A method to improve W-CDMA mobile phone power amplifier linearity with the new method

The third generation mobile communication (3rd-Generation, referred to as 3G) system with greater capacity and better communication quality, higher bandwidth efficiency, these features make it as high-speed and low speed mobile users with voice, data, video conferencing and multimedia and other services. But the outstanding performance of these hardware systems also put forward higher requirements, especially emission power amplifier subsystem (PA) module. W-CDMA (Wideband Code Division Multiple Access) as the third generation mobile communication (3G) has been one of the standards in Europe and Japan has been successful in commercial applications. PA W-CDMA system has the following new features:

First, in the 3G phone, the most critical high-speed data transmission requires a higher bandwidth and transmission power. Used in W-CDMA bandwidth of 3.84MHz pseudo-random noise code (Pseu-do-Noise code, PN code), so the user signal bandwidth is 3.84MHz, IMX from the amplifier nonlinear distortion produced in a wider distribution range.

Second, in order to improve data transmission rates and increased spectrum efficiency, hybrid phase shift keying HPSK (Hybrid Phase ShiftKeying) modulation methods require PA must have good linearity. AM-PM characteristics of the amplifier causes the phase modulation signal distortion, which receives the bit error rate will rise, resulting in lower system communication quality.

Third, the presence of distance effects, PA's output dynamic range. In accordance with the Third Generation Partnership Project (3rd GenerationPartnership Project, referred to as 3GPP) introduced the W-CDMA standards for transmitter control range +24 dBm transmit power to-50dBm, 74dB of dynamic range, if to consider some margin, the transmitter should have more than 80dB of dynamic range. Transmitter chip, dynamic index is often limited to indicators of high power and low ACPR when the noise power when the end of the adjacent channel leakage power of the size and proportional to the cube of input power, adjacent channel to avoid excessive interference with the user generated the largest ACPR output power should not exceed-37dBc.

2 third-generation W-CDMA mobile communication efficiency and linear power amplifier circuit, the relationship between characteristics of

Developed in the 3GPP W-CDMA standard, only the linearity of the PA and the transmit power requirements, they are first. On near-stringent linearity requirements, you need to compromise with other parameters, such as efficiency. W-CDMA system, a non-constant envelope RF signal, which determines the efficiency can only use 25% to 35% range of the linear amplifier, while the non-linear power amplifier of second-generation GSM telephone transmitter Typical efficiency is about 50%. As the W-CDMA RF signal for a variety of business data overlay, so different from the constant envelope signal, RF power amplifiers can not be driven to the compression zone, but must use the power amplifier back-off approach to work in the linear region. Rollback more linear as possible, but the lower power amplifier efficiency. For both linear and efficiency, W-CDMA power amplifier linearization is generally applied to a variety of techniques to achieve a balance between linearity and efficiency.

3 present several commonly used methods to improve the power amplifier linearity

The RF power amplifier linearity of technologies that many common are the following three: power regression (back-off) France, feedforward (feedforward) method and the pre-distortion (predistortion) method.

3.1 power law regression

Among the various linearization techniques, power regression technique is the most commonly used method, that is, the power amplifier input power from the 1dB compression point back back back a few decibels, work in much less than 1dB compression point level on the to power amplifier away from the saturated zone, into the linear working area, thereby improving the power amplifier's third order intermodulation coefficient. That use power tubes for larger governing the use of low power, in fact, at the expense of DC power to improve the amplifier linearity.

Power regression method is simple and easy to achieve, without adding any additional equipment, is to improve the amplifier linearity effective way to the disadvantage of greatly reduced the efficiency of power amplifiers. In addition, when the power fall back to a certain extent, that is, when the third-order intermodulation (IM3) reach-40dBc below to continue to rollback will no longer improve the amplifier linearity. Therefore, the high linearity requirements of the occasion, completely on the power rollback is not enough.

3.2 Forward method

Feedforward technical principles shown in Figure 1. After the RF signal input by the power splitter (Splitter) two routes. All the way into the main amplifier A1, because of its nonlinear distortion, in addition to the need to enlarge the output of the frequency signal, there are third-order intermodulation interference. From the main part of the amplifier output signal coupling with another road through the delay line input signal TD1 delay synthesizer (Subtracter) in the stack, so completely off the main carrier frequency signal, leaving only third-order intermodulation RP . Third-order intermodulation by the auxiliary amplifier, and delay by the delay line TD2 main amplifier output signal at coupler C2 in the superposition of the main amplifier offset the third-order intermodulation interference, which could be linear signal amplification.

