02 Wireless Transmission

8/4/2019 02 Wireless Transmission

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Wireless Transmission 1-1

zgr Koray AHNGZYrd.Do.Dr.

stanbul Kltr niversitesi

DataData CommunicationCommunicationAnalogAnalog vsvs DigitalDigital


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Electromagnetic Signal

Electromagnetic signals(ES) are used as a means totransmit information

ES is a function of time

Can also be expressed as a function of frequency Signal consists of components of different frequencies


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Analog and Digital Waveforms

Speech

Binary 1s and 0s

Analog signal - signalintensity varies in asmooth fashion overtime No breaks or

discontinuities in thesignal

Digital signal - signalintensity maintains aconstant level for some

period of time andthen changes toanother constant level


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Periodic Signals

Periodic Signal; Samesignal patters repeatsover time

s(t+T ) = s(t ) -< t < + where Tis the period of the

signal

T is the smallest value thatsatisfies the equation.


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Time-Domain Concepts

Aperiodic signal - analog or digital signal patternthat doesn't repeat over time

Peak amplitude (A) - maximum value or strength

of the signal over time; typically measured in volts

Frequency (f ) Rate, in cycles per second, or Hertz (Hz) at which the

signal repeats


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Time-Domain Concepts

Period (T ) - amount of time it takes for one repetition ofthe signal

T= 1 /f

Phase () - measure of the relative position in time withina single period of a signal Wavelength () - distance occupied by a single cycle of

the signal Or, the distance between two points of corresponding phase of two

consecutive cycles


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Sine Wave Parameters

General sine waves(t ) = A sin(2ft + )

Figure (next slide)shows the effect ofvarying each of the three parameters (a) A = 1, f= 1 Hz, = 0; thus T= 1s (b) Reduced peak amplitude; A=0.5 (c) Increased frequency; f= 2, thus T= (d) Phase shift; = /4 radians (45 degrees)

note: 2 radians = 360 = 1 period


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Sine Wave Parameters


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Frequency-Domain Concepts

Fundamental frequency - when all frequencycomponents of a signal are integer multiples of onefrequency, its referred to as the fundamentalfrequency

Spectrum - range of frequencies that a signalcontains

Absolute bandwidth - width of the spectrum of asignal

Effective bandwidth (or just bandwidth) - narrow

band of frequencies that most of the signalsenergy is contained in


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Frequency

Components Any electromagnetic

signal can be shown toconsist of a collectionof periodic analog

signals (sine waves) atdifferent amplitudes,frequencies, and phases

The period of the total

signal is equal to theperiod of thefundamental frequency


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Relationship between Data Rate and Bandwidth

The greater the bandwidth, the higher theinformation-carrying capacityAny digital waveform will have infinite bandwidth BUT the transmission system will limit the

bandwidth that can be transmittedAND, for any given medium, the greater thebandwidth transmitted, the greater the cost

HOWEVER, limiting the bandwidth createsdistortions


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Data Communication Terms

Data - entities that convey meaning, or information

Signals - electric or electromagnetic

representations of data

Transmission - communication of data by thepropagation and processing of signals


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Examples of Analog and Digital Data

Analog Video

Audio

Digital Text

Integers


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Analog Signals

A continuously varying electromagnetic wave thatmay be propagated over a variety of media,depending on frequency

Examples of media: Copper wire media (twisted pair and coaxial cable) Fiber optic cable Atmosphere or space propagation

Analog signals can propagate analog and digitaldata


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Conversion of Voice Input into Analog Signal


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Conversion of PC Input to Digital Signal


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Digital Signals

A sequence of voltage pulses that may betransmitted over a copper wire mediumGenerally cheaper than analog signaling Less susceptible to noise interference

Suffer more from attenuation(g azalmas)Digital signals can propagate analog and digital

data


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Attenuation of Digital Signals

A digital signal is a sequence of voltage pulses thatmay be transmitted over a copper wire medium; forexample a constant positive voltage level mayrepresent binary 0 and a constant negative voltage

level may represent binary 1.


