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Wireless Communication

Lecture 1

Wireless Fundamentals

Ammar Karim

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Course Division

1. Fundamentals

Evolution of wireless systems, various impairments in wireless channels.

Spreading: FHSS, DSSS, Spreading sequences

Understanding of FDMA-TDD/FDD, TDMA-FDD/TDD and CDMA-FDD/TDD Systems.

Equalization

2. Wireless Data Networks

Data networks, IEEE 802.11 WLANS their design and operation, Random Access Methods.

Mobile IP.

WLLs: MMDS/LMDS, Wi-MAX

Bluetooth

3. Cellular System

Cellular Fundamentals: Cellular systems, cellular operations, Handoffs & Cluster size Relationship between C/I and Cluster Size, Derivation of expressions to link the Re-Use ratio (D/R) to the Cluster Size (N) , Power control, cellular hierarchy, AMPS and AMPS architecture, Call establishment and control

Frequency planning & re-use, Radio Propagation effects, Adjecent Interference, Cell splitting

Tele traffic engineering

GSM: architecture, entities, channels, signal processing, handoff, call control, roaming, security

CDMA

GPRS

4. Overview of Cutting-edge Technologies: 3G and Beyond

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Recommended Books

David Parsons, The Mobile Radio Propagation Channel, 2nd Edition, John Wiley & Sons; ISBN: 047198857

T. S. Rappaport, Wireless Communications, 2nd Edition, 2002, Pearson Education; ISBN: 81-7808-648-4

Simon Haykin, Communication Systems, 4th edition, May 2000, John Wiley & Sons; ISBN: 0471178691

Lecture Notes

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Evaluation Criterion

Assignments = 5%

Quizzes = 10%

Mids = 30% (15% each)

Final Exam = 55%

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Age of Information Communications

Blackberry 8705g Nokia DVB-H phone Mobile MM

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Evolving Communication Networks

Core and Access Networks

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Wireless communication

Early wireless communication:

in the 400-900 TeraHertz Band!

150 BC smoke signals (Greece)

1794, optical telegraph

What is wireless communication:

Any form of communication that does not require a transmitter and receiver to be in physical contact

Electromagnetic waves propagate through free space

Radar, RF, Microwave, IR, Optical

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Types of Communication

Simplex

one-way communication

radio, TV, etc

Half-duplex:

two-way communication but not simultaneous

push-to-talk radios, etc

Full-duplex:

two-way communication

cellular phones

Frequency-division duplex (FDD)

Time-division duplex (TDD): simulated full-duplex

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Forms of Communication

Analogue & Digital

Which one is Better?

Digital?

Why? Digital Data has inherited frequency reuse property

Lesser noise and interference as compared to analogue communication

Lower transmit power is required

1/0’s can transmit anything : sound, picture, video etc.

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Types of Media/Environments used for Communication

Wireless & Wired

Why Wireless is better than Wired ?

User Mobility

Reduced Cost (cheap infrastructure) Cabling very critical

Developing nations utilize cellular telephony rather than

laying twisted-pair wires to each home

Flexibility Can easily set-up temporary LANs

Disaster situations

Office moves

Only use resources when sending or receiving a signal

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Wired Vs. Wireless Communication

Wired Wireless Each cable is a different channel One media (cable) shared by all

Signal attenuation is low High signal attenuation

No interference High interference

noise; co-channel interference; adjacent

channel interference

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Why wireless different than wired?

Noisy, time-varying channel BER varies by orders of magnitude

Environmental conditions affect transmission

Shared medium Other users create interference

Must develop ways to share the channel

Bandwidth is limited spectrum allocated by state rules

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Classification of Wireless Systems

Mobile Wireless Systems

GSM, TDMA, CDMA

WLAN, Ad-hoc, Bluetooth, Home RF

Fixed Wireless Systems

MMDS, LMDS, Satellite

WiMax(IEEE 802.16a)

Infrastructure Dependent Wireless Systems

Cellular, WLAN,

WLL, WiMAX, Satellite

Ad Hoc Wireless Systems

Packet Radios

Sensor dust, mesh

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Satellite – Wide coverage and high mobility

Cellular networks – High mobility

Wireless LANs, Wireless Local Loop, etc – Low/None mobility

Wireless Networks - Infrastructure

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Wireless Networks - Ad Hoc

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Evolution of Wireless Networks

