Sistemi di Trasmissione Radio. Università di Pavia. Sistemi di Trasmissione Radio

(1)

Sistemi di

(2)

Programma del corso

Tecniche di trasmissione

_{Tecniche di trasmissione}

•

• Modulazioni numericheModulazioni numeriche

•

• Sistemi ad Sistemi ad allargameneto diallargameneto di bandabanda

•

• Sistemi Sistemi multi-tonomulti-tono

•

• Codifica di canaleCodifica di canale

•

(3)

Programma del corso

Sistemi di trasmissione senza filo

_{Sistemi di trasmissione senza filo}

•

• Sistemi Sistemi radiomobili radiomobili cellularicellulari

•

• comunicazioni satellitaricomunicazioni satellitari

•

• Cordless Cordless e e Wireless Local LoopWireless Local Loop

•

• Mobile IP e WAPMobile IP e WAP

•

• WLANWLAN

•

(4)

Testi consigliati

O.

O. Bertazioli

Bertazioli

, L.

Favalli

,

“

GSM

”

.

Hoepli

, Seconda Edizione, 2002.

W.

W. Stallings

Stallings

,

“

Wireless

Communications

and

Networks

”

.

Pearson-Prentice

Hall,

Second

Editions

(5)

Storia delle comunicazioni radio

Guglielmo _GuglielmoMarconi invented the wireless telegraph in_{Marconi invented the wireless telegraph in} 1896

1896 •

• Communication by encoding alphanumeric characters in analogCommunication by encoding alphanumeric characters in analog signal

signal •

• Sent telegraphic signals across the Atlantic OceanSent telegraphic signals across the Atlantic Ocean

Communications satellites launched in 1960s_{Communications satellites launched in 1960s}

Advances in wireless technology_{Advances in wireless technology}

•

• Radio, television, mobile telephone, communication satellitesRadio, television, mobile telephone, communication satellites

More recently_{More recently}

•

(6)

Electromagnetic Signal

Function of time

_{Function of time}

Can also be expressed as a function of

_{Can also be expressed as a function of}

frequency

•

• Signal consists of components of differentSignal consists of components of different frequencies

(7)

Time-Domain Concepts

Analog signal - signal intensity varies in a smooth_{Analog signal - signal intensity varies in a smooth}

fashion over time

•

• No breaks or discontinuities in the signalNo breaks or discontinuities in the signal

Digital signal - signal intensity maintains a_{Digital signal - signal intensity maintains a}

constant level for some period of time and then

changes to another constant level

Periodic signal - analog or digital signal pattern_{Periodic signal - analog or digital signal pattern}

that repeats over time

•

• ss((tt + +T T ) = ) = ss((t t ))

–

(8)

Frequency-Domain Concepts

Spectrum - range of frequencies that a

signal contains

Absolute bandwidth - width of the spectrum

_{Absolute bandwidth - width of the spectrum}

of a signal

Effective bandwidth (or just bandwidth) -

_{Effective bandwidth (or just bandwidth)}

-narrow band of frequencies that most of the

narrow band of frequencies that most of the

signal

(9)

Relationship between Data Rate

and Bandwidth

The greater the bandwidth, the higher the

_{The greater the bandwidth, the higher the}

information-carrying capacity

•

• BUT the transmission system will limit theBUT the transmission system will limit the bandwidth that can be transmitted

bandwidth that can be transmitted

•

• AND, for any given medium, the greater theAND, for any given medium, the greater the bandwidth transmitted, the greater the cost

bandwidth transmitted, the greater the cost

•

• HOWEVER, limiting the bandwidth createsHOWEVER, limiting the bandwidth creates distortions

(10)

Relazione tra data-rate e banda

Sistemi limitati in banda

_{Sistemi limitati in banda}

Sistemi limitati in potenza

(11)

Data Communication Terms

Data - entities that convey meaning, or

_{Data - entities that convey meaning, or}

information

Signals - electric or electromagnetic

_{Signals - electric or electromagnetic}

representations of data

Transmission - communication of data by

_{Transmission - communication of data by}

the propagation and processing of signals

(12)

Segnali analogici e digitali

(13)

Segnali digitali

(14)

