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Keysight Technologies
Envelope Tracking and Digital
Pre-Distortion Power Amplifier Testing
for LTE User Terminal Components
Application Note
Introduction
With the introduction of 4G LTE, mobile communications were able to achieve higher data throughput
than ever before. However that achievement is accompanied by unprecedented battery requirements
for mobile terminals. LTE employs the SC-FDMA modulation format in the uplink and it has a higher
Peak-to-Average Ratio (PAR) than W-CDMA. One of the most power-hungry components of a mobile
terminal is the power amplifier (PA). As shown in Figure 1, the power level of the LTE uplink signal
stays relatively low most of the time and goes to peak power only occasionally, but the PA is designed
to deliver the highest efficiency only at peak power. Since the high power supplied to the PA won't be
used most of the time, it is mostly dissipated as heat and causes battery drain, impacting the
thermal design power (TDP) of the mobile terminals. Envelope tracking (ET) has come to the forefront
as a possible solution for this issue in mobile RF front end design. ET dynamically adjusts DC supply
voltage based on the "envelope" of the PA input signal and delivers higher voltages only when needed,
improving battery consumption and heat dissipation in the PA.
Constant PA supply voltage Envelope tracking voltage
Unused energy
PA input signal Time Time
Figure 1. Time domain view of the input signal and bias voltage.
Another issue facing power amplifiers with high PAR signals, such as LTE, is non-linearity. When a
high power signal is applied to the PA input, the PA can't linearly amplify the signal, resulting in a
situation leading to gain saturation and distortion, ultimately leading to signal quality degradation,
such as higher ACLR and EVM. Techniques such as crest factor reduction (CFR) and/or digital pre-
distortion (DPD) are used to compensate for the non-linear behavior of the PA. DPD applies "inverse"
distortion to the PA input signal to cancel the distortion generated by the PA. There are several types
of DPD methods but a simple look-up-table (LUT)-based DPD that doesn't take memory effect into
account is commonly used in mobile devices. This application note discusses measurement solutions
for PA testing using ET and LUT-based DPD.
03 | Keysight | Envelope Tracking and Digital Pre-Distortion Power Amplifier Testing for LTE User Terminal Components - Application Note
Envelope Tracking Overview
When testing ET devices, you will need an IQ signal for PA input and the associated
envelope signal for modulating the PA power supply. The envelope is generated
from the IQ sample's absolute magnitude, typically with 3x to 6x oversampling
applied in order to create a smoother waveform. The base sample rate of a 20 MHz
LTE signal is 30.72 MHz and the envelope's sample rate will be 92.16 MHz for 3x
oversampling and 184.32 MHz for 6x oversampling. A high sample rate waveform
generator is required to support these high sample rates. Raw envelope data
generated from the IQ signal then goes through the shaping table before being
applied to the ET power supply (ETPS). Typically, the shaping table has a so-called
"de-troughing" function to avoid having the envelope voltage drop down to 0 V.
Shaping tables are also used to maximize PA efficiency or to ensure constant gain
over a wider input power level. This is called "iso-gain". It is not an exaggeration to
say that ET system performance is defined by the shaping table.
Battery
Envelope Shaping Vin Envelope tracking
detector table power supply
|X| (LUT) (ETPS)
Vcc
IQ Upconverter
RFin
PA RFout
Baseband ET front-end
Figure 2. Envelope tracking system simplified block diagram.
ETPS dynamically adjusts the power supply voltage to the PA. As defined in the
MIPITM Alliance's eTrak standard, most ETPS have differential inputs and therefore,
the envelope waveform generator must have differential output. ETPS output is
applied to the PA's supply line. This voltage has to be applied to the PA with precise
timing relative to the input RF signal. For a 100 RB LTE signal, typically the timing
alignment between RF and envelope has to be less than 1 ns. Wider bandwidth LTE
signals, such as carrier aggregation, require even more severe timing control. Rough
timing adjustments between RF and envelope can be achieved with an oscilloscope
before connecting to the ETPA and ETPS. However, due to internal delays in the
ETPS and the RF PA, the final tuning has to be done with the ETPA output using
parameters such as EVM or ACLR. Precise timing adjustments, down to a fraction of
a nanosecond, may be required to optimize ET system performance.
