Digital predistortion of RF amplifiers using baseband injection for mobile broadband communications
2014
Radio frequency (RF) power amplifiers (PAs) represent the most challenging
design parts of wireless transmitters. In order to be more energy efficient, PAs should
operate in nonlinear region where they produce distortion that significantly degrades the
quality of signal at transmitter’s output. With the aim of reducing this distortion and
improve signal quality, digital predistortion (DPD) techniques are widely used. This
work focuses on improving the performances of DPDs in modern, next-generation
wireless transmitters. A new adaptive DPD based on an iterative injection approach is
developed and experimentally verified using a 4G signal. The signal performances at
transmitter output are notably improved, while the proposed DPD does not require large
digital signal processing memory resources and computational complexity. Moreover,
the injection-based DPD theory is extended to be applicable in concurrent dual-band
wireless transmitters. A cross-modulation problem specific to concurrent dual-band
transmitters is investigated in detail and novel DPD based on simultaneous injection of
intermodulation and cross-modulation distortion products is proposed. In order to
mitigate distortion compensation limit phenomena and memory effects in highly
nonlinear RF PAs, this DPD is further extended and complete generalised DPD system
for concurrent dual-band transmitters is developed. It is clearly proved in experiments
that the proposed predistorter remarkably improves the in-band and out-of-band
performances of both signals. Furthermore, it does not depend on frequency separation
between frequency bands and has significantly lower complexity in comparison with
previously reported concurrent dual-band DPDs.
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