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Title: Chapter 2 Wireless Communication Technology (Part Two in textbook)


1
Chapter 2 Wireless Communication
Technology(Part Two in textbook)
2
Outline
  • 2.1Antennas and Propagation(?????)
  • 2.2 Signal Encoding Techniques(??????)
  • 2.3 Spread Spectrum(??)
  • 2.4 Coding and Error Control(????)

3
2.1Antennas and Propagation
Reading material 1Antenna
Tutorial2Chapter 5 in textbook
4
2.1.1 Classifications of Transmission Media (2.4
in textbook)
  • Transmission Medium(????)
  • Physical path between transmitter and receiver
  • Guided Media(????)
  • Waves are guided along a solid medium
  • E.g., copper twisted pair, copper coaxial cable,
    optical fiber
  • Unguided Media
  • Provides means of transmission but does not guide
    electromagnetic signals
  • Usually referred to as wireless transmission
  • E.g., atmosphere, outer space

5
Unguided Media
  • Transmission and reception are achieved by means
    of an antenna
  • Configurations for wireless transmission
  • Directional
  • Omnidirectional

6
?????????
7
General Frequency Ranges
  • Microwave frequency range
  • 1 GHz to 40 GHz
  • Directional beams possible
  • Suitable for point-to-point transmission
  • Used for satellite communications
  • Radio frequency range
  • 30 MHz to 1 GHz
  • Suitable for omnidirectional applications
  • Infrared frequency range
  • Roughly, 3x1011 to 2x1014 Hz
  • Useful in local point-to-point multipoint
    applications within confined areas

8
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9
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10
???????
11
ISM
12
???(1)
13
???(2)
14
???(3)
15
???????
  • ??--LF (Low Frequency)
  • ????--?????,?????
  • ???????????????,?????????????????,???? ?
  • ????????????,??? ,???? ,???? ,????
  • ??????????

16
Broadcast Radio
  • Description of broadcast radio antennas
  • Omnidirectional
  • Antennas not required to be dish-shaped
  • Antennas need not be rigidly mounted to a precise
    alignment
  • Applications
  • Broadcast radio
  • VHF and part of the UHF band 30 MHZ to 1GHz
  • Covers FM radio and UHF and VHF television

17
Microwave
18
Microwave System
19
Terrestrial Microwave
  • Description of common microwave antenna
  • Parabolic "dish", 3 m in diameter
  • Fixed rigidly and focuses a narrow beam
  • Achieves line-of-sight transmission to receiving
    antenna
  • Located at substantial heights above ground level
  • Applications
  • Long haul telecommunications service
  • Short point-to-point links between buildings

20
Satellite Microwave
  • Description of communication satellite
  • Microwave relay station
  • Used to link two or more ground-based microwave
    transmitter/receivers
  • Receives transmissions on one frequency band
    (uplink), amplifies or repeats the signal, and
    transmits it on another frequency (downlink)
  • Applications
  • Television distribution
  • Long-distance telephone transmission
  • Private business networks

21
???
22
??????(Multiplexing)(1)
23
??????(2)
24
??????(3)
25
??????(4)
26
??????(5)
27
??????(6)
28
2.1.2 Introduction to Antennas
  • ?????????????????(An antenna is an electrical
    conductor or system of conductors)
  • Transmission - radiates electromagnetic energy
    into space
  • Reception - collects electromagnetic energy from
    space
  • ???????????????????????(In two-way communication,
    the same antenna can be used for transmission and
    reception)

29
????(Radiation Patterns)
  • ?????????????,?????????????????????????????????
  • ????(Radiation pattern)
  • ?????????????
  • ?????????????????(cross section)
  • ???????????
  • ??????(????)?????
  • ????(Reception pattern)
  • Receiving antennas equivalent to radiation
    pattern

30
????(Radiation Patterns)
??????(????) ????
31
????(Types of Antennas)
  • ??????? (idealized)
  • Radiates power equally in all directions
  • ????(Dipole antennas)
  • ?????? (or ????)
  • 1/4????? (or ?????)??????????????????
  • ??????

32
????(Dipole antennas)
  • ??????????????????????????8????????
    ????????????????????

33
????(Dipole antennas)
  • ??????????
  • ????????
  • ????????????

34
??????(parabolic reflective)
  • ?????? ?????????,???????????
  • ?????????????????????????????????????????(focus),?
    ??????(directrix)

35
??????WL-ANT150????
  • ???

36
??????WL-ANT150????
  • ????

37
??????WL-ANT168????
  • ????

38
??????WL-ANT168????
  • ??????

