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