01 wireless com1 introductionsite.iugaza.edu.ps/ahdrouss/files/2010/03/01-wireless...signal...

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© Ammar Abu-Hudrouss Islamic University Gaza ١

Wireless CommunicationsWireless Communications

Slide 2Wireless Communications

Course SyllabusCourse Syllabus

Text Book Andrea Goldsmith, Wireless Communications, Cambridge

University Press 2005.References1. Rappaport, Wireless Communications: Principles and Practice,

Prentice Hall 2nd Ed 2. D. N. C. Tse and P. Viswanath, Fundamentals of Wireless

Communication, Cambridge, U.K., 2005

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Course SyllabusCourse Syllabus

Course Content:

Statistical multipath channel models (ch. 3).

Capacity of wireless channel: (ch 4).

Diversity techniques for the receiver and the transmitter (ch 7).

Multiple antenna and space time communications (ch. 10)

Orthogonal Frequency Division Multiplexing (OFDM) (ch. 12)

Spread spectrum : DSSS and FSSSS (ch. 13)

Multiple access, Random access, power control (ch. 14)

Cellular systems and infrastructure-based wireless networks (ch. 15)


Course SyllabusCourse SyllabusGrading PolicyThe final course grade will be distributed as follows:

Quizzes and class activity 10 %Programming and simulation assignments 20 %Research topic and presentation 15%Midterm exam 25 %Final exam 30 %

Plagiarism will not be tolerated at any case. Copying homework from your colleagues or project from any source will lead to severe consequences.

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Wireless Communications

Satellite TV Cordless phone Cellular phone Wireless LAN, WIFI Wireless MAN, WIMAX Bluetooth Ultra Wide Band Wireless Laser Microwave GPS Ad hoc/Sensor Networks


Basic Concepts

Simplex, half-duplex, and full duplex Base Station Mobile Station Subscriber Transceiver Mobile Switching centre Control Channel Roamer Handoff Page

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Pager System


Pager System

Broad coverage for short messaging

Message broadcast from all base stations

Simple terminals

Optimized for 1-way transmission

Overtaken by cellular

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Cordless phone

DC2 and DECT standards


Bluetooth

Cable replacement RF technology (low cost) Short range (10m, extendable to 100m) 2.4 GHz band (crowded) 1 Data (700 Kbps) and 3 voice channels, up to 3 Mbps Widely supported by telecommunications, PC, and consumer

electronics companies

Few applications beyond cable replacement

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IEEE 802.15.4 / ZigBee Radios

Low-Rate WPAN Data rates of 20, 40, 250 Kbps Support for large mesh networking or star clusters Support for low latency devices CSMA-CA channel access Very low power consumption Frequency of operation in ISM bands

Focus is primarily on low power sensor networks


Satellite Systems

Cover very large areas Different orbit heights

GEOs (39000 Km) versus LEOs (2000 Km) Optimized for one-way transmission

Most two-way systems struggling or bankrupt Global Positioning System (GPS) use growing

Satellite signals used to pinpoint locationPopular in cell phones, PDAs, and navigation devices

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Cellular System

Mobile identification number (MIN) electronic serial number (ESN)


2G to 3G evolution

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4G and LTE (long term evolution)

OFDM/MIMO Much higher data rates (50-100 Mbps) Greater spectral efficiency (bits/s/Hz) Flexible use of up to 100 MHz of spectrum Low packet latency (<5ms). Increased system capacity Reduced cost-per-bit Support for multimedia


Wireless Local Area Networks (WLANs)

WLANs connect “local” computers (100m range) Breaks data into packets Channel access is shared (random access) Backbone Internet provides best-effort service

Poor performance in some apps (e.g. video)

01011011InternetAccessPoint

0101 1011

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Wifi Networks (Supporting Multimedia)

802.11n++

Wireless HDTVand Gaming

• Streaming video• Gbps data rates• High reliability• Coverage in every room


Wimax (802.16)

