Keysight TechnologiesSolutions for Testing Data Throughput Performance in LTE-A User Equipment

Application Note

Overview

Meeting the demands of today’s evolving wireless industry is a hefty challenge, es-pecially since according to some estimates, mobile data traffic will grow by leaps and bounds in the coming years. In response to this explosion of data consumption, 4G cellular standards like LTE-Advanced (LTE-A) leverage advanced techniques such as Multi-Input Multiple-Output (MIMO) and Carrier Aggregation (CA) to enable operators to squeeze more capacity, better coverage and higher data rates from their existing frequency spectrum. With the higher data throughput, operators can offer customers a better overall experience with integrated services such as voice, video, chat, and file sharing. And that’s why achieving higher data throughput is so critical these days.

LTE-A is a prime example of a standard that has evolved in singular pursuit of that goal. With each new 3GPP standard release it has added support for more bands, more band combinations and more CA Component Carriers (CCs). Designers have responded to the promise of LTE-A helping to solve the mobile data explosion by designing it into their User Equipment (UE). However, that creates a few challenges of its own. Not only do the UE devices become all the more complex just by utilizing LTE-A, they also have to perform in highly dynamic environments. While LTE-A networks are designed to adapt to real-world operating conditions with dynamic link allocation and complex handover scenarios, this variability results in thousands of potential scenarios under which the UE is expected to perform well. That makes real-world functional and RF test (validation) of LTE-A performance a challenging proposition at best.

Problem

The strenuous environment in which LTE-A UE must operate (e.g., characterized by higher data rates, mobility, MIMO, CA, fading, noise, and interference) makes testing UE data throughput performance imperative, as well as challenging. Generally speak-ing, there are a number of physical attributes and other factors, like the type of data to be sent, that will affect data throughput. Some of these physical attributes include: the UE category, cell bandwidth (variable from 1.4 to 20 MHz), use of Single-Input Single-Output (SISO) or MIMO, and whether or not CA is used. In the real world, a number of other channel and network factors may also impact UE data throughput performance including the channel and fading conditions, link loss, noise from other UE, interference from other base stations or radio access technologies, and the number of users. Each UE is defined to be able to operate in the transmission modes shown in Figure 1. Testing must include these parameters.

02 | Keysight | Solutions for Testing Data Throughput Performance in LTE-A User Equipment - Application Note

Simplified, Real-World Functional and RF Test of LTE-A UE Performance Using a Fast, Flexible and Future-Ready One-Box Tester

TM Description Benefits SINR@UE Multipath Correlation

1 Basic SIMO Basic single transmit antenna Low-Med Low High

2 Transmit diversity Low power/SINR signal robustness Low Rural, Low (cell-edge) Rural, High (cell-edge)

3 Open-loop SU-MIMO Throughput increase potential High Urban, High Urban, Low

4 Closed loop SU-MIMO Throughput increase potential High Urban, High Urban, Low

5 MU MIMO Improved UL cell spectral efficiency High Med-High Low-Med

6 Closed-loop Rank=1 pre-coding Beamsteering

Improved signal robustness Low Rural, Low (cell-edge) Rural, High (cell-edge)

7 UE Specific Beamforming Improved signal robustness with non-codebook precoding

Low Rural, Low (cell-edge) Rural, High (cell-edge)

8 Dual Layer UE-specific RS Beamforming

As 7 with throughput increase potential Med-High Urban, High Low-Med

9 Up to 8 Layer UE-specific RS Beamforming

As 8 with throughput increase potential High High Low

Figure 1. This table shows the 9 transmission modes for LTE/LTE-A.

One standard utilized in the testing of LTE-A data throughput is the 3GPP’s TR 37.901 specification. The document specifies test procedures for application layer UE data throughput (end-to-end throughput) performance under simulated realis-tic network scheduling and radio conditions. Its intent is to ensure UE can provide specific data rates to meet data demands without network overload. In the case of LTE-A, those data rates are specified by different categories (Figure 2).

The main challenge is to simply find test equipment that can keep up with the speed of the designs. Also, fully assessing LTE-A UE design performance requires emula-tion of both the network and the wireless channel. Testing devices under both ideal and non-ideal network and channel conditions helps isolate design issues early, and ultimately ensures a high-quality user experience. However, effectively emulating this dynamic and demanding environment requires incorporation of fading, noise, and network emulation, and this can result in a complex test setup with various components that are prone to stability and calibration issues.

03 | Keysight | Solutions for Testing Data Throughput Performance in LTE-A User Equipment - Application Note

Figure 2. Shown here are the various LTE UE data rate categories.

