Department of Signals and Systems
Digital analysis and
compensation of nonlinearities
NEWCOM WPR3 progress report
Nonlinearity in Broadband Wireless Communication Systems
September 22, 2005
Department of Signals and Systems
Multicarrier signal in nonlinearity
ai
Baseband
modulator
Sk
OFDM
modulator
sn
Nonlinear
Amplifier
gT(t)
Channel
âi
•
•
•
Baseband
demodulator
Rk
OFDM
demodulator
rn
r(t)
gR(t)
The block diagram represents a baseband equivalent of the system.
The nonlinear amplifier can be any function, and is assumed known to the receiver.
The input/output relation for the nonlinear amplifier can be written as:
y  x  d ( x)
Nonlinearity in Broadband Wireless Communication Systems
September 22, 2005
Department of Signals and Systems
Compensation at the transmitter
Nonlinearity in Broadband Wireless Communication Systems
September 22, 2005
Department of Signals and Systems
Data predistortion
data predistorter
ai
Baseband
modulator
Sk
OFDM
modulator
sn
gT(t)
Nonlinear
Amplifier
Channel
âi
Baseband
demodulator
Rk
OFDM
demodulator
rn
r(t)
gR(t)
optimal predistorted amplifier model
Nonlinearity in Broadband Wireless Communication Systems
September 22, 2005
Department of Signals and Systems
Nonlinearity Compensation
Performance of some Compensation Methods
10
10
Nonlinear HPA
Predsitorter
Nonlinear Equalizer
Predsitorter + Nonlinear Equ.
Predistorter + Oversampling
Linear HPA
-2
-3
BER
10
-1
10
10
10
-4
-5
-6
5
10
15
Eb /N0 (dB)
Nonlinearity in Broadband Wireless Communication Systems
September 22, 2005
20
Department of Signals and Systems
Tone insertion
Goal: reduce the maximum
amplitudes of the OFDMmodulated signal
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.
.
.
.
.
.
Nonlinearity in Broadband Wireless Communication Systems
OFDM
modulator
(IFFT)
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.
.
.
.
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Method: Insert some subcarriers
(tones) to achieve the goal.
September 22, 2005
Department of Signals and Systems
Compensation at the receiver
Nonlinearity in Broadband Wireless Communication Systems
September 22, 2005
Department of Signals and Systems
Iterative cancellation of nonlinear
distortion
ai
Baseband
modulator
Sk
IFFT
(N points)
sn
Nonlinear
Amplifier
gT(t)
Channel
âi
ˆ (m)
R(m)
FFT
Baseband Rk Hard / Soft k
(N points)
decision
demodulator
IFFT
(N points)
+ rn
+
_
gR(t)
d(.)
• The idea is to recreate the distortion at the receiver using detected symbols.
• The distortion based on the estimation of symbol is used to correct the signal in the time domain.
• Hard or soft decision is being made on the baseband symbols.
Nonlinearity in Broadband Wireless Communication Systems
September 22, 2005
Department of Signals and Systems
Reliability information and softdecision
Nonlinearity in Broadband Wireless Communication Systems
September 22, 2005
Department of Signals and Systems
Reliability information and softdecision
OFDM system with 256 subcarriers and iterative distortion cancellation with soft decision
and reliability information.
0
10
No cancellation
1 iteration
2 iteration
3 iteration
4 iteration
AWGN
-1
10
-2
BER
10
-3
10
-4
10
-5
10
-6
10
0
Nonlinearity in Broadband Wireless Communication Systems
5
10
15
SNR [dB]
20
25
September 22, 2005
30
Department of Signals and Systems
Maximum likelihood detection in
nonlinear systems
•Without the nonlinearity, the standard
receiver makes maximum likelihood
(ML) detection
•With the nonlinearity, the standard
receiver does normally not find the ML
point.
•By testing more than one received
alternative in a ”simulated transmitter”
at the receiver side, the ML point can
be found
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OFDM
demodulator
(FFT)
.
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.
.
1. Find the N ”worst” subcarriers
2. Find the likelihood of the
alternative signal point
3. Choose the point with the largest
likelihood
Nonlinearity in Broadband Wireless Communication Systems
September 22, 2005
Department of Signals and Systems
Estimation of the nonlinearity
•In all methods we assume perfect knowledge of the nonlinearity. How can
we acheive that?
Estimation of the nonlinearity from received data
Feedback of PA output to the digital baseband module
Nonlinearity in Broadband Wireless Communication Systems
September 22, 2005