A method to improve W-CDMA mobile phone power amplifier linearity with the new method

Feedforward technology provides both the advantages of high accuracy calibration, no bandwidth is limited by instability and shortcomings; However, these advantages are paid for with a high cost. As calibrated output, the higher power level, calibration signal be amplified to a higher power level, which requires additional auxiliary amplifier, and requires the auxiliary amplifier distortion characteristics of its feed-forward system should be in indicators above, and because the calibration loop to add a secondary amplifier, which decreased the overall efficiency.

Feedforward power amplifier offset requirement is very high, need to be amplitude, phase and delay matching, if power changes, temperature changes and device aging, etc. will result in failure offset. This requires the system to consider adaptive offset technology to offset the change to keep up with internal and external environment.

3.3 Predistortion Method

Pre-distortion is a nonlinear increase in the power amplifier before the power amplifier circuit to compensate for the nonlinear. Pre-distortion Linearization principle shown in Figure 2.

A method to improve W-CDMA mobile phone power amplifier linearity with the new method

Pre-distortion linearization techniques, it has the advantage of stability does not exist, there is a wider signal bandwidth, can handle with multi-carrier signal. Predistortion lower cost, by a few carefully selected components packaged into a single module, and even between the signal source and amplifier, to form pre-distortion linear amplifier. Handheld mobile Taichung amplifier pre-distortion techniques have been used, it is only a small number of components to reduce the intermodulation products of a few IM dB, but is critical of several dB.

RF pre-distortion technology into pre-distortion and digital baseband pre-distortion are two basic types. RF pre-distortion to achieve the commonly used analog circuit with a circuit of simple structure, low cost, easy-to-high-frequency, broadband applications, advantages, disadvantages that renewable component to improve the low frequency spectrum, high frequency components to offset the more difficult. Baseband pre-distortion as low frequency, can be achieved with digital circuits, adaptable, and can be increased by increasing the sampling rate and the number of ways to quantify the order to offset high order intermodulation distortion, is a promising method . Digital baseband predistortion principle shown in Figure 3.

A method to improve W-CDMA mobile phone power amplifier linearity with the new method

Digital pre-distortion is controlled by a vector composed of the gain regulator, according to the contents of lookup table LUT to control the input signal amplitude and phase pre-distortion of the size of the lookup table to control the input LUT. Once gain regulator vector optimization, will provide a contrast with the amplifier nonlinear characteristics. Ideally, then the output intermodulation products should be two-tone signal through the amplifier output range of the same phase contrast, the adaptive adjustment module is to adjustment lookup table input, thus the input signal and the amplifier output signal of the difference between the minimum . Noted that the envelope of the input signal is a look-up table LUT input, feedback path to sample the output power amplifier distortion, and then through the A / D transform into the DSP in the adaptive regulation, and then to update the look-up table (LUT). The results show that the output power of 27dBm, the application of digital pre-distortion technology, amplifier power added efficiency (PAE) can increase 20%, ACPR can be improved 6dB.

However, both from a cost and size considerations, technical and pre-distortion feed-forward techniques can only be applied to the base station is not suitable for mobile phone power amplifier.

4 using the equivalent low-pass filter model to improve the W-CDMA mobile phone power amplifier linearity with the new method

W-CDMA system uses non-constant envelope modulation method makes the design of the proposed high-power amplifier linearity requirements. Measure the W-CDMA power amplifier linearity index is close to the channel leakage power ratio (ACPR), defined as within the adjacent channel leakage power within the main channel signal power ratio (in dBc) for the. W-CDMA uses 5MHz channel bandwidth, the signal concentrated in the center of the channel within a 3.84MHz bandwidth. For our study the main channel, its sides around 5MHz intervals Department has two channels (corresponding to the lower channel and upper channel). In the actual test is often observed in both left and right channel corresponding ACPR values are different, that is, had ACPR asymmetry. ACPR asymmetry index ACPR will worsen, resulting in a significant reduction in power amplifier linearity. Linearization in digital pre-distortion [7] technology ACPR asymmetry effect is very serious. Therefore, the asymmetry of the causes of ACPR and how to reduce this asymmetry of methods to improve the linearity of the amplifier is of great significance.

Studies have shown that the even the low-frequency-modulation power amplifier linearity factors have important implications for research in this paper, based on the analysis of low frequency factors on the impact of ACPR asymmetry; made according to the actual structure of a power amplifier kind of the equivalent low-pass filter model; simulation shows that the low-pass filter bandwidth and order of the impact on ACPR asymmetry; testing low-pass filter by changing the order of the bandwidth and make ACPR asymmetry improved 3.72dB, while ACPR lower 2.95dB, greatly increased the power amplifier linearity.

