Digital Communications (DCOM)thomas.perso.enseeiht.fr/Slides_DCOM_2018_2019_V4.pdf- Attenuation Absorption, scattering due to atmospheric gases, to clouds, to rain, skin effect for

Digital Communications (DCOM)

Master SATCOM, MoST ESECA

Nathalie Thomas

[email protected]

1

Digital Communications: Introduction

→ Components of the basic digital communication channel

→ AWGN transmission channel

→ Baseband and carrier-modulated transmissions

→ Performance criteria

2

Transmitter

Digitization

Transmission channel

Analog signal

Corrupted analog signal

Receiver

Received Binary information: 0 1 0 1 0 1 1 …

B

A

S

I

C

C

H

A

N

N

E

L

Analog signal : sound, image …

Basic digital transmission channel

Binary information to transmit: 0 1 1 0 0 1 0 1 1 0

3

Transmitter

Digitization


Analog signal


Receiver

B

A

S

I

C

C

H

A

N

N

E

L



Binary information to transmit: 0 1 1 0 0 1 0 1 1 0Transmit a given bit rate Rb

= Number of bits to be

transmitted per second.

4

Transmitter

Digitization


Analog signal


Receiver

B

A

S

I

C

C

H

A

N

N

E

L



Binary information to transmit: 0 1 1 0 0 1 0 1 1 0Transmit a given bit rate Rb

= Number of bits to be

transmitted per second.

Received Binary information: 0 1 0 1 0 1 1 1 1 1BER = Number of erroneous bits

Number of transmitted bits

0 1 1 0 0 1 0 1 1 0

0 1 0 1 0 1 1 1 1 1

BER = 4/10Obtain a given Bit Error Rate:

Example:

DVB example:BER<10-10, (QEF transmission)

Rb 30 à 40 Mbps ~

5

Transmitter

Digitization


Analog signal


Receiver


B

A

S

I

C

C

H

A

N

N

E

L




6

- Wired transmissions: xDSL, optical fiber, cable TV, power Line communications…

Propagation on copper, coaxial cables or optical fibers via electrical or optical signals .

- Wireless transmissions: WiFi, Terrestrial TV, Satellite transmissions, GSM, 3G, 4G …

=> Propagation in free space via radio (or Hertzian): frequencies < 3000 GHz


7

Copper

Fiber

Coaxial cable

- Examples:

8

Wireless transmissions => frequencies regulation

- Depending on the countries : regulatory agencies or a ministriesExamples :

→ In France : ARCEP (Autorité de Régulation des Communications Electroniques), ANRT (Agence Nationale de Régulation des Fréquences), CSA (Conseil Supérieur de l’Audiovisuel)

→ In the United States of America : FCC (Federal Communications Commission)→ In Japan: MIC (Ministry of Internal Affairs and Communications )

- Collaborations between states:Examples :

→ ORECE : Organe des Régulateurs Européens des Communications Electroniques in Europ,→ NARUC : National Association of Regulatory Utility Commissioners (regulators of individual states) in

the United States,→ ARTAC : Association des Régulateurs de Télécommunications de l’Afrique Centrale, in Africa,

- International Telecommunication Union (ITU)→ Responsible for the telecommunications regulation in the world→ 193 member states and 700 associated members (from Information and Communication Technology

sector).→ Forum in which the states and the private sector coordinate together.

- Unlicensed bandwidth→ industrial, scientific and medical (ISM) : (902-928 MHz, 2.400-2.4835 GHz)→ Unlicensed National Information Infrastructure (UNII) : 5 .15-5.25 GHz, 5 .25-5.35 GHz→ UNII-3/ISM : 5.725-5.850 GHz

Shared transmission channel => need for multiplexing methods

Time

FrequencyPower

Density

User 1

User 2

User 3

Time

FrequencyPower

Density

Use

r 1

Time

Frequency

Use

r 2

Use

r 3

Power

Density

Frequency

Time

User 1

User 2

User 3

Power

Density

→ Examples of multiplexing methods

FDM

(Frequency Division Multiplexing)

TDM

(Time Division Multiplexing)

CDM

(Code Division Multiplexing)MF-TDM

(Multi Frequency - Time Division Multiplexing)

Transmission channel => distorsions/constraints

10

- AttenuationAbsorption, scattering due to atmospheric gases, to clouds, to rain, skin effect for

cooper (Increases with increasing frequency),

- One or several paths between the transmitter and the receiver=> flat fading or frequency selective channel

- Fixed or Mobile transmission=> stationnary or non stationnary channel

- Limited allocated bandwidth: channel bandpass

- Carrier modulated or baseband transmission

- Noise→ External noise = other signals received in addition to the useful communication

signal.

