In the past, voice transmission was the main issue in communications. However, new requirements have evolved from voice to data and mobile communications, which demand a larger amount of information and better quality. Because of these requirements, more complex modulation and demodulation schemes and broadband channel bandwidths are necessary in modern mobile communications systems. In general, complex modulation and demodulation schemes require linear transmitters and receivers. In power amplifier design, high linearity, in addition to high efficiency, is a critical issue.
When signals are amplified in an HPA, unwanted harmonics and intermodulation distortion signals are generated, in addition to the amplified desired signals, by the nonlinear characteristics of the power amplifier. These IM signals increase the bit error rate (BER) of the data and the adjacent channel interferences, and decrease the power amplifier efficiency. Predistortion is a linearization technique that connects a circuit having inverse distortion characteristics at the input of the amplifier. Because of the inverse distortion characteristics, the predistortion linearizer can reduce the IM signals generated by the HPA.1 If the third, fifth and higher order IM products can be generated and controlled separately in the predistorter, the IM products of the power amplifier can be reduced more effectively.2 This method presents a problem caused by signal interference in small size circuits and is difficult to implement. In this article, a new type of analog predistortion linearizer is proposed to reduce HPA nonlinearity by generating and controlling individual order IM signals.
Fig. 1 Block diagram of the third-order intermodulation signal generator(IMC3).
Fig. 2 Block diagram of the third-order intermodulation signal generator(IMGh).
Predistorter Operating theory
The nonlinearity of an amplifier can be expressed in terms of a power series
where
vo = output signal
vi = input signal
If the input signal consists of two signals of equal amplitude, as
then the DC, the intermodulation distortion components (w1±w2, 2w1–w2, 2w2–w1, …) and the harmonic components (2w1, 2w2, 3w1, 3w2), besides the desired amplified input signals (w1, w2) appear at the output port because of the HPA nonlinear characteristics. The harmonic generator of the proposed distorter has a simple structure. Figure 1 shows the block diagram of the third-order intermodulation signal generator (IMG3). Its output signals obtained from the IMG3 are the main and third-order intermodulation signals, given by
The desired signals (w1, ?2, 2?1–?2, 2?2–?1) are elements of H13 as shown by Equations 1, 2 and 3. The bias voltage of the small signal transistor is controlled to generate a third-order IM signal as large as possible. An MSA-0386 transistor from HP is used in the IMG3 generator.
Figure 2 shows the block diagram of the higher order intermodulation signals generator (IMGh), which uses as input the output signals from the IMG3. They are given by the expression
IMGh consists of a 3 dB hybrid coupler, two anti-paralleled Shottky diodes for high order intermodulation signal generation, and a resistor and capacitor to control amplitude and phase of the input signal to generate only the desired high order IM terms.3
Figure 3 shows the output of the IMG3 for the case of CW two-tone input signals at 1.854 and 1.855 GHz. Figure 4 shows the output of the IMG3 for the case of CDMA 2FA input signals at 1853.75 and 1.856.25 MHz. Figures 5 and 6 show the IMGh output in the case of CW two-tone and CDMA 2FA signals, respectively.
Fig. 3 IMG3 output for CW two-tone input signals
Fig. 4 IMG3 output for CDMA 2FA input signals.
Fig. 5 IMGh output for CW two-tone input signals.
Fig. 6 IMGh output for CDMA 2FA input signals.
A power amplifier, using the new type of analog predistorter to control the individual order intermodulations, was designed, as shown in Figure 7. It consists of an automatic level controller (ALC) circuit, an IMG3 and IMG3 control block, an IMGh and IMGh control block, and a high power amplifier (HPA). The input signals are divided into the HPA and the ALC paths by a directional coupler. Since the HPA is usually operated over a wide dynamic power range, the ALC generates a constant signal level for effective predistortion signal generation. The ALC circuit, at the input port, stabilizes the device, resulting in a constant IM signal in spite of the changes in input power level. Using this constant ALC output level, IMG3 and IMGh generate the third and higher order IM signals, respectively.
Fig. 7 Block diagram of the new type of analog predistortion power amplifier.
