1. Digital analysis and compensation of nonlinearities
NEWCOM WPR3 progress report

2. Multicarrier signal in nonlinearity
Baseband
modulator
OFDM
Nonlinear
Amplifier
gT(t)
Channel
demodulator
gR(t) ai âi sn Sk
r(t) rn Rk
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:

3. Compensation at the transmitter

4. Data predistortion data predistorter
Baseband
modulator
OFDM
Nonlinear
Amplifier
gT(t)
Channel
demodulator
gR(t) ai âi sn Sk
r(t) rn Rk
optimal predistorted amplifier model

5. Nonlinearity Compensation Performance of some Compensation Methods

6. Tone insertion
OFDM modulator (IFFT)
Goal: reduce the maximum amplitudes of the OFDM-modulated signal
Method: Insert some subcarriers (tones) to achieve the goal. . .

7. Compensation at the receiver

8. Iterative cancellation of nonlinear distortion
Baseband
modulator
IFFT
(N points)
Nonlinear
Amplifier
gT(t)
Channel
FFT
gR(t)
demodulator ai âi sn Sk
d(.) +
Hard / Soft
decision _ rn
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.

9. Reliability information and soft-decision

10. Reliability information and soft-decision
OFDM system with 256 subcarriers and iterative distortion cancellation with soft decision and reliability information.

11. 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
OFDM demodulator (FFT) . .
Find the N ”worst” subcarriers
Find the likelihood of the alternative signal point
Choose the point with the largest likelihood

12. 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