→ Internal Noise = Electronic devices/components inside the receiver.

→ Wireless propagation via radio waves (or Hertzian waves): frequencies < 3000 GHz, in bands

L : 1.4-1.6 GHz, C : 4-6GHz, Ku : 10.7-12.45 GHz and Ka : 20-30 GHz

→ Need for frequency regulation and multiplexing methods

Global frequency regulation: International TelecommunicationUnion (ITU)

Most commonly used multiplexing methods:TDMA, FDMA and MF-TDMA

→ Attenuation by absorption, scattering due to atmospheric gases, to clouds, to rain. Increases with

increasing frequency.

→ Additive noise: → Other signals received by the antenna in addition to the signal from the satellite:

‒ natural sources: atmosphere (lightning, thunder), Earth, sky (Sun, Milky Way)‒ artificial sources: human activity

→ Electronic devices inside the transmitter, the satellite and the receiver: amplifiers, antennas, etc.

In the desert

To speak with the planes

Off shore

In the mountains

To broadcast television

To give Internet to white spots

Line of Sight

(LOS)

11

Transmission channelExample of free space propagation for fixed satellite telecommunication systems

- Carrier-modulated transmission bands L : 1.4-1.6 GHz, C : 4-6GHz, Ku : 10.7-12.45 GHz and Ka : 20-30 GHz


Baseband

modulator

Frequency

transposition

Baseband

signal

Binary

information

Carrier-modulated

signal

Frequency

transposition

12


Us

er1

Frequency

Us

er 2

Us

er 3

Power

Density

BW

GatewayTerrestrial stations

Bandwidth allocated to the satellite

Most commonly used multiplexing methods :

TDMA

Time

FrequencyPower

Density

BW

User

1

Time

FrequencyPower

Density

BW

User

2

Channel 2Channel 1 Channel N

Time

FrequencyPower

Density

BW

User

3

Bandwidth BW

Time


FrequencyPower

Density

BW




BW

Time

Time

FrequencyPower

Density

BW

User 1

Time

FrequencyPower

Density

BW

User 2Time

FrequencyPower

Density

BW

User 3

User 1User 2

User 3


FDMA


Frequency

Power

Density

BW




BWTime

Time

FrequencyPower

Density

BW

Time

Frequency

Power

Density

BW Time

FrequencyPower

Density

BW


MF-TDMA

- Attenuation:

example of the attenuation effect on a DVB-S transmission


16

- Additive White Gaussian Noise (AWGN) channel

y(t) = a x(t - t) + n(t)

x(t)

y(t)

(a , t) , n(t)

Noise,

assumed to be additive,

white and Gaussian

Attenuation and delay

introduced by the channel

Line of Sight

(LOS)

hc(t)

n(t)

x(t) y(t)

→ Channel modelling: filter + noise


|Hc(f)|

f

Arg(Hc(f))

f

17

y(t) = a x(t - t) + n(t)

x(t)

y(t)

(a , t) , n(t)

Noise,

assumed to be additive,

white and Gaussian

Attenuation and delay

introduced by the channel

Line of Sight

(LOS)

hc(t)

n(t)

x(t) y(t)

→ Channel modelling: filter + noise


• White, with PSD = N0/2 whatever is the frequency, and N0=k(Te+Ti) k = Boltzmann constant, Te = external noise temperature, Ti = internal noise temperature

• Additive and Gaussian, with power s2

• Added upstream of the receiver, considering then ideal components,


18

→ A degradation measurement : the signal to noise ratio (SNR)

SNRdB = 10 log Puseful signal

Pnoise

NRZ-type transmitted signal Noisy signal, SNRdB = 10 dB Noisy signal, SNRdB = 0 dB

Examples :


19


- Limited allocated bandwidth = channel bandpass BW


-BW BW

|Hc(f)|

f

Arg(Hc(f))

f-BW BW

hc(t)

n(t)

x(t) y(t)

20

BW

|Hc(f)|

f

Arg(Hc(f))

fp

BW

-fp

BW

ffp

BW

-fp

Carrier-modulated transmission:Baseband transmission:

Transmission channelExample of multi paths transmission channel (WiFi, Digital Terrestrial TV, 4G…)

