1-introduction To Wireless Networks.pdf

Introduction to Wireless Networks

Contents 1. Introduction 2. Applications 3. Wireless transmission 3.1. Frequency 3.2. Antennas 3.3. Signal Propagation 3.4. Multiplexing 3.5. Modulation

Wireless networks

• Access computing/communication services, on the move • Wireless WANs – Cellular Networks: GSM, GPRS, CDMA – Satellite Networks: Iridium

• Wireless LANs – WiFi Networks: 802.11 – Personal Area Networks: Bluetooth

• Wireless MANs – WiMaX Networks: 802.16 – Mesh Networks: Multi-hop WiFi – Adhoc Networks: useful when infrastructure not available Instructor: Prof. Sneha Deshmukh

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Limitations of the mobile environment  Limitations of the Wireless Network

 limited communication bandwidth  frequent disconnections  heterogeneity of fragmented networks

 Limitations Imposed by Mobility

 route breakages  lack of mobility awareness by system/applications

 Limitations of the Mobile Device  short battery lifetime  limited capacities

Instructor: Prof. Sneha Deshmukh

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Mobile communication • Wireless vs. mobile

   

   

Examples stationary computer laptop in a hotel (portable) wireless LAN in historic buildings Personal Digital Assistant (PDA)

• Integration of wireless into existing fixed networks: – Local area networks: IEEE 802.11, ETSI (HIPERLAN) – Wide area networks: Cellular 3G, IEEE 802.16 – Internet: Mobile IP extension

Instructor: Prof. Sneha Deshmukh

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Wireless v/s Wired networks

• Regulations of frequencies

– Limited availability, coordination is required – useful frequencies are almost all occupied

• Bandwidth and delays – Low transmission rates

• few Kbits/s to some Mbit/s. – Higher delays

• several hundred milliseconds – Higher loss rates

• susceptible to interference, e.g., engines, lightning • Always shared medium – Lower security, simpler active attacking – radio interface accessible for everyone – secure access mechanisms important Instructor: Prof. Sneha Deshmukh

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Reference model

Application

Application

Transport

Transport

Network

Network

Data Link

Data Link

Data Link

Data Link

Physical

Physical

Physical

Physical

Radio

Network

Network

Medium

Instructor: Prof. Sneha Deshmukh

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Effect of mobility on protocol stack • Application – new applications and adaptations – service location, multimedia

• Transport – congestion and flow control – quality of service

• Network – addressing and routing – device location, hand-over

• Link – media access and security

• Physical – transmission errors and interference

Instructor: Prof. Sneha Deshmukh

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Perspectives • Network designers: Concerned with cost-effective design – Need to ensure that network resources are efficiently utilized and fairly allocated to different users.

• Network users: Concerned with application services – Need guarantees that each message sent will be delivered without error within a certain amount of time.

• Network providers: Concerned with system administration – Need mechanisms for security, management, fault-tolerance and accounting.

Instructor: Prof. Sneha Deshmukh

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RF Basics

Factors affecting wireless system design • Frequency allocations – What range to operate? May need licenses.

• Multiple access mechanism – How do users share the medium without interfering?

• Antennas and propagation – What distances? Possible channel errors introduced.

• Signals encoding – How to improve the data rate?

• Error correction – How to ensure that bandwidth is not wasted? Instructor: Prof. Sneha Deshmukh

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Frequencies for communication

• • • • •

VLF = Very Low Frequency LF = Low Frequency MF = Medium Frequency HF = High Frequency VHF = Very High Frequency

UHF = Ultra High Frequency SHF = Super High Frequency EHF = Extra High Frequency UV = Ultraviolet Light

• Frequency and wave length:  = c/f • wave length , speed of light c  3x108m/s, frequency f Instructor: Prof. Sneha Deshmukh

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Wireless frequency allocation • Radio frequencies range from 9KHz to 400GHZ (ITU) • Microwave frequency range – – – –

1 GHz to 40 GHz Directional beams possible Suitable for point-to-point transmission Used for satellite communications

• Radio frequency range – 30 MHz to 1 GHz – Suitable for omnidirectional applications

• Infrared frequency range – Roughly, 3x1011 to 2x1014 Hz – Useful in local point-to-point multipoint applications within confined areas Instructor: Prof. Sneha Deshmukh

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Frequencies for mobile communication • VHF-/UHF-ranges for mobile radio – simple, small antenna for cars – deterministic propagation characteristics, reliable connections

• SHF and higher for directed radio links, satellite communication – small antenna, focusing – large bandwidth available