Analog Transmission

Transmit analog signals without regard to

content

Attenuation limits length of transmission

_{Attenuation limits length of transmission}

link

Cascaded amplifiers boost signal

_{Cascaded amplifiers boost signal}

’

_’

s energy

_{s energy}

for longer distances but cause distortion

•

(15)

Digital Transmission

Concerned with the content of the signal_{Concerned with the content of the signal}

Attenuation endangers integrity of data_{Attenuation endangers integrity of data}

Digital Signal_{Digital Signal}

•

• Repeaters achieve greater distanceRepeaters achieve greater distance

•

• Repeaters recover the signal and retransmitRepeaters recover the signal and retransmit

Analog signal carrying digital data_{Analog signal carrying digital data}

•

• Retransmission device recovers the digital data fromRetransmission device recovers the digital data from analog signal

analog signal

•

(16)

About Channel Capacity

Impairments, such as noise, limit data rate

that can be achieved

For digital data, to what extent do

_{For digital data, to what extent do}

impairments limit data rate?

Channel Capacity

_{Channel Capacity}

–

_–

the maximum rate at

_{the maximum rate at}

which data can be transmitted over a given

communication path, or channel, under

(17)

Concepts Related to Channel

Capacity

Data rate - rate at which data can be_{Data rate - rate at which data can be} communicated (bps)

communicated (bps)

Bandwidth - the bandwidth of the transmitted_{Bandwidth - the bandwidth of the transmitted} signal as constrained by the transmitter and the

signal as constrained by the transmitter and the

nature of the transmission medium (Hertz)

Noise - average level of noise over the_{Noise - average level of noise over the} communications path

communications path

Error rate - rate at which errors occur_{Error rate - rate at which errors occur}

•

• Error = transmit 1 and receive 0; transmit 0 andError = transmit 1 and receive 0; transmit 0 and receive 1

(18)

Nyquist

Bandwidth

_Bandwidth

For binary signals (two voltage levels)

_{For binary signals (two voltage levels)}

•

• C C = 2= 2BB

With multilevel signaling

_{With multilevel signaling}

•

• CC = 2 = 2BB log log₂₂ MM

–

(19)

Signal-to-Noise Ratio

Ratio of the power in a signal to the power_{Ratio of the power in a signal to the power}

contained in the noise

Typically measured at a receiver_{Typically measured at a receiver}

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

A high SNR means a high-quality signal, low_{A high SNR means a high-quality signal, low}

number of required intermediate repeaters

SNR sets upper bound on achievable data rate_{SNR sets upper bound on achievable data rate}

power noise

power signal

log 10

)

(20)

Shannon Capacity Formula

Equation:

_Equation:

Represents theoretical maximum that can be

_{Represents theoretical maximum that can be}

achieved

In practice, only much lower rates achieved

_{In practice, only much lower rates achieved}

•

• Formula assumes white noise (thermal noise)Formula assumes white noise (thermal noise)

(

1 SNR

)

log

₂

+

=

B

(21)

Example of

Nyquist

and Shannon

Formulations

Spectrum of a channel between 3 MHz and 4_{Spectrum of a channel between 3 MHz and 4}

MHz;

MHz; SNRSNR_dB_dB = 24 dB= 24 dB

Using Shannon_{Using Shannon}’_’s formula_{s formula}

(

)

251 SNR

SNR

log

10 dB

24 SNR

MHz

1 MHz

3 MHz

4

10 dB

=

B

(

1

251 )

10

8

8 Mbps

log

10

6

₂

+

6

=

(22)

Example of

Nyquist

and Shannon

Formulations

How many signaling levels are required?

