Adaptive Predistortion Using a $\Delta\Sigma$ Modulator for Automatic Inversion of Power Amplifier Nonlinearity

Abstract

This brief demonstrates a new adaptive digital predistortion architecture particularly suited to mobile handset applications. The central idea is to build a lookup table (LUT) that directly captures the static compressive nonlinearity of the power amplifier (PA) and then insert this LUT into the feedback path of a ΔΣ modulator. The oversampled ΔΣ modulator automatically performs both the inversion of the PA nonlinearity and the interpolation between LUT entries, permitting complex modulation strategies to be handled with an absolute minimum of LUT entries and with a dramatically simplified computational structure. The advantages of this architecture over previous methods include: 1) there is no need to explicitly invert the PA nonlinearity, reducing the complexity for the system designer; 2) the LUT training is done with an open-loop method, improving the training speed; 3) there is no need to explicitly employ numerical interpolation between LUT entries; and 4) digital-to-analog converter (DAC)nonlinearity is incorporated into the predistortion, allowing fast low-resolution DACs to be used in the final system. We built a proof-of-concept prototype for a 900-MHz, 27-dBm PA transmitting a 16-ary quadrature amplitude modulation (16-QAM) signal with a bandwidth of 3.4 MHz. The predistortion system reduced out-of-band distortion products by 10 dB and improved the error vector magnitude from 3.5% to 2.0%.

A Polynomial Analog Baseband Predistorter for Compensation of Wireless Amplifier’s Distortion

“…Then the real and imaginary part of distorted amplitude and gain are extracted Fig. 9 shows the normalized output of polynomial vs. the input for a range of coefficients as given in Table (2). This figure illustrates the ability of the proposed 5 th order polynomial circuit in adapting to the behavior of distinct PAs by simply configuring the currents (b1 to b5), which controls the polynomial coefficients.…”

“…A number of different linearization methods including the feed-forward [3], [4], feedback [5], digital PD [6] and analog PD [7], [8] have been widely employed to improve the linear characteristic of the PA. As for implementation, the digital PD will no longer be the optimum candidates. This is because of their bandwidth limitations and the need for high power supply [1], [2]. The challenge in analog PD methods is in designing a circuit that will accurately produce an inverse characteristic of the original PA's distortion [9], see Fig.…”

Inter-band carrier aggregation digital transmitter architecture with concurrent multi-band delta-sigma modulation using out-of-band noise cancellation

“…The filter output is coupled by a directional coupler and used by adaptive digital predistortion subsystem to characterize the transmitter nonlinearity whose impact is different for each frequency band; therefore two quadrature demodulators separately capture each frequency band. Although digital predistortion can be integrated within delta-sigma modulation using a feedback predistortion technique [8], the feedback predistortion requires extra delay to apply digital predistortion on the CMB-DSM signal, which limits the modulation bandwidth. Hence, the digital pre distortion in the proposed architecture is directly applied to the baseband signals.…”

Investigation of Parameter Adaptation in RF Power Amplifier Behavioral Models

“…In order to implement and analyze adaptation algorithms in practice, due to the high cost in equipment and time required for designing a complete closed loop system, two approaches have been taken. One approach has been to develop adaptation techniques with LUT-based DPDs in either the direct path [9]- [12], or both the direct and feedback paths [13]- [15]. These approaches suffer from the huge increase in size of LUTs needed to compensate for the different drifts in PA behavior.…”