• Wireless LANs use frequencies in UHF to SHF spectrum – some systems planned up to EHF – limitations due to absorption by water and oxygen molecules (resonance frequencies)

• weather dependent fading, signal loss caused by heavy rainfall etc. Instructor: Prof. Sneha Deshmukh

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Frequency regulations • Frequencies from 9KHz to 300 MHZ in high demand (especially VHF: 30-300MHZ) • Two unlicensed bands – Industrial, Science, and Medicine (ISM): 2.4 GHz – Unlicensed National Information Infrastructure (UNII): 5.2 GHz

• Different agencies license and regulate – – – –

www.fcc.gov - US www.etsi.org - Europe www.wpc.dot.gov.in - India www.itu.org - International co-ordination

• Regional, national, and international issues • Procedures for military, emergency, air traffic control, etc Instructor: Prof. Sneha Deshmukh

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Wireless transmission Antenna

Antenna

Transmitter

Receiver

• Wireless communication systems consist of: – Transmitters – Antennas: radiates electromagnetic energy into air – Receivers

• In some cases, transmitters and receivers are on same device, called transceivers. Instructor: Prof. Sneha Deshmukh

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Transmitters Amplifier

Mixer

Filter

Antenna Amplifier

Source Oscillator

Transmitter

Suppose you want to generate a signal that is sent at 900 MHz and the original source generates a signal at 300 MHz. •Amplifier - strengthens the initial signal •Oscillator - creates a carrier wave of 600 MHz •Mixer - combines signal with oscillator and produces 900 MHz (also does modulation, etc) •Filter - selects correct frequency •Amplifier - Strengthens the signal before sending it

Instructor: Prof. Sneha Deshmukh

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Antennas

Antennas • An antenna is an electrical conductor or system of conductors to send/receive RF signals – Transmission - radiates electromagnetic energy into space – Reception - collects electromagnetic energy from space

• In two-way communication, the same antenna can be used for transmission and reception

Omnidirectional Antenna (lower frequency)

Instructor: Prof. Sneha Deshmukh

Directional Antenna (higher frequency)

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Antennas: isotropic radiator

• Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission • Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna • Real antennas always have directive effects (vertically and/or horizontally) • Radiation pattern: measurement of radiation around an antenna y

z

z y x

Instructor: Prof. Sneha Deshmukh

x

ideal isotropic radiator 21

Antennas: simple dipoles

• Real antennas are not isotropic radiators

– dipoles with lengths /4 on car roofs or /2 (Hertzian dipole)  shape of antenna proportional to wavelength

• Gain: maximum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power) /4

y

/2

y

x side view (xy-plane)

z

z side view (yz-plane)

x

simple dipole

top view (xz-plane)

Instructor: Prof. Sneha Deshmukh

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Antennas: directed and sectorized

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

y

z

x

z

side view (xy-plane)

x

side view (yz-plane)

top view (xz-plane) z

z

x

x

top view, 3 sector

directed antenna

sectorized antenna

top view, 6 sector

Instructor: Prof. Sneha Deshmukh

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Comparison of omni and directional Issues

Omni

Directional

Spatial Reuse

Low

High

Connectivity

Low

High

Interference

Omni

Directional

Cost & Complexity

Low

High

Instructor: Prof. Sneha Deshmukh

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Signal Propagation and Modulation

• • • •

Signals physical representation of data function of time and location signal parameters: parameters representing the value of data classification – – – –

continuous time/discrete time continuous values/discrete values analog signal = continuous time and continuous values digital signal = discrete time and discrete values

• signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift  – sine wave as special periodic signal for a carrier: s(t) = At sin(2  ft t + t)

Instructor: Prof. Sneha Deshmukh

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Signal propagation ranges • Transmission range – communication possible – low error rate

• Detection range – detection of the signal possible – no communication possible

• Interference range

sender transmission distance detection interference

– signal may not be detected – signal adds to the background noise Instructor: Prof. Sneha Deshmukh

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Attenuation: Propagation & Range

Instructor: Prof. Sneha Deshmukh

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Attenuation • Strength of signal falls off with distance over transmission medium • Attenuation factors for unguided media: – Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal – Signal must maintain a level sufficiently higher than noise to be received without error – Attenuation is greater at higher frequencies, causing distortion

• Approach: amplifiers that strengthen higher frequencies Instructor: Prof. Sneha Deshmukh

