A simple and effective RF predistorter for use in the HPAs of base stations in cellular mobile systems

Abstract

This paper presents the design of using a simple mixer circuit, constructed with Schottky diodes as the nonlinear device, to generate the intermodulation (IM) products for predistortion of high-power amplifiers (HPAs) in the base stations of cellular mobile systems. The design has a simple structure and requires no external dc bias voltage. A complex polynomial is used to model the characteristic of the HPA and the predistorter generates the in-phase and quadrature components of the intermodulation distortion (IMD) products for predistortion of the HPA. Experimental results using a practical HPA show that, for a modulated CDMA (IS-95) signal, the predistorter can reduce the adjacent-channel-power ratio (ACPR) by 15 dB.Index terms Adjacent-channel-power ratio (ACPR), High Power Amplifier, Inter-modulation, Mixer, RF Analog Predistorter. I.INTRODUCTIONThe current mobile radio systems such as the GSM-EDGE, CDMA (IS-95) and 3G require the RF high power amplifiers (RF HPAs) in the base stations to be broadband and have linear amplification. However, due to the inherent nature of RF HPAs (even with class-A or class-AB configurations), the amplification processes are always nonlinear, so some sorts of linearization techniques need to be used in the base station HPAs of these mobile systems. Analog RF predistortion is simple, low cost to implement and having a much wider bandwidth, so it is more suitable than digital predistortion for base station HPAs. An RF predistorter has a distortion characteristic complementary to that of the RF HPA [1, 2] and can be implemented by generating either the harmonicfrequency signals [1,3,4] or the inband intermodulating (IM) products [5-9] and then adding to the original signal at the input of the HPA. The harmonic-frequency technique has an advantage of low insertion loss [4], but the bandwidth of the HPA may block the high-order harmonic-frequency signals, making the technique less useful for high frequency systems. In general, there are two ways, i.e. using the differencefrequency signal or the sum-frequency signal, to generate the inband IM products for HPA predistortion [5,6]. In [5], a diode was used as a nonlinear device to generate the difference-frequency signal. However, for HPAs operating in the microwave frequency band, the difference-frequency signal will be out of the HPA operation frequency band and thus blocked. In [6], two wideband double-balanced mixers capable of operating at twice of the maximum frequency were used to generate the sum-frequency signal in order to generate the IM3 signals. But it is not easy to handle signals with such high frequency, particularly when generating the IM products with higher orders for intermodulation distortion (IMD) products cancellation.This paper proposes to use mixers as the nonlinear devices to generate the inband IM products for RF HPA predistortion and presents the design of a simple mixer circuit to generate the 3 rd -order intermodulation (IM3) products for 3 rd -order IMD cancellation. The mixer circuit employs two mixers c...

Predistortion of a 100‐W base station high‐power amplifiers

“…For higher power efficiency, the HPA has to be operated in the nonlinear region of the characteristic, resulting in severe signal distortion and spectra spreading of the transmitted signal. To minimize signal distortion and spectra spreading yet having high power efficiencies, HPAs have been widely used in conjunction with some sort of linearization techniques [2–17]. One of the promising linearization techniques is predistortion which uses predistorted signals to complement the distortion characteristics of the HPAs.…”

An RF predistorter for base station HPAs of NADC system

“…The hardware implementation of the design is tested using a pi/4-DQPSK signal on a practical HPA. In [8], a predistorter using inband intermodulation frequencies has been proposed with modulated CDMA IS-95 signals spectrum measurements, but the effects of the HPA linearity improvement on the system were not considered. Here, we demonstrate that, at an HPA power level of 30 W, the predistorter can suppress the adjacent-channel-power ratio (ACPR) of the signal spectrum by 20 dB.…”