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Wireless and Mobile Communication

Lecture 1

Wireless Fundamentals

Shibli Nisar

NU-FAST

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Course Division1. Fundamentals

History & Evolution of wireless systems, various impairments in wireless channels. Multi-path signal propagation, Fading and its types, Doppler Effect, Error compensation mechanisms in wireless channels

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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Tentative Grading

Assignments = 5% Quizzes = 10% Mids = 30% (15% each) Final Exam = 55%

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Wireless communication

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

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Forms of Communication

Analogue & Digital Which one is Better?

Digital?Why?

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Good processing techniques are available for digital signals, such as

Data compression (or source coding)

Error Correction (or channel coding)

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Types of Media/Environments used for Communication

Wireless & Wired Why Wireless is better than Wired ?

Reduced Cost (cheap infrastructure) Cabling very critical Developing nations utilize cellular telephony rather

thanlaying twisted-pair wires to each home

Flexibility-Time Can easily set-up temporary LANs Disaster situations

Only use resources when sending or receiving a signal

User Mobility

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Wireless vs. Wired 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

Wireless vs. Wired Communication

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Wireless vs. Wired Communication

Noisy, time-varying channel 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, CDMA Ad-hoc

Fixed Wireless Systems WiMax(IEEE 802.16a) MMDS, LMDS

Infrastructure Dependent Wireless Systems Cellular WLL, WiMAX, Satellite

Ad Hoc Wireless Systems

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Satellite – Wide coverage and high mobility Cellular networks – High mobility Wireless LANs etc – Low/None mobility

Wireless Networks - Infrastructure

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Wireless Networks - Ad Hoc

Adhoc Network

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Cellular Networks

First Generation Analog Systems with Voice Traffic

Analog Modulation, mostly FM

FDMA/FDD multiple access

Second Generation (2G) Digital Systems

Digital Modulation

Voice Traffic

TDMA/FDD and CDMA/FDD multiple access

2.5G Digital Systems

Voice + Low-data rate

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Third Generation Digital

Voice + High-data rate

Multimedia Transmission

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Evolution Path

Gain

An increase in an RF signal's amplitude Usually an active process; an external power

source, such as an RF amplifier, is used to amplify the signal or a high-gain antenna is used to focus the beamwidth of a signal to increase its signal amplitude

Increasing the RF signal's strength may have a positive or a negative result

Typically, more power is better, but when a transmitter is radiating power very close to the legal power output limit, where added power would be a serious problem

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Gain

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Loss

Loss describes a decrease in signal strength Reasons

Resistance of cables and connectors causes loss due to the converting of the AC signal to heat

Impedance mismatches in the cables and connectors can cause power to be reflected back toward the source, which can cause signal degradation

Objects directly in the propagated wave's transmission path can absorb, reflect, or destroy RF signals

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Loss

Measurement and compensation for loss in RF connection or circuit is important because radios have a receive sensitivity threshold

A sensitivity threshold is defined as the point at which a radio can clearly distinguish a signal from background noise

If losses occur between the transmitter and receiver, the problem must be corrected either by removing the objects causing loss or by increasing the transmission power

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Loss

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Radio Propagation Mechanisms

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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)

Multipath Propagation

Reflection Occurs when signal encounters a surface that is

large relative to the wavelength of the signal

Diffraction Occurs at the edge of an impenetrable body

that is large compared to wavelength of radio wave

Scattering Occurs when incoming signal hits an object

whose size in the order of the wavelength of the signal or less

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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): 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)

Multipath

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Multipath is defined as the composition of a primary signal plus duplicate or echoed wave fronts caused by reflections of waves off objects between the transmitter and receiver

The delay between the instant that the first main signal arrives and the instant that the last reflected signal arrives is known as delay spread

Effects of Multipath

Multipath can cause several different conditions, all of which can affect the transmission of the RF signal differently

These conditions include: Decreased Signal Amplitude

(downfade) Corruption Nulling Increased Signal Amplitude (upfade)

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Decreased Signal Amplitude

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Corruption

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Nulling

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Increased Signal Amplitude

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Antenna Diversity

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Characteristics of the Wireless Channels (Impairments)

Susceptible to a variety of transmission impediments/hindrance

Attenuation and attenuation distortion

Path Loss or Free space loss

Noise

Atmospheric absorption

Multipath

Refraction

Thermal noise

These factors restricts the range, data rate, and reliability of the wireless channel