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Analog Signals Carrying Analog and Digital Data


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Digital Signals Carrying Analog and Digital Data


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Analog Transmission

Transmit analog signals without regard to content

Attenuation limits length of transmission link

Cascaded amplifiers boost signals energy forlonger distances but cause distortion Analog data can tolerate distortion

Introduces errors in digital data


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Digital Transmission

Concerned with the content of the signal

Attenuation endangers integrity of data

Digital Signal Repeaters achieve greater distance Repeaters recover the signal and retransmit


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Modulation Digital modulation

digital data is translated into an analog signal (baseband)

ASK, FSK, PSK - main focus in this chapter

differences in spectral efficiency, power efficiency, robustness

Analog modulation shifts center frequency of baseband signal up to the radio carrier

Motivation smaller antennas (e.g., /4)

Frequency Division Multiplexing

medium characteristics

Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM)

Phase Modulation (PM)


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Wireless Transmission1-24

Modulation and demodulation

synchronizationdecision

digitaldataanalog

demodulation

radiocarrier

analogbasebandsignal

101101001 radio receiver

digitalmodulation

digitaldata analog

modulation

radiocarrier

analogbasebandsignal

101101001 radio transmitter


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Digital modulation

Modulation of digital signals known as Shift KeyingAmplitude Shift Keying (ASK):

very simple

low bandwidth requirements

very susceptible to interference

1 0 1

t


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Amplitude Shift Keying (ASK)


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Frequency Shift Keying

Frequency-shift keying (FSK) is a frequencymodulation scheme in which digital information istransmitted through discrete frequency changesof a carrier wave.

needs larger bandwidth


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Frequency Shift Keying (FSK)


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Phase Shift Keying (PSK)

Phase-shift keying (PSK) is a digital modulationscheme that conveys data by changing, ormodulating, the phase of a reference signal (the

carrier wave).

more complex

robust against interference


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PSK Methods


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4 PSK Characteristics


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Quadrature amplitude modulation(QAM)

Quadrature amplitude modulation is a combinationof ASK and PSK so that a maximum contrastbetween each signal unit (bit, dibit, tribit, and soon) is achieved.

Quadrature amplitude modulation (QAM) is amodulation scheme which conveys data by changing(modulating) the amplitude of two carrier waves.These two waves, usually sinusoids, are out of

phase with each other by 90 and are thus calledquadrature carriers


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4 QAM and 8 QAM


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THe domain for 8 QAM


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1 6- QAM constellations


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Bit and Baud


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Assume we need to download text documents at the rate of

100 pages per minute. What is the required bit rate of the

channel?

SolutionA page is an average of 24 lines with 80 characters in each

line. If we assume that one character requires 8 bits, the bit

rate is

Example


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What is the bit rate for high-definition TV (HDTV)?

Solution

HDTV uses digital signals to broadcast high quality video

signals. The HDTV screen is normally a ratio of 16 : 9. There

are 1920 by 1080 pixels per screen, and the screen is renewed

30 times per second. Twenty-four bits represents one color

pixel.

The TV stations reduce this rate to 20 to 40 Mbps through

compression.

Example


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Impairments, such asnoise, limit data rate that canbe achieved

For digital data, to what extent do impairmentslimit data rate?

Channel Capacity the maximum rate at whichdata can be transmitted over a givencommunication path, or channel, under givenconditions

About Channel Capacity


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Concepts Related to Channel Capacity Data rate - rate at which data can be communicated (bps)

Bandwidth - the bandwidth of the transmitted signal asconstrained by the transmitter and the nature of thetransmission medium (Hertz)

Noise - average level of noise over the communicationspath

Error rate - rate at which errors occur Error = transmit 1 and receive 0; transmit 0 and receive 1


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Nyquist Bandwidth

For binary signals (two voltage levels)C= 2B

With multilevel signaling

C= 2B log2MM= number of discrete signal or voltage levels


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Ratio of the power in a signal to the powercontained in the noise thats present at a particularpoint in the transmission

Typically measured at a receiver

Signal-to-noise ratio (SNR, or S/N)

A high SNR means a high-quality signal, lownumber of required intermediate repeaters

SNR sets upper bound on achievable data rate

powernoise

powersignallog10)( 10dB SNR

Signal-to-Noise Ratio


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Equation:

Represents theoretical maximum that can be

achievedIn practice, only much lower rates achieved

Formula assumes white noise (thermal noise)

Impulse noise is not accounted forAttenuation distortion or delay distortion not

accounted for

SNR1log 2 BC

Shannon Capacity Formula


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Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB= 24 dB

Using Shannons formula

251SNR

SNRlog10dB24SNR

MHz1MHz3MHz4

10dB

B

Mbps88102511log10 626 C

Example of Nyquist and Shannon Formulations


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How many signaling levels are required?