_{How many signaling levels are required?}

( )

16 log

4 log

10

2

10

8 log

2

2 2 6 6 2

=

M

B

C

(23)

Wireless transmissions

Transmission and reception are achieved by

_{Transmission and reception are achieved by}

means of antennas

Configurations for wireless transmission

_{Configurations for wireless transmission}

•

• DirectionalDirectional

•

(24)

General Frequency Ranges

Microwave frequency range_{Microwave frequency range}

•

• 1 GHz to 60 GHz1 GHz to 60 GHz

•

• Directional beams possibleDirectional beams possible

•

• Suitable for point-to-point transmissionSuitable for point-to-point transmission

•

• Used for satellite communicationsUsed for satellite communications

Radio frequency range_{Radio frequency range}

•

• 30 MHz to 1 GHz30 MHz to 1 GHz

•

(25)

Terrestrial Microwave

Description of common microwave antenna_{Description of common microwave antenna}

•

• Parabolic "dish", 3 m in diameterParabolic "dish", 3 m in diameter

•

• Fixed rigidly and focuses a narrow beamFixed rigidly and focuses a narrow beam

•

• Achieves line-of-sight transmission to receivingAchieves line-of-sight transmission to receiving antenna

antenna

•

• Located at substantial heights above ground levelLocated at substantial heights above ground level

Applications_Applications

•

• Long haul telecommunications serviceLong haul telecommunications service

•

(26)

Satellite Microwave

Description of communication satellite_{Description of communication satellite}

•

• Microwave relay stationMicrowave relay station

•

• Used to link two or more ground-based microwaveUsed to link two or more ground-based microwave transmitter/receivers

transmitter/receivers

•

• Receives transmissions on one frequency band (uplink),Receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on

amplifies or repeats the signal, and transmits it on

another frequency (downlink)

(27)

Multiplexing

Capacity of transmission medium usually

_{Capacity of transmission medium usually}

exceeds capacity required for transmission

of a single signal

Multiplexing - carrying multiple signals on

_{Multiplexing - carrying multiple signals on}

a single medium

•

(28)

Multiplexing

(29)

Multiplexing Techniques

Frequency-division multiplexing (FDM)

•

• Takes advantage of the fact that the usefulTakes advantage of the fact that the useful

bandwidth of the medium exceeds the required

bandwidth of a given signal

Time-division multiplexing (TDM)

_{Time-division multiplexing (TDM)}

•

• Takes advantage of the fact that the achievableTakes advantage of the fact that the achievable bit rate of the medium exceeds the required

bit rate of the medium exceeds the required

data rate of a digital signal

(30)

Frequency-division Multiplexing

(31)

Time-division Multiplexing

(32)

Antenna Gain

Relationship between antenna gain and effective_{Relationship between antenna gain and effective}

area

–

– G G = antenna gain= antenna gain

–

– AA_e_e = effective area= effective area

–

– f f = carrier frequency= carrier frequency

2 2 2

4

4 c

A

f

A

G

e

₌

(33)

Free Space Loss

Free space loss, ideal isotropic antenna_{Free space loss, ideal isotropic antenna}

–

– PP_t_t = signal power at transmitting antenna = signal power at transmitting antenna

–

– PP_r_r = signal power at receiving antenna = signal power at receiving antenna

–

– = carrier wavelength = carrier wavelength

–

– dd = propagation distance between antennas = propagation distance between antennas

–

– cc = speed of light (» 3 ´ 10 8 m/s) = speed of light (» 3 ´ 10 8 m/s) where

where dd and and are in the same units (e.g., meters) are in the same units (e.g., meters)

( ) (

)

2 2 2 2

4

4 c

fd

d

P

r t

₌

=

(34)

Free Space Loss

Free space loss accounting for gain of other_{Free space loss accounting for gain of other}

antennas

–

– GG_t_t = gain of transmitting antenna= gain of transmitting antenna

–

– GG_r_r = gain of receiving antenna= gain of receiving antenna

( ) ( ) ( )

( )

t r t r t r r t

A

f

cd

A

d

G

d

P

2 2 2 2 2 2

4 =

=

(35)

Categories of Noise

Thermal Noise

_{Thermal Noise}

Intermodulation noise

_{Intermodulation noise}

Crosstalk

_Crosstalk

(36)

Noise Terminology

Intermodulation _{Intermodulation}noise _noise–_– occurs if signals with_{occurs if signals with}

different frequencies share the same medium

•

• Interference caused by a signal produced at aInterference caused by a signal produced at a

frequency that is the sum or difference of original

frequencies

Crosstalk _Crosstalk–_– unwanted coupling between signal_{unwanted coupling between signal}

paths

(37)

