Transcript Wireless Communications and Networks

Wireless Communication
Fundamentals
Elements of a wireless network
network
infrastructure
wireless hosts
r laptop, PDA, IP phone
r run applications
r may be stationary (nonmobile) or mobile
m
wireless does not
always mean
mobility
Elements of a wireless network
network
infrastructure
base station
r typically connected to
wired network
r relay - responsible for
sending packets
between wired
network and wireless
host(s) in its “area”
m e.g., cell towers,
802.11 access
points
Elements of a wireless network
network
infrastructure
wireless link
r typically used to
connect mobile(s) to
base station
r also used as backbone
link
r multiple access
protocol coordinates
link access
r various data rates,
transmission distance
Characteristics of selected wireless link
standards
Data rate (Mbps)
200
54
5-11
4
1
802.11n
802.11a,g
802.11a,g point-to-point
802.11b
data
802.16 (WiMAX)
UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
3G cellular
enhanced
802.15
.384
.056
UMTS/WCDMA, CDMA2000
3G
2G
IS-95, CDMA, GSM
Indoor
Outdoor
10-30m
50-200m
Mid-range Long-range
outdoor
outdoor
200m – 4 Km
5Km – 20 Km
Elements of a wireless network
network
infrastructure
infrastructure mode
r base station connects
mobiles into wired
network
r handoff: mobile
changes base station
providing connection
into wired network
Elements of a wireless network
ad hoc mode
r no base stations
r nodes can only
transmit to other
nodes within link
coverage
r nodes organize
themselves into a
network: route among
themselves
Wireless network taxonomy
single hop
infrastructure
(e.g., APs)
no
infrastructure
host connects to
base station (WiFi,
WiMAX, cellular)
which connects to
larger Internet
no base station, no
connection to larger
Internet (Bluetooth,
ad hoc nets)
multiple hops
host may have to
relay through several
wireless nodes to
connect to larger
Internet: mesh net
no base station, no
connection to larger
Internet. May have to
relay to reach other
a given wireless node
MANET, VANET
Wireless Link Characteristics (1)
Differences from wired link ….
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decreased signal strength: radio signal attenuates
as it propagates through matter (path loss)
interference from other sources: standardized
wireless network frequencies (e.g., 2.4 GHz)
shared by other devices (e.g., phone); devices
(motors) interfere as well
multipath propagation: radio signal reflects off
objects ground, arriving ad destination at slightly
different times
…. make communication across (even a point to point)
wireless link much more “difficult”
Wireless Link Characteristics (2)
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SNR: signal-to-noise ratio
 larger SNR – easier to extract
signal from noise (a “good
thing”)
SNR versus BER tradeoffs
 given physical layer: increase

