The restoration procedure presents two difficulties. The
first concern is the linearity of the restoration control on the
PA through the voltage supply. Analog solutions of polar
modulation require a feedback loop including an envelope
detector and a comparator as demonstrated in [9]. A digital
approach could be made using a predistortion of the envelope
signal. The second tough concern in the envelope processing
path is the design of the wide band modulated supply voltage,
with either a DC-DC converter based on a class S amplification
(classical PWM or ΣΔ PWM [10]) or switching regulator. The
efficiency of the regulator is directly linked to the switching
frequency that is a trade off between the bandwidth of the
envelope signal and the allowed distortions. As an illustration
of the complexity of the power supply generation for the WCDMA,
in term of sampling frequency, we can cite the PWM
buck converter running at 50MHz [11]. To ease the conception
of this supply voltage, crest factor reduction is employed, see
section III-D.