Nonlinearities

The transfer of the system and channel experienced during a wireless transmission is commonly modelled as linear. Although this approach seems valid for the propagation channel, as discussed in Section 2.2, several elements in the transmitter (TX) and receiver (RX) analogue front-end can exhibit a nonlinear transfer. The major sources of nonlinearity are generally the analogue-to-digital (A/D) and digitalto- analogue (D/A) converters, mixers and amplifiers, i.e., the power amplifier (PA) in the TX and the low-noise amplifier (LNA) in the RX.

The influence of these nonlinearities is negligible in conventional systems applying constant modulus signals, making the generally applied linear approximation valid. The time-domain signals in OFDM-based systems, however, exhibit a high peak-to-average power ratio (PAPR), which can easily result in clipping and nonlinear distortion in the different parts of the RF front-end. Two conventional solutions to overcome this problem are either to use highly linear components for the front-end or to apply a large input power backoff (BO) when feeding the signals to the nonlinear components, as such ensuring that the signals experience the linear part of the transfer of the components. The former solution has the disadvantage that the cost of the total front-end will increase. The latter solution is inefficient in terms of power consumption, since the power efficiency of the used components decreases rapidly with increasing BO.