The authors wish to acknowledge Ravi Ramanathan, James ... Norwood,. MA: Artech House, 2000. [7] G. Norris, âApplication of Adaptive Digital Predistortion for.
Efficiency Improvement of a Handset WCDMA PA Module Using Adaptive Digital Predistortion Calogero D. Presti1, Andre G. Metzger2, Hal M. Banbrook2, Peter J. Zampardi2, and Peter M. Asbeck1 1
University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0447, USA 2 Skyworks Solutions, Inc., 2427 W. Hillcrest Drive, Newbury Park, CA 91320, USA
Abstract — Adaptive Digital Predistortion (DPD) is applied to a spec-compliant class-AB GaAs HBT PA module for WCDMA handsets. It is shown that, by using a re-optimized load line, the efficiency can be increased from 37.5% to 47% at nominal 28.5 dBm output power, while maintaining the same excellent linearity of the original PA. The quiescent current consumption is also reduced down to 40 mA at all power levels, enabling up to 49% average dc power savings without dynamic biasing. Under 2:1 VSWR, 7% to 11% higher PAE is demonstrated at 28 dBm, achieving significant ACPR reduction as well. The methodology to tradeoff the linearization capability of DPD to optimize efficiency at a specified power and linearity is discussed in detail. Index Terms — Power Amplifier, Digital Predistortion, GaAs HBT, WCDMA, Efficiency, Decresting
in order to trade this extra power for efficiency, thus achieving a 47% PAE at 28.5 dBm. An 11% efficiency improvement is attained at ~29 dBm, reaching 50.4% PAE at –40 dB ACPR. In the new PAM, the quiescent current is also aggressively reduced by 2.5x, enabling much lower power consumption in back-off, while DPD corrects the resulting linearity degradation. Finally, in section IV we show that the modified PAM outperforms the original part in terms of linearity and efficiency, at 28 dBm, across a 2:1 VSWR mismatch circle. II: DPD APPLIED TO WCDMA HANDSET PA MODULE A. PA Module and Experimental Setup
I. INTRODUCTION
A 4mm x 4mm handset PAM was used to demonstrate DPD operation. This WCDMA PAM is an experimental variant of a 2-stage GaAs HBT Skyworks part, optimized for operation at 1.95GHz. It features high linearity (better than −40 dB ACPR) up to 29 dBm in WCDMA mode. An adaptive DPD system was employed to linearize the PAM for operation beyond 29 dBm. The system was composed by off-the-shelf RF parts, A/D-D/A converters, and a logic analyzer. The AM/AM and AM/PM distortion characteristics are estimated by fitting the sampled experimental points with a polynomial model. Although available, no memory-effect correction was applied, to retain the simplicity that is realistically necessary in handset applications. A WCDMA reference signal with a 3.3-dB peak-to-average ratio (PAPR) was used in all experiments.
The design of power amplifiers generally forces tradeoffs between linearity and efficiency. The exploitation of some linearization technique facilitates the PA design to a great extent, by basically removing linearity from the design goals. Thanks to the cost reduction in Digital Signal Processing techniques, Adaptive Digital Predistortion (DPD) has found growing acceptance in the field of base-station PA linearization. More recently, DPD linearization of handset PAs has been successfully demonstrated [1][2][3]. Adaptive DPD can achieve excellent linearization performance, but without the bandwidth limitations of Cartesian or polar feedback [4]. Being a closed loop method, robustness against load, supply voltage, process, and temperature variations is guaranteed. This is a particularly attractive feature for handset PAs, which experience more severe environmental variations than base-station PAs [2]. It is believed that DPD will be of increasing importance for emerging modulation standards, such as WiMAX and LTE, where the recourse to linearization could be inevitable to meet the stringent linearity requirements. On the other hand, DPD could seem unnecessary when linearity can be met by the PA alone. Here we show that, by revising some common design practices, the augmented linearity gained by DPD can be traded for a significant amount of power and/or efficiency. In this work, we describe a general optimization methodology, which is exploited to improve the performance of a spec-compliant handset WCDMA PA module (PAM). First, in section II, we show that DPD, together with proper baseband processing, considerably increases the maximum PA linear output power. Then, in section III, the PAM is retuned,
978-1-4244-6057-1/10/$26.00 ©2010 IEEE
B. Measurement Results The measured ACPR at 5 MHz offset is shown in Fig. 1, for the original PAM. After DPD, a < –54 dB ACPR is maintained up to ~29 dBm, which overly fulfills the more modest –40 dB requirement. It would be useful to increase the Pout, up to the point where the ACPR meets the specification. However, this is not straightforward. Indeed, due to the high effectiveness of the linearization, after DPD the signal at the PAM output retains the same 3.3-dB PAPR as the input signal. Since the instantaneous peak power is intrinsically limited by the saturated output power Psat, the maximum WCDMA Pout will be limited to Psat – PAPR. The only way to increase the output power is to artificially reduce the signal PAPR through baseband signal decresting. Elaborate decresting techniques [5] enable PAPR reduction without generating significant ACPR. In this work, however,
804
IMS 2010
-30
Original PA Module
5-MHz ACPR (dB)
-35 28.5 dBm −44 dB
-40 -45
+1.8 dB
-50 -55 Before DPD (lower)
Before DPD (upper) -60 After DPD (lower) After DPD (upper)
-65 After DPD + Clipp. (lower) After DPD + Clipp. (upper) 14 16 18 20 Testbench Ideal 32.3-dBm Clipping
22
24
26
28
30
Fig. 3. CW PAE contours of the PAM at fixed Pout = 30.5 dBm, before (a) and after (b) load retuning and bias current reduction.
