Practical aspects of a digital feedforward approach for mitigating non-linear distortions in receivers

This paper addresses the mitigation of non-linearly induced interference in reconfigurable wideband direct down-conversion receivers using digital signal processing techniques. Multi-mode and multi-band transceivers are widely used nowadays. However, due to their aim to be flexible and reconfigurable, they lack of high selectivity and are more sensitive to non-linear distortions beside other RF impairments. The case of blocking through out of band interferer signals is found in practice, where the receiver tries to demodulate a weak signal from a far base station, but a high power wideband signal from a near base station in a different frequency-slot induces the receiver to work in a non-linear region. The spectral regrowth of the interferer signal that falls above the desired one deteriorates the bit error rate performance. In the receiver, the automatic gain control attenuates the interferer, but also reduces the power of the desired signal, thereby decreasing the dynamic range of the receiver. A feedforward algorithm based on a reference model and adaptive filters has been analysed under fading channel conditions in MATLAB and later tested in the lab in a realtime implementation in the FPGA of a software-defined radio device. Results show an enhancement in the bit error rate by reducing the interference and an increase in the dynamic range of the receiver.