Transmitted image

100 200 300 400 500

100

200

300

400

500

0 0.5 110

-20

10-10

100

1010

PSD of the transmitted image (SRRC shaping)

Received image

100 200 300 400 500

100

200

300

400

500

0 0.5 110

-20

10-10

100

1010

PSD of the received image

Transmitted image

100 200 300 400 500

100

200

300

400

500

0 0.5 110

-20

10-10

100

1010

PSD of the transmitted image (SRRC shaping)

Received image

100 200 300 400 500

100

200

300

400

500

0 0.5 110

-20

10-10

100

1010

PSD of the received image

21

Transmitter

Digitization


Analog signal


Receiver

Received Binary information: 0 1 1 0 0 1 0 1 1 0

B

A

S

I

C

C

H

A

N

N

E

L

Analog signal: sound, image …



22


23

t

V

- V

Example :

t

V

- V

SNR = 10 dB :

t

V

- V

SNR = 0 dB :

Analog signal:

Corrupted analog signal:

Transmitter

Digitization


Analog signal


Receiver

Received binary information: 0 1 0 1 0 1 1 1 1 1


SNR

BER = 0.0784

BER = 2.38 10-6

Analo Signal : sound, image …

BER

B

A

S

I

C

C

H

A

N

N

E

L

Transmitter

Digitization


Analog signal


Receiver


B

A

S

I

C

C

H

A

N

N

E

L




Transmission quality is improved:Quality criterion is the Bit Error Rate (BER) which

can be very low even with corrupted receivedanalog signals. Of course BER is a function of SNR.

24

Transmitter

Digitization


Analog signal


Receiver


B

A

S

I

C

C

H

A

N

N

E

L




Transmission quality is improved:Quality criterion is the Bit Error Rate (BER) which

can be very low even with corrupted receivedanalog signals. Of course BER is a function of SNR.

Price to pay: occupied bandwidth is larger for digital transmissions.

But source coding will help on this point !25

Example : fixed phone digitizationBanalog = 3.1 kHz

Bdigital 64 kHz (Fe=8kHz, nb=8 bits) ~


Analog signal

B

A

S

I

C

C

H

A

N

N

E

L

Source codingTRANSMITTER

Physical layer

Binary information to transmit: 0 1 1 0 0 1 0 …

Disturbed analog signal

Source decoding

RECEIVER

Physical layer



26

Message to transmit: EMMENE MOI A LA MER

E M Espace A N I O R L

4/19 4/19 4/19 2/19 1/19 1/19 1/19 1/19 1/19

0000 0001 0010 0011 0100 0101 0110 0111 1000

Natural binary coding:9 different characters = > 4 bits per character (24=16)

19x4 = 76 bits to be sent, example with 2G transmission (9,6 kbps) : 0,79 ms

Smarter code (Huffman) :

12x2+3x4+4x5 = 56 bits to be sent, example with 2G transmission (9,6 kbps) : 0,58 ms

E M Espace A N I O R L

4/19 4/19 4/19 2/19 1/19 1/19 1/19 1/19 1/19

01 10 11 0000 0011 00100 00101 00010 00011

Gain : 26,32 %

Example of source coding : Huffman coding

27

Transmitter

Digitization


Analog signal


Receiver


B

A

S

I

C

C

H

A

N

N

E

L



Binary information to transmit: 0 1 1 0 0 1 0 1 1 0Transmission quality is improved:

Quality criterion is the Bit Error Rate (BER) whichcan be very low even with corrupted received

analog signals. Of course BER is a function of SNR.

Of course there is a price to pay: occupiedbandwidth is larger for digital transmissions.

But source coding will help on this point !

28

Transmitter

Digitization


Analog signal


Receiver


B

A

S

I

C

C

H

A

N

N

E

L



Binary information to transmit: 0 1 1 0 0 1 0 1 1 0Transmission quality is improved:

Quality criterion is the Bit Error Rate (BER) whichcan be very low even with corrupted received

analog signals. Of course BER is a function of SNR.

New functions (digital functions) can be used in the transmission channel, like channel codingallowing to obtain the same BER with a lower

transmitted power.

Of course there is a price to pay: occupiedbandwidth is larger for digital transmissions.

But source coding will help on this point !