power -> increase SNR>decrease BER
given SNR: choose physical
layer that meets BER
requirement, giving highest
thruput
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SNR may change with
mobility: dynamically adapt
physical layer (modulation
technique, rate)
10-1
10-2
10-3
BER
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10-4
10-5
10-6
10-7
10
20
30
SNR(dB)
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
40
Wireless network characteristics (3)
Multiple wireless senders and receivers create additional
problems (beyond multiple access):
A
B
B, A hear each other
r B, C hear each other
r A, C can not hear each other
means A, C unaware of their
interference at B
C
C’s signal
strength
A’s signal
strength
Hidden terminal problem
r
B
A
C
space
Signal attenuation:
r
r
r
B, A hear each other
B, C hear each other
A, C can not hear each other
interfering at B
Limitations and Difficulties of
Wireless Technologies
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Wireless is convenient and less expensive
Limitations and political and technical difficulties
inhibit wireless technologies
Lack of an industry-wide standard
Device limitations
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E.g., small LCD on a mobile telephone can only
displaying a few lines of text
E.g., browsers of most mobile wireless devices use
wireless markup language (WML) instead of HTML
Electromagnetic Signal
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Function of time
Can also be expressed as a function of
frequency
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Signal consists of components of different
frequencies
Time-Domain Concepts
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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
Frequency-Domain Concepts
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Fundamental frequency - when all frequency components of
a signal are integer multiples of one frequency, it’s referred to
as the fundamental frequency
Spectrum - range of frequencies that a signal contains
Absolute bandwidth - width of the spectrum of a signal
Effective bandwidth (or just bandwidth) - narrow band of
frequencies that most of the signal’s energy is contained in
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
Data Communication Terms
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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
About Channel Capacity
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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
Concepts Related to Channel
Capacity
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Data rate - rate at which data can be
communicated (bps)
Bandwidth - the bandwidth of the transmitted
signal as constrained by the transmitter and the
nature of the transmission medium (Hertz)
Noise - average level of noise over the
communications path
Error rate - rate at which errors occur
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Error = transmit 1 and receive 0; transmit 0 and receive 1
Nyquist Bandwidth
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For binary signals (two voltage levels)
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C = 2B
With multilevel signaling
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C = 2B log2 M
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M = number of discrete signal or voltage levels
Signal-to-Noise Ratio
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Ratio of the power in a signal to the power
contained in the noise that’s present at a particular
point in the transmission
Typically measured at a receiver
Signal-to-noise ratio (SNR, or S/N)
( SNR) dB
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signal power
 10 log10
noise power
A high SNR means a high-quality signal, low
number of required intermediate repeaters
SNR sets upper bound on achievable data rate
Shannon Capacity Formula
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Equation:
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Represents theoretical maximum that can be
achieved
In practice, only much lower rates achieved
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C  B log2 1  SNR 
Formula assumes white noise (thermal noise)
Impulse noise is not accounted for
Attenuation distortion or delay distortion not accounted
for
Example of Nyquist and Shannon
Formulations
Multiplexing
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Capacity of transmission medium usually exceeds capacity
required for transmission of a single signal
Multiplexing - carrying multiple signals on a single
medium
 More efficient use of transmission medium
Multiplexing Techniques
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Frequency-division multiplexing (FDM)
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Takes advantage of the fact that the useful bandwidth of the
medium exceeds the required bandwidth of a given signal
Time-division multiplexing (TDM)
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Takes advantage of the fact that the achievable bit rate of the
medium exceeds the required data rate of a digital signal
Communication Networks
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Comparison of basic communication
network technologies
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Circuit switching
Packet switching
Switching Terms
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Switching Nodes:
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Stations:
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Intermediate switching device that moves data
Not concerned with content of data
End devices that wish to communicate
Each station is connected to a switching node
Communications Network:
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A collection of switching nodes
Switched Network
Techniques Used in Switched
Networks
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Circuit switching
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Dedicated communications path between two
stations
E.g., public telephone network
Packet switching
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Message is broken into a series of packets
Each node determines next leg of transmission
for each packet
Phases of Circuit Switching
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Circuit establishment
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Information Transfer
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An end to end circuit is established through switching
nodes
Information transmitted through the network
Data may be analog voice, digitized voice, or binary
data
Circuit disconnect
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Circuit is terminated
Each node deallocates dedicated resources
Characteristics of Circuit
Switching
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Can be inefficient
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Channel capacity dedicated for duration of connection
Utilization not 100%
Delay prior to signal transfer for establishment
Once established, network is transparent to users
Information transmitted at fixed data rate with
only propagation delay
How Packet Switching Works
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Data is transmitted in blocks, called packets
Before sending, the message is broken into
a series of packets
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Typical packet length is 1000 octets (bytes)
Packets consists of a portion of data plus a
packet header that includes control information
At each node en route, packet is received,
stored briefly and passed to the next node
Packet Switching
Packet Switching
Packet Switching Advantages
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Line efficiency is greater
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Packet-switching networks can carry out data-rate
conversion
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Many packets over time can dynamically share the
same node to node link
Two stations with different data rates can exchange
information
Unlike circuit-switching networks that block calls
when traffic is heavy, packet-switching still
accepts packets, but with increased delivery delay
Priorities can be used
Disadvantages of Packet
Switching
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Each packet switching node introduces a delay
Overall packet delay can vary substantially
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Each packet requires overhead information
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This is referred to as jitter
Caused by differing packet sizes, routes taken and
varying delay in the switches
Includes destination and sequencing information
Reduces communication capacity
More processing required at each node
Protocols and the TCP/IP
Protocol Suite
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Protocol architecture
Open systems interconnection (OSI)
reference model
TCP/IP
Antennas and Radio Propagation
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Principles of radio and microwave
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Antenna performance
Wireless transmission modes
Fading
Introduction
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An antenna is an electrical conductor or
system of conductors
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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
Antenna Gain
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Antenna gain
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Power output, in a particular direction,
compared to that produced in any direction by a
perfect omnidirectional antenna (isotropic
antenna)
Effective area
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Related to physical size and shape of antenna
Antenna Gain
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Relationship between antenna gain and effective
area
G
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4Ae
2
4f Ae