16

log4

log102108

log2

2

2

66

2

M

M

MBC

Example of Nyquist and Shannon Formulations


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We need to send 265 kbps over a noiseless channel with abandwidth of 20 kHz. How many signal levels do we need?

Solution

We can use the Nyquist formula as shown:

Since this result is not a power of 2, we need to either

increase the number of levels or reduce the bit rate. If wehave 128 levels, the bit rate is 280 kbps. If we have 64 levels,

the bit rate is 240 kbps.

Example


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Consider a noiseless channel with a bandwidth of 3000 Hz

transmitting a signal with two signal levels. The maximum bit

rate can be calculated as

Example


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Consider the same noiseless channel transmitting a signal

with four signal levels (for each level, we send 2 bits). The

maximum bit rate can be calculated as

Example


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We can calculate the theoretical highest bit rate of a regulartelephone line. A telephone line normally has a bandwidth of

3000Hz. The signal-to-noise ratio is usually 3162. For this

channel the capacity is calculated as

This means that the highest bit rate for a telephone line is

34.860 kbps. If we want to send data faster than this, we can

either increase the bandwidth of the line or improve the

signal-to-noise ratio.

Example


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The signal-to-noise ratio is often given in decibels. Assumethat SNRdB = 36 and the channel bandwidth is 2 MHz. The

theoretical channel capacity can be calculated as

Example


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For practical purposes, when the SNR is very high, we can assume that SNR + 1 is almost the same as SNR. In these

cases, the theoretical channel capacity can be simplified to

For example, we can calculate the theoretical capacity of the

previous example as

Example


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We have a channel with a 1-MHz bandwidth. The SNR for

this channel is 63. What are the appropriate bit rate and

signal level?

Solution

First, we use the Shannon formula to find the upper limit.

Example


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The Shannon formula gives us 6 Mbps, the upper limit. For

better performance we choose something lower, 4 Mbps, for

example. Then we use the Nyquist formula to find the

number of signal levels.

Example


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The Shannon capacity gives us the upper

limit; the Nyquist formula tells us how many

signal levels we need.

Note


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In networking, we use the term bandwidth intwo contexts.

The first, bandwidth in hertz, refers tothe range of frequencies in a

composite signal or the range offrequencies that a channel can pass.

The second, bandwidth in bits per

second, refers to the speed of bittransmission in a channel or link.

Note


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zgr Koray AHNGZYrd.Do.Dr.

stanbul Kltr niversitesi

WirelessWireless TransmissionTransmission


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Frequencies for communication VLF = Very Low Frequency UHF = Ultra High Frequency

LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency

Frequency and wave length = c/f wave length , speed of light c 3x108m/s, frequency f

1 Mm300 Hz

10 km30 kHz

100 m3 MHz

1 m300 MHz

10 mm30 GHz

100 m3 THz

1 m300 THz

visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV

optical transmissioncoax cabletwistedpair


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Frequencies for mobile communication

VHF-/UHF-ranges for mobile radio simple, small antenna for cars deterministic propagation characteristics, reliable connections

SHF and higher for directed radio links, satellitecommunication

small antenna, beam forming large bandwidth available

Wireless LANs use frequencies in UHF to SHF range some systems planned up to EHF

limitations due to absorption by water and oxygen molecules

(resonance frequencies) weather dependent fading, signal loss caused by heavy

rainfall etc.


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Signals

Radio waves are used for multicastcommunications, such as radio and television, andpaging systems.