Wide area wireless network standardSystem architecture similar to cellularHopes to compete with cellular

OFDM/MIMO is core link technology Operates in 2.5 and 3.5 MHz bands

Different for different countries, 5.8 also used.Bandwidth is 3.5-10 MHz

Fixed (802.16d) vs. Mobile (802.16e) WimaxFixed: 75 Mbps max, up to 50 mile cell radiusMobile: 15 Mbps max, up to 1-2 mile cell radius

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Characteristic of Wireless Channel


Channel Impulse Response

)(tx

Channel

)(ty

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Channel Impulse Response Response of channel at t to impulse at t-:

t is time when impulse response is observedt- is time when impulse put into the channelis how long ago impulse was put into the channel for the current observation path delay for MP component currently observed

))(()(),(1

)( tettc n

N

n

tjn

n


Received Signal Characteristics

Received signal consists of many multipath components Amplitudes change slowly Phases change rapidly

Constructive and destructive addition of signal componentsAmplitude fading of received signal (both wideband and narrowband signals)

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Major Categories of Fading

Large Scale Fading :This is the loss that propagation models try to

account for mostly dependant on the distance from the transmitter to the receiver also known as Large Scale Path Loss, Log-Normal Fading or Shadowing

Small Scale Fading :Could be 20-30 dB over a fraction of a wavelength.

It is Caused by the superposition or cancellation of multipath propagation signals, the speed of the transmitter or receiver or the bandwidth of the transmitted signal. It is also known as Multipath Fading or Rayleigh Fading


Small Scale Fading:

The type of fading experienced by a signal propagating through a channel can be determined by the nature of the transmitted signal with respect to the characteristics of the channel.

Factors influencing small scale fading:Factors influencing small scale fading:• Multipath propagation.• Speed of the mobile.• Speed of the surrounding objects.• Transmission bandwidth of the signal.

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Inter symbol interference

A

B

C

D = A+B+C

A


Power delay Profile

Rec

eive

d S

igna

l Lev

el (

dBm

)

-105

-100

-95

-90

-90

0 50 100 150 200 250 300 350 400 450

Excess Delay (ns)

RMS Delay Spread () = 46.4 ns

Mean Excess delay () = 45 ns

Maximum Excess delay < 10 dB = 110 ns

Noise threshold

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Delay Spread:

Mean excess delay RMS delay spread Excess delay spread

Mean excess delay is the first moment of the power delay profile and is defined by the equation

hk

hkk

kk

kkk

P

P

a

a

)(

)(

2

2


Maximum excess delay is defined as the ,where , is the first arriving signal and is the maximum delay at which a multipath component is within X dB of the strongest arriving multipath signal.

RMS delay spread is the square root of the second central moment of the power delay profile and is defined by the equation:

where

22 )(

hk

hkk

kk

kkk

P

P

a

a

)(

)( 2

2

22

2

0 xx0

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Example (Power delay profile)

-30 dB

-20 dB

-10 dB

0 dB

0 1 2 5

Pr()

(µs)

s 38.4]11.01.001.0[

)0)(01.0()2)(1.0()1)(1.0()5)(1(_

22222_

2 07.21]11.01.001.0[

)0)(01.0()2)(1.0()1)(1.0()5)(1(s

s 37.1)38.4(07.21 2

1.37 µs4.38 µs


RMS delay spread and coherence b/w

RMS delay spread and coherence b/w (Bc) are inversely proportional

1cB

.501

cB For 0.9 correlation

.51

cB For 0.5 correlation

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Coherence Bandwidth

)(tx

Time domain view

High correlation of amplitudebetween two different freq.components

Range of freq overwhich response is flat

Bc delay spread

)( fX

Freq. domain view


Time dispersive nature of channel

RMS delay spread () Coherence b/w (Bc)

Time domain view Freq domain view

Delay spread and coherence bandwidth are parameters whichdescribe the time dispersive nature of the channel.

