Wireless Communication

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Wireless Communication

Lecture 1

Wireless Fundamentals

Ammar Karim

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Course Division1. 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

thanlaying 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 WirelessEach 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 PropagationReceived 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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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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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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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)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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0o180o

270o

90o

200o300o

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 tt0 0 0Ts Ts+ττ τ<<Ts

fc

S(f)

ffc

f

R(f)

ffc

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 tt0 0 0Ts Ts+ττ τ<<Ts

fc

S(f)

ffc

f

R(f)

ffc

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