32
WCDMA Output Power (dBm)
load-line impedance, the extra power obtained through DPD can be effectively traded for improved efficiency. Hence, the PAM was retuned, with the final objective of improving WCDMA efficiency at 28.5 dBm, while still meeting a –44 dB ACPR. Load optimization was successfully achieved by simply using single tone (CW) measurements, according to the following procedure. Since the target –44 dB ACPR is obtained at a clipped PAPR = 2 dB (see Fig. 2), the objective of the optimization was to maximize the CW efficiency at 30.5 dBm, i.e. 2 dB beyond the target 28.5-dBm Pout. In this way, we ensure that there will be enough headroom to accommodate the peaks of the decrested signal, and that the PAM will operate in a high efficiency region after DPD. Using a partially automated load-pull system, PAE contour lines were generated at a fixed Pout = 30.5 dBm, where currents of both amplifying stages were included in the PAE calculation. The contour lines are shown in Fig. 3(a) for the original PAM. A point with a peak PAE of 54.0% is noted at the 37.5 + j18 Ω impedance. Using the production part as a basis, a PA variant with a revised die was laid out and placed on the same 4x4 laminate and board of the production part. The revised die featured a simplified bias network with independently adjustable bias currents for each of the two amplifier stages. This modification enables a 2.5x bias current reduction (from 100 mA to 40 mA), while maintaining a sufficient ~25 dB gain at low power. Doing nothing other than changing discrete capacitor tuning elements and bond wire lengths, the PAM was retuned such that the point with maximum PAE was shifted to 50-Ω. The PAE contour lines at constant Pout = 30.5 dBm are shown in Fig. 3(b) for the retuned PAM. A 56.1% maximum PAE is obtained at 50 Ω, as desired. The CW gain and PAE of the original part are compared to those of the retuned / debiased PAM in Fig. 4. As a natural result of the described optimization procedure, the amplifier Psat is reduced by 1.8 dB. At 30.5 dBm, the gain is compressed by 2.6 dB relative to the peak gain, and PAE is maximized.
Fig. 1. ACPRs at 5 MHz offset of original PA module, before and after DPD. Results are compared to ideal clipping at 32.3 dBm. 5-MHz ACPR (dB)
-30
Hard Clipping Low-ACPR Decresting [5]
-40 -50 -60 -70 -80
Constellation EVM (%)
-90
1 7 6 5 4 3 2 1 0 1
1.5
2
2.5
3
3.5
Hard Clipping Low-ACPR Decresting [5]
1.5
2
2.5 PAPR (dB)
3
3.5
Fig. 2. Comparison between different PAPR reduction techniques, applied to the 12.2-kbps WCDMA reference measurement signal.
we chose to limit the signal envelope to the desired PAPR. Envelope limiting, also referred to as clipping, besides being computationally simple, generates very low in-band distortion, if compared to low-ACPR decresting algorithms. This greatly reduces the error-vector magnitude (EVM), as shown in Fig. 2. The measured ACPR, when clipping is engaged, is shown in Fig. 1 as well. At ACPR = –44 dB, an extra 1.8 dB of power is delivered by the PAM. The 10-MHz ACPR and EVM (not shown) are equal to –53 dB and 1.1%, respectively. Interestingly, the nonlinearity at the PA output is now solely and conveniently controlled in the baseband. Indeed, since it is possible to know that in this experiment Psat = 32.3 dBm, we can estimate the amount of ACPR and EVM generated by the clipping itself, which almost perfectly agrees with the measurements (black line in Fig. 1). III. PA MODULE OPTIMIZATION METHODOLOGY AND MEASURED PERFORMANCE ON 50-Ω LOAD A. Load-line Optimization Procedure
B. DPD Applied to the Retuned / De-biased PA Module
Since DPD does not actually modify the PA circuit, but just the input signal, for the same Pout efficiency does not change significantly as a result of predistortion. By increasing the PA
DPD and baseband envelope clipping are applied in a similar fashion to the tuned variant of the PAM. As a result of
978-1-4244-6057-1/10/$26.00 ©2010 IEEE
805
IMS 2010
60 50
TABLE I PERFORMANCE COMPARISON IN WCDMA MODE
29
40
28
30
27
−1.8 dB
20
26
10
25
0
24
70
6
60
8 10 12 14 16 18 20 22 24 26 28 30 32 34
CW Output Power (dBm)
50
Fig. 4. CW Gain and PAE of original PAM, before and after load retuning / bias current reduction.