39
????????
40
????(Antenna Gain)
  • ?????????????
  • ????????????????????
  • ???????????????????????????
  • ???????????????????????,???????????
  • ????Effective area
  • Related to physical size and shape of antenna

41
????(Antenna Gain)
  • ?????????
  • G antenna gain
  • Ae effective area
  • f carrier frequency
  • c speed of light ( 3 108 m/s)
  • ? carrier wavelength

42
????(Antenna Gain)
  • ??????2m???????,?????12GHz,???????????????
  • ???????0.56A,A?????????
  • ?
  • Api, Ae 0.56A, ??0.025m
  • G7pi/(0.0250.025)35186
  • Gdbl0lg3518645.46db

43
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44
2.1.3 Propagation Modes
  • Ground-wave propagation
  • Sky-wave propagation
  • Line-of-sight propagation

45
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46
Ground Wave Propagation
47
Ground Wave Propagation
  • Follows contour of the earth
  • Can Propagate considerable distances
  • Frequencies up to 2 MHz
  • Example
  • AM radio

48
Sky Wave Propagation
49
Sky Wave Propagation
  • Signal reflected from ionized layer of atmosphere
    back down to earth
  • Signal can travel a number of hops, back and
    forth between ionosphere and earths surface
  • Reflection effect caused by refraction
  • Examples
  • Amateur radio
  • CB radio

50
Line-of-Sight Propagation
51
Line-of-Sight Propagation
  • Transmitting and receiving antennas must be
    within line of sight
  • Satellite communication signal above 30 MHz not
    reflected by ionosphere
  • Ground communication antennas within effective
    line of site due to refraction
  • Refraction bending of microwaves by the
    atmosphere
  • Velocity of electromagnetic wave is a function of
    the density of the medium
  • When wave changes medium, speed changes
  • Wave bends at the boundary between mediums

52
Line-of-Sight Equations
  • Optical line of sight
  • Effective, or radio, line of sight
  • d distance between antenna and horizon (km)
  • h antenna height (m)
  • K adjustment factor to account for refraction,
    rule of thumb K 4/3

53
Line-of-Sight Equations
  • Maximum distance between two antennas for LOS
    propagation
  • h1 height of antenna one
  • h2 height of antenna two

54
2.1.4 LOS Wireless Transmission Impairments
  • ?????(Attenuation and attenuation distortion)
  • ??????(Free space loss)
  • ??(Noise)
  • ????(Atmospheric absorption)
  • ??(Multipath)
  • ??(Refraction)
  • ???(Thermal noise)

55
??(Attenuation)
  • ????????????????????????
  • ??????????????,????????????????
  • ?????????????
  • ?????,??????????????????????
  • ??????????,??????

56
??????(Free Space Loss)
  • ????????,?????????,??,????????????????,???????????
  • ??????????,???????????????????????????????????????
    ??
  • ??????????????????????????

57
??????(Free Space Loss)
  • ??????(?????????)
  • Pt signal power at transmitting antenna
  • Pr signal power at receiving antenna
  • ? carrier wavelength
  • d propagation distance between antennas
  • c speed of light ( 3 10 8 m/s)
  • where d and ? are in the same units (e.g., meters)

58
Free Space Loss
  • Free space loss equation can be recast

59
??????(Free Space Loss)
  • ??????(???????)
  • Gt gain of transmitting antenna
  • Gr gain of receiving antenna
  • At effective area of transmitting antenna
  • Ar effective area of receiving antenna

60
Free Space Loss
  • Free space loss accounting for gain of other
    antennas can be recast as

61
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62
????(Categories of Noise)
  • ???(Thermal Noise)
  • ????(Intermodulation noise)
  • ??(Crosstalk)
  • ????(Impulse Noise)

63
???(Thermal Noise)
  • ????????????????.?????????????????,
    ?????????.??????????????,???????.
  • ?????
  • ????????????????,??????????????????.

64
???(Thermal Noise)
  • ?????????1Hz????????
  • N0 noise power density in watts per 1 Hz of
    bandwidth
  • k Boltzmann's constant 1.3803 10-23 J/K
  • T ??,?????(????)??

??T17 ?290K????,?????? No 1.3803 10-23
290410-21(W/Hz) -240(dbw/hz)
65
???(Thermal Noise)
  • ????????
  • ?B???????????????????
  • or, ?????

66
Noise Terminology
  • ??????????????????????,??????????? f1, f2 ,
    f1f2, f1-f2
  • ?? ?????????
  • ????????????????????,???ISM???,????????
  • ???? ???????????????
  • ??????,????,??????????
  • ??????,??????????

67
Expression Eb/N0
  • Ratio of signal energy per bit to noise power
    density per Hertz
  • The bit error rate for digital data is a function
    of Eb/N0
  • Given a value for Eb/N0 to achieve a desired
    error rate, parameters of this formula can be
    selected
  • As bit rate R increases, transmitted signal power
    must increase to maintain required Eb/N0

68
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69
Other Impairments
  • Atmospheric absorption water vapor and oxygen
    contribute to attenuation
  • Multipath obstacles reflect signals so that
    multiple copies with varying delays are received
  • Refraction bending of radio waves as they
    propagate through the atmosphere

70
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71
2.1.5 Fading
  • Fading refers to the time variation of received
    signal power caused by changes in the
    transmission medium or path (s).