Microwaves are used for unicast communication

such as cellular telephones, satellite networks, andwireless LANs

Infrared signals can be used for short- rangecommunication in a closed area using line-of-sight

propagation.


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Signals physical representation of data function of time and location signal parameters: parameters representing the value of

data (what are they?) classification

continuous time/discrete time continuous values/discrete values analog signal = continuous time and continuous values digital signal = discrete time and discrete values

signal parameters of periodic signals:period T, frequency f=1/T, amplitude A, phase shift sine wave as special periodic signal for a carrier:

s(t) = At sin(2 ft t + t)


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Antennas

An antenna is an electrical conductor or system ofconductors Transmission - radiates electromagnetic energy into

space Reception - collects electromagnetic energy from

space In two-way communication, the same antenna can

be used for transmission and reception


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Types of Antennas

Whip Antenna

Isotropic antenna (idealized) Radiates power equally in all directions

Dipole antennas

Designed to receive particular wavelength Parabolic Reflective Antenna

Phased Array


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Whip Antennas

Radio antennas, cell phone antennas, walkie talkies, etc. Simple, cheap, easy to make

Generally low gain antennas

Higher gain off axis from the antenna, low gain on axis(above and below)

Low Sensitivity

High Sensitivity High Sensitivity


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Dipole antennas

These antennas are the simplest practical antennasfrom a theoretical point of view; the currentamplitude on such an antenna decreases uniformlyfrom maximum at the center to zero at the ends.


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Isotropic radiator

Radiation and reception of electromagnetic waves, coupling

of wires to space for radio transmission Isotropic radiator: equal radiation in all directions (three

dimensional) - only a theoretical reference antenna

Real antennas always have directive effects (vertically

and/or horizontally) Radiation pattern: measurement of radiation around an

antenna

zy

x

z

y x ideal

isotropicradiator


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Parabolic Dish Antenna

Satellite receivers, older Radar systems, radiotelescopes, etc.

Strong, and reasonably easy to produce, fairlycheap compared to other antennas with similar gainproperties

Low gain behind antenna and off to the sides, highgain in front of antenna Very Sensitive


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Phased Array Antenna

High gain in a small area, low gain everywhere else Consists of multiple antenna elements arranged in a plane

where the individual elements are wired together tocontrol directional gain

Allows the antenna to scan a volume quickly withoutphysically moving array

Very Sensitive


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Antennas: directed and sectorized

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

top view, 3 sector

x

z

top view, 6 sector

x

z

Often used for microwave connections or base stations for

mobile phones (e.g., radio coverage of a valley)

directedantenna

sectorized

antenna


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Example

What is the appropriate antenna size for a GSM1800MHz mobile phone? Hint: Generally, theantenna size is wavelength/4.

the relation between wavelength (L) and

frequency (f) is l = c / f l = c / f = (3x108)/(1800x106) = 1/6 mUsually, the antenna size is wavelength/4

so the appropriate size for a GSM 1800 mobilephone is 1/24 m,which is about 4. 1 67 cm.


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Example

What is the wavelength of 300 MHz frequencyis?

What is the frequency of 30 cm wavelength is ?


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Propagation Modes

Ground-wave propagation

Sky-wave propagation

Line-of-sight propagation


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Ground Wave Propagation

Follows contour of the earth

Can propagate considerable distances

Frequencies up to 2 MHz Example: AM radio (during the day)


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Sky Wave Propagation

Signal reflected from ionized layer of atmosphere back down to earth

Signal can travel a number of hops, back and forth betweenionosphere and earths surface Examples: amateur radio, CB radio, AM (at night)


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Line-of-Sight Propagation

Transmitting and receiving antennas must be within line ofsight Satellite communication signal above 30 MHz not reflected by

ionosphere Ground communication antennas within effectiveline of site due torefraction


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Signal propagation ranges

Transmission range communication possible low error rate

Detection range detection of the signal

possible

no communicationpossible

Interference range signal may not be

detected

signal adds to thebackground noise

distance

sender

transmission

detection

interference


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Real world example


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Multipath propagation Signal can take many different paths between sender and receiver due

to reflection, scattering, diffraction

Time dispersion: signal is dispersed over time interference with neighbor symbols, Inter Symbol Interference (ISI)

The signal reaches a receiver directly and phase shifted

distorted signal depending on the phases of the different parts

signal at sender

signal at receiver

LOS pulsesmultipathpulses


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Multiplexing

In telecommunications and computer networks, multiplexing

is a term used to refer to a process where multiple analogmessage signals or digital data streams are combined intoone signal over a shared medium. The aim is to share anexpensive resource. For example, in telecommunications,several phone calls may be transferred using one wire.