4.1 ACPR low frequency factors on the impact of asymmetric

ACPR asymmetry is essentially third-order intermodulation IM3 asymmetry. IM3 asymmetry on the views of many causes, mainly boils down to two: Steve c. Cripps that the effect of AM-AM and AM-PM effect is to produce the phase shift between the IM3 asymmetry causes; also study suggests that low-frequency input impedance of the circuit and the second harmonic impedance is to produce the direct cause of IM3 asymmetry. In this paper, the even factor in the low-frequency intermodulation staff, a new W-CDMA mobile phones affect the power amplifier ACPR asymmetry mechanism, that is the equivalent low-pass filter model, simulation and test verification of this model for improving the ACPR asymmetry and raise the effectiveness of power amplifier linearity.

5MHz W-CDMA system using broadband modulated signals within the main channel can be seen as an n sound signal, the n audio signals in order intermodulation IM3 just fall on both sides of the main channel near the channel, thereby forming a close channel power leakage. Adjacent channel leakage power and the main channel signal power ratio is the ACPR.

To simplify the calculation, at the same time without losing the general discussion, we first analyze the two-tone signal produced by intermodulation effects, and then extended to multi-tone signals. Suppose there are two main channels are the input amplitude A, input frequencies were ω1 and ω2 (so ω2> ω1) of the two-tone signal

A method to improve W-CDMA mobile phone power amplifier linearity with the new method

From the above we can see that, due to the presence of third party items in the output signal produced (2ω1-ω2) and (2ω2-ω1) two third-order intermodulation component, left and right sides of the third-order intermodulation components are equal . Also noted that there is also in the Shu Chu component due to the secondary side in the item 產生 second order intermodulation (ω2-ω1), 此 order intermodulation frequency is very low, Jie Jinyu DC component. On the surface, second order intermodulation component generated from the two main low frequency signal ω1 and ω2 very far, does not appear to affect the main signal, but this factor along the power line frequency and bias network feedback to the input to re-generate the input signal modulation, low frequency factors shown in Figure 4 shall be fed back to amplifier input and the input signal a return of second-order intermodulation of the diagram.

Frequency (ω2-ω1) of low-frequency factor and the two main signals were re-ω1 and ω2 produce intermodulation (To simplify the calculation, only taking into account second order effects) output for the following:

A method to improve W-CDMA mobile phone power amplifier linearity with the new method

The output from the above results show that low frequency factor (ω2-ω1) input and feedback to happen again ω1 and ω2 order intermodulation, will produce (2ω1-ω2) and (2ω2-ω1) of the third order transfer component, which will have two second-order intermodulation third order intermodulation. It can be drawn, left and right sides of the third-order intermodulation (2ω1-ω2) and (2ω2-ω1) is composed of two parts: the first third-order intermodulation component and two second-order intermodulation components, Figure 5 show shall be composed of two components IM3 diagram.

W-CDMA power amplifier for multi-tone signal that is the case, near the channel power leakage is composed of two parts: First, within the channel between the various signals generated by a third-order intermodulation component; second channel signal generation within the leakage of the second order intermodulation component to the input, and the recurrence of the input signal generated by second-order intermodulation third order intermodulation.

4.2 The equivalent low-pass filter model bias

Figure 6 shows the W-CDMA handset power amplifier in a typical circuit used. Amp DC through DC bias added to the amplifier tube; in PA Tubes of the collector with microstrip Xian Yi Ban Hui Lai as equivalent inductance of RF power to prevent Dianyuanxielou, Tong Chang also in 微帶 plus a number between line and ground decoupling capacitor to eliminate the power of the clutter. Shown in Figure 6, the collector bias on the microstrip lines and decoupling capacitors to form a typical low-pass filter model. The low-pass filter model will be of even low-frequency-modulation filtering effect produced factors, thus changing the factors of the low-frequency amplitude and phase, resulting in the low-frequency feedback back to the input factors and the recurrence of the input signal will produce second-order intermodulation on both sides Third-order intermodulation asymmetry (amplitude and phase), the asymmetric component and a third-order intermodulation component superposed on both sides will create the third-order intermodulation asymmetry. For W-CDMA power amplifier is close to the channel that is adjacent channel leakage power within the third-order intermodulation are the result of integration, asymmetry of the third-order intermodulation component led to both sides of the main channel near the channel leakage power within the different, resulting in ACPR asymmetry on both sides.

