adaptive modulation schemes for frequency selective fading channels mª carmen aguayo torres, josé...

Adaptive modulation schemes for frequency selective fading channels

Mª Carmen Aguayo Torres, José Paris Angel,

José Tomás Entrambasaguas Muñoz

Universidad de MálagaEscuela Técnica Superior de Ingeniería de Telecomunicación

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Conclusions

Conclusions

AOFDM,ADFE,ATHP

AOFDM,ADFE,ATHP

Contents

Adaptive QAM

(AQAM)

Adaptive QAM

(AQAM)

OFDM, DFE and THP

OFDM, DFE and THP

Introduction

Introduction

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Introduction

• Time spread– Frequency selectivity

• Multipath– Time selectivity or fading

Time and frequency

selective channel

At an instant of time, distinct frequencies have different gains and phases

At a single frequency: the channel change with time

Mobile environment

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Time selectivity

Adaptive transmission techniques

Simple way to face fading: use enough power or transmit slowly enough

Inefficient

Possibility: Modify the transmitted signal depending on the instantaneous channel conditions

Any signal parameter can be modified: power, symbol period, modulation scheme...

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Adaptive QAMHigh bit rates when channel is suitableSlow down the transmission when the channel gets worse

Time selectivity

Modify constellation sizePower and symbol period are kept

Adaptive modulation levelR

ece

ive

d s

ign

al

time

Used constellatio

n: BPSK

Used constellatio

n: BPSK

Used constellatio

n:16QAM

Used constellatio

n:16QAM

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Frequency selectivity

Several techniques against frequency selectivity

Equalization

Decision feedback equalization

Problem:error propagation

Tomlinson-Harashima pre-equalization

Linear equalization enhances noise

Disadvantage:transmitter needs knowledge about the channelTDD

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Other possibility:

splitting up the whole band width in narrow flat subbands

Frequency selectivity

Overlapped but orthogonal spectra

Multicarrier modulation or OFDM

Problem:time selective channels destroy the orthogonality

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Time and frequency selective channel

Time and frequency selective channel

Adaptive DFE, THP or

OFDM

Adaptive DFE, THP or

OFDM

Adaptive QAM

Adaptive QAM

FadingchannelFadingchannel

Adaptive OFDM, DFE and THP

DFE, THP orOFDM

DFE, THP orOFDM

Frequency selective channel

Frequency selective channel

Adaptive modulation can be used over each subcarrier in OFDM or over the equivalent channel after equalization

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Time and frequency selective channel

Conclusions

Conclusions

AOFDM,ADFE,ATH

AOFDM,ADFE,ATH

Fixed constellationFrequency selective

Flat fading channel

Contents

OFDM, DFE and THP

OFDM, DFE and THP

Adaptive QAM

(AQAM)

Adaptive QAM

(AQAM)

Introduction

Introduction

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Channel model

System model

Efficiency and BER

Conclusions

Conclusions

AOFDM,ADFE,ATH

AOFDM,ADFE,ATH

Contents

OFDM, DFE and THP

OFDM, DFE and THP

Adaptive QAM

(AQAM)

Adaptive QAM

(AQAM)

Introduction

Introduction

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Flat fading channel model

Propagation model

The signal arrives through a multitude of rays of different gains and phases but similar lengths

Channel model

System model

Efficiency and BER

Flat fading channel

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Flat fading channel model

x(t)

h(t)

y(t)

FLAT FADING CHANNEL

Flat fading channel

Propagation model

The signal arrives through a multitude of rays of different gains and phases but similar lengths

Complex Gaussian distributionTime variations: fD

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AQAM system model

Modify the bit rate to get closer the variable channel capacity

Channel model

System model

Efficiency and BER

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AQAM system model

Return channel

R[n] bits/segSymbol period: T

b[n] x[n]

h[n] nAWGN[n]

Adaptive transmitter

Adaptive receiver

Channel

y[n] [n]b̂

Modify the bit rate to get closer the variable channel capacity

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AQAM transmitter model

m[n+1]

1

U2

Adaptivemodulator

x[n]

m[n]

b[n] To the direct channel

From return channel

S/P

m[n] is variable and under receiver controlU constellations and no-transmission

U+1 modulation regions

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AQAM receiver model

m[n]

1 S/P

y[n]

U2

Adaptivedetector

m[n]

m[n+1]To the return channel

From the direct

channel

[n]

Grid adjust

Channel estimation

Contellation selector

[n]b̂

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Adaptation algorithm

• 5 modulation regions– No transmission– QPSK– 16QAM– 64QAM– 256QAM

ber(

m,

)

10-6

10-5

10-4

10-3

10-2

10-1

100

5 10 15 20 25 30 35

(dB)

m = 2

QPSK

Average BER

Po

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Adaptation algorithm

ber(

m,

)

10-6

10-5

10-4

10-3

10-2

10-1

100

5 10 15 20 25 30 35

(dB)

ber() m()

2

4

6

8

0

1 2 43

442

4332

3222

2112

1

M

M

M

M

00

)m

)(log

)(log

)(log

)(log

(

• 5 modulation regions– No transmission– QPSK– 16QAM– 64QAM– 256QAM

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Average efficiency and BER

Average efficiency

Average of m()

Channel model

System model

Efficiency and BER

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Average efficiency and BER

10-6

10-5

10-4

10-3

10-2

10-1

100

5 10 15 20 25 30 35 (dB)

2

4

6

8

0

ber() m()

Average efficiency

Average of m()

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Average efficiency and BER

Po = 10-2

Average efficiency

Average of m()

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Average BER

Average of the instantaneous BER ber()

Average efficiency and BER

10-6

10-5

10-4

10-3

10-2

10-1

100

5 10 15 20 25 30 35 (dB)