04 | Keysight | Envelope Tracking and Digital Pre-Distortion Power Amplifier Testing for LTE User Terminal Components - Application Note
Keysight's ET test solution
Keysight Technologies, Inc. offers two envelope tracking signal generation and
analysis test solution configurations, which are shown in Figures 3a and 3b. The
one in Figure 3a is made up of benchtop test instruments, while the one in Figure 3b
is based on PXI test equipment.
Keysight's N7624/25B Signal Studio for LTE/LTE-Advanced software's envelope
tracking option enables generation of the envelope waveform from LTE and
LTE-Advanced IQ waveforms. An arbitrary oversampling ratio of up to 32x can be
specified for envelope generation (The sample rate is also limited by envelope gen-
erator). The software's built-in shaping table manager allows for creating, importing
and selecting multiple shaping tables quickly and easily. Generated IQ and envelope
waveforms are then downloaded to the RF signal generator and arbitrary waveform
generator (AWG) directly. The 33600/33500 series AWG can be used as an enve-
lope waveform generator. After downloading waveforms, it is possible to adjust
RF and envelope timing with 1 ps resolution in real-time, without re-generating
the waveforms. This simplifies timing optimization. The 33600/33500 series AWG
with its 2 ch output configuration supports differential output operation and has
low wideband noise, making it an ideal function generator that can be connected
to the ETPS input directly without filtering. Generated IQ and envelope waveforms
can be saved and used in the other platforms, such as PXI, for higher throughput
production testing.
It is also possible to use ET (and DPD) with non-LTE waveforms, such as
W-CDMA or MATLAB, using the library waveform capability in the
N7624B/25B Signal Studio software.
On the analysis side, Keysight's 89600 VSA software can be used with X-Series
signal analyzers or a PXI VSA, as shown in Figure 3. The 89600 VSA software's
multi-measurement and powerful graph function enables easy measurement of
AM-AM, AM-PM and gain compression, by comparing PA input and output signals
measured alternately a signal analyzer's single input.
05 | Keysight | Envelope Tracking and Digital Pre-Distortion Power Amplifier Testing for LTE User Terminal Components - Application Note
DC power 89600 VSA software
supply
Signal Studio's envelope tracking
control window 33600/33500 series AWG
Envelope
ETPS
Trigger
Timing
alignment
RF
ETPA
X-Series signal generator
DUT X-Series signal generator
Figure 3a. ET signal generation and analysis test system with bench-top test instruments.
DC power
89600 VSA software
supply
33600/33500 series AWG
Envelope
Reference PA test
application ETPS
Trigger
Timing
alignment
RF
ETPA
Signal Studio's envelope tracking DUT
control window
M9381A PXIe VSG M9391A PXIe VSA
Figure 3b. ET signal generation and analysis test system with PXI test instruments.
06 | Keysight | Envelope Tracking and Digital Pre-Distortion Power Amplifier Testing for LTE User Terminal Components - Application Note
Digital Pre-Distortion Overview
The LTE uplink signal could have as high as +7 dB PAR which is 2 to 3 dB higher
than WCDMA/HSPA signals. As discussed earlier, the PA is required to have higher
linearity to amplify high PAR signals. However, due to the limitations of cost, power
consumption, and physical space, it is not easy to find higher linearity components
that meet all of a particular system's requirements. Classic measures of PA linearity
are AM-AM and AM-PM, comparing the input signal and the PA output signal.
Figure 4's AM-AM curve shows reasonably linear characteristics with some minor
deviation. On the other hand, AM-PM shows a relatively large phase shift over the
range of the input power. If you take curve-fitting data and calculate the inverse
characteristics to create a linear AM-AM curve and a flat AM-PM curve, the result
is exactly what is needed for the DPD lookup table data. Figure 5 shows the DPD
LUT calculation. During evaluation, you can typically repeat this measurement and
the DPD LUT calculation several times to get optimum performance. This process is
called DPD closed-loop iteration.