Thermal Noise

Thermal noise due to agitation of electrons

_{Thermal noise due to agitation of electrons}

Present in all electronic devices and

_{Present in all electronic devices and}

transmission media

Cannot be eliminated

_{Cannot be eliminated}

Function of temperature

_{Function of temperature}

Particularly significant for satellite

_{Particularly significant for satellite}

communication

(38)

Thermal Noise

Amount of thermal noise to be found in a_{Amount of thermal noise to be found in a}

bandwidth of 1Hz in any device or conductor is:

–

– NN₀₀ = noise power density in watts per 1 Hz of bandwidth = noise power density in watts per 1 Hz of bandwidth

–

– k =k = Boltzmann's Boltzmann's constant = 1.3803 ´ 10constant = 1.3803 ´ 10-23-23_J/K_J/K

(

W/Hz

)

k

0

T

(39)

Thermal Noise

Noise is assumed to be independent of frequency_{Noise is assumed to be independent of frequency}

Thermal noise present in a bandwidth of _{Thermal noise present in a bandwidth of}B_B Hertz_Hertz (in watts):

(in watts):

or, in decibel-watts

TB

N

=

k

B

T

N

=

10 log

k

+

10 log

+

10 log

B

T

10 log

log

10 dBW

6 .

228 +

+

(40)

Expression

E

_E

_b_b

/

_/

N

_N

₀₀

Ratio of signal energy per bit to noise power_{Ratio of signal energy per bit to noise power}

density per Hertz

The bit error rate for digital data is a function of_{The bit error rate for digital data is a function of}

E

E_b_b//NN₀₀

•

• Given a value for Given a value for EE_b_b//NN₀₀to achieve a desired error rate,to achieve a desired error rate, parameters of this formula can be selected

parameters of this formula can be selected

TR

S

N

R

S

N

E

_b

k

/

0 0

=

(41)

Other Impairments

Atmospheric absorption

_{Atmospheric absorption}

–

_–

water vapor and

_{water vapor and}

oxygen contribute to attenuation

Multipath

_Multipath

–

_–

obstacles reflect signals so that

_{obstacles reflect signals so that}

multiple copies with varying delays are

received

Refraction

_Refraction

–

_–

bending of radio waves as they

_{bending of radio waves as they}

propagate through the atmosphere

(42)

Multipath

(43)

Multipath Propagation

Reflection - occurs when signal encounters a_{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_{Diffraction - occurs at the edge of an impenetrable}

body that is large compared to wavelength of

radio wave

Scattering _Scattering–_– occurs when incoming signal hits an_{occurs when incoming signal hits an}

object whose size in the order of the wavelength of

the signal or less

(44)

The Effects of Multipath

Propagation

Multiple copies of a signal may arrive at

different phases

•

• If phases add destructively, the signal levelIf phases add destructively, the signal level relative to noise declines, making detection

relative to noise declines, making detection

more difficult

Intersymbol interference (ISI)

_{Intersymbol interference (ISI)}

•

(45)

Types of Fading

Fast fading

_{Fast fading}

Slow fading

_{Slow fading}

Flat fading

_{Flat fading}

Selective fading

_{Selective fading}

Rayleigh

_Rayleigh

fading

_fading

(46)

Error Compensation Mechanisms

Forward error correction

_{Forward error correction}

Adaptive equalization

_{Adaptive equalization}

(47)

Forward Error Correction

Transmitter adds error-correcting code to data_{Transmitter adds error-correcting code to data}

block

•

• Code is a function of the data bitsCode is a function of the data bits

Receiver calculates error-correcting code from_{Receiver calculates error-correcting code from}

incoming data bits

•

• If calculated code matches incoming code, no errorIf calculated code matches incoming code, no error occurred

occurred

•

• If error-correcting codes donIf error-correcting codes don’’t match, receiver attemptst match, receiver attempts to determine bits in error and correct

(48)

Adaptive Equalization

Can be applied to transmissions that carry analog_{Can be applied to transmissions that carry analog}

or digital information

•

• Analog voice or videoAnalog voice or video

•

• Digital data, digitized voice or videoDigital data, digitized voice or video

Used to combat _{Used to combat}intersymbol _intersymbolinterference_interference