72
Multipath Propagation
  • ??(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 in the order of the
    wavelength of the signal or less

73
??
74
??
75
??
76
Multipath Propagation
77
The Effects of Multipath Propagation
  • Multiple copies of a signal may arrive at
    different phases
  • If phases add destructively, the signal level
    relative to noise declines, making detection more
    difficult
  • Intersymbol interference (ISI)
  • One or more delayed copies of a pulse may arrive
    at the same time as the primary pulse for a
    subsequent bit

78
???????
???????
???????
???????
???????
???????????????????????,????????????????????????.
???????,???????????????????????
79
Types of Fading
  • Fast fading
  • Slow fading
  • Flat fading
  • Selective fading
  • Rayleigh fading
  • Rician fading

80
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81
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82
??????(Error Compensation Mechanisms)
  • ????(Forward error correction)
  • ?????(Adaptive equalization)
  • ????(Diversity techniques)

83
????(Forward Error Correction)
  • ???????????????????????????????????
  • ???????????????????????????????
  • Transmitter adds error-correcting code to data
    block
  • Code is a function of the data bits
  • Receiver calculates error-correcting code from
    incoming data bits
  • If calculated code matches incoming code, no
    error occurred
  • If error-correcting codes dont match, receiver
    attempts to determine bits in error and correct

84
????(Forward Error Correction)
  • ????????????????
  • ????????,??????????????????23??
  • ?????,????????????????????

85
?????( Adaptive Equalization)
  • Can be applied to transmissions that carry analog
    or digital information
  • Analog voice or video
  • Digital data, digitized voice or video
  • Used to combat intersymbol interference
  • Involves gathering dispersed symbol energy back
    into its original time interval
  • Techniques
  • Lumped analog circuits
  • Sophisticated digital signal processing algorithms

86
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87
????( Diversity Techniques)
  • Diversity is based on the fact that individual
    channels experience independent fading events
  • Space diversity techniques involving physical
    transmission path
  • Frequency diversity techniques where the signal
    is spread out over a larger frequency bandwidth
    or carried on multiple frequency carriers
  • Time diversity techniques aimed at spreading
    the data out over time

88
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89
2.2 Signal Encoding Techniques
Reading material 1Chapter 6 in textbook
90
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91
Reasons for Choosing Encoding Techniques
  • Digital data, digital signal
  • Equipment less complex and expensive than
    digital-to-analog modulation equipment
  • Analog data, digital signal
  • Permits use of modern digital transmission and
    switching equipment

92
Reasons for Choosing Encoding Techniques
  • Digital data, analog signal
  • Some transmission media will only propagate
    analog signals
  • E.g., optical fiber and unguided media
  • Analog data, analog signal
  • Analog data in electrical form can be transmitted
    easily and cheaply
  • Done with voice transmission over voice-grade
    lines

93
2.2.1 Signal Encoding Criteria
  • What determines how successful a receiver will be
    in interpreting an incoming signal?
  • Signal-to-noise ratio
  • Data rate
  • Bandwidth
  • An increase in data rate increases bit error rate
  • An increase in SNR decreases bit error rate
  • An increase in bandwidth allows an increase in
    data rate

94
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95
Factors Used to CompareEncoding Schemes
  • Signal spectrum
  • With lack of high-frequency components, less
    bandwidth required
  • With no dc component, ac coupling via transformer
    possible
  • Transfer function of a channel is worse near band
    edges
  • Clocking
  • Ease of determining beginning and end of each bit
    position

96
Factors Used to CompareEncoding Schemes
  • Signal interference and noise immunity
  • Performance in the presence of noise
  • Cost and complexity
  • The higher the signal rate to achieve a given
    data rate, the greater the cost

97
2.2.2 Digital data, analog signals
98
Basic Encoding Techniques
  • Digital data to analog signal
  • Amplitude-shift keying (ASK)
  • Amplitude difference of carrier frequency
  • Frequency-shift keying (FSK)
  • Frequency difference near carrier frequency
  • Phase-shift keying (PSK)
  • Phase of carrier signal shifted

99
Basic Encoding Techniques
100
Amplitude-Shift Keying
  • One binary digit represented by presence of
    carrier, at constant amplitude
  • Other binary digit represented by absence of
    carrier
  • where the carrier signal is Acos(2pfct)

101
Amplitude-Shift Keying
  • Susceptible to sudden gain changes
  • Inefficient modulation technique
  • On voice-grade lines, used up to 1200 bps
  • Used to transmit digital data over optical fiber

102
Binary Frequency-Shift Keying (BFSK)
  • Two binary digits represented by two different
    frequencies near the carrier frequency
  • where f1 and f2 are offset from carrier frequency
    fc by equal but opposite amounts

103
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104
Binary Frequency-Shift Keying (BFSK)
  • Less susceptible to error than ASK
  • On voice-grade lines, used up to 1200bps
  • Used for high-frequency (3 to 30 MHz) radio
    transmission
  • Can be used at higher frequencies on LANs that
    use coaxial cable