40
-30
30
Retuned PA Module
5-MHz ACPR (dB)
Retuned PA w/ DPD 28.53 dBm –42.8 / –43.6 dB –56.7 / –54.3 dB 1.11% 27.5 dB –24 dBm / –36 dBm 46.9 %
5-MHz ACPR = −40 dB
Output Power (dBm) Gain (dB) PAE (%) EVM (%)
50.4
48.2
39.5 29.0 28.5
29.3
28.8 27.4
26.1
20
-35
10 28.5 dBm −43 dB
-40
1.37
1.46
1.52
0 Original PA Module
-45
Retuned PA w/ DPD (2.6-dB compression)
Retuned PA w/ DPD (4-dB compression)
Fig. 7. Performance of the original PA module, compared with the retuned PA module, driven into lower and higher compression.
-50 -55
In Fig. 6 significant PAE improvements are shown at all power levels. The EVM is lower than 3% up to 29.3 dBm, for the modified PA. At high power, the measured EVM is equal to what is created through clipping in the baseband. Table I shows that, at Pout= 28.5 dBm, the modified PAM is identical to the original part, while efficiency is significantly improved. It has been shown so far that, when DPD is engaged, the linearity performance of the PA is ultimately limited by the peak envelope power at the output. It is possible to improve performance even further, by driving the PA more deeply into compression, at the expense of gain. Fig. 7 shows the results of another experiment conducted at a fixed –40 dB ACPR. By allowing the signal peaks to reach the 4-dB compression point, Pout is increased from 28.8 dBm to 29.3 dBm, while PAE reaches an outstanding 50.4%, at the cost of a 1.3-dB gain reduction. EVM still remained at 1.5%.
Before DPD (lower)
-60Before DPD (upper)
After DPD (lower) After DPD (upper) -65After DPD + Clipp. (lower) After DPD + Clipp. (upper) 14 16 18 20 22 24 26 28 30 Testbench WCDMA Output Power (dBm) Ideal 30.5-dBm Clipping
32
Fig. 5. ACPRs at 5 MHz offset of retuned PA module, before and after DPD. Results are compared to ideal clipping at 30.5 dBm. 60
4
50 45 40 35
3
30
2
25 20 15
1
10
Constellation EVM (%)
Original PA, PAE Retuned PA, PAE Retuned PA w/ Clipp., PAE Original PA, EVM Retuned PA, EVM Retuned PA w/ Clipp., EVM Testbench Ideal 30.5-dBm Clipping
55
PAE (%)
Original PA 28.50 dBm –43.8 / –43.9 dB –55.1 / –54.4 dB 1.06 % 28.6 dB –26 dBm / –38 dBm 37.5 %
Output Power ACPR at +/– 5 MHz ACPR at +/– 10 MHz Constellation EVM Gain 2nd / 3rd harmonic PAE
Gain (dB)
PAE (%)
30
Retuned PA, PAE Original PA, PAE Retuned PA, Gain Original PA, Gain
C. Average Power Consumption in Back-Off In WCDMA networks, the PA has highest probability of operating at low power levels (0 to 10 dBm). Therefore power consumption is often dominated by the bias current [6]. The efficiency improvements so far demonstrated at high power levels would be useless, if not combined with the 2.5x bias current reduction implemented in the modified PAM. To quantify the achievable efficiency improvement, the average power consumption has been calculated both for the original1 and the modified PAM, assuming CDMA power usage profiles [6]. The results, shown in Fig. 8, indicate that a 49%
5 0
0 14
16
18
20
22
24
26
28
30
32
WCDMA Output Power (dBm)
Fig. 6. Efficiency and EVM of retuned PA w/ DPD, compared to original PA module. EVM produced by ideal clipping is also shown.
the lower bias current, the measured ACPR before DPD (Fig. 5) does not meet the specification, not even at low power. However, after DPD, linearity is easily recovered. When the input signal is decrested down to a 2-dB PAPR, a 28.5-dBm Pout is obtained, with a –43 dB ACPR, as expected. Also in this case, the ACPR is solely what is purposely introduced through the digital clipping of the baseband signal.
978-1-4244-6057-1/10/$26.00 ©2010 IEEE
1
It should be noted that several bias steps could be used for the original PAM. In this comparison, however, a constant 100-mA bias current is chosen.