29


Analog signal

B

A

S

I

C

C

H

A

N

N

E

L

Channel coding


Physical layer



Source decoding

RECEIVER

Channel decoding

Physical layer



30

RECEIVER

Physical layer

TRANSMITTER

Physical layer

Example of channel coding

Binary information to transmit: 0 1 1 0

Channel coding

Coded Binary information: 0 0 0 1 1 1 1 1 1 0 0 0


Modulation

Demodulation

Analog signal


Decoded Binary information: 0 0 1 1

Channel decoding

Corrupted coded binary information: 0 0 1 1 0 0 1 1 1 1 1 1

Adding redundancyCoding rate = 1/3

Correction ability: 1 error

Detection ability: 2 errors

31


B

A

S

I

C

C

H

A

N

N

E

L

Channel coding


Physical layer

Modulation


Source decoding

RECEIVER

Demodulation

Channel decoding

Physical layer



32

Analog signal


TRA

NSMIT

TER

Physical layer


Channel coding

Coded Binary information: 0 0 0 1 1 1 1 1 1 0 0 0

BasebandModulation

Analog signal

Example of Modulation

Digital signal

tV

-V

( FrequencyTransposition )

DI

GITAL

MODULATI

ON

Digital to Analog Convertor

Modulation

Example : NRZ signal

RECEIVER

Physical layer

Retrieved binary information: 0 0 1 1

Channel decoding

Corrupted coded Binary information: 0 0 1 1 0 0 1 1 1 1 1 1

BasebandDemodulation

Corrupted Analog signal

Corrupted Digital signal

tV

-V

( Downconversion)

DIGITAL

DE

MODULATION

Analog To Digital Concvertor

Demodulation


0 0 0 1 1 1 1 1 1 0 0 0

SNR=0 dB

BER=2/4

33


Analog signal

W

H

O

L

E

B

A

S

I

C

C

H

A

N

N

E

L

Channel coding


Physical layer

Modulation



Source decoding

RECEIVER

Demodulation

Channel decoding



Synchronization

DAC

ADC

34

!! Need for synchronization !!

tV

-V


Signal:

Ts : symbol duration

Time 0

- On the clock

- On the carrier for carrier-modulated transmissions

35

Transmitter


Analog signal


Receiver

Received Binary information:0 1 0 1 0 1 1 …

B

A

S

I

C

C

H

A

N

N

E

L

Performance criteria

Binary information to transmit: 0 1 1 0 0 1 0 1 1 0- Transmit a given bit rate Rb

= Number of bits to betransmitted per second.

- Achieve a given Bit Error Rate

Channel transmission is

designed to:It will cost in terms of:

- Needed bandwidth in the

transmission channel.

- Needed SNR at the receiver

input=> needed transmitted power.

BER = <1Number of erroneous bits

Number of transmitted bits 36

Performance criteria

Channel transmission is

designed to:It will cost in terms of:

- Needed bandwidth B

in the transmission

channel

Spectral Efficiency:

needed bandwidth B to transmit wanted Rb

Power Efficiency:

needed SNR per bit at the receiver input to

achieve wanted BER

- Transmit a given bit

rate Rb = Number of

bits to be transmitted

per second.

- Achieve a given Bit

Error Rate



- Needed SNR at the

receiver input => needed

transmitted power.

Two main transmission

channel performance criteria

37


Analog signal

W

H

O

L

E

B

A

S

I

C

C

H

A

N

N

E

L

Channel coding


Physical layer

Modulation



Source decoding

RECEIVER

Demodulation

Channel decodingPhysical layer



Synchronization

DAC

ADC

Bit rate Rb

Needed transmission

bandwidth B

Needed SNR

Bit Error Rate (BER)

Spectral Efficiency:

needed bandwidth B to transmit wanted Rb

Power Efficiency:

needed SNR per bit at the receiverinput to achieve wanted BER

38

Channel transmission is designed to: It will cost in terms of:

- Needed bandwidth B

in the transmission

channel

- Transmit a given bit rate

Rb = Number of bits to

be transmitted per

second.

- Achieve a given Bit

Error Rate



- Needed SNR at the

receiver input => needed

transmitted power.

39

Rb SNRB

DVB-S example: satellite broadcasting for muti-media contentsQuasi Error Free (QEF) transmission:

BER < 10 -10

Basic digital transmission channel: example

Baseband Modulation/demodulation:

joint optimization

→ Signal generation, Spectral efficiency→ Inter Symbol Interference(ISI), Nyquist criterion

→ Matched filtering,

→ BER computation, Power efficiency

40

Binary information: 0 1 1 0 0 1 0 1 1 0

BasebandModulation

Baseband Digital Modulation/Demodulation

x(t) Retrievedbinary information: 0 1 0 1 0 1 1 1 1 1



hc(t)

n(t)

r(t)

Bit rate Rb =1/Tb

BER

SNR per bit:

Eb/N0 ?