Involves gathering dispersed symbol energy back_{Involves gathering dispersed symbol energy back}

into its original time interval

(49)

Diversity Techniques

Diversity is based on the fact that individual_{Diversity is based on the fact that individual}

channels experience independent fading events

Space diversity _{Space diversity}–_– techniques involving physical_{techniques involving physical}

transmission path

Frequency diversity _{Frequency diversity}–_– techniques where the signal_{techniques where the signal}

is spread out over a larger frequency bandwidth

or carried on multiple frequency carriers

Time diversity _{Time diversity}–_– techniques aimed at spreading_{techniques aimed at spreading}

the data out over time

(50)

Signal Encoding Techniques

What determines how successful a receiver will be_{What determines how successful a receiver will be}

in interpreting an incoming signal?

•

• Signal-to-noise ratioSignal-to-noise ratio

•

• Data rateData rate

•

• BandwidthBandwidth

An increase in data rate increases bit error rate_{An increase in data rate increases bit error rate}

An increase in SNR decreases bit error rate_{An increase in SNR decreases bit error rate}

(51)

Factors Used to Compare

Encoding Schemes

Signal spectrum_{Signal spectrum}

•

• With lack of high-frequency components, lessWith lack of high-frequency components, less bandwidth required

bandwidth required

•

• With no dc component, ac coupling via transformerWith no dc component, ac coupling via transformer possible

possible

•

• Transfer function of a channel is worse near bandTransfer function of a channel is worse near band edges

edges

Clocking_Clocking

•

• Ease of determining beginning and end of each bitEase of determining beginning and end of each bit position

(52)

Factors Used to Compare

Encoding Schemes

Signal interference and noise immunity_{Signal interference and noise immunity}

•

• Performance in the presence of noisePerformance in the presence of noise

Cost and complexity_{Cost and complexity}

•

• The higher the signal rate to achieve a given data rate,The higher the signal rate to achieve a given data rate, the greater the cost

(53)

Basic Encoding Techniques

Digital data to analog signal

_{Digital data to analog signal}

•

• Amplitude-shift keying (ASK)Amplitude-shift keying (ASK)

–

– Amplitude difference of carrier frequencyAmplitude difference of carrier frequency

•

• Frequency-shift keying (FSK)Frequency-shift keying (FSK)

–

– Frequency difference near carrier frequencyFrequency difference near carrier frequency

•

• Phase-shift keying (PSK)Phase-shift keying (PSK)

–

(54)

Basic Encoding Techniques

(55)

Amplitude-Shift Keying

One binary digit represented by presence of_{One binary digit represented by presence of}

carrier, at constant amplitude

Other binary digit represented by absence of_{Other binary digit represented by absence of}

carrier

–

– where the carrier signal is where the carrier signal is AAcoscos(2(2ff_c_ctt))

( )

= t

s Acos

(

2

fct

)

0

1 binary

0 binary

(56)

Amplitude-Shift Keying

Susceptible to sudden gain changes

_{Susceptible to sudden gain changes}

Inefficient modulation technique

_{Inefficient modulation technique}

On voice-grade lines, used up to 1200 bps

_{On voice-grade lines, used up to 1200 bps}

Used to transmit digital data over optical

_{Used to transmit digital data over optical}

fiber

(57)

Binary Frequency-Shift Keying

(BFSK)

Two binary digits represented by two different_{Two binary digits represented by two different}

frequencies near the carrier frequency

–

– where where ff₁₁ and and ff₂₂ are offset from carrier frequency are offset from carrier frequency ff_c_c by equalby equal but opposite amounts

but opposite amounts

( )

= t

s Acos

(

2

f1t

)

t f

Acos 2

₂

1 binary

0 binary

(58)

Binary Frequency-Shift Keying

(BFSK)

Less susceptible to error than ASK

_{Less susceptible to error than ASK}

On voice-grade lines, used up to 1200bps

_{On voice-grade lines, used up to 1200bps}

Used for high-frequency (3 to 30 MHz)

_{Used for high-frequency (3 to 30 MHz)}

radio transmission

Can be used at higher frequencies on LANs

_{Can be used at higher frequencies on LANs}

that use coaxial cable

(59)

Multiple Frequency-Shift Keying

(MFSK)

