Download - 1. Principles of Wireless Communication

8/3/2019 1. Principles of Wireless Communication

1/26

1

1. Principles of wireless communication

Wireless communications:

overview & applications


2/26

2001,

TelindusHigh-TechInstitute

2


Course overview


2. Fixed wireless

3. Wireless LAN

4. Neighbourhood telepoint systems

5. Cellular systems

6. Satellite systems


3/26

3


1. Principles of wireless

communication


4/26

2001,


4


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 (


5/26

2001,


5


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


6/262001,


6


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


7/262001,


7


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


8/262001,


8


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


9/26

2001,


9


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


10/26

2001,


10


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


11/26

2001,


11


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


12/26

2001,


12


Free space propagation

Attenuation of received radio wave in free space (Pr ~ 1/d2)

Free space propagation

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

1 11 21 31 41 51 61 71 81 91 101

Distance d (km)

Relativereceivedpower(dB)


13/26

2001,


13


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)

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

1 11 21 31 41 51 61 71 81 91 101

distance (km)

relativereceivedpow

er(dB)

n=2

n=4


14/26

2001,


14


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


15/26

2001,


15


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


16/26

2001,


16


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


17/26

2001,


17


Multiple access schemes

Frequency Division Time Division

Multiple Access (FDMA) Multiple Access (TDMA)

frequency

time

power

frequency

time

power

M lti l


18/26

2001,


18


Multiple access

Code Division Multiple Access (CDMA)

Example of spread spectrum communication

frequency

timepower

19Modes of transmission


19/26

2001,


19


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


20/26

2001,


20


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


21/26

2001,


21


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


22/26

2001,


22


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


23/26

2001,


23


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


24/26

2001,


24


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


25/26

2001,


25


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


26/26

2001,



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