Occupied

bandwidth B ?

Baseband signal :

Spectrum aroundfrequency0

0

Sx(f)

Performance criteria:

→ Spectral Efficiency: Needed bandwidth B to transmit wanted bit rate Rb

→ Power Efficiency: Needed SNR per bit at the receiver input to achieve wanted Bit

Error Rate (BER)

→ Robustness to non linearities; Signal with constant envelop ?

41

→ Elementary coding with independent symbols→ Coding using a level :

→Unipolar NRZ :

→Polar NRZ :

→ Coding with an edge→Biphase :

→ Bloc coding with independent symbols→ Coding using a level :

→ Several levels NRZ :

1 0 1 0 1 1 0 0 1 1

Ts

t

+V

0

t

+V

0-V

t

+V

0

-V

Baseband Digital ModulationSome signal examples

Ts=2Tb

t

+3V

0-V

+V

-3V42


BasebandModulation

Bit rate Rb =1/Tb

Example (NRZ, M=4):

t

-3

-1

+1

+3

…

h (t)

t

+1

sT

200 250 300 350-4

-2

0

2

-5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 50000

500

1000

1500

2000

Bit Rate (bits/s)Symbol rate

(symbols/s or bauds)

Baseband Digital ModulationGeneral model

Binary information 0 1 1 0 0 1 0 1 1 0

M-ary Symbols Shaping filterh(t)

Mapping

Baseband Modulation

x(t)

Symbol rate = number of symbolstransmitted per seconds:

M=number of possible symbols

Occupied

bandwidth B ?

sT43

Baseband Digital ModulationExample on Matlab

Generation of a polar NRZ

%Symbol duration in number of samplesTs=4; %Number of generated bitsnb_bits=100; %Bit generationbits=randint(1,nb_bits);%Symbol generation : 0->-1, 1->1Symboles=2*bits-1;%Weighted Dirac delta function seriesDiracs=kron(Symboles, [1 zeros(1,Ts-1)]); %Shaping filter impulse response (for NRZ)h=ones(1,Ts)%Shaping filteringy=filter(h,1,Diracs);%Signal display plot(y);axis([0 nb_bits-1 -1.5 1.5]);

44


BasebandModulation

Bit rate Rb =1/Tb

Baseband Digital ModulationGeneral model

Binary information 0 1 1 0 0 1 0 1 1 0


Mapping

Baseband Modulation

x(t)

Occupied

bandwidth B ?

where : ; ;

Cyclostationnarysignal

f

Example (NRZ, M=4):

Baseband Pulse Amplitude Modulation : M-PAM = Linear Modulation, spectrum around frequency 0

45

Baseband Digital ModulationSome spectrum examples

→ Two level NRZ (GPS waveform)

x(t)

TS

h (t)

+1

t

46


→ Biphase or Manchester (Ethernet waveform : IEEE802.3)

x(t)

h(t)

TS

+1

-1t

47


→ Square root raised cosine shaping (DVB-C, DVB-S waveform)

x(t)

TS

h(t)

- TSt

48

Baseband Digital ModulationSpectral efficiency

→ Bandwidth definition:

▪Definition 1 : frequency bandwidth B concentrating x % of the signal energy (typical

values : 95 à 99 %)

▪ Definition 2 : frequency bandwidth beyond which the minimum rejection is of x dB

(typical values: 20 à 30 dB)

→ Spectral Efficiency (bits/s/Hz):

49

M-ary symbols

Baseband Digital DemodulationJoint optimization with the modulation

Binaryinformation 0 1 1 0 0 1


Mapping

Baseband Modulation

x(t)

RetrievedBinary

information 0 1 0 1 0 1

Receiver filterhr(t)Mapping-1

Baseband Demodulation

r(t)Decisions

Sampling

Transmission channelhc(t)

n(t)

Decisions

ISI at sampling

instantsNoise

(filtered and sampled)

Term of

Interest

(Inter Symbol Interference)50


→ Interference visualization at the sampler input : example

t0 2TS

t

TS

-Ts

0

Ts

TS

Ts

Interference (ISI)

on the following symbolInstants where ISI=0

-1 +1 -1 -1 +1 +1 -1

On the signal :

Eye diagram

TFwith

→ Interference suppression at t0+mTs : Nyquist criterion

▪ Time domain expression:

