1. Principles of Wireless Communication
Transcript of 1. Principles of Wireless Communication
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1. Principles of wireless communication
Wireless communications:
overview & applications
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Course overview
1. Principles of wireless communication
2. Fixed wireless
3. Wireless LAN
4. Neighbourhood telepoint systems
5. Cellular systems
6. Satellite systems
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1. Principles of wireless
communication
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History
1896: Guglielmo Marconi
First demonstration of wireless telegraphy
Built on work of Maxwell and Hertz to send and receive Morse Code
Long wave (>> 1 km) transmission, high transmitting power necessary (>200 kW)
1907
Commercial transatlantic connections
Huge base stations (30 100m high antennas)
Beginning of the end for cable-based telegraphy
1920: Marconi discovers shortwave (
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Electromagnetic waves
Electricity can be static
Like what holds a balloon to the wall or makes your hair stand on end
Magnetism can also be static Like a refrigerator magnet
But when they change or move together, they make waves - electromagneticwaves
Radio waves, television waves, and microwaves are all types ofelectromagnetic waves
They only differ from each other in wavelength and frequency
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Electromagnetic waves
Electromagnetic radiation is normally considered to consist of a sine wavewhich has the properties of
Wavelength and frequency
Amplitude
900 MHz for example corresponds to a wavelength of approximately 33 cm
positionamplitudewavelength
Velocity = 300 000 km/sFrequency= wavelength
velocity
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Antenna
Converts time-varying voltage to time-varying propagating electromagneticfield
OR Converts time-varying propagating electromagnetic field to time-varying
voltage
Time varying voltage
Propagatingelectromagnetic fieldproportional to timevarying voltage
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Transmitting information by modulating a carrier
Baseband information (e.g. voice signal) is superimposed on high frequencycarrier
Information is carried by introducing variations in this carrier signal
Binary data
RF Carrier
~
Demodulation
Binary data
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Electromagnetic spectrum
Frequency f Wavelength Band
30 300 Hz 10000 1000 km ELF
3 30 kHz 100 10 km VLF
30 300 kHz 10 1 km LF
300 3000 kHz 1000 100 m MF
3 30 MHz 100 10 m HF
30 300 MHz 10 1 m VHF
300 3000 MHz 100 10 cm UHF
3 30 GHz 10 1 cm SHF
30 300 GHz 10 1 mm EHF
300 3000 GHz 1 0.1 mm
Radiowaves
Microwaves
Mm-waves
Electromagnetic waves do not require a medium, per se
Radio waves can travel through a vacuum, for example, outer space
Mechanical waves (e.g. sound waves), on the other hand, require the presence of amaterial medium in order to transport their energy from one location to another
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Frequency bands behaviour
The atmosphere surrounding the earth attenuates and refracts radio signals
How much depends on the frequency
As a general rule, the lower the frequency, the less the attenuation, or loss of signal
Below 300 kHz radio waves follow the curvature of the earth for greatdistances.
