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Testing LTE - Where to Begin?Optimizing LTE Test for IQxstream®

WHITEPAPER

© 2012 LitePoint, A Teradyne Company. All rights reserved.

Optimizing LTE Test for IQxstream 2

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


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.


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

Measurement3GPP TS 36.521-1

ReferenceDiscussion

TX Power6.2, 6.3

Performance on LTE networks, like most modern air interfaces, are highly dependent upon accurate power control across a wide range of power settings and over rapidly changing channel parameters.

Error Vector Magnitude6.5.2.1 6.5.2.4

This is the primary TX quality measurement. EVM detects distortions in the waveform that will ultimately degrade the ability for the signal to be received accurately.

Frequency Error 6.5.1Frequency accuracy is critically important to avoid interference on the uplink and for successful decoding at the base station.

Adjacent Channel Leakage Ratio 6.6.2.1

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 working channel. In LTE, this measurement is concerned with both LTE and WCDMA potential adjacent channels.

Occupied Bandwidth 6.6.1Another measure of signal quality, this measurement confirms that the signal is being confined within its required bandwidth.

Spectrum Emission Mask 6.6.2.3This measurement insures that the signal in adjacent channels is falling off in a manner that minimizes interference.

Carrier Leakage 6.5.2.2This measurement looks for the presence of the carrier frequency on the output which is normally suppressed. This is an indication that there is some mismatch in the I-Q modulator of the mobile’s transmitter.

TX Time Mask 6.3.4

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 are shared both in frequency and time, being accurate in the time domain is just as important as being accurate in the frequency domain.

In-band emissions for non allocated Resource Blocks (RB)

6.5.2.3LTE subsets the uplink signal into Resource Blocks assigned on an individual User Equipment (UE) basis. This test verifies that UE does not produce power outside of 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.


The following two tests are used to verify RX performance:

UE Receiver Measurements

Measurement 3GPP TS 36-521-1 Discussion

RX BER 7.3, 7.4RX BER is a fundamental test of a Receiver’s ability to decode the inbound signal. Typically this measurement is made at the RX threshold and at a maximum input power.

RSSI N/A

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 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.


LTE Test Plan

Derived from 3GPP Test Specification

Test ConfigurationParameters 1 2 3 4 5 6 7 8 9 10 11TX 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 11Power √ √ √ √ √

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

Test ConfigurationParameters 12 13 14 15 16 17 18 19 20 21TX 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 21Power √ √ √ √ √ √ √

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

Upper and Lower Extremes of RB Offsets

Variations in RB Offset for RB=1 QPSK channel

Variations in Power level for RB=12 QPSK channel

QPSK RB=50 @ Min and Max

Power

Test of Absolute Power Setting

16QAM RB=12 @ Min and Max

Power

16QAM RB=50 @ Min and Max

Power

Upper and Lower Extremes of RB Offsets


In 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 Configuration

Discussion

1 - 3

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 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.

4In configuration 4 the author increases the RB allocation to a mid range number of 12 at zero offset and tests for power, EVM, ACLR and SEM. These tests provide good coverage of overall transmitter performance.

5 - 7

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 following the recommendations of the 3GPP test spec. EVM is also measured at -40 dBm in accordance with the recommendations of the test spec.

8 - 11These next 4 configurations repeat the measurements of configurations 4-7 but for an RB offset of 38 – the upper edge of the channel assignment. Similar to tests 1-3, they are looking for deviations due to non-uniform components across the band.

12

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 bandwidth, this configuration would be expected to have the most issues with signal quality measurements for QPSK modulation.

13This configuration uses the max RB allocation of configuration 12 and goes to -40 dBm (minimum power) to measure Power and EVM. With this test configuration complete we have made a comprehensive exploration of the various parameters for the QPSK modulation.

14, 15Switching gears completely, these two configurations evaluate the ability of the mobile to set power at two 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.

16, 17, 18, 19At configuration 16, the modulation shifts up to 16QAM and the RB assignment falls back to 12. The test 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.

20, 21

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 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.