A method to improve W-CDMA mobile phone power amplifier linearity with the new method

Collector in the power tube bias, the microstrip lines and decoupling capacitors forming a low-pass filter equivalent. The order of the low-pass filter by the number of decoupling capacitors determine the bandwidth by the decoupling capacitor size and location decisions. The equivalent low-pass filter bandwidth and order of the frequency factors will have a major impact: a second-order intermodulation frequency signal after the first through the low-pass filter, and then leak through the bias circuit to the power control input. It can be clearly seen that the transmission characteristics of low pass filter will have a significant impact on low leakage, and thus impact on ACPR asymmetry. By our own design and silicon success of the W-CDMA power amplifier for mobile phone testing, validation of the equivalent low-pass filter model for improving the ACPR asymmetry and raise the effectiveness of power amplifier linearity.

4.3 Simulation and test results

We used Advanced Design System (ADS) simulation software Envelope simulator equivalent low-pass filter of the W-CDMA power amplifiers on both sides of the margin of ACPR. Shown in Figure 7, the input by adding W-CDMA uplink signal, the collector filter model used to facilitate behavioral change the filter bandwidth and the order. Figure 8 shows the low-pass filter with the ideal case, with the filter bandwidth (0.5Mz-5Mz) changes in ACPR margin on both sides of the curve. From the figure it is evident that the filter bandwidth on the ACPR has great influence on the difference between both sides, this is because the low-pass filter to filter low-frequency factors change the role of leakage along the collector bias to the amplifier's low frequency input factor amplitude and phase, resulting in the asymmetry of third-order intermodulation component, resulting ACPR asymmetry on both sides. Figure 9 shows the fixed filter bandwidth of 5MHz circumstances, with the filter order n changes in ACPR margin changes on both sides. Observation chart curve indicated that the number of higher order in the filter case (more than 3 bands), ACPR small difference on both sides, that is the case of large filter capacitor on the improvement of ACPR asymmetry good.

A method to improve W-CDMA mobile phone power amplifier linearity with the new method

In order to verify the correctness of the theoretical model, we tested the W-CDMA handset power amplifier in an output spectrum use. Figure 10 shows the W-CDMA power amplifier test system, Figure 11 shows the power amplifier chip PCB test card. Test conditions: Vbb = 2.85V, Vref = 3.2V, Vcc = 3.2V, the center frequency f0 = 1.95GHz, the output power Pout = 16.7dBm. Test, the order low-pass filter by the number of decoupling capacitors determine the bandwidth size of the decoupling capacitors and their location decisions in the microstrip line.

A method to improve W-CDMA mobile phone power amplifier linearity with the new method

Test results show that by changing the number of decoupling capacitors, size, and adjust its position in the microstrip line, can significantly change the difference between both sides of ACPR. Figure 12 shows the non-regulated decoupling capacitors, the output ACPR test chart. Can be seen from the figure, ACPR values of both sides were-52.44dBc and-48.38dBc, the difference is 4.06dB.

Changing the number of decoupling capacitors, size, and adjust its location in the microstrip line, ACPR asymmetry was significantly improved: as shown in Figure 13, ACPR values of both sides and -51.67, respectively-51.33dBc dBc, the difference is only 0.34dB. Comparison of Figure 12 and Figure 13, known to regulate the asymmetry before and after the ACPR improves 3.72dB, while noting that a power amplifier ACPR from-48.38dBc down to-51.33dBc (ACPR high value of the decision by the left and right sides), by smaller 2.95dB, thus greatly improving the linearity of the amplifier.

A method to improve W-CDMA mobile phone power amplifier linearity with the new method

Shall be adjusted as shown in Figure 11, the final outcome of the microstrip line consists of three capacitors, and capacitive microstrip line on behalf of time to achieve a third-order low-pass filter, to achieve an effective improvement of ACPR asymmetry on both sides of the effect of and the test results and the previous simulation results fit together well. Thus proving that the low-pass filter bandwidth and directly determines the order of ACPR asymmetry on both sides, adjusting the low-pass filter bandwidth and the order may well improve the ACPR asymmetry, thus substantially improving power amplifier linearity.

5 Conclusion

In this paper, the impact of a new W-CDMA handset power amplifier ACPR asymmetry with the mechanism - the equivalent low-pass filter model. As the low-pass filter of even the low-frequency-modulation factors exist filtering effect, resulting in low frequency of the input signal after the recurrence of second-order intermodulation third order intermodulation asymmetry component, and thus make ACPR asymmetry generated on both sides, lead to lower power amplifier linearity. In this paper, the ADS software simulation of low-pass filter bandwidth and order on both sides of the asymmetry of ACPR, and verified by the actual power amplifier test result of the simulation. The results show that changing the low-pass filter bandwidth and the order can be improved ACPR asymmetry on both sides of 3.72dB, while ACPR decreases 2.95dB, which effectively improved amplifier linearity.

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