Testing LTE - Where to Begin? - Optimizing LTE Test for IQxstream
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
WHITEPAPER
Testing LTE - Where to Begin?
Optimizing LTE Test for IQxstream®
© 2012 LitePoint, A Teradyne Company. All rights reserved.
Table of Contents
Testing LTE – Where to Begin? Optimizing LTE Test for IQxstream®.......................... 3
Physical Layer Measurements....................................................................................... 4
LTE Test Plan Development.......................................................................................... 5
Compacting the Test Plan............................................................................................. 8
Filling out the LTE Test Plan........................................................................................ 11
The Direct Approach................................................................................................... 13
Further Refinement .................................................................................................... 14
Conclusion .................................................................................................................. 14
Optimizing LTE Test for IQxstream 2
Testing LTE - Where to Begin?
Optimizing LTE Test for IQxstream®
With any new technology one of the greatest
challenges for the production engineer is what to
test and why. This is particularly troublesome for
devices as complex as the modern smartphone
or tablet. For LTE, the degree of complexity
is unprecedented and to test it all would have
devices sitting on the tester all day long.
In production, the basic assumption has to be that
the design handed off from engineering meets
all the requirements of the customer and when
assembled correctly will do so consistently. While
supporting this assumption places a burden on
the engineering team and its processes, without
this assurance, the dimension of tests is simply too
large to examine all the possibilities with today’s
extremely complex devices. The production floor
is not the place to be verifying millions of lines of
firmware nor the hardware functionality associated
with a multi-million gate DSP/ASIC design.
In production test, the primary goal is to exercise
the mobile as much as possible to identify
manufacturing defects while minimizing test time. The software and digital designs have been proven during engineering and
conformance testing. The digital ICs have already gone through extensive testing during their production processes. Digital
failures, when they occur, will typically be catastrophic resulting in the phone not powering up, not producing an output or not
being able to receive a signal. They can often best be found through techniques like simple internal power up tests and through
the use of checksums without any tester involvement at all. Therefore the optimal production tests focus on physical layer
measurements, the area that exhibits the largest degree of variability associated with the manufacturing process.
The following sections discuss testing for LTE and how to optimize testing for physical layer testers such as LitePoint’s IQxstream.
Optimizing LTE Test for IQxstream 3
Physical Layer Measurements
Physical layer testing focuses on the lowest layer of the air interface. It seeks to determine conformance with the key parameters
essential to the successful transmission of a signal over the air. Transmit power, the quality of the TX waveform, the accuracy of the
TX frequency, are all key to a mobile station’s performance. On the receive side, the ability of the mobile to successfully decode the
received signal at the lowest and highest signal levels defines its successful operation in the network.
The 3GPP test spec for LTE contains a large number of different tests meant to determine compliance with the LTE specifications.
Many of these tests have some degree of overlap and given the degree of implementation within the digital domain many of
the measurements will not vary from one mobile to another. The following tests can generally be considered sufficient to detect
problems within the production environment.
UE Transmitter Measurements
3GPP TS 36.521-1
Measurement Discussion
Reference
Performance on LTE networks, like most modern air interfaces, are highly dependent
6.2, 6.3
TX Power upon accurate power control across a wide range of power settings and over rapidly
changing channel parameters.
6.5.2.1 This is the primary TX quality measurement. EVM detects distortions in the waveform
Error Vector Magnitude
6.5.2.4 that will ultimately degrade the ability for the signal to be received accurately.
Frequency accuracy is critically important to avoid interference on the uplink and for
Frequency Error 6.5.1
successful decoding at the base station.
ACLR is one of several measurements associated with not interfering with other users
and systems. ACLR measures undesired power in the immediate channel beside the
Adjacent Channel Leakage Ratio 6.6.2.1
working channel. In LTE, this measurement is concerned with both LTE and WCDMA
potential adjacent channels.
Another measure of signal quality, this measurement confirms that the signal is being
Occupied Bandwidth 6.6.1
confined within its required bandwidth.
This measurement insures that the signal in adjacent channels is falling off in a
Spectrum Emission Mask 6.6.2.3
manner that minimizes interference.
This measurement looks for the presence of the carrier frequency on the output
Carrier Leakage 6.5.2.2 which is normally suppressed. This is an indication that there is some mismatch in the
I-Q modulator of the mobile’s transmitter.