channel 1

channel 2

channel 3

Sig

nal

Cha

nnel

Symbol Time (Ts) Signal bandwidth (Bs)

signal 1

signal 2

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Revisit Example (Power delay profile)

-30 dB

-20 dB

-10 dB

0 dB

0 1 2 5

Pr()

(µs)

s 38.4_

2_2 07.21 s

s 37.1

1.37 µs4.38 µs

kHzBcoherence c 146.51)%50(

Signal bandwidth for Analog Cellular = 30 KHzSignal bandwidht for GSM = 200 KHz


Coherence bandwidth:

It is the range of frequencies over which two frequency components have a potential for amplitude correlation.

If two sinusoids with a frequency separation of greater than Bc are propagating in the same channel, they are affected quite differently by the channel.

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Doppler Effect


Doppler Shift

cosvf v

Doppler shift

Example- Carrier frequency fc= 1850 MHz (i.e. = 16.2 cm)- Vehicle speed v = 60 mph = 26.82 m/s

- If the vehicle is moving directly towards the transmitter

- If the vehicle is moving perpendicular to the angle of arrival of the transmitted signal

Hzf 165162.082.26

0f

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Doppler Spread and Coherence Time

Doppler spread and Coherence Time take into account the relative motion between mobile and base station, or by movements of objects in the channel.

They describe the time varying nature of the channel in a small scale region.


Doppler Spread Bd:When a signal of frequency fc is transmitted,

the received signal spectrum, called the Doppler spectrum, will have components fc - fd to fc + fd, where fd is the Doppler shift.

Coherence time Tc:It is used to characterize the time varying

nature of the frequency depressiveness of the channel in the time domain

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For high correlation

For correlation above 0.5

Mean of the previous two equation is usually used in digital communication systems

mc fT 1

mc fT

169

mc fT 423.0


Time varying nature of channel

Coherence Time (TC) Doppler spread (BD)

Symbol Time (TS) Signal bandwidth (BS)

Time domain view Freq domain view

Doppler spread and coherence time are parameters whichdescribe the time varying nature of the channel.

channel 1

channel 2

channel 3

Sig

nal

Cha

nnel

signal 1

signal 2

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Small Scale Fading:

Different types of transmitted signals undergo different types of fading depending upon the relation between the

Signal Parameters: Bandwidth, Symbol Periodand

Channel Parameters: RMS Delay Spread,Doppler Spread

In any mobile radio channel a wave can be dispersed either in Time or in Frequency.

These time and frequency dispersion mechanisms lead to four possible distinct effects which depend on the nature of transmitted signal, the channel and the velocity.


Flat Fading:

A received signal is said to have underwent Flat Fading if “The Mobile Radio Channel has a constant gain and linear phase response over a Bandwidth which is greater than the Bandwidth of the transmitted Signal”

Fading in which all frequency components of a received radio signal vary in the same proportion simultaneously

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Here the multipath structure of the channel is such that spectral characteristics of the transmitted signal are preserved at the receiver But due to the fluctuations in the gain of the channel caused by multipath, the signal strength varies with time


From the figure we can note that if the channel gain varies with time, a change of amplitude of the received signal occurs.

From the figure we can note that the spectrum of the received signal r (t) is preserved even though there is a change in gain.

Flat fading channels are also referred as amplitude varying channels or narrow band channels, since the bandwidth of the applied signal is narrow as compared to the channel flat fading bandwidth.

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Typical Flat fading channels cause deep fades To achieve low bit error rates during times of deep fades, Flat fading channels operate at 20 to 30dB more transmitter power compared to the systems operating over non-fading channels.

Rayleigh distribution is the most common amplitude distribution.

According to this distribution, Rayleigh Flat fading channel model assumes that the channel induces an amplitude which varies in time.