105
Multiple Frequency-Shift Keying (MFSK)
  • More than two frequencies are used
  • More bandwidth efficient but more susceptible to
    error
  • f i f c (2i 1 M)f d
  • f c the carrier frequency
  • f d the difference frequency
  • M number of different signal elements 2 L
  • L number of bits per signal element

106
Multiple Frequency-Shift Keying (MFSK)
  • To match data rate of input bit stream, each
    output signal element is held for
  • TsLT seconds
  • where T is the bit period (data rate 1/T)
  • So, one signal element encodes L bits

107
Multiple Frequency-Shift Keying (MFSK)
  • Total bandwidth required
  • 2Mfd
  • Minimum frequency separation required 2fd1/Ts
  • Therefore, modulator requires a bandwidth of
  • Wd2L/LTM/Ts

108
Multiple Frequency-Shift Keying (MFSK)
109
Phase-Shift Keying (PSK)
  • Two-level PSK (BPSK)
  • Uses two phases to represent binary digits

110
Phase-Shift Keying (PSK)
  • Differential PSK (DPSK)
  • Phase shift with reference to previous bit
  • Binary 0 signal burst of same phase as previous
    signal burst
  • Binary 1 signal burst of opposite phase to
    previous signal burst

111
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112
Phase-Shift Keying (PSK)
  • Four-level PSK (QPSK)
  • Each element represents more than one bit

113
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114
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115
Phase-Shift Keying (PSK)
  • Multilevel PSK
  • Using multiple phase angles with each angle
    having more than one amplitude, multiple signals
    elements can be achieved
  • D modulation rate, baud
  • R data rate, bps
  • M number of different signal elements 2L
  • L number of bits per signal element

116
Performance
  • Bandwidth of modulated signal (BT)
  • ASK, PSK BT(1r)R
  • FSK BT2?F(1r)R
  • R bit rate
  • 0 lt r lt 1 related to how signal is filtered
  • ?F f2-fcfc-f1

117
Performance
  • Bandwidth of modulated signal (BT)
  • MPSK
  • MFSK
  • L number of bits encoded per signal element
  • M number of different signal elements

118
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119
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120
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121
Quadrature Amplitude Modulation
  • QAM is a combination of ASK and PSK
  • Two different signals sent simultaneously on the
    same carrier frequency

122
Quadrature Amplitude Modulation
123
2.2.3 Analog data, analog signals
124
Reasons for Analog Modulation
  • Modulation of digital signals
  • When only analog transmission facilities are
    available, digital to analog conversion required
  • Modulation of analog signals
  • A higher frequency may be needed for effective
    transmission
  • Modulation permits frequency division multiplexing

125
Basic Encoding Techniques
  • Analog data to analog signal
  • Amplitude modulation (AM)
  • Angle modulation
  • Frequency modulation (FM)
  • Phase modulation (PM)

126
Amplitude Modulation
  • Amplitude Modulation
  • cos2?fct carrier
  • x(t) input signal
  • na modulation index
  • Ratio of amplitude of input signal to carrier
  • a.k.a (also known as) double sideband transmitted
    carrier (DSBTC)

127
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128
Spectrum of AM signal
129
Amplitude Modulation
  • Transmitted power
  • Pt total transmitted power in s(t)
  • Pc transmitted power in carrier

130
Single Sideband (SSB)
  • Variant of AM is single sideband (SSB)
  • Sends only one sideband
  • Eliminates other sideband and carrier
  • Advantages
  • Only half the bandwidth is required
  • Less power is required
  • Disadvantages
  • Suppressed carrier cant be used for
    synchronization purposes

131
Angle Modulation
  • Angle modulation
  • Phase modulation
  • Phase is proportional to modulating signal
  • np phase modulation index

132
Angle Modulation
  • Frequency modulation
  • Derivative of the phase is proportional to
    modulating signal
  • nf frequency modulation index

133
Angle Modulation
  • Compared to AM, FM and PM result in a signal
    whose bandwidth
  • is also centered at fc
  • but has a magnitude that is much different
  • Angle modulation includes cos(? (t)) which
    produces a wide range of frequencies
  • Thus, FM and PM require greater bandwidth than AM

134
Angle Modulation
  • Carsons rule
  • where
  • The formula for FM becomes

135
2.2.4 Analog data, digital signals
136
Basic Encoding Techniques
  • Analog data to digital signal
  • Pulse code modulation (PCM)
  • Delta modulation (DM)

137
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138
Analog Data to Digital Signal
  • Once analog data have been converted to digital
    signals, the digital data
  • can be transmitted using NRZ-L
  • can be encoded as a digital signal using a code
    other than NRZ-L
  • can be converted to an analog signal, using
    previously discussed techniques

139
Pulse Code Modulation
  • Based on the sampling theorem
  • Each analog sample is assigned a binary code
  • Analog samples are referred to as pulse amplitude
    modulation (PAM) samples
  • The digital signal consists of block of n bits,
    where each n-bit number is the amplitude of a PCM
    pulse