This type of propagation is called a ground wave
Radio communications over distances up to several thousand kilometres arepossible
Above 300 kHz to about 30 MHz, the ionosphere will sometimes reflect and/orrefract the radio signals
This type of propagation is called sky waves
When returned to earth, they are received hundreds or even thousands of miles
away
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Frequency bands behaviour
In rural areas, VHF signals have the best range because they tend to followthe curvature of the earth
UHF and 800/900 MHz bands are general most suited for urban usage
More bandwidth is available at these higher frequencies
Smaller range makes frequency reuse possible used in cellular systems
Microwave transmissions operate in the 2-40 GHz range
Microwaves can be used for highly directional transmission
Long-haul telecommunications, in the TV and radio broadcasting, satellite and
space communications, and even for specialized LANs
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Free space propagation
Attenuation of received radio wave in free space (Pr ~ 1/d2)
Free space propagation
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Distance d (km)
Relativereceivedpower(dB)
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Attenuation in mobile propagation paths
Received signal loss does not decrease with the square of the distance, butwith higer exponent values
Decay with distance d-n (n=2..4)
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distance (km)
relativereceivedpow
er(dB)
n=2
n=4
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Antenna directivity
Ability of an antenna to
Focus energy in a particular direction when transmitting
Receive energy better from a particular direction when receiving
Directional antennas focus energy in a particular direction
As the frequencies go higher and the wavelengths get shorter, antennas canmore easily focus the energy
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Multipath propagation
At high frequencies, the radio propagation resembles the propagation of light
Wavelength @ 900 MHz:
30 cm
direct path(line-of-sight)
reflections
Direct
Reflected
Total
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Typical signal in a channel with fading
Diversity may be used to overcome adverse effects of fading
E.g. space diversity: two antennas separated by at least half a wavelength
M lti l h
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Multiple access schemes
Frequency Division Time Division
Multiple Access (FDMA) Multiple Access (TDMA)
frequency
time
power
frequency
time
power
M lti l
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Multiple access
Code Division Multiple Access (CDMA)
Example of spread spectrum communication
frequency
timepower
19Modes of transmission
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Modes of transmission
Simplex = messages go in one direction only (e.g. pagers)
Half duplex = two way transmission but only one direction at a time (e.g., pushto talk radios)
Full duplex = two way simultaneous transmission (e.g. cellular phones)
20Full duplex transmission methods
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Full duplex transmission methods
Frequency division duplex (FDD)
Time division duplex (TDD)
Data must be digitized (no analog voice)
Guard times must be used to account for variable propagation delays
Asymmetric allocations possible
uplink downlink
Powerdensity
frequency
FDD
time
Powerdensity
TDD
21Standardisation bodies
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Standardisation bodies
Worldwide
ITU International Telecommunications Union
Europe
ETSI European telecommunications Standardisation Institute
Belgium
BIPT Belgian Institute of Postal services and Telecommunications
IEEE Institute of Electrical and Electronics Engineers
22BIPT - Frequency management
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BIPT - Frequency management
Managing the Belgian radio-electric spectrum
Ensuring the correct application of the various international agreements
signed by Belgium concerning the use of the electro-magnetic spectrum Co-ordination of works with a series of international entities
ARFA (NATO), UIT-R, ERC, ...
Exclusive allocation frequencies and common, collective and temporaryallocations in band 29.7 at 960 MHz for fixed service and terrestrial mobile
service Co-ordination of the frequencies
for satellite links (terrestrial stations, networks, etc.)
as well as by hertzian beams
Management of the maritime mobile service frequencies
23Mobile wireless
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Mobile wireless
Wireless versus Mobile
Both terms tend to be used interchangeably, but are not the same
Fixed wireless
Mobility
Network needs to know where the users are located
Degrees of mobility
Cordless phones have low mobility Cellular have higher mobility
24Wireless systems
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Wireless systems
Cordless systems
Low power, low mobility (low range and low speed)
Provide higher quality of speech than cellular systems, up to that achieved with fixed
networks
Cordless standards primarily offer an access technology rather than fully specified
networks
Cellular systems
High mobility (high speed and wide-range coverage), two-way voice
communications
Transmitter power generally on the order of 100 times that of cordless telephones
25Mobility - Throughput paradigm
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1. Principles of wireless communication
Fixed wireless(chapter 2)
Satellitesystems (chapter 6)
Wireless LAN(chapter 3)
Mobility
throughput
Cellularsystems
(chapter 5)
10kb/s 100k/s 10Mb/s 100Mb/s
Global
Rural
Urban
Indoor
1Mb/s
Cordlesssystems (chapter 4)
26Wireless technologies overview
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g
PANPersonal
Area Network
LANLocal
Area Network
MANMetropolitan
Area Network
WANWide
Area Network
Bluetooth 802.11b802.11a802.11g
802.11MMDSLMDS
GSMGPRSUMTS
Low data rates Higher rates Higher rates Lower rates
Short distances Medium distances Med-longerdistances
Longer distances
Cable replacement (Inter)networkconnectivity
Fixed, last mileaccess
PDA devices andhandhelds