Mixed BB-IF Predistortion of OFDM Signals in Non-linear Channels

It is well known that power amplifier induced nonlinear distortions produce a signal spectral regrowth at the transmitter output of digital radio communication systems. This effect is responsible for both adjacent channel interference and BER degradation. Signal predistortion is a technique that counteracts such phenomena. Technological advances in the last decade, renewing the interest in this technique, led to the realization of digital baseband (BB) predistorters that overcome the performance of the existing analog IF (Intermediate Frequency) ones. However, the substitution of an analog IF predistorter with a digital BB one forces to partially redesign the system architecture. An alternative approach is proposed in this paper, based on digital and analog techniques, which combines the precision of the digital BB solution with the practicality of an IF architecture. This solution is particularly interesting to substitute an old analog IF predistorter simply plugging-in the new digital one, without further changes in the transmitter architecture. Critical aspects, predistortion algorithms and simulation performance are presented with respect to a Digital Video Broadcasting system which is based on an OFDM modulation and is very sensitive to nonlinear distortions because of the adopted multicarrier modulation.