This measurement looks at the signal in time, verifying that the PA is turning on and
off at the correct time without producing any extraneous signals. Since LTE signals
TX Time Mask 6.3.4
are shared both in frequency and time, being accurate in the time domain is just as
important as being accurate in the frequency domain.
LTE subsets the uplink signal into Resource Blocks assigned on an individual User
In-band emissions for non
6.5.2.3 Equipment (UE) basis. This test verifies that UE does not produce power outside of
allocated Resource Blocks (RB)
its assigned RB(s) but within the bandwidth of the uplink signal.
To a large extent ACLR, Occupied Bandwidth and SEM are all chasing the same problem. Typically something is degraded in the
final portion of the analog output chain or there is a noise source within the DUT producing spurious signals. For this reason very
often only one of these measurements will be specified as part of a test plan.
Unlike the TX chain where the final output is presented at the antenna connector for evaluation, the RX signal remains buried
within the DUT until the signal is fully decoded. The fortunate part of this equation is that while there are many components in the
RX chain that can degrade, virtually all degradation will show up in a RX Bit Error Rate measurement at or near the RX threshold.
Physical layer testers are generally dependent upon the DUTs ability to report results on RX testing. Since RX quality monitoring is
a critical component of modern air interface operation, it is a straight forward problem to route this data to the external terminal
interface. Most, if not all, IC manufacturers provide support for BER testing in one form or the other.
Optimizing LTE Test for IQxstream 4
The following two tests are used to verify RX performance:
UE Receiver Measurements
Measurement 3GPP TS 36-521-1 Discussion
RX BER is a fundamental test of a Receiver’s ability to decode the inbound signal.
RX BER 7.3, 7.4
Typically this measurement is made at the RX threshold and at a maximum input power.
Receive signal strength is a parameter that is often measured as part of calibration.
Since the initial TX power level is calculated based on the measured RSSI, accuracy in a
RSSI N/A
DUT’s RSSI measurement is key to producing just the right amount of power when first
communicating with a base station.
Given the above set of measurements, the challenge now becomes how to apply them to the near infinite number of possible
mobile configurations.
LTE Test Plan Development
There are many approaches to test plan development including:
• Looking for likely failure modes in the design
• Using the standards body’s recommendations
• IC manufacturer’s recommendations
• Past history of similar devices in production
Unfortunately with new technologies such as LTE there may be very limited history upon which to base a test plan, the detailed
internals of a design may not be exposed by the various part manufacturers and the manufacturers themselves may have limited
experience with a relatively new design.
So often, manufacturers will be on their own to develop a test plan and may fall back to the standards body’s test spec as
a baseline.
The tables on the following page represents a test plan developed for an LTE User Equipment transmitter. There are a few more
tests we’ll want to perform before declaring a Device Under Test (DUT) ‘passed’ in terms of production test, but for purposes of
this discussion this subset is useful.
Each column in the tables, moving from left to right represents a test configuration as specified by the Parameters at the top
of each column. In general when we talk about a test configuration we talk about a steady state that the DUT is placed in eg.
constant modulation rate, constant power level, etc. The lower portion of each column denotes the measurements to be made
for each configuration.
