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Transcript
```CSE 6806:
Wireless and Mobile Communication
Networks
1
The physical layer
for
Mobile Communications
2
Electromagnetic Signal
• Function of time
• Can also be expressed as a function of
frequency
– Signal consists of components of different
frequencies
3
Time-Domain Concepts
• Analog signal - signal intensity varies in a smooth
fashion over time
– No breaks or discontinuities in the signal
• Digital signal - signal intensity maintains a
constant level for some period of time and then
changes to another constant level
• Periodic signal - analog or digital signal pattern
that repeats over time
–
s(t +T ) = s(t )
-ɑ ≤ t ≤ +ɑ
• where T is the period of the signal
4
Analog and Digital Signal
5
Types of signals
(a) continuous time/discrete time
(b) continuous values/discrete values
– analog signal = continuous time, continuous values
– digital signal = discrete time, discrete values
• Signal parameters of periodic signals:
period T, frequency f=1/T, amplitude A, phase shift 
– sine wave as special periodic signal for a carrier:
s(t) = At sin(2  ft t + t)
6
Time-Domain Concepts
• Aperiodic signal - analog or digital signal
pattern that doesn't repeat over time
• Peak amplitude (A) - maximum value or
strength of the signal over time; typically
measured in volts
• Frequency (f )
– Rate, in cycles per second, or Hertz (Hz) at
which the signal repeats
7
Time-Domain Concepts
• Period (T ) - amount of time it takes for one
repetition of the signal
– T = 1/f
• Phase () - measure of the relative position in time
within a single period of a signal
• Wavelength () - distance occupied by a single
cycle of the signal
– Or, the distance between two points of corresponding
phase of two consecutive cycles
8
Sine Wave Parameters
• General sine wave
– s(t ) = A sin(2ft + )
• The effect of varying each of the three parameters
(see next slide)
–
–
–
–
(a) A = 1, f = 1 Hz,  = 0; thus T = 1s
(b) Reduced peak amplitude; A=0.5
(c) Increased frequency; f = 2, thus T = ½
(d) Phase shift;  = /4 radians (45 degrees)
• note: 2 radians = 360° = 1 period
9
Sine Wave Parameters
10
Frequency-Domain Concepts
• Any electromagnetic signal can be shown to
consist of a collection of periodic analog
signals (sine waves) at different amplitudes,
frequencies, and phases
• The period of the total signal is equal to the
period of the fundamental frequency
11
The underlying mathematics
Fourier representation of periodic signals


1
g (t )  c   an sin( 2nft)   bn cos( 2nft)
2
n 1
n 1
1
1
0
0
t
ideal periodic signal
t
real composition
(based on harmonics)
12
Freq. components of Square Wave
13
Freq. components of Square Wave
14
Frequency domain
• Fundamental frequency: when all frequency
components of a signal are integer multiples of one
frequency, it is the fundamental frequency
• Spectrum - range of frequencies that a signal
contains
• Absolute bandwidth - width of the spectrum of a
signal
• Effective bandwidth - narrow band of frequencies that
most of the signal’s energy is contained in
15
Data Rate and Bandwidth
• The greater the bandwidth, the higher the
information-carrying capacity
• Conclusions
– Any digital waveform will have infinite bandwidth
– BUT the transmission system will limit the bandwidth
that can be transmitted
– AND, for any given medium, the greater the bandwidth
transmitted, the greater the cost
– HOWEVER, limiting the bandwidth creates distortions
16
Data Communication Terms
• Data - entities that convey meaning, or
information
• Signals - electric or electromagnetic
representations of data
• Transmission - communication of data by
the propagation and processing of signals
17
Analog and Digital Data
• Analog
– Video
– Audio
• Digital
– Text
– Integers
18
Analog Signals
• A continuously varying electromagnetic wave that
may be propagated over a variety of media,
depending on frequency
• Examples of media:
– Copper wire media (twisted pair and coaxial cable)
– Fiber optic cable
– Atmosphere or space propagation
• Analog signals can propagate analog and digital
data
19
Digital Signals
• A sequence of voltage pulses that may be
transmitted over a copper wire medium
• Generally cheaper than analog signaling
• Less susceptible to noise interference
• Suffer more from attenuation
• Digital signals can propagate analog and
digital data
20
Analog Signaling
21
Digital Signaling
22
Reasons for Choosing Data and
Signal Combinations
• Digital data, digital signal
– Equipment for encoding is less expensive than digital-toanalog equipment
• Analog data, digital signal
– Conversion permits use of modern digital transmission and
switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog
signals
– Examples include optical fiber and satellite
• Analog data, analog signal
– Analog data easily converted to analog signal
23
Analog Transmission
• Transmit analog signals without regard to
content
• Attenuation limits length of transmission
• Cascaded amplifiers boost signal’s energy
for longer distances but cause distortion
– Analog data can tolerate distortion
– Introduces errors in digital data
24
Digital Transmission
• Concerned with the content of the signal
• Attenuation endangers integrity of data
• Digital Signal
– Repeaters achieve greater distance
– Repeaters recover the signal and retransmit
• Analog signal carrying digital data
– Retransmission device recovers the digital data from
analog signal
– Generates new, clean analog signal
25
• Impairments, such as noise, limit data rate
that can be achieved
• For digital data, to what extent do
impairments limit data rate?
