Introduction & Wireless Transmission

1

Simplified Reference Model

Application

Transport

Network

Data Link

Physical

Medium

Data Link

Physical

Application

Transport

Network

Data Link

Physical

Data Link

Physical

Network Network

Radio

2

Reference Model

Physical Layer :Bit Stream to signal conversionFrequency selectionGeneration of carrier frequencyData modulation over carrier frequencyData encryption

3

Reference Model

Data Link Layer :Data Multiplexing Error detection and correctionMedium Access

In essence :Reliable point-to-point transfer of data

between sender and receiver.

4

Reference Model

Network Layer :Connection setupPacket routingHandover between networksRoutingTarget device locationQuality of service (QoS)

5

Reference Model

Transport Layer :Establish End-to-End ConnectionFlow controlCongestion controlTCP and UDPApplications – Browser etc.

6

Reference Model

Application Layer:Multimedia applicationsApplications that interface to various

kinds of data formats and transmission characteristics

Applications that interface to various portable devices

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Overlay Networks - the global goal

regional

metropolitan area

campus-based

in-house

verticalhandover

horizontalhandover

integration of heterogeneous fixed andmobile networks with varyingtransmission characteristics

8

Frequency Ranges

WIRELESS TRANSMISSION

1 Mm300 Hz

10 km30 kHz

100 m3 MHz

1 m300 MHz

10 mm30 GHz

100 m3 THz

1 m300 THz

visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV

optical transmissioncoax cabletwisted pair

VLF = Very Low Frequency UHF = Ultra High Fequency

LF = Low Frequency SHF = Super High Frequency

MF = Medium Frequency EHF = Extra High Frequency

HF = High Frequency UV = Ultraviolet Light

VHF = Very High Frequency

wave length , speed of light c 3x108m/s, frequency f

9

Frequencies

kHz Range (Low and Very Low frequencies)

Used for short distances using twisted copper wires

Several KHz to MHZ (Medium and High Frequencies)

For transmission of hundreds of radio stations in the AM and FM mode

Use co-axial cables Transmission power is several kW.

10

Frequencies

Several MHz to Terra Hz Range (VHF and UHF)Typically 100 MHz to 800 MHz and

extending to terraHz) Conventional Analog TV (174-230 MHz

and 470-790 MHz) DAB Range (220 – 1472 MHz) DTV (470 – 872 MHz)Digital GSM (890-960MHz)

11

Frequencies

3G Mobile Systems (1900-2200 MHz)

Super High(SH) and Extremely Super High(ESH) Hundreds of GHz Fixed Satellite Services Close to infra-red.

12

Frequencies

For Several TerraHz : Optical Transmission

Why do we need very high transmission frequencies?

The information content in video, satellite data etc is enormous.

If we need to accommodate many signals simultaneously, we need a high bit rate which in turn demands high frequency.

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REGULATIONS

International Telecommunications Union (ITU), Geneva responsible for world-wide coordination of telecommunications activity.

ITU – R (Radio Communications sector) handles standardization in Wireless sector.

14

REGULATIONS

ITU-R

Region-1

Europe, Middle East, Former Russia, Africa

Region-2

Greenland, N & S America

Region-3

Australia, New Zealand

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Frequency Allocation

Europe USA Japan

Cellular Phones

GSM 450-457, 479-486/460-467,489-496, 890-915/935-960, 1710-1785/1805-1880 UMTS (FDD) 1920-1980, 2110-2190 UMTS (TDD) 1900-1920, 2020-2025

AMPS, TDMA, CDMA 824-849, 869-894 TDMA, CDMA, GSM 1850-1910, 1930-1990

PDC 810-826, 940-956, 1429-1465, 1477-1513

Cordless Phones

CT1+ 885-887, 930-932 CT2 864-868 DECT 1880-1900

PACS 1850-1910, 1930-1990 PACS-UB 1910-1930

PHS 1895-1918 JCT 254-380

Wireless LANs

IEEE 802.11 2400-2483 HIPERLAN 2 5150-5350, 5470-5725

902-928 IEEE 802.11 2400-2483 5150-5350, 5725-5825

IEEE 802.11 2471-2497 5150-5250

Others RF-Control 27, 128, 418, 433, 868

RF-Control 315, 915

RF-Control 426, 868

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REGULATIONS

PDC : Personal Digital Cellular

NMT : Nordic Mobile Telephone

DECT : Digital Enhanced Cordless Telephone

PACS : Personal Access Communications System

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SIGNALS

A sine wave is represented as

g(t) = At sin (ω.t + ø)

Here, At : Maximum amplitude

w : angular frequency = 2πf

ø : Phase Displacement

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SIGNALS

Different representations of signals amplitude (amplitude domain) frequency spectrum (frequency domain) phase state diagram (amplitude M and phase in

polar coordinates)

A [V]

I= M cos

Q = M sin

A [V]

t[s]

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Signals

According to fourier series, it is possible to reconstruct the original signal using the sine and cosine functions.