1st generation: analog - voice

AMPS with manual roaming

Cordless phones

Packet radio

2nd generation: digital - voice, data

Cellular & PCS with seamless roaming and integrated paging (IS-95, IS-136, GSM)

Multizone digital cordless

wireless LANs (IEEE 802.11), MANs (Metricom), and WANs (CDPD)

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The 3nd Generation

Wide-area mobile voice/data

2.5G: GPRS, EDGE

3G standards: UMTS,/IMT2000, Wideband CDMA, CDMA2000

Wireless Local Loop (IEEE 802.16)

LMDS (local multipoint distribution) 24-28GHz

MMDS below 5 GHz

WiMAX

Higher-speed WLAN

802.11b (2.4GHz, 11 Mbps), IEEE 802.11a (5GHz, 54 Mbps & higher)

HyperLAN

Personal area networks

Bluetooth, 802.15

Wireless device networks

Sensor networks, wirelessly networked robots

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Evolution Path

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Attributes of Wireless Access

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New Paradigm in Wireless Design

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Wireless Channel

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Wireless Channel

Pr ~ 1/r2

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Multi-path Propagation

Received signal is made up of several paths which can be classified as:

1. Direct Path

2. Reflected Path

3. Scattered Path

4. Diffracted Path

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2

3

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Line Of Sight (LOS) Non Line Of Sight (NLOS)

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Other Basic Propagation Mechanisms

Reflection: It occurs when a propagating electromagnetic wave intrudes upon an object which has very large dimensions when compared to the wavelength of the propagating wave. Reflection occurs from the surface of the earth and from buildings and walls.

Diffraction: It occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities (edges). The secondary waves resulting from the obstructing surface are present throughout the space and even behind the obstacle, giving rise to a bending of waves around the obstacle, even if the line of sight path does not exist between the transmitter and the receiver.

Scattering: It occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength, and where the number of obstacles per unit volume is large. Scattered waves are produced by rough surfaces, small objects, or by other irregularities in the channel.

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LOS & NLOS Scenarios

LOS (Line of Sight): The equations shown below hold only for LOS scenarios, where direct paths of electromagnetic rays exist. Since, the received signal is directly received at the receiver the effects such as reflection, diffraction and scattering doesn’t affect the signal reception that much.

NLOS (Non Line of Sight): When the direct LOS between transmitter and receiver is lost the effects such as reflection, diffraction and scattering become very important as in the absence of direct path they become the main contributors to the received signal at the receiver.

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2

3

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Line Of Sight (LOS) Non Line Of Sight (NLOS)

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Shadowing – Slow Fading

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Slow Fading (Shadowing)

Shadowing: It is the term given to the slow variations in received signal power as the user moves through the environment, especially behind large buildings or near by hills. These variations occur approx. 1 -2 times per second, that’s why Slow Fading!

Reflected

Scattered Path

Diffracted Path

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4

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Shadowing: Behavior Prediction and

Mathematical Modeling Behavior of the Constraint

P & 1/d4

Equipment Developed

Receiver and transmit Antennas

Amplifier (at the transmitter to increase the power)

Factors affecting this behavior

PT (Transmit power)

GT (Transmit Antenna Gain)

GR (Receiver Antenna Gain)

Effective Area of Antenna

Note: This effect can be mitigated by increasing the power using Amp.

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Slow Fading (Shadowing)

PR= PT GT GR (λ / 4 π) 2 x 1/d4

PT = Transmit power (Watts)

PR = Received Power (watts)

GT = Transmit Antenna Gain – relative to isotropic source (no unit)

GR = Receiver Antenna Gain – relative to isotropic source (no unit)

λ = Carrier’s Wavelength (λ = c / f) (meters)

d = Distance between transmitter and receiver (meters)

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Non Line Of Sight (NLOS)

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The Effects of Multipath Propagation

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Types of Fading

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Fast Fading in Mobile terrestrial Channel

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2

3

4

0o 180o

270o

90o

200o 300o

This can be attributed to the phasor addition of various multi-path signals.100-200 times/sec, that’s why Fast Fading!

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Fast Fading in Mobile terrestrial Channel

Constructive interference takes place when two or more rays arrive in-phase (or almost in-phase) with each other

Destructive interference takes place when two or more rays arrive anti-phase (or almost out-of-phase) with each other. This also means rays arriving 180o apart from each other

Semi-constructive/destructive

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Non-line-of-sight case (k=0)

Rayleigh Fading

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Line-of-sight case (k>1)

Rician Fading

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K-Factor

K-Factor is the ratio of power of a dominant (LOS) path to the power of the random components (/scatter)

For cases where LOS component is week (Rayleigh), the K-factor will be small (in some cases negative). However, if the line of sight dominates (Rician), the K-factor will normally take positive values between 5 and 10 dB.