140
Pulse Code Modulation
141
Pulse Code Modulation
  • By quantizing the PAM pulse, original signal is
    only approximated
  • Leads to quantizing noise
  • Signal-to-noise ratio for quantizing noise
  • Thus, each additional bit increases SNR by 6 dB,
    or a factor of 4

142
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143
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144
Delta Modulation
  • Analog input is approximated by staircase
    function
  • Moves up or down by one quantization level (?) at
    each sampling interval
  • The bit stream approximates derivative of analog
    signal (rather than amplitude)
  • 1 is generated if function goes up
  • 0 otherwise

145
Delta Modulation
146
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147
Delta Modulation
  • Two important parameters
  • Size of step assigned to each binary digit (?)
  • Sampling rate
  • Accuracy improved by increasing sampling rate
  • However, this increases the data rate
  • Advantage of DM over PCM is the simplicity of its
    implementation

148
Reasons for Growth of Digital Techniques
  • Growth in popularity of digital techniques for
    sending analog data
  • Repeaters are used instead of amplifiers
  • No additive noise
  • TDM is used instead of FDM
  • No intermodulation noise
  • Conversion to digital signaling allows use of
    more efficient digital switching techniques

149
2.3 Spread Spectrum
Reading material 1Chapter 7 in textbook
150
2.3.1 The Concept of Spread Spectrum
151
??????
152
Spread Spectrum
  • Input is fed into a channel encoder
  • Produces analog signal with narrow bandwidth
  • Signal is further modulated using sequence of
    digits
  • Spreading code or spreading sequence
  • Generated by pseudonoise, or pseudo-random number
    generator
  • Effect of modulation is to increase bandwidth of
    signal to be transmitted

153
Spread Spectrum
  • On receiving end, digit sequence is used to
    demodulate the spread spectrum signal
  • Signal is fed into a channel decoder to recover
    data

154
Spread Spectrum
155
Spread Spectrum
  • What can be gained from apparent waste of
    spectrum?
  • Immunity from various kinds of noise and
    multipath distortion
  • Can be used for hiding and encrypting signals
  • Several users can independently use the same
    higher bandwidth with very little interference

156
?????????(1)
  • ?????????????1941????????Hedy Lamarr ????George
    Antheil????
  • 1949????????????????????????,Derosa?Rogoff????????
    ???????????????,??????New Jersey?California???????
    ??
  • 1950?Basore?????????????NOMACS(Noise Modulation
    and Correlation Detection System)??????????????
  • 1951???,??????????MIT???????????NOMACS??,?????????
    ???????????????????MIT???????
  • 1952??????????P9D?NOMACS ??,???????

157
?????????(2)
  • 1955??????????????,??????????????????,???????Phat
    om(??,??)? Hush-Up(??),???????Blades(??),????
    ?????????
  • 1976??????????????Spread Spectrum Systems???
  • 1978??????????????????(CCIR)??????????????

158
?????????(3)
  • 1982??????????????????????????????,???????????????
    ???,?????????????
  • ???????????????Coherent Spread Spectrum ???
  • 1985??????????????
  • ?????,??????????????????,??????????????????????

159
2.3.2 Frequency Hopping Spread Spectrum
160
Frequency Hoping Spread Spectrum (FHSS)
  • Signal is broadcast over seemingly random series
    of radio frequencies
  • A number of channels allocated for the FH signal
  • Width of each channel corresponds to bandwidth of
    input signal
  • Signal hops from frequency to frequency at fixed
    intervals
  • Transmitter operates in one channel at a time
  • Bits are transmitted using some encoding scheme
  • At each successive interval, a new carrier
    frequency is selected

161
Frequency Hoping Spread Spectrum
  • Channel sequence dictated by spreading code
  • Receiver, hopping between frequencies in
    synchronization with transmitter, picks up
    message
  • Advantages
  • Eavesdroppers hear only unintelligible blips
  • Attempts to jam signal on one frequency succeed
    only at knocking out a few bits

162
Frequency Hoping Spread Spectrum
163
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164
FHSS Using MFSK
  • MFSK signal is translated to a new frequency
    every Tc seconds by modulating the MFSK signal
    with the FHSS carrier signal
  • For data rate of R
  • duration of a bit T 1/R seconds
  • duration of signal element Ts LT seconds
  • Tc ? Ts - slow-frequency-hop spread spectrum
  • Tc lt Ts - fast-frequency-hop spread spectrum

165
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166
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167
FHSS Performance Considerations
  • Large number of frequencies used
  • Results in a system that is quite resistant to
    jamming
  • Jammer must jam all frequencies
  • With fixed power, this reduces the jamming power
    in any one frequency band

168
2.3.3 Direct Sequence Spread Spectrum
169
Direct Sequence Spread Spectrum (DSSS)
  • Each bit in original signal is represented by
    multiple bits in the transmitted signal
  • Spreading code spreads signal across a wider
    frequency band
  • Spread is in direct proportion to number of bits
    used
  • One technique combines digital information stream
    with the spreading code bit stream using
    exclusive-OR (Figure 7.6)