806
IMS 2010
454
500 400
-35
371 269
300
189
200 100 0
Original PA Module
Output Power = 28 dBm, 2:1 VSWR
-30
Sub-urban
5-MHz ACPR (dB)
Average Power Consumption (mW)
Urban
600
Retuned PA w/ DPD
-40 -45 -50 -55 Original PA (lower) Retuned PA w/ Clipp. (lower) Retuned PA (lower) Testbench
-60
Fig. 8. Average power consumption, assuming CDMA power usage profiles. Retuned PA with DPD is compared to original PA.
Original PA (upper) Retuned PA w/ Clipp. (upper) Retuned PA (upper) Effect of Clipping
-65 0
(41%) reduction is achieved in the urban (sub-urban) scenario, without exploiting multiple bias steps at different power levels. Power savings between 180 mW (in back off) and 370 mW (at 28.5 dBm) are achieved. This reduced consumption should be largely sufficient to overcome the estimated ~20 mW power required to implement DPD in a transceiver [7].
100
200
Load Phase (deg)
300
Fig. 9. ACPR at 5-MHz offset, under 2:1 VSWR, vs. load phase. Retuned PA is compared to original PA module. Pout = 28 dBm. Output Power = 28 dBm, 2:1 VSWR
70 60 50 PAE (%)
IV. PERFORMANCE UNDER LOAD MISMATCH As already mentioned, DPD has the potential of strongly improving performance under load mismatch. To investigate this aspect, both the original and the retuned PAM have been tested under 2:1 VSWR in WCDMA mode, at a fixed Pout of 28 dBm. In these experiments Pout indicates the forward power, measured through a directional coupler. As regards the modified PAM, proper clipping is introduced at those phases where insufficient headroom is detected. The measured ACPR is reported in Fig. 9. Thanks to DPD, the modified PAM exceeds the linearity of the original part at all load angles. Fig. 10 shows that this linearity improvement is attained together with a 7% to 11% PAE enhancement. For the retuned PA, the EVM (not shown) varies between 0.65% and 3%, while the gain is between 25.1 dB and 29.6 dB.
40 30 20
Original PA Retuned PA w/ Clipp. Retuned PA
10 0 0
100
200 Load Phase (deg)
300
Fig. 10. PAE under 2:1 VSWR, vs. load phase. Retuned PA is compared to original PA module. Pout = 28 dBm.
as the support and guidance of the applications and product development teams at Skyworks. Special thanks to Jonmei Yan of UCSD for valuable help with measurements.
V. SUMMARY
REFERENCES
In this work, we have discussed how DPD can significantly improve the efficiency and linearity of handset PAs. DPD was applied to a WCDMA PAM, thus increasing the output power by 1.8 dB. Then, the load-line of the PAM was retuned to trade this extra power for efficiency. PAE improvements of the order of 10% were achieved in various load conditions, up to 2:1 VSWR, while exhibiting better or equal linearity than the original part. A peak 50.4% PAE is demonstrated at 29.3 dBm, while maintaining a –40 dB ACPR and 1.5% EVM. A 2.5x reduced bias current also enabled > 40% average power savings in back off.
[1] N. Ceylan, et al., “Optimization of EDGE terminal power amplifiers using memoryless digital predistortion,” IEEE Trans. Microw. Theory Tech., vol. 53, pp. 515–522, 2005. [2] G. Norris, et al., “Application of Digital Adaptive Pre-distortion to Mobile Wireless Devices”, in 2007 IEEE RFIC Symposium, Digest of Papers, June 2007, pp. 247-250. [3] A. Afsahi, et al., ‘‘Fully Integrated Dual-Band Power Amplifiers with on-chip Baluns in 65nm CMOS for an 802.11n MIMO WLAN SoC’’, in 2009 IEEE RFIC Symposium, Digest of Papers, June 2009, pp. 368-365. [4] T. Sowlati, et al., “Quad-band GSM/GPRS/EDGE polar loop transmitter,” IEEE J. Solid-State Circuits, vol. 39, pp. 2179-2189, 2004. [5] H.Y. Jian, L. Mathe, “Reducing the peak-to-average power ratio of a communication signal,” U.S. Patent 7266354, Sep. 4, 2007 [6] J. Groe and L. Larson, CDMA Mobile Radio Design. Norwood, MA: Artech House, 2000. [7] G. Norris, “Application of Adaptive Digital Predistortion for EDGE, W-CDMA, and LTE Mobile Transmitters”, in RFIC Workshop WSJ Handouts, June 2008.
ACKNOWLEDGEMENT The authors wish to acknowledge Ravi Ramanathan, James Young, Gene Tkachenko, and Chandra Mohan of Skyworks Solutions Inc. for the financial support of this project, as well
978-1-4244-6057-1/10/$26.00 ©2010 IEEE
807
IMS 2010