Summary

Signal undergoes Flat Fading if: Bs<<Bc

where Bs is bandwidth and Bc is the coherence bandwidth of the channel

And Ts>>

whereTs is the reciprocal bandwidth and rms delay spread.

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Frequency Selective Fading:

The channel creates frequency selective fading on the received signal when the channel possesses a constant gain and linear phase response over a bandwidth, which is smaller than the bandwidth of the transmitted signal

Under these conditions the channel impulse response has a multipath delay spread which is greater than the reciprocal bandwidth of the transmitted message waveform So the received signal includes multiple versions of the transmitted waveform, which are attenuated and delayed in time, and hence the received signal is distorted.


Frequency selective fading is much difficult to model than flat fading channels because each multipath signal must be modeled and the channel must be considered to be a linear filter.

It is for this reason that wideband multipath measurements are made and models are developed from these measurements.

When analyzing mobile communication systems, statistical impulse response models such as the 2-ray Rayleigh model or computer generated or measured impulse responses are generally used for analyzing frequency selective small-scale fading.

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For frequency selective fading, the spectrum S(f) of the transmitted signal has a bandwidth which is greater than the coherence bandwidth Bc of the channel.

Frequency selective fading is caused by multipath delays which approach or exceed the symbol period of the transmitted symbol.

These channels are also known as wideband channels since the bandwidth of the signal s(t) is wider than the bandwidth of the channel impulse response.

As time varies, the channel varies in gain and phase across the spectrum of s(t),resulting in time varying distortion in the received signal r(t)

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Summary

Signal undergoes Frequency Selective Fading if:

Bs>Bc

where

Bs is bandwidth and

Bc is the coherence bandwidth of the channel

And

Ts<where

Ts is the reciprocal bandwidth and

rms delay spread.


Small scale fading

Multi path time delay

Doppler spread

Flat fading BC

BS

Frequency selective fading BC

BS

TC

TSSlow fading

Fast fading TC

TS

fading

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Rayleigh Fading Distribution:

Rayleigh Fading Distribution in mobile radio channels is commonly used to describe the statistical time varying nature of the received envelope of a flat fading signal or the envelope of an individual multipath component.

where is the rms value of the received voltage signal before

envelope detection,is the time-average power of the received signal before

envelope detection.

2

0 (r < 0)

)0(2

exp)( 2

2

2

rrrrp


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The variance of the Rayleigh distribution is given byr

which represents the ac power in the signal envelope.

The rms value of the envelope is 2

2)(][][

2

0

2222

drrprrErEr

22 4292.02

2


The median value of r is found by solving

Note: It is customary to use median values instead of the mean values, since fading data are usually measured in the field and a particular distribution cannot be assumed. By using median values instead of mean values it is easier to compare different fading distributions which have widely varying means

medianr

drrp0

)(21

177.1medianr

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Ricean Fading Distribution:

When there is a dominant stationary (nonfading) signal component present, such as a line-of-sight propagation path, the small scale-scale fading envelope distribution is Ricean.

Random multipath components arriving at different angles are superimposed on a stationary dominant signal

At the output of an envelope detector this has the effect of adding a dc component to the random multipath


The effect of a dominant signal arriving with many weaker multipath signals gives rise to Ricean distribution

As the dominant signal becomes weaker, the composite signal gives resembles a noise signal which has an envelope that is Rayleigh

Thus, the Ricean distribution degenerates to a Rayleigh distribution when the dominant component fades away

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The Ricean distribution is given by

Where A denotes peak amplitude of the dominant signalIo(.) is the modified Bessel function of the first kind and

zero order

202

)(

22

22

)(

ArIerrpAr For A 0 ,r 0

0 For r< 0


Ricean Factor K completes determines the Riceandistribution.

As A 0, K - dB, and as the dominant path decreases in amplitude, the Ricean distribution degenerates to Rayleigh distribution

01 wireless com1 introductionsite.iugaza.edu.ps/ahdrouss/files/2010/03/01-wireless...signal...

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