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DSSS Using BPSK
  • Multiply BPSK signal,
  • sd(t) A d(t) cos(2? fct)
  • by c(t) takes values 1, -1 to get
  • s(t) A d(t)c(t) cos(2? fct)
  • A amplitude of signal
  • fc carrier frequency
  • d(t) discrete function 1, -1
  • At receiver, incoming signal multiplied by c(t)
  • Since, c(t) x c(t) 1, incoming signal is
    recovered

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DSSS Using BPSK
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2.3.4 Code Division Multiple Access
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Code-Division Multiple Access (CDMA)
  • Basic Principles of CDMA
  • D rate of data signal
  • Break each bit into k chips
  • Chips are a user-specific fixed pattern
  • Chip data rate of new channel kD

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CDMA Example
  • If k6 and code is a sequence of 1s and -1s
  • For a 1 bit, A sends code as chip pattern
  • ltc1, c2, c3, c4, c5, c6gt
  • For a 0 bit, A sends complement of code
  • lt-c1, -c2, -c3, -c4, -c5, -c6gt
  • Receiver knows senders code and performs
    electronic decode function
  • ltd1, d2, d3, d4, d5, d6gt received chip pattern
  • ltc1, c2, c3, c4, c5, c6gt senders code

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CDMA Example
  • User A code lt1, 1, 1, 1, 1, 1gt
  • To send a 1 bit lt1, 1, 1, 1, 1, 1gt
  • To send a 0 bit lt1, 1, 1, 1, 1, 1gt
  • User B code lt1, 1, 1, 1, 1, 1gt
  • To send a 1 bit lt1, 1, 1, 1, 1, 1gt
  • Receiver receiving with As code
  • (As code) x (received chip pattern)
  • User A 1 bit 6 -gt 1
  • User A 0 bit -6 -gt 0
  • User B 1 bit 0 -gt unwanted signal ignored

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CDMA for Direct Sequence Spread Spectrum
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2.3.5 Generation of Spreading Sequences
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Categories of Spreading Sequences
  • Spreading Sequence Categories
  • PN sequences
  • Orthogonal codes
  • For FHSS systems
  • PN sequences most common
  • For DSSS systems not employing CDMA
  • PN sequences most common
  • For DSSS CDMA systems
  • PN sequences
  • Orthogonal codes

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PN Sequences
  • PN generator produces periodic sequence that
    appears to be random
  • PN Sequences
  • Generated by an algorithm using initial seed
  • Sequence isnt statistically random but will pass
    many test of randomness
  • Sequences referred to as pseudorandom numbers or
    pseudonoise sequences
  • Unless algorithm and seed are known, the sequence
    is impractical to predict

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Important PN Properties
  • Randomness
  • Uniform distribution
  • Balance property
  • Run property
  • Independence
  • Correlation property
  • Unpredictability

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Linear Feedback Shift Register Implementation
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Properties of M-Sequences
  • Property 1
  • Has 2n-1 ones and 2n-1-1 zeros
  • Property 2
  • For a window of length n slid along output for N
    (2n-1) shifts, each n-tuple appears once, except
    for the all zeros sequence
  • Property 3
  • Sequence contains one run of ones, length n
  • One run of zeros, length n-1
  • One run of ones and one run of zeros, length n-2
  • Two runs of ones and two runs of zeros, length
    n-3
  • 2n-3 runs of ones and 2n-3 runs of zeros, length 1

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Properties of M-Sequences
  • Property 4
  • The periodic autocorrelation of a 1
    m-sequence is

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Definitions
  • Correlation
  • The concept of determining how much similarity
    one set of data has with another
  • Range between 1 and 1
  • 1 The second sequence matches the first sequence
  • 0 There is no relation at all between the two
    sequences
  • -1 The two sequences are mirror images
  • Cross correlation
  • The comparison between two sequences from
    different sources rather than a shifted copy of a
    sequence with itself

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Advantages of Cross Correlation
  • The cross correlation between an m-sequence and
    noise is low
  • This property is useful to the receiver in
    filtering out noise
  • The cross correlation between two different
    m-sequences is low
  • This property is useful for CDMA applications
  • Enables a receiver to discriminate among spread
    spectrum signals generated by different
    m-sequences

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Gold Sequences
  • Gold sequences constructed by the XOR of two
    m-sequences with the same clocking
  • Codes have well-defined cross correlation
    properties
  • Only simple circuitry needed to generate large
    number of unique codes
  • In following example (Figure 7.16a) two shift
    registers generate the two m-sequences and these
    are then bitwise XORed

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Orthogonal Codes
  • Orthogonal codes
  • All pairwise cross correlations are zero
  • Fixed- and variable-length codes used in CDMA
    systems
  • For CDMA application, each mobile user uses one
    sequence in the set as a spreading code
  • Provides zero cross correlation among all users
  • Types
  • Welsh codes
  • Variable-Length Orthogonal codes