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Signal propagation

• Propagation in free space always like light (straight line) • Receiving power proportional to 1/d² (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

shadowing

reflection

refraction

scattering

Instructor: Prof. Sneha Deshmukh

diffraction 32

Multipath propagation • Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction multipath LOS pulses pulses

signal at sender signal at receiver • Time dispersion: signal is dispersed over time •  interference with “neighbor” symbols, Inter Symbol Interference (ISI) • The signal reaches a receiver directly and phase shifted •  distorted signal depending on the phases of the different parts

Instructor: Prof. Sneha Deshmukh

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Effects of mobility • Channel characteristics change over time and location – signal paths change – different delay variations of different signal parts – different phases of signal parts •  quick changes in the power received power

(short term fading)

long term fading

• Additional changes in – distance to sender – obstacles further away

short term fading

t

•  slow changes in the average power

received (long term fading) Instructor: Prof. Sneha Deshmukh

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Propagation modes

Signal

Transmission Antenna

Earth

a) Ground Wave Propagation

Receiving Antenna

Ionosphere

Signal b) Sky Wave Propagation

Earth

Signal c) Line-of-Sight Propagation Earth Instructor: Prof. Sneha Deshmukh

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Multiplexing Mechanisms

Multiplexing

channels ki k1

• Multiplexing in 4 dimensions – – – –

space (si) time (t) frequency (f) code (c)

k2

k3

k4

k5

k6

c t

c t

s1

f

s2

f

c

• Goal: multiple use of a shared medium

t

• Important: guard spaces needed!

Instructor: Prof. Sneha Deshmukh

s3

f

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• • • •

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 k1 k2 k3 k4 k5 k6 • works also for analog signals c

f

• Disadvantages: • waste of bandwidth if the traffic is distributed unevenly • inflexible • guard spaces t

Instructor: Prof. Sneha Deshmukh

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•

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

k1

k2

k3

k4

k5

k6

c f

• Disadvantages: • precise synchronization necessary t Instructor: Prof. Sneha Deshmukh

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Time and frequency multiplex • Combination of both methods • A channel gets a certain frequency band for a certain amount of time • Example: GSM k1 k2 k3 k4 k5 k6 • Advantages: – better protection against tapping – protection against frequency selective interference – higher data rates compared to code multiplex

c f

• but: precise coordination t required Instructor: Prof. Sneha Deshmukh

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Code multiplex

• Each channel has a unique code k1 • All channels use the same spectrum at the same time • Advantages:

k2

k3

k4

k5

k6

c

– bandwidth efficient – no coordination and synchronization necessary – good protection against interference and tapping

f

• Disadvantages: – lower user data rates – more complex signal regeneration

• Implemented using spread spectrum technology Instructor: Prof. Sneha Deshmukh

t

41

CDMA Example – D = rate of data signal – Break each bit into k chips

• Chips are a user-specific fixed pattern – Chip data rate of new channel = kD

• If k=6 and code is a sequence of 1s and -1s – For a ‘1’ bit, A sends code as chip pattern

• – For a ‘0’ bit, A sends complement of code

• <-c1, -c2, -c3, -c4, -c5, -c6> • Receiver knows sender’s code and performs electronic decode function

Su •d   d 3=received c3  d 4 chip c4 pattern d 5  c5  d 6  c6 • = sender’s code Instructor: Prof. Sneha Deshmukh

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CDMA Example • User A code = <1, –1, –1, 1, –1, 1> – To send a 1 bit = <1, –1, –1, 1, –1, 1> – To send a 0 bit = <–1, 1, 1, –1, 1, –1>

• User B code = <1, 1, –1, – 1, 1, 1> – To send a 1 bit = <1, 1, –1, –1, 1, 1>

• Receiver receiving with A’s code – (A’s code) x (received chip pattern) • User A ‘1’ bit: 6 -> 1 • User A ‘0’ bit: -6 -> 0 • User B ‘1’ bit: 0 -> unwanted signal ignored Instructor: Prof. Sneha Deshmukh

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•

Modulation Digital modulation – digital data is translated into an analog signal (baseband) – ASK, FSK, PSK – differences in spectral efficiency, power efficiency, robustness

• Analog modulation – shifts center frequency of baseband signal up to the radio carrier

• Motivation – smaller antennas (e.g., /4) – Frequency Division Multiplexing – medium characteristics

• Basic schemes – Amplitude Modulation (AM) – Frequency Modulation (FM) – Phase Modulation (PM) Instructor: Prof. Sneha Deshmukh

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Modulation and demodulation digital data 101101001

digital modulation

analog baseband signal

analog modulation

radio transmitter

radio carrier

analog demodulation

analog baseband signal

synchronization decision

digital data 101101001

radio receiver

radio carrier

Instructor: Prof. Sneha Deshmukh

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Digital modulation 1

0

• Modulation of digital signals known as Shift Keying • Amplitude Shift Keying (ASK): – very simple – low bandwidth requirements – very susceptible to interference