Optimizing LTE Test for IQxstream 5LTE Test Plan
Derived from 3GPP Variations in RB Offset Variations in Power level
Test Specification for RB=1 QPSK channel for RB=12 QPSK channel
Test Configuration
Parameters 1 2 3 4 5 6 7 8 9 10 11
TX Power 23 23 23 23 3.2 -30 -40 23 3.2 -30 -40
Modulation QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK
RB 1 1 1 12 12 12 12 12 12 12 12
RB Offset 0 24 49 0 0 0 0 38 38 38 38
RX Power -57 -57 -57 -57 -57 -57 -57 -57 -57 -57 -57
Measurements 1 2 3 4 5 6 7 8 9 10 11
Power √ √ √ √ √
EVM √ √ √ √
EVM Flatness
Frequency Accuracy
Carrier feed through √ √ √ √ √ √
TX Time Mask
Occupied Bandwidth Upper and Lower
Extremes of RB Offsets
ACLR √ √
SEM √ √
In-band emissions for non
√ √ √ √ √ √
allocated Resource Block (RB)
Spurious Response
QPSK RB=50 16QAM RB=12 16QAM RB=50
@ Min and Max Test of Absolute @ Min and Max @ Min and Max
Power Power Setting Power Power
Test Configuration
Parameters 12 13 14 15 16 17 18 19 20 21
TX Power 23 -40 6.4 -5.6 23 -40 23 -40 23 -40
Modulation QPSK QPSK QPSK QPSK 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM
RB 50 50 50 50 12 12 12 12 50 50
RB Offset 0 0 0 0 0 0 38 38 0 0
RX Power -57 -57 -57 -57 -57 -57 -57 -57 -57 -57
Measurements 12 13 14 15 16 17 18 19 20 21
Power √ √ √ √ √ √ √
EVM √ √ √ √ √
EVM Flatness √
Frequency Accuracy √
Carrier feed through
TX Time Mask
Occupied Bandwidth √ Upper and Lower
Extremes of RB Offsets
ACLR √ √ √ √
SEM √ √ √ √
In-band emissions for non
allocated Resource Block (RB)
Spurious Response
Optimizing LTE Test for IQxstream 6In the following table we will walk through the development of this test plan section by section. The test developer in this case had
a good knowledge of LTE, a good understanding of the 3GPP test specification for LTE and was considered an expert in testing
mobiles for other technologies.
Test
Discussion
Configuration
The test plan author started with the most basic of configuration: maximum TX power, modulation set to
QPSK, and a single Resource Block assigned. The author then begins to explore deviations in output power
1-3 for different RB offsets of 0, 24 and 49 - both channel edges and the center. He is looking for variations in
output power, most likely to occur at the channel edges. These variations could be driven by some imbalance
or misalignment in TX filtering or by a PA that has a weakness at one of the edges of the band.
In configuration 4 the author increases the RB allocation to a mid range number of 12 at zero offset and tests
4
for power, EVM, ACLR and SEM. These tests provide good coverage of overall transmitter performance.
As follow-on to configuration 4, during these three configurations, the test author holds modulation, RB
allocation and RB offset constant and verifies carrier leakage and inband emissions at 3.2, -30 and -40 dBm
5-7
following the recommendations of the 3GPP test spec. EVM is also measured at -40 dBm in accordance with
the recommendations of the test spec.
These next 4 configurations repeat the measurements of configurations 4-7 but for an RB offset of 38 – the
8 - 11 upper edge of the channel assignment. Similar to tests 1-3, they are looking for deviations due to non-uniform
components across the band.
In this configuration, the RB allocation is increased to the maximum value of 50 and a fairly comprehensive set
of measurements made. With the full channel occupied, RB offset is not applicable. At full power and wide
12
bandwidth, this configuration would be expected to have the most issues with signal quality measurements for
QPSK modulation.
This configuration uses the max RB allocation of configuration 12 and goes to -40 dBm (minimum power) to
13 measure Power and EVM. With this test configuration complete we have made a comprehensive exploration
of the various parameters for the QPSK modulation.
Switching gears completely, these two configurations evaluate the ability of the mobile to set power at two
14, 15 specific test points driven by the 3GPP test spec. This can be considered a verification of the RSSI/Power
Amplifier output calibration that would typically have been performed earlier in the production process.
At configuration 16, the modulation shifts up to 16QAM and the RB assignment falls back to 12. The test
16, 17, 18, 19 author then goes on to explore min and max power and both edges of the channel in terms of RB assignment
measuring power, ACLR, SEM and EVM selectively.
Finally the author checks the maximum uplink rate using 16QAM and 50 RB at maximum and minimum power.
At full power and wide bandwidth, configuration 20 would be expected to have the most issues with signal
20, 21
quality measurements while configuration 21 would explore any deviations in the transmit chain at minimum
power settings.
Note that for all tests the author uses a RX Power level of -57 dBm. Since RX power should have no direction relationship on TX
measurements, it shouldn’t matter what level it is set to.
Note: It is generally assumed that this test plan will be applied uniformly over a variety of bands/channels in line with the
capabilities of the DUT. 3GPP recommends that a device be tested at low, mid and high channels for each band. In some band
allocations this may mean that only a single channel is to be tested.