(60)

Phase-Shift Keying (PSK)

Two-level PSK (BPSK)

•

• Uses two phases to represent binary digitsUses two phases to represent binary digits

( )

= t

s Acos

(

2

fct

)

f t +

Acos 2 _c

1 binary 0 binary

= Acos

(

2

fct

)

(

)

1 binary

0 binary

(61)

Phase-Shift Keying (PSK)

Differential PSK (DPSK)

_{Differential PSK (DPSK)}

•

• Phase shift with reference to previous bitPhase shift with reference to previous bit

–

– Binary 0 Binary 0 –– signal burst of same phase as previous signal burst of same phase as previous signal burst

signal burst

–

– Binary 1 Binary 1 –– signal burst of opposite phase to signal burst of opposite phase to previous signal burst

(62)

Phase-Shift Keying (PSK)

Four-level PSK (QPSK)

_{Four-level PSK (QPSK)}

•

• Each element represents more than one bitEach element represents more than one bit

( )

=

t

s

₊ 4 2

cos f t

A _c ₁₁

₊ 4 3 2

cos f t

A _c

3

01

00

(63)

Phase-Shift Keying (PSK)

Multilevel PSK_{Multilevel PSK}

•

• Using multiple phase angles with each angle havingUsing multiple phase angles with each angle having

more than one amplitude, multiple signals elements can

be achieved

–

– DD = modulation rate, baud = modulation rate, baud

–

– RR = data rate, bps = data rate, bps

–

– MM = number of different signal elements = 2 = number of different signal elements = 2LL

M

R

L

R

D

2

log

=

(64)

Performance

Bandwidth of modulated signal (

_{Bandwidth of modulated signal (}

B

_B

_T_T

)

₎

•

• ASK, PSKASK, PSK BB_T_T=(1+=(1+rr))RR •

• FSKFSK BB_T_T=2=2DDF+F+(1+(1+rr))RR

–

– RR = bit rate = bit rate

–

– 0 < r < 1; related to how signal is filtered0 < r < 1; related to how signal is filtered

–

(65)

Performance

Bandwidth of modulated signal (_{Bandwidth of modulated signal (}B_B_T_T)₎

•

• MPSKMPSK

•

• MFSKMFSK

–

– LL = number of bits encoded per signal element = number of bits encoded per signal element

–

– MM = number of different signal elements = number of different signal elements

R

M

r

R

L

r

B

_T

+

=

+

=

2

log

1

1 ( )

_R

M

r

B

_T

+

=

2

log

1

(66)

Quadrature

Amplitude

Modulation

QAM is a combination of ASK and PSK

_{QAM is a combination of ASK and PSK}

•

• Two different signals sent simultaneously onTwo different signals sent simultaneously on the same carrier frequency

the same carrier frequency

( ) ( )

t

d

t

f

t

d

( )

t

f

t

(67)

Quadrature

Amplitude

Modulation

(68)

Reasons for Analog Modulation

Modulation of digital signals

_{Modulation of digital signals}

•

• When only analog transmission facilities areWhen only analog transmission facilities are

available, digital to analog conversion required

Modulation of analog signals

_{Modulation of analog signals}

•

• A higher frequency may be needed for effectiveA higher frequency may be needed for effective transmission

(69)

Pulse Code Modulation

Based on the sampling theorem

Each analog sample is assigned a binary

_{Each analog sample is assigned a binary}

code

•

• Analog samples are referred to as pulseAnalog samples are referred to as pulse amplitude modulation (PAM) samples

amplitude modulation (PAM) samples

The digital signal consists of block of

n

bits,

where each

n

-bit number is the amplitude of

a PCM pulse

(70)

(71)

Pulse Code Modulation

By quantizing the PAM pulse, original signal is_{By quantizing the PAM pulse, original signal is}

only approximated

Leads to quantizing noise_{Leads to quantizing noise}

Signal-to-noise ratio for quantizing noise_{Signal-to-noise ratio for quantizing noise}

Thus, each additional bit increases SNR by 6 dB,_{Thus, each additional bit increases SNR by 6 dB,}

or a factor of 4

dB 76 . 1 02 . 6 dB 76 . 1 2 log 20