Figure 4. 89600 VSA software showing AM-AM and AM-PM curves.
Response (PA output) Pre-distortion curve Ideal (linear response)
+ =
Figure 5. DPD LUT calculation.
07 | Keysight | Envelope Tracking and Digital Pre-Distortion Power Amplifier Testing for LTE User Terminal Components - Application Note
Keysight's DPD solution overview
To apply digital pre-distortion, it is necessary to change amplitude and phase, based
on an IQ sample's magnitude. Signal Studio for LTE supports LUT-based pre-distor-
tion applied to IQ samples. As with the ET shaping table, multiple look-up-tables
can be edited, imported, and switched with just a few clicks needed to apply DPD.
You can also observe how DPD affects IQ samples by looking at a CCDF curve or
spectrum before and after applying DPD, as shown in Figure 6.
Figure 6. Signal Studio's digital pre-distortion tool panel.
To calculate the DPD LUT, compare the non-pre-distorted linear signal and the
distorted PA output signal, plot the AM-AM and AM-PM curves, and generate curve
fitting polynomial coefficients. The 89600 VSA software graph capability displays
AM-AM, AM-PM, and gain compression traces as well as curve-fitting polynomials
with a user-definable polynomial order up to 12. By taking these coefficients and
calculating the inverse characteristics, it is possible to generate a DPD LUT that can
be used in Signal Studio, with spreadsheet tools such as Excel.
The required measurement bandwidth for acquiring signals will be determined by
the bandwidth to be optimized. For example, to cover the channel adjacent to a 20
MHz LTE signal, 60 MHz bandwidth must be available for capture. To cover an alter-
nate channel, an additional 40 MHz BW would be required, which means 100 MHz
BW would be needed overall. In case contiguous carrier aggregation is used with
a 2x 20 MHz LTE carrier, 120 MHz bandwidth would be needed to cover adjacent
and alternate channels. PXA and MXA X-Series signal analyzers and the M9391A
PXIe VSA all support a maximum 160 MHz bandwidth, which provides adequate
bandwidth for analyzing DPD-applied waveforms.
08 | Keysight | Envelope Tracking and Digital Pre-Distortion Power Amplifier Testing for LTE User Terminal Components - Application Note
Combined use of DPD and ET
When applying DPD, in order to linearize AM-PM characteristics, the envelope is
typically generated based on pre-distorted IQ waveforms. This means that at each
DPD closed-loop iteration, both the pre-distorted IQ and the envelope waveforms
need to be re-generated based on the updated LUT. Typically, ET will be applied and
optimized before DPD, so that the DPD closed-loop iteration can be performed to
compensate for amplitude and phase non-linearity, in order to understand if perfor-
mance improvements are due to ET or DPD. Signal Studio for LTE and LTE-Advanced
has integrated GUI support for both ET and DPD, allowing generation of ET-enabled
DPD waveforms.
ET/DPD control software
Signal Studio
PXA signal analyzer
33522B
ETPS+
ETPA
MXG-B
Polynomial coefficients
89600 VSA software
Figure 7. Sample software program performing a DPD closed-loop iteration on an ETPA.
This sample program, ET/DPD control software, shown in Figure 7, controls both
Signal Studio and 89600 VSA software to extract curve-fitting coefficients of the
AM-AM and AM-PM graphs, calculates the LUT, and applies digital pre-distortion.
The process is then repeated for a specified number of iterations. Figure 8 and 9
show example results with 10 MHz LTE uplink with QPSK modulation applied, 5x
oversampling for IQ, and 3x oversampling for envelope. The polynomial order is set
to 5 for both AM-AM and AM-PM. After applying DPD, both AM-AM and AM-PM
curves are linearized. Figure 9 shows the spectrum of the ET PA output with and
without DPD. Without DPD, it has approximately