▪ Frequency domain expression:

51


→ Interference suppression at t0+mTs : frequency domain Nyquist criterion

▪ Example

▪ Nyquist bandwidth

f

……

f

……

⇒ Maximum symbol rate without interferences

at time sampling instants : 52

Nyquist Bandwidth


→ Example of Nyquist filter : raised cosine filter

53



Some typical values: α=0.22 (UMTS), α=0.35 (DVB-S), α=0.15 (DVB-C) 54



Some eyediagrams without noise (on 2Ts)

Without noise, different roll off at the transmitter and receiver:

55


Decisions

ISI at sampling

instantsNoise

(filtered and sampled)

Term of

Interest

(Inter Symbol Interference)

→ Interference suppression at t0+mTs: Nyquist criterion

Maximize ⬄Maximize

Matched FilterFT-1

→ SNR maximisation at t0+mTs: matched filter (to the received waveform)

Filtered and sampled noise:wm, variance σ2

Term of Interest

( Cauchy-Schwarz inequality: , equality for )

for

56

Baseband Digital DemodulationDecision block

→ Decision rule: Maximum A Posteriori

Binary case:

Nyquist criterion is fulfilled:

(Threshold detector or slicer)

4-ary case:

for equally likely symbols

57

Baseband Digital M-PAM TransmissionPerformance

→ Symbol Error Rate (SER)Matched filtering

▪ Binary case:

▪ M-ary case:

Obtained for a M-PAM modulation (Baseband), in a Nyquist channel, with matched filtering

One erroneous symbol = 2 erroneous bits

One erroneous symbol = 1 erroneous bit

« Natural » mapping: GRAY Mapping

P1>P2

Matched filtering

→ Bit Error Rate (BER): Mapping optimization

D

Example for V=1, N0=10-3 V2/Hz, Rb=1kbps:

58

Baseband Digital M-PAM Transmission

→ BER = f(Eb/N0) for M-PAM transmissions

Obtained results for a M-ary baseband modulations (PAM), in a Nyquist channel, with matched filtering and Gray mapping

BER0

Power efficiency Spectral efficiency

Power efficiency

59

Linear Carrier Modulations:

→ One or two dimensional modulations, → Complex envelop,

→ Equivalent lowpass channel,

→ Performance

The complex envelop associated to the transmitted signal linearly depends

on the message

60

BasebandModulation

Linear Carrier ModulationOne-dimensionnal

Frequency transposition

Coherent

demodulation

Down conversion

LPF

M-ASK (Amplitude Shift Keying)

Binaryinformation:

0 1 1 0 0


RetrievedBinary

information: 0 0 1 0 1

61

Linear Carrier ModulationOne-dimensionnal

M-ASK (Amplitude Shift Keying)

Example : 4-ASK, rectangular shaping

f

Signal modulated on fp :

62

BasebandModulation

Linear Carrier ModulationTwo-dimensionnal


Binaryinformation:

0 1 1 0 0 BasebandModulation

+

-


Downconversion

Binaryinformation:

0 1 1 0 0 BasebandDeodulation

LPF

LPF

Coherent demodulation Orthogonal signals

63

BasebandModulation

Linear Carrier ModulationComplex envelop

Binaryinformation:


I(t)In Phase Component

Q(t)Quadrature Component

Complex envelop associated to x(t)


+

-

64

BasebandModulation



Binaryinformation:


+

-

h (t)

Complex envelop

associated to x(t):Complex symbols

Bits Mapping

Complex baseband modulationFrequency

transposition

65


h (t)

Complex envelop


Bits Mapping

Complex message generation with a baseband modulatorFrequency

transposition

→ The PSD of the carrier-modulated signal:

is obtained from the PSD of its associated complex envelop (known baseband spectrum):

→ But also :

Re-use the results obtained for baseband modulations

66

Linear Carrier ModulationTwo main classes of two-dimensionnal modulations

h (t)

Complex envelop


Bits Mapping


transposition

→ ak and bk:: M-ary independent symbols {+/- 1, +/- 3, …, +/- ( M-1)}

square M-QAM (Quadrature Amplitude Modulation)

→

M-PSK (Phase Shift Keying)

67

Linear Carrier ModulationConstellation

ak

bk

0 1 3 5 7 …… -7 -5 -3 -1

1

3

5

7

.

.

.

-7

-5

-3

-1

.

.

