2. LITERATURE REVIEW
2.9 Common Linearization Methods
2.9.3 Predistortion
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The separate I and Q signal inputs will be the filtered, or smoothed, binary symbol sequences. The signals are fed through differential correcting amplifiers into vector modulators that form the actual RF signal S(t), where ωc being the RF carrier frequency, resulting in
t t
Q t t
I t
S( ) ( )cosc ( )sinc (2.48) The signal S(t) is then fed into the RF power amplifier, emerging with some distortion. A small portion of the output is coupled into a downconverter and retrieves the now distorted I and Q signals, which are then directly compared with the undistorted input baseband signals. The gain of the input differential amplifiers will force the loop into generating an output signal that closely tracks the original I and Q signals. The effectiveness of the Cartesian loop depends on the ratio of the feedback loop bandwidths to the I and Q input bandwidths and the linearity of the demodulators.
One of the benefits of the Cartesian loop over the polar loop is the symmetry of gain and bandwidth in the two quadrature signal processing paths. This will reduce the tendency to introduce phase shifts between the AM-AM bandwidth and stability will limit the capability to handle multicarrier signals. With the widespread availability of low-cost quadrature modulators and demodulators, the overall system becomes a simple linearized transmitter architecture. Cartesian loop transmitters that operate up to 900 MHz for relatively narrow band signals (<5 kHz bandwidth) with excellent results have been constructed [92]. The Cartesian loop also forms an entry point for digital linearization techniques.
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it also tends to be cheap, typically consisting of simple module containing a few carefully optimized components. Predistortion techniques are superior in terms of its wideband performance [93] and RF predistorters based on diode or transistor devices have been implemented for wideband systems [78]-[80].
Predistortion techniques attempt to modify the incoming signal to complement and cancel the nonlinear effects in PAs. Historically, these techniques has been aimed primarily at AM-PM correction, particularly in TWTs, which to this day perform broadband amplification feats untouched by the solid state revolution in military ECM applications. There is an extensive literature on the subjects [94]-[95], but only limited to applications in narrowband systems. Predistortion can be classified as gain and phase, nonlinear generators or baseband predistortion.
1) Gain and phase predistortion
The simplest predistorters use expansive networks to compensate for the gain compression experienced by a power amplifier as the operating point approaches the 1 dB compression point. These networks are typically diode- resistor networks and can achieve 5 to 15 dB reduction in third order IM distortion [96]. An RF level-dependent resistor, combined with a fixed capacitor, can give a suitable AM-to-AM and AM-to-PM characteristic that opposes the distortion of the amplifier in the pre-compression zone. This can result in as much as a 5 dB improvement in ACP [97].
An alternative approach is to place voltage-controlled attenuators and phase shifters in the input path and use the envelope of the RF signal to dynamically adjust the settings. Typical improvements in third order product are up to 10 dB, but as with all open-loop correction systems, it is sensitive to temperature, amplifier gain and some form of adaptation is often required.
2) Predistortion using nonlinear generators
Using nonlinear generators, complementary IMD can be generated and used to cancel the PA distortion [17]. Several ways are used to implement this method with the simplest being a diode or transistor network in series with the main
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signal path. Another alternative is to apply feedforward predistortion using a low power transistor with similar distortion characteristics as the main amplifier. With care, it is possible to achieve 15 dB reduction in third order products.
3) Baseband predistortion
By using digital signal processing (DSP), predistortion to the baseband modulation signals can be applied prior to upconversion and greater precision can be obtained [98]. The mapping based predistorter uses a large look-up table to map the input I and Q signals to new predistorted values [99]. Another form of baseband predistortion is by using the gain-based predistorter. It uses the envelope level to modify the complex output signal with interpolation between stored values. The third alternative is analog-based predistortion where the correction is applied to the baseband signals with analog circuits.
PA 90o
90o
LO
Linearized Output
Attenuator IOut
QOut
IIn
QIn
Look-up table &
DAC DSP
ADC
Figure 2.25: Digital adaptive predistortion system
Fig. 2.25 shows a block diagram of an adaptive digital predistortion loop. In a normal operation, the system works as an open-loop predistortioner, with the lookup table providing a preprogrammed I-Q output pair for each input envelope sample. The
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I-Q output pair contains the appropriate phase and amplitude correction required to compensate for the PA nonlinearity at the current signal level. Because of the precision of the DSP, such systems have been reported to give impressive corrections, as measured in reduced IM or ACP levels [100], but even in open-loop mode they operate too slowly for some applications. The DSP circuitry also can consume too much power for lower power PA applications.
Digital predistortion is a dominant choice in contemporary base station amplifier designs. It provides very good performance and great flexibility by using an adaptive digital closed-loop control [101]. Advanced algorithms such as the genetic algorithm can be used to improve computation efficiency in the digital domain. However, using digital predistortion for handset amplifiers is still not popular due to the concerns of circuit complexity and cost.
Analog predistortion is still the only viable ways to achieve linearity improvement in a compact handset power amplifier, although it only achieves moderate linearity improvement and usually cannot adapt over a wide dynamic power range or wide temperature change. A diode based analog predistortioner was presented in [102]. If the nonlinear gain and phase transfer functions introduced by the diode are inverse to those of the power amplifier following it, the input signal can be properly predistorted. This method is really popular in practice because of its simplicity and its ability to enhance maximum output power by 1-2 dB, where efficiency and linearity of the amplifier matter the most. By having the same output power, a 1-2 dB enhancement from the predistortioner translates to 25-50% saving of active device area.