Introduction to Wireless Communication

CS5440 Wireless Access Networks

Dilum Bandara

Dilum.Bandara@uom.lk

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Outlines Elements of a wireless system

Transmitter Frequency spectrum Modulation Antenna

Medium Propagation Attenuation

Receiver Antenna Demodulation

Issues & constraints

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Wireless Communication Transfer of data between 2+ points that aren’t

connected by an electrical conductor Typically use electromagnetic waves

Why wireless? Running cables not always possible Low footprint Rapid (re)configuration Low cost

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Wireless History Ancient systems – Smoke Signals, Carrier Pigeons, etc. Radio invented in 1880s by Marconi Many sophisticated military radio systems developed

during & after WW2 Exponential growth in Cellular systems since 1988

Ignited wireless revolution Voice, data, & multimedia ubiquitous 6.8 billion subscribers worldwide as of Feb. 2013 (source ITU) Use in 3rd-world countries growing rapidly

3.5 billion subscribers in Asia Pacific in 2013

Wi-Fi enjoying tremendous success & growth Wide area networks (e.g., WiMax) & short-range systems other

than Bluetooth (e.g., UWB) less successful

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Future Wireless Networks

Next-generation CellularWireless Internet AccessWireless MultimediaSensor Networks Smart Homes/SpacesAutomated HighwaysIn-Body NetworksIoT

Ubiquitous communication among People & Devices

Source: Andrea Goldsmith, “Cross Layer Design in Wireless Networks”, Stanford University

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Wireless Services Telemetry control & traffic control systems Infrared & ultrasonic remote control devices Professional LMR (Land Mobile Radio) & SMR (Specialized

Mobile Radio) Used by business, industrial & public safety entities

Consumer 2-way radio Airband & radio navigation equipment Amateur Radio Service (Ham radio) Cellular telephones & pagers Global Positioning System (GPS) Cordless computer peripherals Cordless phones Satellite television

Source: www.access.kth.se

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Medium

Elements of a Wireless System

Transmitter

Receiver 1

Receiver 2

Receiver n

Source: www.mikroe.com/old/books/rrbook/chapter2/chapter2.htm

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Transmitter

Elements depend on transmission technology Frequency & wavelength, c = f λ Modulation Antenna

AM Transmitter

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Exercise Wavelength of an electromagnetic wave travelling

in space is 60 cm. What is its frequency? Assume speed of light is 3×108 m/s

a) 500 MHz

b) 3 GHz

c) 5 GHz

d) 15 GHz

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Frequency Spectrum Range of available frequencies To avoid interference, various wireless

technologies use distinct frequency bands Signal power is well controlled Assigned by regulatory agencies

e.g., FCC, ITU, TRC

Source: www.cosmosportal.org

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Government license not requiredIndustrial, Scientific, & Medical (ISM) band

VLF – very low frequencyLF – low frequencyMF – medium frequencyHF – high frequencyVHF – very high frequencyUHF – ultra-high frequencySHF – super-high frequencyEHF – extremely high frequency

Source: P. Zheng et al., Wireless Networking Complete

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Key Frequency Bands AM – 520 - 1650.5 KHz FM – 87.5 - 108 MHz Direct broadcast satellite – 10.9 - 12.75 GHz Global System for Mobile (GSM)

890 - 960 MHz & 1710 - 1880 MHz Referred as 900 & 1800 bands

Code Division Multiple Access (CDMA) 900 & 1800 bands

3G wideband CDMA (UMTS) 1900 - 1980, 2020 - 2025, & 2110 - 2190 MHz bands

Wireless LAN (IEEE 802.11) 902 - 928 MHz, 2400 - 2483 MHz, 5.15 - 5.725 GHz ISM band – 2.4 GHz in US

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Key Frequency Bands (Cont.) Bluetooth – 2.402 - 2.480 GHz in US WiMax – 2 - 11 GHz (includes both licensed &

unlicensed) Ultra-wideband (UWB) – 1.1 - 10.6 GHz Radio-frequency IDentification (RFID)

LF (120 – 140 KHz), HF (13.56 MHz), UHF (868 – 956 MHz), & Microwave (2.4 GHz)

IrDA – 100 GHz Wireless sensors

300 - 1000 MHz & 2.4 GHz ISM band Global Positioning System (GPS)

1575.42 MHz (referred to as L1) & 1227.60 MHz (L2)

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Antenna Converts signal to electromagnetic waves Size must be consistent with wavelength Types

Directional Satellite communication

Omnidirectional Cell phones, car radios

MIMO Wireless routers

Source: www.flann.com

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Antenna Gain How well an antenna converts input power into

radio waves headed in a specified direction Depends on antenna's directivity & electrical

efficiency Gain

Ratio of power produced by antenna to power produced by a hypothetical lossless isotropic antenna

Unitless Usually expressed in decibels (dB) Directional high gain Omnidirectional low gain

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Attenuation Reduction in signal strength with distance,

propagation medium, & atmospheric conditions Typically high for high frequencies Friis free-space equation

PR, PT – Power at receiver (in Watts or Milliwatts)

GT, GR – gain of antenna λ – wavelength (in meters) d – distance (in meters)

