Compensation of nonlinear distortions with memory effects in OFDM transmitters

The OFDM modulation scheme is characterized by the Gaussian-like signal behavior with a relatively high peak-to-average power ratio (PAPR). As a result, it is very sensitive to nonlinear distortions, which arise mainly from the high power amplifier (HPA). This paper proposes an algebraic solution to compensate at the transmitter for nonlinearity of the HPA with memory effects, where the HPA behavioral model is represented by the Hammerstein structure, a cascade of a memoryless nonlinear block followed by a linear filter. In particular, a frequency domain parameter identification methodology is developed that first estimates the parameters of the unknown nonlinearity, which is modelled through a polynomial expansion. The frequency response of the unknown filter is then calculated, in order to capture the memory effects in the system. Using the identified nonlinear system parameters, an inverse Wiener structure, consisting of a cascade of the inverse filter and the inverse memoryless nonlinearity, is constructed preceding the HPA, in order to predistort the input signals so as to achieve overall linear transmitter characteristics.