Let’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 two things stand out. The table is quite sparse and there are a significant number of configurations. Given that an IQxstream supports a methodology where data capture is separate from analysis, test time is largely determined by the configure-capture cycle and not by the number of measurements calculated per capture. This suggests that a test plan optimized for throughput would minimize the number of configurations while expanding the number of measurements. This favors a narrower table with greater density to the measurements.

Let’s also examine how the test engineer chose the different configurations. In the above example, the tests explore completely one set of parameters and then move orthogonally to the next set. Tests 1-3 explore the variations in RB offset fully and then change RB block size and go on to explore the effects of different offsets again. In a lab environment, such control is essential to tracking down the source of an unacceptable variation in the design but in a manufacturing test environment, this orthogonality is far less important.

Compacting the Test Plan

When looking at the original test plan, compacting is an appropriate term to use to improve it for execution on the IQxstream. Total test time will largely be determined by the number of test configurations and with the analysis portion decoupled from data capture, we can make far more measurements for a given 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.

Simple Defect Example

Let’s look at how a deficiency in analog performance might play out with a simple example. Let’s assume the post modulation analog filter was offset in frequency with the cut off frequency intruding into the upper edge of the channel. The result would be that the power output would be low at the upper edge of the channel. This failure would show up in both a 1 RB test and a 12 RB test on the upper side of the band i.e. power measurements in test configurations 3 and 8. It might also show up in an EVM flatness measurement for a 50 RB block.

Remember in production we are just trying to find determine if a DUT is ‘good’ or ‘bad’. Once identified as ‘bad’ we can put that DUT off to the side for further investigation and repair. We need not and should not burden the production line with tests needed to isolate problems if that isolation adds significantly to test times.

Logically then, either configuration 3 or 8 in the test table could be deleted since they both offer similar coverage. These types of duplications often show up throughout a test plan and while some duplication may be desirable or necessary, it should not be wasteful.


LTE Test Plan – Reductions

Test ConfigurationParameters 1 2 3 4 5 6 7 8 9 10 11TX 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 11Power √ √ √ √ √

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

Test ConfigurationParameters 12 13 14 15 16 17 18 19 20 21TX 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 21Power √ √ √ √ √ √ √

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 Configuration 21

Delete RB Offset Variations

Covered by Configuration 21

Use Mid Power Test Point of 5 for Absolute Power

Covered by Configurations 20, 21

Delete RB Offset Variations


While 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 T7TX 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 T7Power √ √ 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 √


A 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 down, measuring power output at each step. Midway through the test a change in RB allocation is made confirming the ability of the mobile to move from a max RB allocation to a minimum RB allocation without a dramatic change in RF power out.

Similarly, the ’Power – Control Up’ test takes the mobile down to minimum power and then steps it back up to full power, changing the RB assignment midway 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.

Parameters PC-1Modulation QPSK

RX Power -57

Test 1

Power - Control Down RB (50..1) √

Power - Control UP RB (1..25) √


Receiver 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 base station will typically have switched to the simpler modulation to maximize range at low input power levels.

RX2: A maximum input power test will use 16 QAM modulation simulating a mobile being in close proximity to a base station.

RX3: A mid power test point is used to verify the accuracy of RSSI.

RX BER tests tend to be lengthy due need for statistically valid measures of errors. SER measurements (see sidebar) have some benefit as they expose the largest number of errors to the tester for a given threshold.

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

BER – Bit Error Rate

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 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.

FER – Frame Error Rate

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 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.

SER – Symbol Error RateThis refers to the symbol detected at baseband that may contain one or more bits of information depending 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.

Parameters RX1 RX1 RX1TX Power 23 18.5 18.5

Modulation QPSK 16QAM 16QAM

UL RB 25 25 25

UL RB Offset 0 0 0

DL Modulation QPSK 64QAM 64QAM

DL RB 50 50 50

RB Offset 0 0 0

RX Power -94 -25 -60

Diversity Y Y Y

Test 1 2 3

RX Error Rate √ √

RX Level √ √ √


The 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:

ParameterRange ofInterest

GreatestChallenge

Notes

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 Offset Max, Min Max, MinRB offsets using small RB allocations will show the difference in PA performance from one 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.


Following 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.


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