From the perspective of test coverage, the author of this test plan did a good job of testing the bounds of performance of the
DUT. Minimum and maximum RB assignments, minimum and maximum modulation rates and minimum and maximum power
levels are tested. Variation across a channel is explored in terms of RB assignments. The tests are drawn from and match well with
the recommendations of the 3GPP test spec. It is unlikely that many problems if any will slip by this test plan in production.
Optimizing LTE Test for IQxstream 7Let’s examine this test plan from a test throughput perspective as after all, our
goal is to effectively test DUTs as quickly as possible. When you look at the plan Simple Defect Example
two things stand out. The table is quite sparse and there are a significant number
Let’s look at how a deficiency in analog
of configurations.
performance might play out with a simple
example. Let’s assume the post modulation
Given that an IQxstream supports a methodology where data capture is separate analog filter was offset in frequency with the cut
from analysis, test time is largely determined by the configure-capture cycle and off frequency intruding into the upper edge of
not by the number of measurements calculated per capture. This suggests that a the channel. The result would be that the power
test plan optimized for throughput would minimize the number of configurations output would be low at the upper edge of the
while expanding the number of measurements. This favors a narrower table with channel. This failure would show up in both a
greater density to the measurements. 1 RB test and a 12 RB test on the upper side
of the band i.e. power measurements in test
Let’s also examine how the test engineer chose the different configurations. In configurations 3 and 8. It might also show up in
the above example, the tests explore completely one set of parameters and then an EVM flatness measurement for a 50 RB block.
move orthogonally to the next set. Tests 1-3 explore the variations in RB offset
Remember in production we are just trying to
fully and then change RB block size and go on to explore the effects of different
find determine if a DUT is ‘good’ or ‘bad’. Once
offsets again. In a lab environment, such control is essential to tracking down the
identified as ‘bad’ we can put that DUT off to
source of an unacceptable variation in the design but in a manufacturing test the side for further investigation and repair. We
environment, this orthogonality is far less important. need not and should not burden the production
line with tests needed to isolate problems if that
isolation adds significantly to test times.
Compacting the Test Plan Logically then, either configuration 3 or 8 in the
test table could be deleted since they both offer
When looking at the original test plan, compacting is an appropriate term to similar coverage. These types of duplications
use to improve it for execution on the IQxstream. Total test time will largely be often show up throughout a test plan and while
determined by the number of test configurations and with the analysis portion some duplication may be desirable or necessary,
decoupled from data capture, we can make far more measurements for a given it should not be wasteful.
capture with minimal cost. So our goal should be to reduce the number of test
configurations while making more measurements for each capture. Let’s walk
through such a compacting exercise.
While we will want to check all the modulation schemes in the phone as they typically take different data paths through the
circuitry, we probably don’t need to validate all the variations since they are generally produced within the digital domain and are
not affected by analog variations.
Let’s start by picking what we definitely want to keep. Configurations 1, 12 and 20 explore the extremes of modulation and RB
assignments and configuration 4 provides a moderate, perhaps typical test of RB assignments. These 4 configurations are logical
candidates for retention.
For TX quality measurements the emphasis should generally be in the maximum power measurements since these will typically
challenge the high power circuits the most. If there are going to be variations in output power from one edge of the band/channel
to the other, they will show up in the single RB allocation measurements. So configuration 3 with its maximum RB offset should also
be retained as a complement for the minimum RB offset of configuration 1.
Configuration 2 is a candidate for being cut since it only tests the middle RB offset of the channel. Since analog problems will
usually exhibit themselves either across the band or at band edges there is little to be gained from this middle measurement.
With configurations 1 and 3 we have tested the analog variations at band edges for different RB offsets so we can safely eliminate 8
through 11 since they only differ from 4 to 7 by the RB offset.
