Digital analysis and compensation of nonlinearities NEWCOM WPR3 progress report

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:

Compensation at the transmitter

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

Nonlinearity Compensation Performance of some Compensation Methods

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

Compensation at the receiver

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.

Reliability information and soft-decision

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

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

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