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Walsh Codes
  • Set of Walsh codes of length n consists of the n
    rows of an n n Walsh matrix
  • W1 (0)
  • n dimension of the matrix
  • Every row is orthogonal to every other row and to
    the logical not of every other row
  • Requires tight synchronization
  • Cross correlation between different shifts of
    Walsh sequences is not zero

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Typical Multiple Spreading Approach
  • Spread data rate by an orthogonal code
    (channelization code)
  • Provides mutual orthogonality among all users in
    the same cell
  • Further spread result by a PN sequence
    (scrambling code)
  • Provides mutual randomness (low cross
    correlation) between users in different cells

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2.4 Coding and Error Control
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Coping with Data Transmission Errors
  • Error detection codes
  • Detects the presence of an error
  • Automatic repeat request (ARQ) protocols
  • Block of data with error is discarded
  • Transmitter retransmits that block of data
  • Error correction codes, or forward correction
    codes (FEC)
  • Designed to detect and correct errors

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2.4.1 Error Detection
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Error Detection Probabilities
  • Definitions
  • Pb Probability of single bit error (BER)
  • P1 Probability that a frame arrives with no bit
    errors
  • P2 While using error detection, the probability
    that a frame arrives with one or more undetected
    errors
  • P3 While using error detection, the probability
    that a frame arrives with one or more detected
    bit errors but no undetected bit errors

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Error Detection Probabilities
  • With no error detection
  • F Number of bits per frame

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Error Detection Process
  • Transmitter
  • For a given frame, an error-detecting code (check
    bits) is calculated from data bits
  • Check bits are appended to data bits
  • Receiver
  • Separates incoming frame into data bits and check
    bits
  • Calculates check bits from received data bits
  • Compares calculated check bits against received
    check bits
  • Detected error occurs if mismatch

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Error Detection Process
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Parity Check
  • Parity bit appended to a block of data
  • Even parity
  • Added bit ensures an even number of 1s
  • Odd parity
  • Added bit ensures an odd number of 1s
  • Example, 7-bit character 1110001
  • Even parity 11100010
  • Odd parity 11100011

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Cyclic Redundancy Check (CRC)
  • Transmitter
  • For a k-bit block, transmitter generates an
    (n-k)-bit frame check sequence (FCS)
  • Resulting frame of n bits is exactly divisible by
    predetermined number
  • Receiver
  • Divides incoming frame by predetermined number
  • If no remainder, assumes no error

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CRC using Modulo 2 Arithmetic
  • Exclusive-OR (XOR) operation
  • Parameters
  • T n-bit frame to be transmitted
  • D k-bit block of data the first k bits of T
  • F (n k)-bit FCS the last (n k) bits of T
  • P pattern of nk1 bits this is the
    predetermined divisor
  • Q Quotient
  • R Remainder

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CRC using Modulo 2 Arithmetic
  • For T/P to have no remainder, start with
  • Divide 2n-kD by P gives quotient and remainder
  • Use remainder as FCS

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CRC using Modulo 2 Arithmetic
  • Does R cause T/P have no remainder?
  • Substituting,
  • No remainder, so T is exactly divisible by P

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CRC using Polynomials
  • All values expressed as polynomials
  • Dummy variable X with binary coefficients

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CRC using Polynomials
  • Widely used versions of P(X)
  • CRC12
  • X12 X11 X3 X2 X 1
  • CRC16
  • X16 X15 X2 1
  • CRC CCITT
  • X16 X12 X5 1
  • CRC 32
  • X32 X26 X23 X22 X16 X12 X11 X10
    X8 X7 X5 X4 X2 X 1

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CRC using Digital Logic
  • Dividing circuit consisting of
  • XOR gates
  • Up to n k XOR gates
  • Presence of a gate corresponds to the presence of
    a term in the divisor polynomial P(X)
  • A shift register
  • String of 1-bit storage devices
  • Register contains n k bits, equal to the length
    of the FCS

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Digital Logic CRC
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2.4.2 Block Error Correction Codes
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Wireless Transmission Errors
  • Error detection requires retransmission
  • Detection inadequate for wireless applications
  • Error rate on wireless link can be high, results
    in a large number of retransmissions
  • Long propagation delay compared to transmission
    time

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Block Error Correction Codes
  • Transmitter
  • Forward error correction (FEC) encoder maps each
    k-bit block into an n-bit block codeword
  • Codeword is transmitted analog for wireless
    transmission
  • Receiver
  • Incoming signal is demodulated
  • Block passed through an FEC decoder

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Forward Error Correction Process
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FEC Decoder Outcomes
  • No errors present
  • Codeword produced by decoder matches original
    codeword
  • Decoder detects and corrects bit errors
  • Decoder detects but cannot correct bit errors
    reports uncorrectable error
  • Decoder detects no bit errors, though errors are
    present