Optimizing LTE Test for IQxstream 8LTE Test Plan – Reductions
Test Configuration
Parameters 1 2 3 4 5 6 7 8 9 10 11
TX Power 23 23 23 23 3.2 -30 -40 23 3.2 -30 -40
Modulation QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK
RB 1 1 1 12 12 12 12 12 12 12 12
RB Offset 0 24 49 0 0 0 0 38 38 38 38
RX Power -57 -57 -57 -57 -57 -57 -57 -57 -57 -57 -57
Measurements 1 2 3 4 5 6 7 8 9 10 11
Power √ √ √ √ √
EVM √ √ √ √
EVM Flatness
Frequency Accuracy
Carrier feed through √ √ √ √ √ √
TX Time Mask
Occupied Bandwidth
ACLR √ √
SEM √ √
In-band emissions for non
√ √ √ √ √ √
allocated Resource Block (RB)
Spurious Response
No Need for Mid-Channel RB Offset Covered by Delete RB Offset Variations
Configuration 21
Test Configuration
Parameters 12 13 14 15 16 17 18 19 20 21
TX Power 23 -40 6.4 -5.6 23 -40 23 -40 23 -40
Modulation QPSK QPSK QPSK QPSK 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM
RB 50 50 50 50 12 12 12 12 50 50
RB Offset 0 0 0 0 0 0 38 38 0 0
RX Power -57 -57 -57 -57 -57 -57 -57 -57 -57 -57
Measurements 12 13 14 15 16 17 18 19 20 21
Power √ √ √ √ √ √ √
EVM √ √ √ √ √
EVM Flatness √
Frequency Accuracy √
Carrier feed through
TX Time Mask
Occupied Bandwidth √
ACLR √ √ √ √
SEM √ √ √ √
In-band emissions for non
allocated Resource Block (RB)
Spurious Response
Covered by Use Mid Power Covered by Delete RB Offset
Configuration 21 Test Point of 5 for Configurations 20, 21 Variations
Absolute Power
Optimizing LTE Test for IQxstream 9While configuration 20 represents a form of stress test on the PA and other high power circuits, configuration 21 actually represents
the most realistic scenario of maximum rate operation. Maximum rate operation is only possible when you are close to the base
station so typically the PA will be set to a low power setting. At least one TX quality measure should be made at low power since
the Power Amplifier will be operating in a very different mode at 63 dB below its max power setting. So configuration 21 remains a
valuable test configuration.
Configurations 14 and 15 are there to test specific absolute power setting capability but given that this ability should hold true
over the full range of operation any intermediate power measurement will do. So we will retain configuration 5 as a measure of the
ability to set an intermediate power level and delete 14 and 15.
Let’s walk through the remaining configurations to see what is left.
Configuration 6 and 7 drop the power down to -30 dBm and -40 dBm respectively but there is no reason to believe that simpler
modulation and reduced RB allocations of these tests will highlight any problem not found by the more complex waveforms of
configuration 21 nor should there be much of a difference between -30 dBm and -40 dBm. So these tests can be eliminated.
A similar argument holds for test configuration 13, again a simpler configuration than test 21.
Likewise for test configuration 16, 17, 18 and 19, these tests validate operation at variations of RB offset and power for the
simpler 12 RB allocation operating at 16QAM. We’ve already verified different RB offsets earlier in configurations 1 and 3 and the
modulation scheme is proven out in configurations 20 and 21. So these four tests become candidates for deletion.
During this process we eliminated a large number of test configurations but as we said earlier we were looking at a sparse matrix of
tests. Since there is little or no cost associated with adding measurements to a given test configuration, let’s fill in some of
the blanks.
The following test plan represents adding more tests in and making a few further adjustments.
Compact Test Plan
Test Configuration
Parameters T1 T2 T3 T4 T5 T6 T7
TX Power 23 23 23 23 3.2 23 -40
Modulation QPSK QPSK QPSK QPSK QPSK 16QAM 16QAM
RB 1 1 12 50 12 12 50
RB Offset 0 49 24 0 24 0 0
RX Power -90 -90 -90 -90 -57 -90 -25
Measurements T1 T2 T3 T4 T5 T6 T7
Power √ √ MPR MPR √ MPR √
EVM √ √ √ √ √ √ √
EVM Flatness √ √ √ √ √ √ √
Frequency Accuracy √ √ √ √ √ √ √
Carrier feed through √ √ √ √ √ √ √
TX Time Mask √
Occupied Bandwidth √ √ √ √ √ √ √
ACLR √ √ √ √ √ √ √
SEM √ √ √ √ √ √ √
In-band emissions for non allocated
√ √ √ √
Resource Block (RB)
Spurious Response √
Optimizing LTE Test for IQxstream 10A few notes:
• The tests marked in red including their levels are generally traceable to the 3GPP standard.