• Channel Capacity – the maximum rate at
which data can be transmitted over a given
communication path, or channel, under
given conditions
26
Signals, channels and systems
• What is a signal?
– Modulation
– Bandwidth
– Transmission/reception
• What is a channel?
– Bandwidth
– Noise
– Loss?
• What is a communication system?
27
28
Bandwidth
• Of a signal
• Of a channel
29
Bit rates, channel capacity
• Noise limits data rate that can be achieved
• For digital data, to what extent do these
impairments limit data rate?
• Channel Capacity: The maximum rate at
which data can be transmitted over a given
communication path, or channel, under given
conditions
30
Nyquist Bandwidth
• For binary signals (two voltage levels)
C = 2B
(in bps)
• Data rate = B Hz
• With multilevel signaling
C = 2B log2 M
(in bps)
• M = number of discrete signal or voltage levels
31
Signal-to-Noise Ratio
• Ratio of the power in a signal to the power contained in
the noise that is present at a particular point in the
transmission
• Typically measured at a receiver
signal power
( SNR) dB  10 log 10
noise power
• A high SNR means a high-quality signal, low number of
required intermediate repeaters
• SNR sets upper bound on achievable data rate
32
Shannon Capacity Formula
• Equation:
C  B log 2 1  SNR
• Represents theoretical maximum that can be achieved
• In practice, only much lower rates achieved
– Formula assumes white noise (thermal noise)
– Impulse noise is not accounted for
– Attenuation distortion or delay distortion not accounted for
33
Example of Nyquist and Shannon
Formulations
• Spectrum of a channel between 3 MHz and 4
MHz ; SNRdB = 24 dB
B  4 MHz  3 MHz  1 MHz
SNR dB  24 dB  10 log 10 SNR 
SNR  251
• Using Shannon’s formula, Channel capacity
C  10  log 2 1  251  10  8  8Mbps
6
6
34
Example of Nyquist and Shannon
Formulations
• How many signaling levels are required?
C  2 B log 2 M
 
8 10  2  10  log 2 M
6
6
4  log 2 M
M  16
35
Classifications of Transmission
Media
• 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
Unguided Media
• Transmission and reception are achieved by
means of an antenna
• Configurations for wireless transmission
– Directional
– Omnidirectional
General Frequency Ranges
• Microwave frequency range
–
–
–
–
1 GHz to 100 GHz
Directional beams possible
Suitable for point-to-point transmission
Used for satellite communications
– 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
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
• Alternative to Coaxial cable or optical fiber
– Short point-to-point links between buildings
• CCTV
Satellite Microwave
• Description of communication satellite
– Microwave relay station
– Used to link two or more ground-based microwave
amplifies or repeats the signal, and transmits it on
• Applications
– Television distribution
– Long-distance telephone transmission
– Omnidirectional
– Antennas not required to be dish-shaped
– Antennas need not be rigidly mounted to a precise
alignment
• Applications
• VHF and part of the UHF band; 30 MHZ to 1GHz
• Covers FM radio and UHF and VHF television
Frequencies for wireless communication
•
•
•
VLF = Very Low Frequency
LF = Low Frequency
MF = Medium Frequency
UHF = Ultra High Frequency
SHF = Super High Frequency
EHF = Extra High Frequency
•
•
HF = High Frequency
VHF = Very High Frequency
UV = Ultraviolet Light
• Frequency and wave length
–  = c/f
– wave length , speed of light c  3x108 m/s, frequency f
twisted
pair
coax cable
1 Mm
300 Hz
10 km
30 kHz
VLF
LF
optical transmission
100 m
3 MHz
MF
HF
1m
300 MHz
VHF
UHF
10 mm
30 GHz
SHF
EHF
100 m
3 THz
infrared
1 m
300 THz
visible light UV
42
Modulation
• Why?
• How?
