 subtraction/addition.
􏰣 The result is the checksumed frame’s polynomial , T(x).
𝑇𝑥 􏰽𝑋􏱄𝑀𝑋􏱆𝑅􏱁𝑋􏱂
􏰣 The frame corresponding to T(x) is transmitted.
T(X)􏰠 XnM(X)􏱆R(x)􏰠[D(X)􏱅G(X)􏱆R(x)]􏱆R(X)􏱋
T ( X ) G(X) divides
􏱋 T ( X ) 􏰠 D( X ) 􏱅 G( X ) 􏱆 O 􏱋 G( X ) 􏰠 D( X )perfectly T(X)
(remainder = O) !
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CRC: an Example
• Frame: 1101011011
• Generator: 10011
CRC Error Detection
􏰶 Let us assume that some t’ransmission errors occur 􏰶 Instead of receiving T(x), the receiver will receive
H(x)=T(x)􏱆E(x)
􏰶 If there are k “1” bits in E(x), (it is most probable that) k
errorswill bedetected.
􏰶single‐bit error means E(X ) 􏰠 X m􏰵1, where 0 􏱌 𝑚 􏱍 𝑛 􏰸 𝑘
bit errors have occurred
􏰶 the receiver computes (T(x)
&􏱆 E(x))/G(x)=E(x)/G(x) 􏰶 If G(x) contains two or more terms, (i.e. n>1) all single
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Singleerrorand G(x)=Xn 􏰣H(X)􏰠T(X)􏱆E(X) where
reversed (0 􏱌 𝑚 􏱍 𝑛 􏰸 𝑘; the larger the value of m is, the more !’
􏰣E(X ) 􏰠 O if no bit errors occur
􏰣E(X) 􏰠 X m􏰵1 if only the m-th bit of the [k+n]-bit long frame is
significant the location of the bit within the frame is) 􏰣E(X) 􏰠 L(X)􏱅G(X)􏱆 F(X)
􏰣􏰣 H(X)􏰠T(X)􏱆E(X)􏰠T(X)􏱆L(X)􏱅G(X)􏱆F(X)
H ( X ) 􏰠 D( X ) 􏱆 L( X ) 􏱆 F ( X ) G(X) G(X)
Error will be detected if F(X ) 􏱎 O G ( X”)
For m-1>=n,L(X ) 􏰠 X m􏰵n􏰵1 and F(X ) 􏰠 O . Error is not detected. F(X) G(X)
For m-1

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Payload Type Identifier
 Used to identify the following cells:
 Network generated cells
 used for maintenance & control of network
 used for call set‐up, loopbacks and keep alives
 Customer generated cells  user information
Cell Loss Priority
 CLP in the header
CLP=0: Highpriority,lastlikelytobediscarded
CLP=1: Lowpriority,maybediscardedduringcongested intervals
 CLP can be set:
 by the terminal
 by the ATM switch
 CLP determines the class of service or service contract
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Virtual Connections
 Permanent Virtual Circuits (PVC)  network operator connects endpoints
 Switched Virtual Circuits (SVC)  can be switched like PSTN
 Call set‐up routine
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Service Categories (1)
 Quality of Service (QoS) are parameters that are set for end‐to‐end network performance
 Cell Transfer Delay (CTD): delay between start & finish of cell
 Peak to Peak Cell Delay Variation (CDV): difference between
maximum CTD and minimum CTD
 Cell Loss Ratio (CLR): % of cells lost
 Sustained Cell Rate (SCR): average rate of transmitted cells
 Bust Tolerance (BT): maximum burst size at PCR
 Maximum Burst Size (MBS): maximum No. of cells sent at PCR
 Minimum Cell Rate (MCR)
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Service Categories (2)
 ATM is divided into (5 + 1=) 6 service categories
 Constant Bit Rate (CBR).
 CTD & CDV are tightly constrained, low CLR
 Real ‐ Time Variable Bit Rate ( rt ‐ VBR)  CTD & CDV are tightly constrained
 Non‐Real ‐ Time Variable Bit Rate ( nrt ‐ VBR)  CTV is tightly constrained
 Available Bit Rate (ABR)
 Minimize CTD, CDV and CLR
 Unspecified Bit Rate (UBR)
 No CTD, CVD or CLR constraints
 Guaranteed Frame Rate (guarantees delivery of “x”% of frames to user)
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Available Bit Rate (ABR)
Itguaranteestothesourcesaminimumrate. Itis based on adaptation of the source rate to use resources when available and reduce transmission rate when resource are scarce, in order to avoid congestion.
Its primaryusewastocarryInternettraffic
 Two main categories  Rate Based Control
 Credit Based Control
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ATM Bit Rate Services
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Traffic Shaping(1)
 Traffic shaping is forcing your traffic to conform to certain specified behaviour
 Each service has a contract
 If the contract is violated, the network has the right
to discard the cells
The ATM switch monitors the traffic flow
 The shaping and policing is based on the Generic Cell Rate Algorithm (Leaky Bucket)
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Traffic Shaping(2)
 Traffic parameters
 Mean Cell Rate (Sustained Rate)  Peak Cell Rate
 Burst Frequency
 Burst Length
 Cell‐loss Priority
 Cell‐loss Rate
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Leaky Bucket
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ATM Layers
AAL adapts from other protocols to ATM.
ATM-L is responsible for routing and switching the cells.
PL is responsible for sending and receiving bits on the medium.
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Higher Layer
ATM Adaptation Layer (AAL)
ATM Layer (ATM-L)
Physical Layer (PL)
ATM Layers (2)
ATM Layer
 Generic Flow Control (applied to UNI to alleviate short term overload)
 Cell header generation/extraction
 Cell Virtual Path Identifier(VPI)/Virtual Circuit Identifier
(VCI) translation
 Cell multiplexing/demultiplexing.
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ATM Layers (3)
 ATM Adaptation Layer is subdivided into:  Segmentation And Reassembly (SAR)
 Convergence Service Specific (CS)
CS – interfaces with the upper layer protocol information – provides padding and CRC checking.
SAR – generates the ATM payload.
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CS
SAR
ATM Adaptation Layers
Standardized ATM Adaptation Layers
 AAL1 ‐ provides connection oriented Constant Bit Rate
services that have timing and delay requirements
 AAL2 ‐ provides connection oriented Variable Bit Rate services that have timing and delay requirements
 AAL3/4 ‐ provides connection‐oriented Variable Bit Rate services with no timing requirements (e.g., frame relay)
 AAL5 ‐ provides connectionless Variable Bit Rate services with no timing requirements
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ATM Interfaces
 User to Network Interface (UNI)
 specifies how cells come to a public network
 Broadband Inter‐Carrier Interface (B‐ICI)
 specifies how two carriers interact their services  defines the traffic contract between two carriers
 ATM Data Exchange Interface (DXI)  protocol between router and CSU/DSU
 Network to Network Interface (NNI)  connection between switches
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END
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