• The power tests marked MPR are relative power measurements against the T1, T2 power data. Maximum Power Reduction
(MPR) measures changes in power level associated with changing modulation or RB allocations. Since such changes are
common in operation, it is important that they do not produce spikes or drops in power level which could negatively affect
operation.
• The TX Time Mask measurement is made only once as it involves a dynamic measurement of the mobile passing from an off
state to on and back to off. This requires some synchronization between the tester and the DUT, not necessary in the other tests.
• RX Power levels for the tests where TX power is at maximum have been moved closer to the RX threshold. This is typical of
actual operation in the field and helps to insure that the Receiver is adequately tracking the RX frequency. Any failure in this case
would generally show up in the frequency accuracy measurements. Similarly for when TX power is at minimum, the RX power
can be expected to be the strongest hence the increase on configuration T7.
• T6 uses a 12 RB allocation simply to get a 12 RB test in at 16QAM. It could be argued that an eighth configuration is required to
include both a 12 RB and 50 RB configuration at full power.
This new test plan outputs 62 measurements over 7 configurations whereas the original test plan yielded 48 measurements over 21
configurations. Based on configuration/capture times, the new test plan should run in roughly 1/3 the test time.
Should it be found that the new test time is dominated by analysis calculations, it would be perfectly acceptable to reduce or
even eliminate some of the overlapping tests. In this regard the Occupied Bandwidth calculation as well as well as one of ACLR
or SEM could be eliminated for some of the configurations. Likewise for the In-band emissions for non-allocated Resource Block
(RB) measurement, performance is dominated by the implementation in the digital domain so the number of configurations this is
calculated for could be reduced.
Filling out the LTE Test Plan
The previous analysis focused on static TX measurements. Missing is a discussion of power control and RX measurements so we
will add these in now.
Power Control Tests: The ability of a mobile to respond linearly to power control commands is critical to network performance
yet the dynamic range required in LTE challenges transmitter designers who often have to resort to segmented designs to cover
the full -40 dBm to +23 dBm typical range of output power. The boundaries of these segmented designs present challenges to the
linearity of the output power of the DUT.
The following tests measure a DUT’s ability to respond to power commands received on the downlink and to step accordingly in
the presence of changing RB assignments.
The ‘Power – Control Down’ test takes the mobile to full power and then steps it Parameters PC-1
down, measuring power output at each step. Midway through the test a change in
Modulation QPSK
RB allocation is made confirming the ability of the mobile to move from a max RB
RX Power -57
allocation to a minimum RB allocation without a dramatic change in RF power out.
Test 1
Similarly, the ’Power – Control Up’ test takes the mobile down to minimum power Power - Control Down RB (50..1) √
and then steps it back up to full power, changing the RB assignment midway Power - Control UP RB (1..25) √
through the test.
These two tests are examples of using waveforms sent to the mobile that contain signaling commands (power up, power down)
which the mobile reacts to. In this case the tester has no specific knowledge of the signaling. As far it is concerned it is simply
playing back a recorded waveform. This is one of the defining characteristics of a physical layer tester – the ability to support
signaling but generally only through a playback mechanism.
Optimizing LTE Test for IQxstream 11Receiver Tests: To test the receiver we use three test configurations as shown in the following table:
RX1: At minimum input power in, QPSK modulation is used since the
Parameters RX1 RX1 RX1
base station will typically have switched to the simpler modulation to
TX Power 23 18.5 18.5
maximize range at low input power levels.
Modulation QPSK 16QAM 16QAM
RX2: A maximum input power test will use 16 QAM modulation UL RB 25 25 25
simulating a mobile being in close proximity to a base station. UL RB Offset 0 0 0
DL Modulation QPSK 64QAM 64QAM
RX3: A mid power test point is used to verify the accuracy of RSSI. DL RB 50 50 50
RB Offset 0 0 0
RX BER tests tend to be lengthy due need for statistically valid RX Power -94 -25 -60
measures of errors. SER measurements (see sidebar) have some Diversity Y Y Y
benefit as they expose the largest number of errors to the tester for a
Test 1 2 3
given threshold.
RX Error Rate √ √
RX Level √ √ √
In this whitepaper we discussed the development of an optimized
test plan for an IQxstream Physical Layer tester. This was done by
modifying a test plan that had been developed in line with the 3GPP TS recommendations without taking into account the ‘capture
once, measure many’ capabilities of the IQxstream. The result of the optimized test plan will be roughly a 3x improvement in test
throughput.