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Multiplexing in 4 dimensions space (si) time (t) frequency (f) code (c)

Goal: multiple useof a shared medium

Important: guard spaces needed!

s2

s3

s1

Multiplexing

f

t

c

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

channels ki


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Space Division Multiple Access

Space division multiplexing: In wirelesscommunication, a SDM implies a separate senderfor each communication channel with wide enoughdistance between senders.

For example, This type of multiplexing is used atFM radio stations where the transmission range islimited to a certain region. Many radio stationsaround the world can use the same frequencywithout interference.


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Space Division Multiple Access channels are mapped onto dedicated (sufficiently distant)

space so that different users can access the mediumwithout interfering with each other SDMA can be achieved through:

a) use of spot beam antennasb) by controlling transmission power, in case of omnidirectional

antennas SDMA is typically used in combination with FDMA, TDMA

or CDMA example: cellular networks (e. g. SDMA + FDMA)

network coverage area is divided into a number of small areas called

cells frequencies are reused in different cells users using same

frequency are so far away that they do not interfere with eachother


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Frequency multiplex

Separation of the whole spectrum into smaller frequency

bands A channel gets a certain band of the spectrum for the

whole time

Advantages

no dynamic coordination necessary works also for analog signals

Disadvantages waste of bandwidth

if the traffic isdistributed unevenly

inflexible

k2 k3 k4 k5 k6k1

f

t

c


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Frequency multiplex

FDM is an analog multiplexingtechnique that combines signals.

F l l


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FDM Multiplexing

FDM D l l


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FDM Demultiplexing

E l


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Assume that a voice channel occupies a bandwidth of 4 KHz.We need to combine three voice channels into a link with a

bandwidth of 12 KHz, from 20 to 32 KHz. Show the

configuration using the frequency domain without the use of

guard bands.

SolutionSolution

Shift (modulate) each of the three voice channels to a different

bandwidth.

Example

S l i


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Solution

E l


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Five channels, each with a 100-KHz bandwidth, are to bemultiplexed together. What is the minimum bandwidth of the

link if there is a need for a guard band of 10 KHz between the

channels to prevent interference?

SolutionSolution

For five channels, we need at least four guard bands. This

means that the required bandwidth is at least

5 x 100 + 4 x 10 = 540 KHz,

Example


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E l


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Four data channels (digital), each transmitting at 1 Mbps, use asatellite channel of 1 MHz. Design an appropriate

configuration using FDM

SolutionSolution

The satellite channel is analog. We divide it into four

channels, each channel having a 250-KHz bandwidth. Each

digital channel of 1 Mbps is modulated such that each 4 bits

are modulated to 1 Hz. One solution is 16-QAM modulation.

Example

S l ti


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Solution

A l Hi h


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Anolog Hierarchy

E l


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The Advanced Mobile Phone System (AMPS) uses two bands.

The first band, 824 to 849 MHz, is used for sending; and 869

to 894 MHz is used for receiving. Each user has a bandwidth

of 30 KHz in each direction. The 3-KHz voice is modulated

using FM, creating 30 KHz of modulated signal. How many

people can use their cellular phones simultaneously?

SolutionSolution

Each band is 25 MHz. If we divide 25 MHz into 30 KHz, we

get 833.33. In reality, the band is divided into 832 channels.