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BER for Various Fading Conditions

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Types of Small-scale Fading

Fading Effects Due to Multipath Time Delay Spread-Flat fading

It is the most common type of fading described in the technical literature.

The spectral characteristics of the transmitted signals are preserved at the receiver, however the strength of the received signal changes with time.

Flat fading channels are known as amplitude varying channels or narrow-band channels.

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Types of Small-scale Fading

Fading Effects Due to Multipath Time Delay Spread- Frequency Selective Fading

Frequency selective fading is due to time dispersion of the transmitted symbols within the channel. Thus the channel brings on inter-symbol-interference.

Computer generated impulse responses are used for analyzing frequency selective small-scale fading.

Frequency selective fading channels are known as wideband channels since the BW of the signal is wider than the BW of the channel impulse response.

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Types of Small-scale Fading

Flat fading channel characteristics

,ths(t) r(t)

s(t) ,th

r(t)

t t t 0 0 0 Ts Ts+τ τ τ<<Ts

fc

S(f)

f fc

f

R(f)

f fc

H(f)

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Types of Small-scale Fading

Frequenecy selective fading channel characteristics.

,ths(t) r(t)

s(t) ,th

r(t)

t t t 0 0 0 Ts Ts+τ τ τ<<Ts

fc

S(f)

f fc

f

R(f)

f fc

H(f)

Ts

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Doppler Effect/Shift

The Doppler effect, named after Christian Doppler, is the change in frequency and wavelength of a wave that is perceived by an observer moving relative to the source of the waves.

For waves, such as sound waves, that propagate in a wave medium, the velocity of the observer and of the source are reckoned relative to the medium in which the waves are transmitted.

The total Doppler effect may therefore result from either motion of the source or motion of the observer.

Example: As the train approaches the station sound pitch is increased and as it leaves pitch starts decreasing.

This Phenomenon of sound waves was discovered by the Dutch scientist Christoph Hendrik Diederik Buys Ballot in 1845. Later Hippolyte Fizeau discovered independently the same phenomenon on electromagnetic waves in 1848.

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Doppler Effect/Shift

For waves that travel at the speed of light, the mathematical model of this phenomenon is as follows:

fdoppler = fv/c cos θ

f’= f + fdoppler

Where

f’ = observed frequency (Hz)

fdoppler = Doppler Frequency (Hz)

V = the velocity of the transmitter relative to the receiver (meters/second)

θ = Arrival angle (degrees)

c = speed of light = 3 x 108 (meters/second)

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Doppler Effect

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Error Compensation Mechanisms in Wireless Channels

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Adaptive Equalization

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ARQ & FEC

Error detection codes

Detects the presence of an error

Automatic repeat request (ARQ) protocols

Block of data with error is discarded

Transmitter retransmits that block of data

ACK and NACK

Retransmissions --> excessive delay

Retransmission strategy not conveniently implemented

Error correction codes, or forward correction codes (FEC)

Designed to detect and correct errors

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Error Detection Process

Transmitter

For a given frame, an error-detecting code (check bits) is calculated from data bits

Check bits are appended to the data bits

Receiver

Separates incoming frame into data bits and check

bits

Calculates check bits from received data bits

Compares calculated check bits against received check bits

Detected error occurs if mismatch is found

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Error Detection Process

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Code Rate and Redundancy

In case of block codes, encoder transforms each k-bit data block into a larger block of n-bits called code bits or channel symbol

The (n-k) bits added to each data block are called redundant bits, parity bits or check bits

They carry no new information

Ratio of redundant bits to data bits: (n-k)/k is called redundancy of code

Ratio of data bits to total bits, k/n is called code rate

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Block Error Correction Codes

Transmitter

Forward error correction (FEC) encoder maps each

k-bit block into an n-bit block codeword

Codeword is transmitted

Receiver

Incoming signal is demodulated

Block passed through an FEC decoder

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Forward Error Correction Process

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FEC Decoder Outcomes

No errors present

Codeword produced by decoder matches original codeword

Decoder detects and corrects bit errors

Decoder detects but cannot correct bit errors;

reports un-correctable error

Decoder detects no bit errors, though errors are present