The IMG3 and IMGh control blocks consist of a variable attenuator and a variable phase shifter to control the magnitude and phase of the third and higher order inverse IM signals to the HPA. The delay circuit in the power amplifier path compensates for the delay in the IM generators and IM control circuits. Adaptive control of the magnitude and phase of the third and higher IM signals is necessary to match those of the intermodulation signals generated by the HPA over the dynamic power range.4–6
Experimental Results
In order to show the validity of the proposed predistorter, a power amplifier (STA1800-37 from Sewon Teletec Inc.), employed in base stations and repeaters of the Korean Personal Communications Service (KPCS), was used in the experiment. Its gain and 1 dB compression power are 50 dB and 37 dBm, respectively. The delay circuit is realized with a coaxial delay line of 14 ns. The variable attenuators and variable phase shifters are of the reflection type in order to obtain good reflection characteristics. The variable phase shifter uses a varactor diode 1T362 from Sony and a variable attenuator, a PIN diode HSMP-4810 from HP.
Fig. 8 ACPR impovements versus input power for a CDMA 1FA signal.
Fig. 9 C/I characteristics of HPA (a) without and (b) with the proposed predistortion linearizer in the case of a two-tone signal (P0 = 37.7 dBm/tone).
Fig. 10 ACPR characteristics of the HPA (a) without and b) with the proposed predistortion linearizer in the case of a CDMA 1FA signal (P0 = 37.0 dBm).
Figure 8 shows the ACPR improvements as a function of input power level for a CDMA 1FA signal. Figure 9 compares the carrier to intermodulation distortion (C/I) ratio of the HPA with and without the proposed predistortion circuit for the case of CW two-tone signals, where the output power is 37.7 dBm/tone. The test frequencies are 1.854 and 1.855 GHz. The improvements of IMD3 and IMD5 are 42.87 and 23.95 dB, respectively. Figure 10 shows the CDMA 1FA adjacent power ratio (ACPR) of the HPA with and without the proposed predistortion circuit. The output power is 37 dBm and the test frequency is 1.855 GHz. Figure 11 compares the CDMA 4FA ACPR of the HPA with and without the proposed predistortion circuit, where the output power is 30 dBm/FA and the carrier frequencies are 1.8525, 1.85375, 1.85625 and 1.8575 GHz, respectively. The improvements in ACPR are 10.25, 8.6 and 9.37 dB at f0 ±885 kHz, f0 ±1.25 MHz and f0 ±2.25 MHz, respectively. The test signal generator is ESG4433B from Agilent Technologies. A signal similar to a carrier leakage was observed at the center frequency with this equipment.
Fig. 11 ACPR characteristics of the HPA (a) without and b) with the proposed predistortion linearizer in the case of a CDMA 4FA signal (P0 = 30.0 dBm/FA).
Conclusion
In this article, a new type of analog predistortion linearizer for controlling individual intermodulation distortion signals is proposed. The signal generators generate and control predistorted third and higher IM component signals separately. The high order IM signals are generated by using the main and third-order IM signals at the input of the generator. The proposed predistorter structure offers the advantage of little signal interference in small size circuits and easy implementation. Using the predistorted signals, the HPA’s intermodulation distortion signals are suppressed effectively. The test results show that IMD3 and IMD5 are reduced by more than 40 and 23 dB for CW two-tone signals, respectively. The predistorter improves the ACPR by more than 10 dB for CDMA (IS-95) 4FA signals.
Acknowledgment
This work was supported by the Research Fund, Kumoh National Institute of Technology, Gyungbuk, Korea.
References
1. P.B. Kenington, High Linearity RF Amplifier Design, Artech House Inc., Norwood, MA, 2000, pp. 341–420.
2. Y.C. Jeong, “A Design of Predistortion Linearizer by Individual Order Control of Intermodulation Distortion Signals,” PhD Dissertation, Sogang University, 1996.
3. T. Nojima and T. Konno, “Cuber Predistortion Linearizer for Relay Equipment in 800 MHz Band Land Mobile Telephone System,” IEEE Transactions on Vehicular Technology, Vol. 34, No. 4, Nov.1985.
4. S.A. Maas, Nonlinear Microwave Circuits, Artech House Inc., Norwood, MA, 1988.
5. T.T. Ha, Solid-state Microwave Amplifier Design, John Wiley & Sons Inc., Somerset, NJ, 1981.
6. S.C. Cripps, RF Power Amplifiers for Wireless Communications, Artech House Inc., Norwood, MA, 1999.