Effects depends upon the environmental conditions and the mobility of the transmission and receiver

Attenuation

Strength of signal falls off with distance over transmission medium

Guided media Attenuation is generally logarithmic and typically expresses

as a constant number of decibels per unit distance Unguided media

Attenuation is a more complex function of distance makeup of atmosphere

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

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Path Loss Or Free Space Loss

For any type of wireless communication the signal disperses with distance

This form of attenuation is also expressed as Free space loss

Important for designing and deploying the wireless communication networks

Dependent of Radio frequency used

Nature of the Territory Distance

Different estimation (model) for different environment

Noise

For any data transmission event, the received signal will consists of Transmitted signal, modified by the various distortions imposed

by the transmission systems Additional unwanted signals that are inserted somewhere

between transmission and reception

These unwanted signals are referred as noise Thermal Noise Intermodulation noise Crosstalk Impulse Noise

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Thermal Noise

Thermal noise due to agitation of electrons Present in all electronic devices and transmission

media Cannot be eliminated Function of temperature Particularly significant for satellite communication

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Noise

Intermodulation noise – occurs if signals with different frequencies share the same medium Interference caused by a signal produced at a frequency that is the

sum or difference of two original frequencies or multiple of those frequencies

Crosstalk – unwanted coupling between signal paths, can also occur when unwanted signals are picked up by microwave antennas Often dominates in ISM bands

Impulse noise – irregular pulses or noise spikes Caused by external electromagnetic disturbances, or faults and flaws

in the communications system

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Other Impairments

Atmospheric absorption – water vapor and oxygen contribute to attenuation

Multipath propagation – obstacles reflect signals so that multiple copies with varying delays are received

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Multipath Propagation

Reflection Occurs when signal encounters a surface that is

large relative to the wavelength of the signal

Diffraction Occurs at the edge of an impenetrable body

that is large compared to wavelength of radio wave

Scattering Occurs when incoming signal hits an object

whose size in the order of the wavelength of the signal or less

Multipath Propagation

Effects of Multipath Propagation

Multiple copies of a signal may arrive at different phases If phases add

destructively, the signal level relative to noise declines, making detection more difficult

Inter-symbol interference (ISI) One or more delayed

copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit

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Fading

Fluctuation in signal strength when received at the receiver

Four types Fast fading Or small-scale fading Slow fading Or large-scale fading Flat fading Selective fading

Fixed environment Affected by changes in environmental conditions, such as

rainfall or electric noise Mobile environment

Effect of fading is more complex - mobility Effects

Add signals constructively or destructively at the receiver, causing a variation in the power level of the received signal

Introduce bit error rate and packet error rate

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Fast Fading

Rapid fluctuations in the amplitude, phase, or multipath delays of the received signal

Reasons Due to the interference between multiple version (copies)

of the same transmitted signal arriving at the receiver at slightly different times

Delayed Spread Time between the reception of the first version of the

signal and the last echoed signal is called delayed spread

Reasons Occurs because of the three mechanism, reflection,

diffraction, and scattering

Error control coding, interleaving, frequency hopping and diversity is used to mitigate the effect 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

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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. 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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Slow Fading or Shadow Fading

Cause the received signal power to vary, though the distance between the transmitter and receiver remains the same

Occurs when objects that partially absorb the transmission lie between the transmitter and receiver

It is called Slow Fading because the variations are much slower as compare to other fading phenomena

Also referred to as Shadow Fading since the objects that cause the fade, (buildings or other structures), block the direct transmission path from the transmitter to the receiver

A fade margin is added as an additional signal power to overcome the problem

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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)

Two More Types of Fading

Flat Fading All frequency components of the

received signal fluctuate in the same proportions simultaneously

Selective Fading Affects unequally the different spectral

components of a radio signal

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Interference

Adjacent channel interference Reason

Signals in nearby frequencies have components outside their allocated ranges, these components may interferes with on-going transmissions in the adjacent frequencies

Avoidance Introduce guard bands between the allocated

frequency ranges

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Interference

Co-channel interference/narrow band interference Reason

Due to other nearby systems using the same transmission frequency

Avoidance Can be minimized with the use of multiuser

detection mechanisms, directional antennas, and dynamic channel allocation methods