BER Test Types: Depending upon the modem IC manufacturer, you may see one or more of the terms SER, FER, or BER associated with receiver
testing. They are explained in the following table.
Error Measure Notes
This is typically a measure of the data delivered to the user in one form or the other. It is usually measured after
all error correction techniques have been applied. In some systems BER may be reported by extrapolating from
BER – Bit Error Rate
the results of error correction as opposed to an exact bits-in vs. bits-out comparison. In such systems BER is a
statistical estimate based on the number of corrections attempted by the error correction circuitry.
This refers to frames received from the base station that are received in error as detected by error checking/
correcting codes. Systems are usually designed to operate at some non-zero level of FER as means of insuring
FER – Frame Error Rate
that the system is operating at either maximum range or maximum capacity. Frames that contain errors can
often be corrected for in the baseband error correction algorithms.
This refers to the symbol detected at baseband that may contain one or more bits of information depending
SER – Symbol Error Rate upon the modulation scheme. Typically symbol errors are reported before error correction techniques are
applied and hence it provides the most direct insight into the behavior of the receiver.
In general while the actual measurements of error rate are different, any of the three can be used as a means of verifying RX performance however
it is usually important to understand which measure is being used in order to use the correct threshold and/or limits.
Optimizing LTE Test for IQxstream 12The Direct Approach
An experienced test designer familiar with the ‘capture once, measure many’ capabilities might chose to take a more direct path to
IQxstream test plan development. The following provides some insight into what such an approach might look like.
The greatest challenge is developing a test plan is in getting a handle as to what parameters are important to fully exercising
a DUT. In the complex air interfaces such as LTE and WCDMA narrowing these parameters down to a useful subset and further
determining what part of any given range will be most important is the key to not having an ‘expensive’ test plan.
Let’s start by listing the parameters that exercise a DUT, the range of parameters of interest and the values within that range that
will most challenge the performance of the DUT. In the case of LTE they are:
Range of Greatest
Parameter Notes
Interest Challenge
TX Power Max, Mid, Min Max Maximum power will stress the linearity of the power amplifier
Modulation Max, Min Max Maximum modulation scheme will be the most sensitive to TX quality
RB Allocation Max, Mid, Min Max Maximum RB allocation will test the linearity of the TX over the widest bandwidth.
RB offsets using small RB allocations will show the difference in PA performance from one
RB Offset Max, Min Max, Min edge of a channel to the other. The use of offsets does not need to be repeated across all
configurations.
If we were to fully explore the range of interest of each of the parameters orthogonally, we would wind up with 30 test
configurations (there is no RB offset for an max RB allocation) and a correspondingly slow test time. That is why we identify the
greatest challenge and take note of items like the comment in the section on RB offsets.
1. Since we know most distortions will be at their worst at high power, our emphasis will be on high power testing with probably
only a single sample from the mid and low power settings.
2. For Modulation we will want to test over the range keeping in mind that the most complex modulation rate will be the most
sensitive to distortion. We also recognize that maximum modulation will be used when the mobile is closest to the base
station so max rate modulation is suitable for testing at the low power setting.
3. RB allocation will want to be tested but again the largest allocations will present the greatest challenges. We also know that
most of this is synthesized inside already tested digital hardware so we should not expect too much deviation by changing
allocations. On the other hand given that the uplink is a shared resource full allocations of all 50 RBs will be rare. While single
RB allocations will be common when web browsing, a shared uplink with some capacity needed will present a frequent
challenge to the network as users upload photos and video so moderate RB allocations probably represent a common
usage scenario.
4. RB offset is synthesized inside digital hardware so we will actually use this to validate performance of the PA from one edge of
the band to the other. Power measurements will be most sensitive at the band edges using a single RB allocation.