43
Multiplexing
• Multiplexing in 4 dimensions
–
–
–
–
space (si)
time (t)
frequency (f)
code (c)
• Goal: multiple use
of a shared medium
channels ki
k1
k2
k3
k4
k5
k6
c
t
c
t
s1
f
s2
f
c
t
• Important: guard spaces needed!
s3
f
44
Frequency multiplexing
• Separation of the whole spectrum into smaller
frequency bands
• A channel gets a certain band of the spectrum for the
whole time
– no dynamic coordination
necessary
– works also for analog signals
k1
k2
k3
k4
k5
k6
c
f
– waste of bandwidth
if the traffic is
distributed
unevenly
– inflexible
t
45
Time division multiplexing
• A channel gets the whole spectrum for a certain
amount of time
– only one carrier in the
medium at any time
– throughput high even
for many users
k1
k2
k3
k4
k5
k6
c
f
– precise
synchronization
necessary
t
46
Time and frequency multiplex
• Combination of both TDM and FDM
• A channel gets a certain frequency band for a certain
amount of time
• Example: GSM
k1
– better protection against
tapping
– protection against frequency
selective interference
k2
k3
k4
k5
k6
c
f
• but: precise coordination
required
t
47
Code multiplex
• Each channel has a unique code
k1
k2
• All channels use the same spectrum
at the same time
k3
k4
k5
k6
c
– bandwidth efficient
– no coordination and synchronization
necessary
– good protection against interference
and tapping
f
– varying user data rates
– more complex signal regeneration
t
48
Example
• Lack of coordination requirement is an
49
Modulation
• Digital modulation
– digital data is translated into an analog signal (baseband)
– differences in spectral efficiency, power efficiency, robustness
• Analog modulation
– shifts center frequency of baseband signal up to the radio carrier
• Motivation
– smaller antennas (e.g., /4)
– Frequency Division Multiplexing
– medium characteristics
• Basic schemes
– Amplitude Modulation (AM)
– Frequency Modulation (FM)
– Phase Modulation (PM)
50
Modulation and demodulation
digital
data
101101001
digital
modulation
analog
baseband
signal
analog
modulation
carrier
analog
demodulation
analog
baseband
signal
synchronization
decision
digital
data
101101001
carrier
51
Digital modulation
• Modulation of digital signals known as Shift
Keying
1
0
1
– very simple
– low bandwidth requirements
– very susceptible to interference
t
1
0
1
• Frequency Shift Keying (FSK):
– needs larger bandwidth
• Phase Shift Keying (PSK):
– more complex
– robust against interference
t
1
0
1
t
52
Signal propagation basics
Many different effects have to be
considered
53
Signal propagation ranges
• Transmission range
– communication possible
– low error rate
• Detection range
– detection of the signal
possible
– no communication
possible
• Interference range
– signal may not be
detected
background noise
sender
transmission
distance
detection
interference
54
Signal propagation
• Propagation in free space always like light (straight line)
• Receiving power proportional to 1/d² in vacuum – much more in real
environments (d = distance between sender and receiver)
• Receiving power additionally influenced by
– reflection at large obstacles
– refraction depending on the density of a medium
– scattering at small obstacles
– diffraction at edges
reflection
refraction
scattering
diffraction
55
Multipath Propagation
56
Multipath propagation
• Signal can take many different paths between sender
and receiver due to reflection, scattering, diffraction
multipath
LOS pulses pulses
signal at sender
• Time dispersion: signal is dispersed over time
• The signal reaches a receiver directly and phase
shifted
– distorted signal depending on the phases of the different
parts
57
Atmospheric absorption
• Water vapor and oxygen contribute most
• Water vapor: peak attenuation near 22GHz, low
below 15Ghz
• Oxygen: absorption peak near 60GHz, lower
below 30 GHz.
• Rain and fog may scatter (thus attenuate) radio
waves.