Example

Tim m ltipl x


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f

t

c

k2 k3 k4 k5 k6k1

Time multiplex

A channel gets the whole spectrum for a certain amount of

time

Advantages only one carrier in the

medium at any time

throughput high evenfor many users

Disadvantages

precisesynchronizationnecessary


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TDM Frames


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TDM Frames

Solution


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Solution

Interleaving


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Interleaving

Example


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Four 1-Kbps connections are multiplexed together. A unitis 1 bit. Find

(1) the duration of 1 bit before multiplexing,

(2) the transmission rate of the link,

(3) the duration of a time slot, and

(4) the duration of a frame?

Example

Solution


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Solution

We can answer the questions as follows:

1. The duration of 1 bit is 1/1 Kbps, or 0.001 s (1 ms).

2. The rate of the link is 4 Kbps.

3. The duration of each time slot 1/4 ms or 0.250 ms.4. The duration of a frame 1 ms.


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In a TDM, the data rate of the link is nIn a TDM, the data rate of the link is n

times faster, and the unit duration is ntimes faster, and the unit duration is ntimes shorter.times shorter.

Example


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Four channels are multiplexed using TDM. If each channelsends 100 bytes/s and we multiplex 1 byte per channel, show

the frame traveling on the link,

the size of the frame,

the duration of a frame,

the frame rate,

and the bit rate for the link.

Example

Solution


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Solution

Example


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A multiplexer combines four 100-Kbps channels using a timeslot of 2 bits.

Show the output with four arbitrary inputs.

What is the frame rate?

What is the frame duration?

What is the bit rate?

What is the bit duration?

Example

Solution


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Solution

Example


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We have four sources, each creating 250 characters per

second. If the interleaved unit is a character and 1

synchronizing bit is added to each frame, find

(1) the data rate of each source,

(2) the duration of each character in each source,

(3)the frame rate,

(4) the duration of each frame,

(5) the number of bits in each frame, and

(6) the data rate of the link.

Example

Solution


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We can answer the questions as follows:

1. The data rate of each source is 2000 bps = 2 Kbps.

2. The duration of a character is 1/250 s, or 4 ms.

3. The link needs to send 250 frames per second.4. The duration of each frame is 1/250 s, or 4 ms.

5. Each frame is 4 x 8 + 1 = 33 bits.

6. The data rate of the link is 250 x 33, or 8250 bps.

Solution

Example


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Two channels, one with a bit rate of 100 Kbps and another

with a bit rate of 200 Kbps, are to be multiplexed. How this

can be achieved? What is the frame rate? What is the frame

duration? What is the bit rate of the link?

Example

Solution


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Solution

We can allocate one slot to the first channel and

two slots to the second channel. Each frame carries 3 bits. The frame rate is 100,000 frames per second

because it carries 1 bit from the first channel.

The frame duration is 1/100,000 s, or 10 ms. The bit rate is 100,000 frames/s x 3 bits/frame,

or 300 Kbps.

DS Hierarchy


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DS Hierarchy

Time and frequency multiplex


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f

Time and frequency multiplex Combination of both methods

A channel gets a certain frequency band for a certain amount oftime

Example: GSM

Advantages

better protection againsttapping

protection against frequencyselective interference

but: precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Code multiplex


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Code multiplex Each channel has a unique code

All channels use the same spectrumat the same time

k2 k3 k4 k5 k6k1

f

t

c


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CDMA Advantages


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CDMA Advantages

in CDMA, each user is far more liberated from other users,

than in case of TDMA flexibility in timing and quality oftransmission

CDMA has soft capacity limit there is no absolute limiton the number of users; rather, system performancegradually degrades for all users as # of users an additional user can be added by sacrificing somewhat the link

quality of other users

degradation of performance with an increasing # of users isgraceful, as opposed to hard limits of FDMA / TDMA

soft- handoff can be performed by the MSC, which cansimultaneously monitor a particular user from 2 or morebase stations

CDMA Disadvantages


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CDMA Disadvantages

self-jamming spreading sequence of different

users may not be exactly orthogonal; causingproblems in despreading of particular code

near-far problem occurs at a CDMA receiver if anundesired user has a high detected power

compared to the desired user

Multiplexing analogy


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Mult plex ng analogy

CDMA


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Wireless Transmission 1-120Courtesy of Suresh Goyal & Rich Howard

DM

CDMA


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CDMA


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CDMA


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Access method CDMA


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What is a good code for CDMA?A good autocorrelation (the absolute value of the inner

product of a vector multiplied by itself should be large)

Orthogonal to other codes (Two vectors are calledorthogonal if their inner product is 0.