.

ak

bk

Representation of possible dks in the (ak, bk) plane = « constellation » of the modulation

QAM ConstellationsPower efficient

(DVB-C, DVB-T, xDSL)

PSK ConstellationsRobust to non linearities

(DVB-S)

Hybrid modulations : APSK(DVB-S2, DVB-S2X)

68

Linear Carrier ModulationExamples

→ Two-dimensionnal linear modulations : M-QAM

Independent and

Example : 4-QAM or QPSK (DVB-S)

I(t) Q(t)

x(t)

69


→ Two-dimensionnal linear modulations : M-QAM

Independent and

Example : 16-QAM (DVB-C)

x(t)

* *0111

*0110

0101

*0100

+1 +3

*0010

*0011

*0000

*0001

-3 -1

*

1110

*1111

*

1101

*1100

-1

-3

+3

+1

*1010

*1011

*1000

*1001

ak

bk

I(t) Q(t)

70


→ Two-dimensionnal linear modulations : M-PSK

and are linked

Example : 8-PSK (DVB-S2)

x(t)

I(t) Q(t)

Zoom

71


→ Hybrid modulations : M-APSK (DVB-S2)

16-QAM

32-APSK (4-12-16 APSK)

M-APSK

16-APSK (4-12 APSK)

ak

* *0111

*0110

0101

*0100

+1 +3

*0010

*0011

*0000

*0001

-3 -1

*

1110

*1111

*

1101

*1100

-1

-3

+3

+1

*1010

*1011

*1000

*1001

72


→ Hierarchical modulations : DVB-T and T2, DVB-H, DVB-S2

Example 1 : hierarchical 16-QAM (DVB-T or H)

Internalinterleaver

Mapping

I

Q

HP

BP

* *0111

*0110

0101

*0100

+2 +4

*0010

*

0011

*0000

*

0001

-4 -2

*

1110

*1111

*

1101

*1100

-4

-2

+4

+2

*1010

*1011

*1000

*1001

I

Q

Example 2 : hierarchical 8-PSK (DVB-S2)

73

h (t)

Complex symbols

Bits Mapping

→ M-ASK:

→ M-QAM:

→ M-PSK:

74

Linear Carrier ModulationTransmitter


transposition

Complex envelop associated to :

Linear Carrier ModulationReceiver

LPF

LPF

j

Bits Receiver filterhr(t)Mapping-1


Decisions

Sampling

Downconversion

M-ASK :

M-QAM :

M-PSK :

j

75

Downconversion

jBits

h (t)

Symboles complexes

Bits Mapping

Baseband modulation

Transmissionchannel

hc(t)

n(t)

Receiverfilterhr(t)

Demapping Decisions

SamplingBandPassfilter

Fe > 2 Fmax

Fmax = 2fp +Be

Example of used bandwidth for satellite broadcasting:L: 1.4-1.6 GHz, C: 4-6 GHz, Ku: 10.70-12.75 GHz, Ka: 20-30 GHz.

76

Lowpass

Lowpass

Linear Carrier ModulationEquivalent lowpass channel to reduce the processing time for digital implementations



(Bande Be)


Downconversion

jBits

h (t)

Symboles complexes

Bits Mapping

Baseband modulation

Receiverfilterhr(t)

Demapping Decisions


77

Lowpass

LowpassBaseband Demodulation


Fmax = Be

Lower sampling frequencies

Equivalent lowpasschannel

(Bande Be)


Linear Carrier ModulationEquivalent lowpass channel to reduce the processing time for digital implementations

ffp-fp

2

ffp-fp

1

(remark: the channel is assumed to be ideal in the figure)

→ Complex envelop associated to the bandpass channel:

78

Linear Carrier ModulationEquivalent lowpass channel: construction

Transmissionchannel

hc(t)

n(t)

Equivalentlow pass channel

Carrier modulated signal:

Associated complex envelop:

HBPF(f)

f

N0

f2

N0

2

Sn(f)

-fp fp

fp-fp

2N0

ffp-fp

→ Bandpass filtering:

→ Complex envelop associated to the filtered noise:

79


f

Equivalentlow pass channel

Equivalent low pass channel

80


h (t)

Symboles complexes

Bits Mapping

Baseband modulationFrequency

transposition

Downconversion

jBits

Receiverfilterhr(t)

Demapping Decisions


Lowpass

LowpassBaseband Demodulation

(Bande Be)


h (t)Bits Mapping

Baseband SER computations can be re-used

Symboles complexes

81

Bits

Recieverfilterhr(t)

Demapping Decisions

Sampling

Matched filtering

Fullfill Nyquist criterion on:

Equivalent low pass channel

Baseband modulation


(Bande Be)


Linear Carrier ModulationEquivalent lowpass channel

Linear Carrier ModulationPerformance (Hypothesis : Nyquist + Matched filtering)

hr(t)h (t)Bits Mapping Mapping-1

⬄ two independent M- -PAM transmissions

But !! Es = physical parameter = average symbol energy at the receiver input (M symbols dk) !!

