Mobile Computing and Wireless Networking

Lec 02

03/03/2010

ECOM 6320

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Outline

Characteristic of wireless channels Antennas and Signal Propagation Multiplexing

3

)2cos()2sin(2

1)(

11

nftbnftactgn

nn

n

1

0

1

0

t t

ideal periodical digital signal

decomposition

Fourier Transform: Every Signal Can be Decomposed as a Collection of Harmonics

- Two representations: - time domain; frequency domain

- Knowing one can recover the other

Time domain Frequency domain

Examples

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Try spectrum1.m and spectrum2.m

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Objective encode digital data into analog signals at the

right frequency range

Recap: Modulation

Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

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Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK):

Frequency Shift Keying (FSK):

Phase Shift Keying (PSK):

1 0 1

t

1 0 1

t

1 0 1

t

Modulation

Example

Suppose fc = 1 GHz(fc1 = 1 GHz, fc0 = 900 MHzfor FSK)

Bit rate is 1 Mbps Encode one bit at a time Bit seq: 1 0 0 1 0

Q: How does the wave look like for each scheme?

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t

1 0 1

t

1 0 1

t

1 0 1

t

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BPSK (Binary Phase Shift Keying): bit value 0: sine wave bit value 1: inverted sine wave very simple PSK

Properties robust, used e.g. in satellite systems

Q

I01

Phase Shift Keying: BPSK

Q: What is the spectrum usage of BPSK?

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Spectral Density of BPSK

b

Rb =Bb = 1/Tbb

fc : freq. of carrier

fc

Spectral Density =

bit rate-------------------

width of spectrum used

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Phase Shift Keying: QPSK

11 10 00 01

Q

I

11

01

10

00

A

t

QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol symbol determines shift of sine

wave often also transmission of relative,

not absolute phase shift: DQPSK - Differential QPSK

Antennas and Signal Propagation

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Isotropic radiator: a single point equal radiation in all directions (three dimensional) only a theoretical reference antenna

Radiation pattern: measurement of radiation around an antenna

zy

x

z

y x idealisotropicradiator

Antennas: Isotropic Radiator

Q: how does power level decrease as a function of d, the distancefrom the transmitter to the receiver?

Antennas: directed and sectorized

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

top view, 3 sector

x

z

top view, 6 sector

x

z

directedantenna

sectorizedantenna

Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley)

Signal propagation ranges

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

distance

sender

transmission

detection

interference

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Free-Space Isotropic Signal Propagation

In free space, receiving power proportional to 1/d² (d = distance between transmitter and receiver)

Suppose transmitted signal is x,received signal y = h x, where h is proportional to 1/d²

2

4

dGG

P

Ptr

t

r

Pr: received power

Pt: transmitted power

Gr, Gt: receiver and transmitter antenna gain

(=c/f): wave length

Sometime we write path loss in log scale: Lp = 10 log(Pt) – 10log(Pr)

Free Space Signal Propagation

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1 0 1

t

1 0 1

t

1 0 1

t

at distance d

?

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Real Antennas

Real antennas are not isotropic radiators Some simple antennas: quarter wave /4 on car roofs or

half wave dipole /2 size of antenna proportional to wavelength for better transmission/receiving

/4/2

Q: Assume frequency 1 Ghz, = ?

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Dipole: Radiation Pattern of a Dipole

http://www.tpub.com/content/neets/14182/index.htmhttp://en.wikipedia.org/wiki/Dipole_antenna

Why Not Digital Signal (revisited) Not good for spectrum usage/sharing The wavelength can be extremely large

to build portal devices e.g., T = 1 us -> f=1/T = 1MHz ->

wavelength = 3x108/106 = 300m

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Receiving power additionally influenced by shadowing (e.g. through a wall or a door) refraction depending on the density of a medium reflection at large obstacles scattering at small obstacles diffraction at edges

reflection

scattering

diffraction

shadow fadingrefraction

Signal Propagation

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Signal Propagation: Scenarios

Details of signal propagation are very complicated

We want to understand the key characteristics that are important to our objective

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Shadowing

Signal strength loss after passing through obstacles

Some sample numbers

i.e. reduces to ¼ of signal10 log(1/4) = -6.02

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Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction

Multipath

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Example: reflection from the ground: received power decreases proportional to 1/d4 instead of 1/d² due to the destructive interference between the direct signal and the signal reflected from the ground

Multipath Can Reduce Signal Strength

ground

For detail, see page 9: http://www.eecs.berkeley.edu/~dtse/Chapters_PDF/Fundamentals_Wireless_Communication_chapter2.pdf

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signal at sender

Multipath Can Spread Delay

signal at receiver

LOS pulsemultipathpulses

LOS: Line Of Sight

Time dispersion: signal is dispersed over time

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signal at sender

Multipath Can Cause ISI

signal at receiver

LOS pulsemultipathpulses

LOS: Line Of Sight

dispersed signal can cause interference between “neighbor” symbols, Inter Symbol Interference (ISI)

Assume 300 meters delay spread, the arrival time difference is 300/3x108 = 1 msif symbol rate > 1 Ms/sec, we will have serious ISI

In practice, fractional ISI can already substantially increase loss rate

Multiplexing in 4 dimensions space (si) time (t) frequency (f) code (c)

Goal: multiple use of a shared medium

Important: guard spaces needed!

s2

s3

s1

Multiplexing

f

t

c

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

channels ki

Frequency multiplex

Separation of the whole spectrum into smaller frequency bands

A channel gets a certain band of the spectrum for the whole time

Advantages no dynamic coordination

necessary works also for analog signals

Disadvantages waste of bandwidth

if the traffic is distributed unevenly

inflexible

k2 k3 k4 k5 k6k1

f

c

f

c

k2 k3 k4 k5 k6k1

Time multiplex

A channel gets the whole spectrum for a certain amount of time

Advantages only one carrier in the

medium at any time throughput high even

for many users

Disadvantages precise

synchronization necessary

f

Time and frequency multiplex Combination of both methods A channel gets a certain frequency band for a certain

amount of time Example: GSM

Advantages better protection against

tapping protection against frequency

selective interference but: precise coordination

required

t

c

k2 k3 k4 k5 k6k1

Code multiplex

Each channel has a unique code

All channels use the same spectrum at the same time

Advantages bandwidth efficient no coordination and synchronization

necessary good protection against interference

and tapping Disadvantages

varying user data rates more complex signal regeneration

Implemented using spread spectrum technology

k2 k3 k4 k5 k6k1

f

t

c