G(t) = ½ C + )2cos()2sin(11

nftbnftann

nn

In the above eqn, C represents the DC component.

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Signals

As n varies, increasing number of harmonics are added to the signal representation.

As n approaches infinity, the original signal is truly represented.

The given signal has to be modulated over a career frequency.

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Antenas

An Antenna aids in transforming a wired medium to a wireless medium

Antennas couple electromagnetic energy to the space and from the space TO and FROM a wire/coaxial cable.

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ISOTROPIC RADIATOR ANTENNA

Theoretical reference antenna is the isotropic radiator.

It emits equal power in all directions.

zy

x

z

y x

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Antennas

Practical Antennas Exhibit Directional properties.

Thin Centre-fed Dipole:

λ/2

• Dipole consists of two collinear conductors separated by a small feeding gap.

• Generally, the length of the Dipole is half the wavelength of the signal to be transmitted/received.(λ = C/f where is is the speed of light {3*10 8 m/s)

24

Wavelength

Forms of electromagnetic radiation like radio waves, light waves or infrared (heat) waves make characteristic patterns as they travel through space. Each wave has a certain shape and length. The distance between peaks (high points) is called wavelength.

25

Dipole Antenna

•When the signal is obstructed by mountains, buildings etc, the power of the sinal gets weak.

• It can be boosted by additional devices.

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Directional Antenna

Several directional antennas can be combined to form a sectored antenna.

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Signal Propagation Range

distance

sender

transmission

detection

interference

Transmission range communication possible low error rate

Detection range detection of the signal

possible no communication

possible Interference range

signal may not be detected

signal adds to the background noise

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Path Loss during Transmission

Propagation in free space is always in a straight line like that of light. Receiving power proportional to 1/d² in vacuum – much more in

real environments (d = distance between sender and receiver)

Receiving power additionally influenced by Fading (frequency dependent) shadowing reflection at large obstacles refraction depending on the density of a medium scattering at small obstacles diffraction at edges

29

Path Loss Effects

reflection scattering diffractionshadowing refraction

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Signal Propagation effects

Signal Penetration through objects : At lower frequency, the penetration is higher. At very high frequencies, the transmission

behavior of the wave is close to that of light,

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Propagation behavior of waves

Ground Wave (<2 MHz): Can follow earth’s surface and can propagate long distances

[Submarine communication, AM Radio etc] Sky Wave (2-30 MHz) : Waves are reflected.

They can bounce back and forth between ionosphere and earth’s surface and can travel around the world.

Line of Sight [>30 MHz) : The waves are bent by refraction.

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Multipath Propagation

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Multipath Propagation

Radio waves sent from the sender to the receiver can travel in a straight line as well as may reach the destination after being reflected by several obstacles.

The signal arrives at different times at the receiver. THIS EFFECT IS CALLED DELAY SPREAD

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Multipath Propagation

The original signal gets a spread signalThe order of delays is 2 to 12 micro

secs.

36

Effects of delay spread

Short-pulse signals will be spread into a broader impulse or several weaker pulses.

In the fig, the impulse at the sender is received as three smaller pulses at the receiver.

Also, the power level of the received pulses will be low. So, they will be perceived as noise.

37

Effects-2 of delay spread

Inter Symbol Interference :The second symbol is separated from

the first in the transmitted signal.At the receiver, they overlap because of

delays.If the pulses represent symbols, they will

interfere with each other and there will be INTER SYMBOL INTERFERENCE.

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One possible solution

Receiver should know the delay characteristics of different paths.

Receiver can compensate for the distortion

Receiver can equalize the signals based on the channel characteristics.

39

Effects of mobility

Channel characteristics change over time and location signal paths changedifferent delay variations of different signal

partsdifferent phases of signal parts

quick changes in the power received (short term fading) short term fading

long termfading

t

power

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Solution for Long Term Fading

Senders can increase/decrease power on a regular basis so that the received power is within certain bounds.

41

Long Term Fading

Additional changes indistance to senderobstacles further away

slow changes in the average power received (long term fading)