1

t

1

0

1

• Frequency Shift Keying (FSK): – needs larger bandwidth

• Phase Shift Keying (PSK): – more complex – robust against interference

t

1

0

1

t

• Many advanced variants Instructor: Prof. Sneha Deshmukh

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Spread spectrum technology • Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference • Solution: spread the narrow band signal into a broad band signal using a special code - protection against narrow band interference power

interference

spread signal

power

signal

detection at receiver

• Side effects:

spread interference

f

f

– coexistence of several signals without dynamic coordination – tap-proof

• Alternatives: Direct Sequence, Frequency Hopping Instructor: Prof. Sneha Deshmukh

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Spread-spectrum communications

Instructor: Prof. Sneha Deshmukh

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Source: Intersil

Effects of spreading and interference dP/df

dP/df

i)

user signal broadband interference narrowband interference

ii) f sender dP/df

f

dP/df

dP/df

iii)

iv) f

receiver

v) f

Instructor: Prof. Sneha Deshmukh

f

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DSSS properties

Instructor: Prof. Sneha Deshmukh

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Source: Intersil

•

DSSS (Direct Sequence) XOR of the signal with pseudo-random number (chipping sequence)

– many chips per bit (e.g., 128) result in higher bandwidth of the signal tb

• Advantages – reduces frequency selective fading – in cellular networks

user data 0

1 tc

• base stations can use the same frequency range 01101010110101 • several base stations can detect and recover the signal • soft handover 01101011001010 • Disadvantages – precise power control necessary

Instructor: Prof. Sneha Deshmukh

XOR chipping sequence = resulting signal

tb: bit period tc: chip period

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DSSS Transmit/Receive spread spectrum signal

user data

transmit signal

X

modulator

chipping sequence

radio carrier transmitter

correlator lowpass filtered signal

received signal demodulator

radio carrier

sampled sums

products

data X

integrator

decision

chipping sequence receiver

Instructor: Prof. Sneha Deshmukh

52

Frequency Hopping Spread Spectrum (FHSS) • Signal is broadcast over seemingly random series of radio frequencies • Signal hops from frequency to frequency at fixed intervals • Channel sequence dictated by spreading code • Receiver, hopping between frequencies in synchronization with transmitter, picks up message • Advantages – Eavesdroppers hear only unintelligible blips – Attempts to jam signal on one frequency succeed only at knocking out a few bits

Instructor: Prof. Sneha Deshmukh

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FHSS (Frequency Hopping) • Discrete changes of carrier frequency – sequence of frequency changes determined via pseudo random number sequence

• Two versions – Fast Hopping: several frequencies per user bit – Slow Hopping: several user bits per frequency

• Advantages – frequency selective fading and interference limited to short period – simple implementation – uses only small portion of spectrum at any time

• Disadvantages – not as robust as DSSS – simpler to detect Instructor: Prof. Sneha Deshmukh

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Slow and Fast FHSS tb user data 0

1

f

0

1

1

t

td

f3

slow hopping (3 bits/hop)

f2 f1 f

t

td

f3

fast hopping (3 hops/bit)

f2 f1 t

tb: bit period

td: dwell time

Instructor: Prof. Sneha Deshmukh

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FHSS Transmit/Receive user data modulator

modulator

frequency synthesizer

transmitter

hopping sequence

narrowband signal

received signal

data demodulator

hopping sequence

spread transmit signal

narrowband signal

demodulator

frequency synthesizer

Instructor: Prof. Sneha Deshmukh

receiver

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OFDM (Orthogonal Frequency Division) • Parallel data transmission on several orthogonal subcarriers with lower rate c

f

k3

t

Maximum of one subcarrier frequency appears exactly at a frequency where all other subcarriers equal zero 

superposition of frequencies in the same frequency range

Amplitude

subcarrier: sin(x) SI function= x

f Instructor: Prof. Sneha Deshmukh

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• Properties

OFDM

– Lower data rate on each subcarrier  less ISI – interference on one frequency results in interference of one subcarrier only – no guard space necessary – orthogonality allows for signal separation via inverse FFT on receiver side – precise synchronization necessary (sender/receiver)

• Advantages – no equalizer necessary – no expensive filters with sharp edges necessary – better spectral efficiency (compared to CDM)

• Application – 802.11a, HiperLAN2, ADSL Instructor: Prof. Sneha Deshmukh

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