Mobile Wireless Communications - Overview - … · 02-10-2014 · Multiple-AccessCellular...

Multiple-Access Cellular Communications Propagation Characteristics Improving Signal Quality Satellite-Mobile Communications Use of Arrays

Mobile Wireless Communications - Overview

S. R. Zinka

[email protected]

October 16, 2014

Mobile Wireless Communications - Overview CT531, DA-IICT

mailto:[email protected]


First of all ...

Which frequencies we can use for wireless communications?



Atmospheric Attenuation of EM Waves

Gamma rays, X-rays and ultravioletlight blocked by the upper atmosphere(best observed from space).

Visible lightobservablefrom Earth,with someatmosphericdistortion.

Most of theinfrared spectrumabsorbed byatmosphericgasses (bestobservedfrom space).

Radio waves observablefrom Earth.

Long-wavelengthradio wavesblocked.

0 %

50 %

100 %

0.1 nm 1 nm 10 nm 100 nm 1 µm 10 µm 100 µm 1 mm 1 cm 10 cm 1 m 10 m 100 m 1 km

Wavelength

Atm

ospheric

opacity



Outline

1 Multiple-Access

2 Cellular Communications

3 Propagation Characteristics

4 Improving Signal Quality

5 Satellite-Mobile Communications

6 Use of Arrays



FDMA

Shared BandwidthSubchannels

FrequencyF1 F2 F3 F10 F11 F12



TDMA

InputChannels

1

2

3

4

5

1 2 3 4 5Time

Scanner Output



TDMA ... Contd

1 2 3 4

101010000100011101

Gaurd Periods

Data stream dividedinto frames

Frames divided intotime slots. Each user is allocated one slot.

Time slot contain datawith a gaurd period ifneeded for syncronisation.



CDMA

Data Signal

Pseudorandom Code

Transmitted signal:Data Signal XOR withthe Pseudorandom

T

T

b

c



FDMA vs TDMA vs CDMA

Carrier Frequency 1

Carrier Frequency 2

Carrier Frequency 3

Carrier Frequency 4

Carrier Frequency 5

Carrier Frequency 6

Carrier Frequency 1T S1

T S2

T S3

Carrier Frequency 2T S1

T S2

T S3

Carrier Frequency 1

Code 1Code 2Code 3Code 4Code 5Code 6

FDMA

TDMA

CDMA



SDMA

f1 f2

f3

f5

f4

f6 f7

Frequency Reuse Distance

R = Cell RadiusN = Cluster Size



SDMA ... Contd

F1F2 F3

F4



Outline

1 Multiple-Access





6 Use of Arrays



Frequency Reuse

SwitchingCenter

Public SwitchedTelephone Network

BaseStation Base

Station

BaseStation

BaseStation

DownlinkUplink

f1 f2

f3

f5

f4

f6 f7

Frequency Reuse Distance

R = Cell RadiusN = Cluster Size



Classification of Cells

Pico CellIn-BuildingCoverage

Micro CellNeighbhourhoodCoverage

Marcro CellSub Urban Coverage

Global Coverage



Calling from a Mobile

Step 1: Scan for the strongest control channel and establish authorization

Step 2: Switching center assignsan available traffic channel

Switching Center Switching Center

Step 3: Switching center setups audio connection through PSTN

Switching Center PSTN

Scan co

ntrol channels

request

authorization



Calling a Mobile

Step 1: Switching center sends page alert message signal to last known mobile location

Step 2: Switching center assignsan available traffic channel

Switching Center Switching Center

Step 3: Switching center setups audio connection through PSTN

Switching Center PSTN

Paging



Handoff / Handover

BS 1 BS 2 BS 1 BS 2

+



Cell Splitting



Cell Sectoring



Outline

1 Multiple-Access





6 Use of Arrays



Friss Transmission Equation

R

Transmitter Reciever

Pt

Pr = PtGtGr

(λ

4πR

)2



Inverse Square Law

A AAr

2r

S

3r



Multipath Propagation

Reflection

Scattering

Diffraction

Direct LOS Path



Multipath Propagation ... Contd

Scattering Reflection Diffraction

Refraction

Direct pathTransmitterReceiver



Fading

Slow fading

–10

–12

–14

–16

–18

–20

–22

–24

–26

–28

–300 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Fast fading

Distance (km)

Rec

eive

d po

wer

Fading is defined as the waxing and waning of the received signal strength.



Delay Spread

Reflection

Scattering

Diffraction

Direct LOS Path

A

B

C

D

A B C D

Delay Spread

Time



Delay Spread ... Contd

Direct Signal

Reflected and Delayed Signal

Resulting Recieved Signal

Time

Time

Time



Doppler Spread

Moving towards

Moving away

Stationary ormoving across

Scattering object is:

Transmitted pulse

Recieved signal



Outline

1 Multiple-Access





6 Use of Arrays



Equalization

hc(t) heq(t)x(t) y(t) x̂(t)

Channel impulse response Equalizer impulse response

Transmittedsignal

Receivedsignal

Heq (f ) =1

H∗c (−f )



Adaptive Equalization

Equalizerheq (t)

y(t) )(ˆ tx

r(t)

Reference signal

e(t)

Error signal



Diversity

• Space Diversity• Polarization Diversity• Time Diversity

• Frequency Diversity



Channel Coding

Channel coding deliberately introduces redundancies (e.g., parity bits) intothe data to allow for correction of the errors caused by dispersion. These re-dundant symbols can be used to detect errors and/or correct the errors in thecorrupted data. Thus, the channel codes can fall into two types:

• Error detection codes (automatic repeat request (ARQ) codes)

• Error correction codes (forward error correcting (FEC) codes)



MIMO

Transmitter Receiver•••

•••

M-antenna elements N-antenna elements

Channel

H

s x

1

2

M – 1

M

1

2

N – 1

N

x̄ = H̄ · s̄ + n̄



Outline

1 Multiple-Access





6 Use of Arrays



Types of Satellites

• Astronomical satellites (Hubble Space Telescope)• Communications satellites (INSAT, GSAT)• Navigational satellites (GPS, GLONASS, IRNSS)• Reconnaissance satellites• Earth observation satellites (IRS, Cartosat, Oceansat, Megha-Tropiques)

• Space stations (Skylab, Mir, ISS)



Types of Orbits

LEO

MEO

GEO

HEO

Van AllenInner Belt

(1000-6000 kmabove the surface

of the Earth)

Van AllenOuter Belt

(13000-60000 kmabove the surface

of the Earth)



Van Allen Radiation Belts

InnerRadiationBelt Inner

RadiationBelt

OuterRadiationBelt

OuterRadiationBelt

MagneticAxis

RotationalAxis



Disadvantages of GEOS

• Two-way propagation delay on the order of 0.6 s encountered by asignal makes it unacceptable for voice communication.

• It is difficult to cover the area of the globe far south and far north of theequator using GEOS.

• GEOS requires high-power transmitters with large antennas toovercome the propagation loss suffered by the signal due to a largedistance.



Disadvantages of LEOS

• A large number of satellites are required for global coverage.• There is a requirement of fast handoffs, as the satellites move rapidly

and are in view only for a short time.

• There is also a concern of limited lifetime (five-ten years) due to orbitaldecay, requiring regular replacement..



Shaped Beam vs Spot Beams



Outline

1 Multiple-Access





6 Use of Arrays



Adaptive Beams



Dynamic Cells



Adaptive Beams in Satellite Communications (TX)



Adaptive Beams in Satellite Communications (RX)



Satellite-to-Satellite Communication

Jammer


Mobile Wireless Communications - Overview - … · 02-10-2014 · Multiple-AccessCellular...

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