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Block Code Principles
  • Hamming distance for 2 n-bit binary sequences,
    the number of different bits
  • E.g., v1011011 v2110001 d(v1, v2)3
  • Redundancy ratio of redundant bits to data bits
  • Code rate ratio of data bits to total bits
  • Coding gain the reduction in the required Eb/N0
    to achieve a specified BER of an error-correcting
    coded system

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Hamming Code
  • Designed to correct single bit errors
  • Family of (n, k) block error-correcting codes
    with parameters
  • Block length n 2m 1
  • Number of data bits k 2m m 1
  • Number of check bits n k m
  • Minimum distance dmin 3
  • Single-error-correcting (SEC) code
  • SEC double-error-detecting (SEC-DED) code

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Hamming Code Process
  • Encoding k data bits (n -k) check bits
  • Decoding compares received (n -k) bits with
    calculated (n -k) bits using XOR
  • Resulting (n -k) bits called syndrome word
  • Syndrome range is between 0 and 2(n-k)-1
  • Each bit of syndrome indicates a match (0) or
    conflict (1) in that bit position

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Cyclic Codes
  • Can be encoded and decoded using linear feedback
    shift registers (LFSRs)
  • For cyclic codes, a valid codeword (c0, c1, ,
    cn-1), shifted right one bit, is also a valid
    codeword (cn-1, c0, , cn-2)
  • Takes fixed-length input (k) and produces
    fixed-length check code (n-k)
  • In contrast, CRC error-detecting code accepts
    arbitrary length input for fixed-length check code

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BCH Codes
  • For positive pair of integers m and t, a (n, k)
    BCH code has parameters
  • Block length n 2m 1
  • Number of check bits n k mt
  • Minimum distancedmin 2t 1
  • Correct combinations of t or fewer errors
  • Flexibility in choice of parameters
  • Block length, code rate

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Reed-Solomon Codes
  • Subclass of nonbinary BCH codes
  • Data processed in chunks of m bits, called
    symbols
  • An (n, k) RS code has parameters
  • Symbol length m bits per symbol
  • Block length n 2m 1 symbols m(2m 1) bits
  • Data length k symbols
  • Size of check code n k 2t symbols m(2t)
    bits
  • Minimum distance dmin 2t 1 symbols

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Block Interleaving
  • Data written to and read from memory in different
    orders
  • Data bits and corresponding check bits are
    interspersed with bits from other blocks
  • At receiver, data are deinterleaved to recover
    original order
  • A burst error that may occur is spread out over a
    number of blocks, making error correction possible

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Block Interleaving
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2.4.3 Convolutional Codes
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Convolutional Codes
  • Generates redundant bits continuously
  • Error checking and correcting carried out
    continuously
  • (n, k, K) code
  • Input processes k bits at a time
  • Output produces n bits for every k input bits
  • K constraint factor
  • k and n generally very small
  • n-bit output of (n, k, K) code depends on
  • Current block of k input bits
  • Previous K-1 blocks of k input bits

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Convolutional Encoder
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Decoding
  • Trellis diagram expanded encoder diagram
  • Viterbi code error correction algorithm
  • Compares received sequence with all possible
    transmitted sequences
  • Algorithm chooses path through trellis whose
    coded sequence differs from received sequence in
    the fewest number of places
  • Once a valid path is selected as the correct
    path, the decoder can recover the input data bits
    from the output code bits

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2.4.4 Automatic Repeat Request
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Automatic Repeat Request
  • Mechanism used in data link control and transport
    protocols
  • Relies on use of an error detection code (such as
    CRC)
  • Flow Control
  • Error Control

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Flow Control
  • Assures that transmitting entity does not
    overwhelm a receiving entity with data
  • Protocols with flow control mechanism allow
    multiple PDUs in transit at the same time
  • PDUs arrive in same order theyre sent
  • Sliding-window flow control
  • Transmitter maintains list (window) of sequence
    numbers allowed to send
  • Receiver maintains list allowed to receive

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Flow Control
  • Reasons for breaking up a block of data before
    transmitting
  • Limited buffer size of receiver
  • Retransmission of PDU due to error requires
    smaller amounts of data to be retransmitted
  • On shared medium, larger PDUs occupy medium for
    extended period, causing delays at other sending
    stations

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Flow Control
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Error Control
  • Mechanisms to detect and correct transmission
    errors
  • Types of errors
  • Lost PDU a PDU fails to arrive
  • Damaged PDU PDU arrives with errors

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Error Control Requirements
  • Error detection
  • Receiver detects errors and discards PDUs
  • Positive acknowledgement
  • Destination returns acknowledgment of received,
    error-free PDUs
  • Retransmission after timeout
  • Source retransmits unacknowledged PDU
  • Negative acknowledgement and retransmission
  • Destination returns negative acknowledgment to
    PDUs in error

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Go-back-N ARQ
  • Acknowledgments
  • RR receive ready (no errors occur)
  • REJ reject (error detected)
  • Contingencies
  • Damaged PDU
  • Damaged RR
  • Damaged REJ

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