2

4

6

8

0

ber() m()

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Average efficiency and BER

Average BER

Average of the instantaneous BER ber()

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Average efficiency and BER

At 23 dB, QPSK has the same BER than AQAM, but half efficiency

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Conclusions

Conclusions

AOFDM,ADFE,ATH

AOFDM,ADFE,ATH

Contents

OFDM, DFE and THP

OFDM, DFE and THP

Adaptive QAM

(AQAM)

Adaptive QAM

(AQAM)

Introduction

Introduction

Channel model

OFDM

DFE and THP

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Frequency selective fading channel model

Small symbol period propagation paths can be distinguished

frequency selective fading channel

Channel model

OFDM

DFE and THP

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Frequency selective fading channel model

x(t) y(t)

FREQUENCY SELECTIVE FADING CHANNEL

h0 (t)

Retardo 0Retardo 0

hL-1 (t)

Retardo L-1Retardo L-1

Channel output comes from adding L different echos which passed through a flat fading channel

Small symbol period propagation paths can be distinguished

frequency selective fading channel

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Frequency selective fading channel model

number, delay and average power of each ray

0 1 2 3 4 50

0.1

0.2

0.3

0.4

Ray delay, l (s)

Nor

mal

ized

pow

er

Typical Urban Channel

Power profile

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Frequency selective fading channel model

Frequency responseAt each frequency, the response is different and variable in time

In average, SNR is the same for all

frequencies

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OFDM system model

s0a

,ˆ

s1a

,ˆ

s1Na

,ˆ

IDF

Ta1,s

x[n]

nW [n]

h[n,i]y[n]P/S

+Ext

DF

TExt+

S/P

aN-1,s

a0,s

Channel model

OFDM

DFE and THP

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OFDM system model

s0a

,ˆ

s1a

,ˆ

s1Na

,ˆ

IDF

Ta1,s

x[n]

nW [n]

h[n,i]y[n]P/S

+Ext

DF

TExt+

S/P

aN-1,s

a0,s

Cyclic extension

Eliminates ISI

Linear convolution with the channel appears as circular

Total period Tt = T + TgUseful period: N Ts

mmmmNHaa ˆ

For a constant channel, OFDM can be considered as N parallel channels Fourier transform of the channel

impulse response sampled at each subcarrier frequency

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OFDM system model

s0a

,ˆ

s1a

,ˆ

s1Na

,ˆ

IDF

Ta1,s

x[n]

nW [n]

h[n,i]y[n]P/S

+Ext

DF

TExt+

S/P

aN-1,s

a0,s

f

f = fs/N

f

Fourier transform of the pulse

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m

1N

mk0kmkkmmmm

NHaHaa,

,,ˆ Intercarrier Interference (ICI)

Doppler spread effects

f

The transference function Hm,m results from the average over a symbol period

Received pulses over fading channels are not orthogonal

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Interference analysis

Signal to Interference Ratio, SIR

1N

mk0k

2

mk

2

mm

H E

H E

,.

.

SIR depends on fDTbut not on the power profile

Useful signal power

Interference of the k subcarrier on the m one

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High SNR are limited by interference

Interference analysis

Intercarrier interference added Gaussian noise

Signal to Noise and Interference ratio, SNIR 1

11

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Performance results

BER

QPSKRayleigh channel

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DFE system model

Channel model

OFDM

DFE and THP

x[n]b[n]

DFE TX

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nW [n]

y[n]h[n,i]

DFE system model

[n]b̂Feedforward Filter

Feedback FilterDFE RX

x[n]b[n]

DFE TX

Error propagation:Depends on specific channel

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DFE system model

ZF DFE

For FIR channels, only feedback filter is necessary

[n]b̂

Feedback Filter

ZF DFE RX

nW [n]

y[n]h[n,i]

x[n]b[n]

DFE TX

Grid adjustment

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THP system model

Moving the feedback filter to the transmitter: no error propagation

Preequalizer Filter

s[n]b[n]

THP TX

xy[n]

Gk

x[n]

v[n]

x

GainControl

+u[n]

Mod{·}

Gk-1

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THP system model

nW [n]

r[n]h[n,i]

y[n]

ZF THP RX

x

GainControl

Mod{·}[n]b̂

A[n]^

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THP system model

Moving the feedback filter to the transmitter: no error propagation

Preequalizer Filter

s[n]b[n]

THP TX

xy[n]

Gk

x[n]

v[n]

x

GainControl

+u[n]

Mod{·}

Gk-1

Transmitted signal is like-QAM but continuous

Power penalty QPSK:Average: 1.0 dBMaximum: 5.6 dB

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5 10 15 20 25 30 35 10-5

10-3

10-2

10-1

10-0 B

ER

(dB)

DFE-I

DFE

THP

10-4

Performance results

QPSKTU channel

BER

Power penalty

Error propagation

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Adaptive QAM

(AQAM)

Adaptive QAM

(AQAM)

AOFDM

ADFE

ATHP

Conclusions

Conclusions

AOFDM,ADFE,ATH

AOFDM,ADFE,ATH

Contents

OFDM, DFE and THP

OFDM, DFE and THP

Introduction

Introduction

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System model

Return channel

R[n] bits/sec

Adaptive receiver

Adaptivetransmitter

x[n]b[n]

nAWGN[n]

h[n,i]y[n]

Channel

[n]b̂

Modifying the bits per second transmitted depending on the instantaneous channel conditions

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Adaptive OFDM

Signal properties can be selected depending on gain and noise at each subcarrier

OFDM splits up the whole bandwidth in parallel flat channels

AOFDM

ADFE

ATHP

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Adaptive OFDM

f

SNR(f)

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