Optimizing LTE Test for IQxstream 13Following this logic we end up with a desire for:
Test Configurations Number of Tests
1 test for 50 RB and one for 12 RB at QPSK at high power 2
1 test for 50 RB at 16 QAM at high power 1
2 tests for 1 RB with different offsets at QPSK 2
An extra test at low power, 16 QAM and 50 RB block 1
An extra test at mid power, QPSK, 12 RB 1
Totals 7
Following this to conclusion results in a Test Plan almost identical to the Compact Test Plan from page 9. The slight difference from
the earlier result in that the high power test at 16 QAM is being done with 50 RB. There is no hard and fast rule and it may actually
be desirable to add an eighth configuration to perform both a 12 and 50 RB 16 QAM test. Keep in mind though that full power at
50 RB and 16 QAM is not a realistic operational scenario, so there is no strong basis for one versus the other.
Further Refinement
While the above approach assumes that all tests will be performed at all frequencies of interest, further refinement of the test plan
is possible by creating a subset of the test plan and applying the subset selectively. Learning from test experience in production
or from direct knowledge of the failure modes possible in a specific design may suggest that very limited or even singular test
configurations for most bands/frequencies are adequate to detect failures.
As a hypothetical example of how subsets could be used, a DUT with support for E-UTRA bands 5 and 8 may share a single TX and
RX design except for a switched duplexer. In that case it may be adequate to simply check fully the lower channel in band 5 and
the upper channel in band 8 and then perform a small subset of tests on the upper band 5 channel and the lower band 8 channels.
Conclusion
In production test, the primary goal is to exercise the mobile as much as possible to identify manufacturing defects while
minimizing test time. To this end, leveraging the ‘capture once, measure many’ ability of IQxstream has great benefits both in
terms of test speed but also overall test coverage.
When compared against a 3GPP test spec centric plan, a compacted plan will run in 1/3 the time with similar test coverage.
With complex air interfaces such as LTE, it is important in test plan development to narrow the dimensions of each of the
parameters recognizing what parameters stress a DUT and those that are locked into the digital design for which IC testing and lab
testing has already proven out.
IQxstream represents a fundamentally new value proposition when discussing production test as compared to the more familiar
lab test environment. Its multi-DUT capability and ‘capture once, measure many’ capability combined with an architecture that
separates data capture from analysis makes for throughputs and flexibility never thought possible in a manufacturing environment.
Optimizing LTE Test for IQxstream 14Copyright © 2012 LitePoint, A Teradyne Company.
All rights reserved
RESTRICTED RIGHTS LEGEND TRADEMARKS
No part of this document may be LitePoint and the LitePoint logo,
reproduced, transmitted, transcribed, IQxstream, IQview, IQflex, IQnxn, and
stored in a retrieval system, or translated IQmax are registered trademarks and
into any language or computer IQnxnplus, IQsignal, IQwave, IQfact,
language, in any form or by any means, IQcheck, IQdebug, IQmeasure, IQtest,
electronic, mechanical, magnetic, IQexpress, IQturbo, IQultra, IQxel,
optical, chemical, manual, or otherwise, IQ2015, IQ2012, IQ201X, IQ2011,
without the prior written permission of IQ2011q IQ2010, TrueChannel, and
LitePoint Corporation. TrueCable are trademarks of LitePoint
Corporation. Microsoft Windows is
a registered trademark of Microsoft
DISCLAIMER
Corporation in the United States and/
LitePoint Corporation makes no
or other countries. All trademarks or
representations or warranties with
registered trademarks are owned by
respect to the contents of this manual or
their respective owners.
of the associated LitePoint Corporation
products, and specifically disclaims any
implied warranties of merchantability CONTACT INFORMATION
or fitness for any particular purpose. LitePoint Corporation
LitePoint Corporation shall under no 575 Maude Court
circumstances be liable for incidental Sunnyvale, CA 94085-2803
or consequential damages or related United States of America
expenses resulting from the use of this
product, even if it has been notified of Telephone: +1.408.456.5000
the possibility of such damages. Facsimile: +1.408.456.0106
LITEPOINT TECHNICAL SUPPORT
If you find errors or problems with this www.litepoint.com/support
documentation, please notify LitePoint Telephone: +1.408.456.5000
Corporation at the address listed Available: weekdays 8am to 6pm,
below. LitePoint Corporation does not Pacific Standard Time.
guarantee that this document is error- E-mail: support@litepoint.com
free. LitePoint Corporation reserves the
right to make changes in specifications
and other information contained in this Doc: 1075-0033-001
document without prior notice. June 2012 Rev. 1
Optimizing LTE Test for IQxstream 15You can also read