c2
2
G = antenna gain
Ae = effective area
f = carrier frequency
c = speed of light (» 3 x 108 m/s)
 = carrier wavelength
Some Examples
LOS Wireless Transmission
Impairments
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Attenuation and attenuation distortion
Free space loss
Noise
Atmospheric absorption
Multipath
Refraction
Thermal noise
Attenuation
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Strength of signal falls off with distance over
transmission medium
Attenuation factors for unguided media:
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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
Free Space Loss
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Free space loss, ideal isotropic antenna
Pt 4d  4fd 
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
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 x 10 8 m/s)
where d and  are in the same units (e.g., meters)
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Free Space Loss
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Free space loss equation can be recast:
Pt
 4d 
LdB  10 log  20 log

Pr
  
 20log   20logd   21.98 dB
 4fd 
 20log
  20log f   20logd   147.56 dB
 c 
Categories of Noise
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Thermal Noise
Intermodulation noise
Crosstalk
Impulse Noise
Thermal Noise
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Thermal noise due to agitation of electrons
Present in all electronic devices and
transmission media
Cannot be eliminated
Function of temperature
Particularly significant for satellite
communication
Noise Terminology
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Intermodulation noise – occurs if signals with
different frequencies share the same medium
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Interference caused by a signal produced at a frequency
that is the sum or difference of original frequencies
Crosstalk – unwanted coupling between signal
paths
Impulse noise – irregular pulses or noise spikes
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Short duration and of relatively high amplitude
Caused by external electromagnetic disturbances, or
faults and flaws in the communications system
Other Impairments
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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
Multipath Propagation
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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
Attributes of Radio Wave Propagation
Three attributes:
1. Reflection
2. Diffraction
3. Scattering
Reflection, Diffraction and Scattering of radio wave
Multipath Propagation
The Effects of Multipath
Propagation
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Multiple copies of a signal may arrive at
different phases
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If phases add destructively, the signal level
relative to noise declines, making detection
more difficult
Intersymbol interference (ISI)
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One or more delayed copies of a pulse may
arrive at the same time as the primary pulse for
a subsequent bit
Types of Fading
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Fast fading
Slow fading
Flat fading
Selective fading
Rayleigh fading
Rician fading
Fading
Fast Fading
(Short-term fading)
Slow Fading
(Long-term fading)
Signal
Strength
(dB)
Path Loss
Distance
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Slow Fading
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Slow fading is caused by movement over
distances large enough to produce gross variations
in the overall path between transmitter and
receiver.
The long-term variation in the mean level is
known as slow fading (shadowing or log-normal
fading). This fading caused by shadowing.
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Doppler Shift
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Doppler Effect: When a wave source and a receiver are moving towards
each other, the frequency of the received signal will not be the same as the
source.
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When they are moving toward each other, the frequency of the received signal
is higher than the source.
When they are opposing each other, the frequency decreases.
Thus, the frequency of the received signal is
f R  fC  f D
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where fC is the frequency of source carrier,
fD is the Doppler frequency.
Doppler Shift in frequency:
fD 
v

cos 
where v is the moving speed,
 is the wavelength of carrier.

Moving
speed v
MS
Signal
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Delay Spread
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When a signal propagates from a transmitter to a
receiver, signal suffers one or more reflections.
This forces signal to follow different paths.
Each path has different path length, so the time of
arrival for each path is different.
This effect which spreads out the signal is called
“Delay Spread”.
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Delay Spread
Signal Strength
The signals from
close by reflectors
The signals from
intermediate reflectors
The signals from
far away reflectors
Delay
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Questions????