Examples of a good CDMA code:The Baker code (+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1)

a good autocorrelation: the inner product is large, 11.(+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1) (+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1) = 11

Orthogonal to other codes

(+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1) (+1, +1, -1, +1, -1, -1, +1, +1, +1, -1, + 1) = 1 used for ISDN and IEEE 802.11.

CDMA in Theory


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y

Sender A

sends Ad = 1, key Ak = 010011 (assign: 0 = -1, 1= +1) sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)

Sender B sends Bd = 0, key Bk = 110101 (assign: 0 = -1, 1 = +1)

sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)

Both signals superimpose in space interference neglected (noise etc.)

As + Bs = (-2, 0, 0, -2, +2, 0)

Receiver wants to receive signal from sender A

apply key Ak bitwise (inner product) Ae = (-2, 0, 0, -2, +2, 0) Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6

result greater than 0, therefore, original bit was 1

receiving B Be = (-2, 0, 0, -2, +2, 0) Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. 0

CDMA Code Division Multiple Access


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p

(a) Binary chip sequences for fourstations

(b) Bipolar chip sequences(c) Six examples of transmissions(d) Recovery of station Cs signal


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CDMA Code Division Multiple Access


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Advantages


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g

Flexible network planning (planning is no longer needed)This is huge code space compared to frequency

spaceGreater coverage (larger area for a given amount of power )

High capacity(greater coverage capacity)

Cost(larger profit for providers due to increased capacity, less infrastructure) Clarity Customer satisfaction (privacy, better call quality longer battery life due to less power

consumption,prevent cross talks)

Compatibility (dual mode analog and digital)

Disadvantages


130/132


SynchronizationDifficulty to satisfy synchronization requirements.

Self jammingSelf jamming is a steep deterioration of performance as a result of poorsynchronization. Poor synchronization causes partial-correlation with the codesof other users and the result will be a vast increase of the interference.

Near- far problempower control is necessary for mitigating the Near-far problem. There aresome factors for imperfect power control such as: feedback delays, imperfectpower estimates, traffic conditions, errors in the feedback channel.

Network complexityComplex network support is needed for implementing soft handoff, and also forcountering multipath and fading effects.

ThroughputLow throughput efficiency for large number of users.


131/132


CDMA ha s many a dvantages, but it is still sec ond tec hnology

CDMA System eng ineer should adop t to use global network

system broadly to become a ma jor telec omm unication

in 3G ma rket

Adopting hando ver requirement of GSM and will be

beneficial lead CDMA to g loba l telecommunication system

Conclusion

Comparison SDMA/TDMA/FDMA/CDMA


132/132

Approach SDMA TDMA FDMA CDMA

Ideasegment space intocells/sectors

segment sendingtime into disjointtime-slots, demanddriven or fixedpatterns

segment thefrequency band intodisjoint sub-bands

spread the spectrumusing orthogonal codes

Terminals only one terminal canbe active in onecell/one sector

all terminals areactive for shortperiods of time onthe same frequency

every terminal has itsown frequency,uninterrupted

all terminals can be activeat the same place at thesame moment,uninterrupted

Signalseparation

cell structure, directed

antennas

synchronization in

the time domain

filtering in the

frequency domain

code plus special

receivers

Advantages very simple, increasescapacity per km

established, fullydigital, flexible

simple, established,robust

flexible, less frequencyplanning needed, softhandover

Dis-advantages

inflexible, antennastypically fixed

guard spaceneeded (multipathpropagation),synchronization

difficult

inflexible,frequencies are ascarce resource

complex receivers, needsmore complicated powercontrol for senders

Comment only in combinationwith TDMA, FDMA orCDMA useful

standard in fixednetworks, togetherwith FDMA/SDMAused in many

typically combinedwith TDMA(frequency hoppingpatterns) and SDMA

still faces some problems,higher complexity,lowered expectations; willbe integrated with

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