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2

)4(=)(

d

GGPdP RTT

R

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Example Transmission frequency is 881.52 MHz & antenna gains

are 8 dB & 0 dB for base station & mobile station What is the signal attenuation at a distance of 1,500 m?

c = 299 792 458 m/s

Solution c = f λ λ = 299 792 458/881.52×106 = 0.34 m 8 dB = 100.8 = 6.3 0 dB = 100 = 1 Loss = PT – PR

Loss = 86.89 dB

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9-

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2

22

2

22

2

22

2

10×8788.4=)(

10×0497.2=)(

1500)4(

34.0×1×3.6=

)(

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)(

)4(=

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)4(=)(

dP

P

P

dP

P

dP

P

dP

d

GG

P

dP

d

GGPdP

R

T

T

R

T

R

T

R

RT

T

R

RTTR

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Attenuation (Cont.) Based on empirical evidence, more reasonable to

model PR as a log-distance path-loss model

np – path loss exponent

Xσ – zero-mean Gaussian random variable with STD σ All power values are in dBm

)/log(10)()( 000 ddndPdP pR

Source: S. Rao, “Estimating the ZigBee transmission-range ISM band,” EDN, May 2007.

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Complex Attenuation When signal encounters obstacles High-frequency signals experience

1. Absorption

2. Shadowing When object >> λ

3. Reflection When object >> λ

4. Refraction

5. Diffraction

6. Scattering When object ≤ λ

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Complex Attenuation (Cont.)

Source: http://computer-help-tips.blogspot.com/2011/04/radio-frequency-behaviors.html

Absorption

Source: http://elmag.org/en/propagation-modeling-of-shadowing-by-vegetation-for-mobile-satcom-%26-satnav-systems

Shadowing

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Complex Attenuation (Cont.)

ReflectionSource: http://wireless.navigator.co.uk/radio_link.htm

Refraction

Source: http://computer-help-tips.blogspot.com/2011/04/radio-frequency-behaviors.html

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Complex Attenuation (Cont.)

Scattering

Diffraction

Source: http://newhorizons.bg/blog/2010/12/wireless-101-terminology-part-2-implementing-cisco-unified-wireless-networking-essentials-iuwne/

Source: http://www.astrosurf.com/luxorion/qsl-propa.htm

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Example – Attenuation Experienced by Mobile Phones

Source: www.intechopen.com/books/matlab-a-fundamental-tool-for-scientific-computing-and-engineering-applications-volume-2/mobile-radio-propagation-prediction-for-two-different-districts-in-mosul-city

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Exercise Reflection of wireless signals occurs when

a) wavelength is constant

b) object size << wavelength

c) object size ≈ wavelength

d) object size >> wavelength

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Noise Disturbances introduced to wireless signals

Source: www.cisco.com/en/US/prod/collateral/video/ps8806/ps5684/ps2209/prod_white_paper0900aecd805738f5.html

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Noise (Cont.) Sources

Thermal (white) noise From electronic circuit PThermal = KTB K – Boltzmann constant, T - Ambient temperature, B - receiver BW

Intermodulation noise When 2 frequencies of signals are transmitted over same medium

2 signals at 270 & 275 MHzSource: http://en.wikipedia.org/wiki/Intermodulation

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Noise (Cont.) Crosstalk between channels Impulse noise

Due to instantaneous electromagnetic changes

Source: http://volpefirm.com/impact-of-impulse-noise-on-adaptive-pre-equalization-part-ii/impulse-noise/

Source: www.chalmers.se/en/departments/s2/research/ Pages/Hardware-constrained-communication.aspx

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Signal-to-Noise Ratio To cope with noise, transmitted signal > noise

High Signal-to-Noise Ratio (SNR)

Or use spread spectrum technology Embed signal over wide range of frequencies with low power

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Example PT = 10 W, free space loss 117 dB, antenna gains 8 dB &

0 dB, total system losses 8 dB, receiver antenna temperature 290 K, & receiver bandwidth 1.25 MHz

Find PR

Find thermal noise, K = 1.38×10-23 W/Kelvin-Hz Find SNR at receiver Solution

PR = -107 dBW

PThermal = KTB = 1.38×10-23 × 290 × 1.25×106 = -143 dBW SNR = -107 + 143 = 36 dB

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Multipath Propagation Receive same signal through different paths

Different arrival times Inter Symbol Interference (ISI)

Different levels of attenuation Different levels of distortion

Source: http://www.ni.com/white-paper/6427/en/

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Signal Propagation Amplitude domain

Amplitude change with time

Frequency domain Frequency change

with time Phase domain

Phase change with time

Frequency & phase modulation require high-frequency carriers

Source: www.ni.com/white-paper/4805/en

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AM & FM

Amplitude Modulation

Frequency Modulation

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Phase Modulation

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Phase Modulation (Cont.) Amplitude-Shift Keying (ASK)

Binary ASK 1 – By presence of a signal 0 – No signal

Pros Bandwidth efficient Simple to implement

Cons Low power efficiency Susceptible to noise & multipath propagation Unclear absence of a signal vs. binary 0