• Low frequency band usage helps…
58
Propagation Modes
• Ground-wave propagation
• Sky-wave propagation
• Line-of-sight propagation
59
Ground Wave Propagation
60
Ground Wave Propagation
•
•
•
•
Follows contour of the earth
Can Propagate considerable distances
Frequencies up to 2 MHz
Example
61
Sky Wave Propagation
62
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
earth’s surface
• Reflection effect caused by refraction
• Examples
63
Line-of-Sight Propagation
64
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
65
Line-of-Sight Equations
• Optical line of sight
d  3.57 h
• Effective, or radio, line of sight
d  3.57 h
• d = distance between antenna and horizon
(km)
• h = antenna height (m)
• K = adjustment factor to account for
refraction, rule of thumb K = 4/3
66
Line-of-Sight Equations
• Maximum distance between two
antennas for LOS propagation:

3.57 h1  h2

• h1 = height of antenna one
• h2 = height of antenna two
67
LOS Wireless Transmission
Impairments
•
•
•
•
Attenuation and attenuation distortion
Free space loss
Atmospheric absorption
Multipath (diffraction, reflection,
refraction…)
• Noise
• Thermal noise
68
Attenuation
• Strength of signal falls off with distance over
transmission medium
• Attenuation factors for unguided media:
– Received signal must have sufficient strength so
that circuitry in the receiver can interpret the signal
– Signal must maintain a level sufficiently higher
than noise to be received without error
– Attenuation is greater at higher frequencies,
causing distortion
69
Free Space Loss
• Free space loss, ideal isotropic antenna

Pt 4d 
4fd 


2
2
Pr

c
2
2
• 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)
70
Free Space Loss
• Free space loss equation can be recast:
Pt
 4d 
LdB  10 log  20 log 

Pr
  
 20 log    20 log d   21.98 dB
 4fd 
 20 log 
  20 log  f   20 log d   147.56 dB
 c 
71
• What?
• Why?
• How?
72
channel
quality
1
2
5
3
6
narrowband channels
4
frequency
narrow band
signal
guard space
channel
quality
1
spectrum
2
2
2
2
2
frequency
73
74
• Input is fed into a channel encoder
– Produces analog signal with narrow
bandwidth
• Signal is further modulated using
sequence of digits
– Generated by pseudonoise, or pseudorandom number generator
• Effect of modulation is to increase
bandwidth of signal to be transmitted
75
• At the receiving end, digit sequence is used
to demodulate the spread spectrum signal
• Signal is fed into a channel decoder to
recover data
76
(DSSS)
• Each bit in original signal is represented
by multiple bits in the transmitted signal
wider frequency band
– Spread is in direct proportion to number of
bits used
• One technique combines digital
code bit stream using exclusive-OR
77
DSSS illustration
DSSS Using BPSK
Spectrum (FHSS)
• Signal is broadcast over seemingly random
– 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
80
FHSS - contd
• Channel sequence dictated by spreading
code
• Receiver, hopping between frequencies in
synchronization with transmitter, picks up
message
– Eavesdroppers hear only unintelligible blips
– Attempts to jam signal on one frequency
succeed only at knocking out a few bits
81
FHSS - illustration
82
FHSS details - 1
• Discrete changes of carrier frequency
– sequence of frequency changes determined via pseudo random
number sequence
• Two versions
– Fast Hopping:
several frequencies per user bit
– Slow Hopping:
several user bits per frequency
– frequency selective fading and interference limited to short period
– simple implementation
– uses only small portion of spectrum at any time
– not as robust as DSSS
– simpler to detect
83
FHSS - iIIustration
tb
user data
0
1
f
0
1
1
t
td
f3
slow
hopping
(3 bits/hop)
f2
f1
f
t
td
f3
fast
hopping
(3 hops/bit)
f2
f1
t
tb: bit period
td: dwell time
84
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
85
FHSS and Retransmissions
• What happens when a packet is corrupt
and has to be retransmitted?
• IEEE 802.11: max time of each hop:
400ms, max packet length: 30 ms.
86
FHSS and WLAN access points
• IEEE 802.11 FHSS WLAN specifies 78
hopping channels separated by 1 MHz in 3
groups
• (0,3,6,9,…, 75), (1,4,7,…, 76), (2,5,8,…,77)
• Allows installation of 3 AP’s in the same
area.
87
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
88
CDMA Example
• If k=6 and code is a sequence of 1s and -1s
– For a ‘1’ bit, A sends code as chip pattern
• <c1, c2, c3, c4, c5, c6>
– For a ‘0’ bit, A sends complement of code
• <-c1, -c2, -c3, -c4, -c5, -c6>
• Receiver knows sender’s code and performs
electronic decode function
Su d   d1 c1  d 2  c2  d 3  c3  d 4  c4  d 5  c5  d 6  c6
• <d1, d2, d3, d4, d5, d6> = received chip pattern
• <c1, c2, c3, c4, c5, c6> = sender’s code
89
CDMA Example
• User A code = <1, –1, –1, 1, –1, 1>
– To send a 1 bit = <1, –1, –1, 1, –1, 1>
– To send a 0 bit = <–1, 1, 1, –1, 1, –1>
• User B code = <1, 1, –1, – 1, 1, 1>
– To send a 1 bit = <1, 1, –1, –1, 1, 1>
• Receiver receiving with A’s code
– (A’s code) x (received chip pattern)
• User A ‘1’ bit: 6 -> 1
• User A ‘0’ bit: -6 -> 0
• User B ‘1’ bit: 0 -> unwanted signal ignored
90
CDMA for DSSS
```
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