Bits

hr(t)h (t)Bits Mapping Mapping-1 Bits

→ M-ASK

→ Squared M-QAM

→ M-PSK

82

Linear Carrier ModulationBER comparison for M-QAM and M-PSK

BER0

Power efficiency for PSKSame spectral efficiency

PSK

QAM

83

Example of physical layer on an AWGN channel:

Satellite Digital Video Broadcasting : DVB-S (1994)

84

Mux Adaptationand energydispersal

Outercode Interleaver Mapper

Shapingfilter

PhysicalInterface

Video Coder

Audio Coder

Data Coder

Source coding and multiplexing

ES PES

Program 1

Video Coder

Audio Coder

Data Coder

ES PES

Program N

…

PCRSTC 1

PCRSTC N

Program Information

Transport Stream

Reed SolomonRS(204,188, t=8)

Forney ConvolutionnalInterleaving

QPSK SRRCF α = 0.35

MPEG-2 SystemTransport stream

generation

Innercode

Convolutional Code (7,1/2)

To RF Satellite Channel

Physical layer

85

Satellite Digital Video Broadcasting : DVB-SPhysical layer

Bitrates

Digital TV transmission must be « Quasi Error

Free » (QEF) TEB < 10-10


Video Coder

Audio Coder

Data Coder

Codage source et multiplexage

ES PES

Program 1

Video Coder

Audio Coder

Data Coder

ES PES

Program N

…

PCRSTC 1

PCRSTC N

Program Information

Train transport


Scrambling

Example on an image

PSD of theunscramble signal :

PSD of thescramble signal :

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.510

-10

10-8

10-6

10-4

10-2

100

102

104

Fréquences normalisées

DS

P

Partie positive de la DSP du signal émis

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.510

-12

10-10

10-8

10-6

10-4

10-2

100

102

Fréquences normalisées

DS

P

Partie positive de la DSP du signal émis

86


Outercode Interleaver

Video Coder

Audio Coder

Data Coder


ES PES

Program 1

Video Coder

Audio Coder

Data Coder

ES PES

Program N

…

PCRSTC 1

PCRSTC N

Program Information

Train transport



Innercode

Convolutional code

(7,1/2)

A digital TV transmission must be « Quasi Error Free » (QEF) :

TEB < 10-10


Forward Error Correction (FEC)

-4 -3 -2 -1 0 1 210

-5

10-4

10-3

10-2

10-1

100

Eb/N

0 (dB)

TE

B

TEB théorique non codé

TEB simulé non codé

TEB simulé, codage convolutif

TEB simulé, codes concaténés sans entrelaceur

TEB simulé, codes concaténés avec entrelaceur

87


Outercode Interleaver Mapper

Shapingfilter

PhysicalInterface

Video Coder

Audio Coder

Data Coder


ES PES

Program 1

Video Coder

Audio Coder

Data Coder

ES PES

Program N

…

PCRSTC 1

PCRSTC N

Program Information

Train transport



QPSK SRRCF α = 0.35

Innercode

Convolutional code

(7,1/2)

To RF Satellite Channel


Modulation

)(1fpour

pour

pour

N

a

aaa

a

f

fffFf

f

ff

fH NN

N

N

N

0

)1()1(2

sin2

1

2

1

)1(1

)(

2/1

AWGN channelwith non linearities

88

References

→ Digital Communications, J. G. Proakis, Mac Graw Hill Book Cie

→ Telecommunications system engineering, Lindsay and Simon, Prentice Hall

→ Digital communication by satellite, J.J. Spilker, Prentice Hall

→ Digital Video Broadcasting (DVB): Framing structure, channel coding and modulation for 11/12 GHz satellite services, norme ETSI EN 300 421.

→ Digital Video Broadcasting (DVB): User guidelines for the second generation system for broadcasting, interactive services, news gathering and other broadband satellite applications (DVB-S2), norme ETSI EN 102 376.

89

Documents

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