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Phase Modulation (Cont.) Frequency-Shift Keying (FSK)

Binary FSK 1 – High frequency 0 – Low frequency

Pros Better SNR Simple decoding Long distance

Cons Slightly less bandwidth efficient than ASK & PSK More complicated circuitry than ASK

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Phase Modulation (Cont.) Phase-Shift Keying (PSK)

Encode based on phase of carrier wave Binary PSK

1 – 180o

0 – 0o

Quadrature PSK 0o, 90o, 180o, 270o

Pros Power efficient

Cons Low-bandwidth efficiency More complicated circuitry than FSK

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Phase Modulation (Cont.) Quadrature Amplitude Modulation (QAM)

Combines ASK & PSK Widely used

Source: www.physics.udel.edu/~watson/scen103/projects/96s/thosguys/qam.html

Source: www.scicos.org/ScicosModNum/modnum_web/src/modnum_421/interf/scicos/help/eng/htm/MODQAM_f.htm

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Multiplexing Transmitting multiple signals simultaneously

Maximize capacity Time Division Multiplexing (TDM)

Multiple channels occupy same frequency in alternating slices

Frequency Division Multiplexing (FDM) Use different carrier frequencies

Code Division Multiplexing (CDM) Same frequency & same time but different codes Code – like Tx & Rx speak different languages

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Multiplexing (Cont.)

Source: http://m.ztopics.com/Time%20division%20multiple%20access/

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CDMA

Source: http://electronicdesign.com/communications/fundamentals-communications-access-technologies-fdma-tdma-cdma-ofdma-and-sdma

Source: www.umtsworld.com/technology/cdmabasics.htm

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Exercise Which of the following multiplexing technique

allow signals to use different frequencies at the same time?a) Amplitude Division Multiplexing

b) Frequency Division Multiplexing

c) Code Division Multiplexing

d) Time Division Multiplexing

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Narrowband Transmission

Pros Efficient use of frequency

Cons Require regulation Easier to intercept & jam

Source: www.tapr.org

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Spread Spectrum

Spread signal over a large range of frequencies Low power density (power per frequency) Signal appear as background noise

www.intercomsonline.com/Spread-Spectrum-Technology_a/162.htm

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Spread Spectrum (Cont.) Only receivers that know the spreading scheme

can reconstruct original signal Spreading scheme defined by a code

Only designated receiver knows the code Pros

Improved channel capacity Resistance against interference Security against tapping & jamming

Cons Complex circuits

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Signal With & Without Noise

Source: www.sciencedirect.com/science/article/pii/S0888327009003756

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Types of Spread Spectrum Systems

1. Direct Sequence Spread Spectrum (DSSS)

2. Frequency-Hopping Spread Spectrum (FHSS)

3. Orthogonal Frequency-Division Multiplexing (OFDM)

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Direct Sequence Spread Spectrum (DSSS)

Spread signal over broader frequency band

Chipping technique to spread signal

Transmitter & receiver needs to be synchronized

Used in WiFi

Source: www.maximintegrated.com/app-notes/index.mvp/id/1890

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Frequency-Hopping Spread Spectrum (FHSS)

Hoping sequence of frequencies Only subset of the available

frequencies are used to hop Transmitter & receiver needs

to be synchronized Relatively simple to implement

than DSSS Relatively easier to recover Tx

signal than DSSS Relatively less robust to signal

distortion & multipath effects Used in Bluetooth

Source: www.maximintegrated.com/app-notes/index.mvp/id/1890

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Orthogonal Frequency-Division Multiplexing (OFDM)

Utilize orthogonal multiple subcarriers in parallel Much higher data rates Low multipath interference Used in IEEE 802.11 a/g

Source: http://wiki.hsc.com//Main/OFDM

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Challenges Wireless channels are a difficult & capacity-

limited communications medium Typically less efficient Traffic patterns, user locations, & network

conditions are constantly changing Applications are heterogeneous with hard

constraints that must be met by networks

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More Challenges

Network challenges Scarce spectrum Demanding/diverse applications Reliability Ubiquitous coverage Seamless indoor/outdoor operation

Device challenges Size, power, cost Multiple antennas in Silicon Multi-radio Integration Coexistance

Cellular

AppsProcessor

BT

MediaProcessor

GPS

WLAN

Wimax

DVB-H

FM/XM

53Growth in mobile data, massive spectrum deficit & stagnant revenues require technical & political breakthroughs for ongoing success of cellular

Careful what you wish for…

Exponential Mobile Data

Growth

Leading to massive spectrum deficit

Source: Unstrung Pyramid Research 2010Source: FCC

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Software-Defined (SD) Radio

Wideband antennas & A/Ds span BW of desired signals DSP programmed to process desired signal: no specialized

HW

Cellular

AppsProcessor

BT

MediaProcessor

GPS

WLAN

Wimax

DVB-H

FM/XM A/D

A/D

DSPA/D

A/D

Is this the solution to the device challenges?

Today, this isn’t cost, size, or power efficient

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Summary Bandwidth & QoS is in demand

Many applications & services Spectrum is scare

Many elements & solutions Still not enough It’s only going to be even more interesting...