UE category

RoundedMax. Data rate

(DL / UL) (Mbps)

Downlink Uplink

Max. # DL-SCH TB bits/TTI Max. # DL-SCH bits/TB/TTI

Total. soft channel

bits

Max. #. spatial layers

Max.# UL-SCH TB

bits / TTI

Max. # UL-SCH bits/TB/

TTISupport

for 64QAM

Cat 0 1/1 1000 1000 25344 1 1000 1000 No

Cat 1 10 / 5 10296 10296 250368 1 5160 5160 No

Cat 2 50 / 25 51024 51024 1237248 2 25456 25456 No

Cat 3 100 / 50 102048 75376 1237248 2 51024 51024 No

Cat 4 150 / 50 150752 75376 1827072 2 51024 51024 No

Cat 5 300 / 75 299552 149776 3667200 4 75376 75376 Yes

Cat 6 300 / 50 301504 149776 (2 layers)75376 (4 layers)

3667200 2 or 4 51024 51024 No

Cat 7 300 / 100 301504 149776 (2 layers)75376 (4 layers)

3667200 2 or 4 102048 102048 No

Cat 8 3000/1500 2998560 299856 35982720 8 1497760 1497760 Yes

Cat 9 450 / 50 452256 149776 (4 layers)75376 (2 layers)

5481216 2 or 4 51024 51024 No

Cat 10 450 / 100 452256 149776 (4 layers)75376 (2 layers)

5481216 2 or 4 102048 51024 No

Cat 11 600 / 50 603008 149776 (4 layers, 64QAM)195816 (4 layers, 256QAM)75376 (2 layers, 64QAM)

97896 (2 layers, 256QAM)

7308288 2 or 4 51024 51024 No

Cat 12 600 / 100 603008 149776 (4 layers, 64QAM)195816 (4 layers, 256QAM)75376 (2 layers, 64QAM)

97896 (2 layers, 256QAM)

7308288 2 or 4 102048 51024 No

Cat 13 391 / 50 391632 195816 (4 layers)97896 (2 layers)

3654144 2 or 4 51024 51024 No

Cat 14 391 / 100 391632 195816 (4 layers)97896 (2 layers)

3654144 2 or 4 102048 51024 No

Cat 15 3916/1500 3916560 391656 47431680 8 1497760 149776 Yes

04 | Keysight | Solutions for Testing Data Throughput Performance in LTE-A User Equipment - Application Note

Solution

Effectively testing data throughput performance to TR 37.901 requires test equip-ment that offers more than just the processing power necessary to keep up with the speed of modern LTE-A UE. The equipment also needs integrated test capabilities to enable full/maximum IP data measurements, integrated fading and noise, and RF/Layer 1 measurements. Finally, any test equipment used to measure data through-put performance must be flexible. As with any standard, LTE-A will evolve over time. As it does, so too will the TR 37.901 specification; it will evolve to cover higher throughput scenarios such as CA, higher-order modulation in both downlink (DL) and uplink (UL) and higher MIMO schemes. Consequently, the test equipment has to be able to support these different scenarios.

One test solution that effectively delivers on these capabilities is the UXM Wireless Test Set from Keysight Technologies (Figure 3). The UXM is a highly integrated signal-ling test set created for functional and RF design validation. By providing integrated base station and channel emulation along with flexible control, measurement capa-bility, and diagnostic ability, it eliminates the complexity generally associated with measuring data throughput. The UXM supports 450 Mbps DL/50 Mbps UL (Category 9) end-to-end IP data throughput, multiple cells, DL and UL CA, MIMO up to 4x2, and integrated fading—all of which allows users to assess design readiness with greater confidence.

The UXM also features the processing power needed to test today’s high-speed data, with two fully independent 4x2 cells that provide dedicated resources for sig-nal and protocol processing. Additionally, the UXM features ample digital bandwidth to transport the large volumes of data that comes from measuring high-speed data. The bandwidth stems from the combination of highly integrated broadband MIMO-enabled RF (100-MHz bandwidth + UL/DL support) dedicated to each cell and multiple high-speed interconnects for large digital baseband flows.

To support the needs of future UE, the UXM is specifically designed for extensibil-ity so that more cells can be added without degrading IP data performance. Its fully synchronized architecture supports either a single instrument or multiple instru-ments. A modular design, lots of expansion slots and interconnects via a switch fabric further support expansion as UE design evolves and test needs change.

Figure 3. The UXM one-box tester provides integrated sources for receiver testing, analyzers for trans-mitter testing, standards-based measurements, and a host of built-in functions to validate the UE’s functional and RF performance while on a connection. It not only provides users new insights for LTE-A, but is flexible enough to scale to meet the demands of both 4G and beyond.

TR 37.901 Data Throughput Measurement

To better understand how the UXM can be used for end-to-end data throughput testing as defined in TR 37.901, it’s critical to first understand exactly what tests are needed. The TR 37.901 document specifies application layer UE data through-put performance measurements for both HSPA and LTE. All measurements include fading and noise to simulate testing in real-world environments. The specific tests covered include File Transfer Protocol (FTP)/Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

TCP/IP is measured using FTP because it effectively tests all forms of the TCP/IP underlying protocol (e.g., SFTP and HTTP). For TR 37.901, it is specified to be tested in the DL only, UL only or alternating bi-directional. Although the most stressful UE throughput test would be concurrent bi-directional FTP, it is difficult to obtain reliable comparisons between UE’s because of the influence of TCP/IP ACK/NACK’s competing for capacity with the actual FTP data flows. UDP is mea-sured because it can be tested bi-directionally without the other direction interfer-ing, and can be used to characterize the Real-time Transport Protocol (RTP). The test can be performed in the DL only, UL only or bi-directional (concurrent).

Because these tests are used to compare different UE, there are no test limits and no pass/fail requirements are imposed. TR 37.901 also defines signal levels, various noise levels, interference, and various fading profiles. The fading profiles are used to mimic different channel conditions and can be combined with varying geometry to test a large variety of situations. TR 37.901 currently does not support UL or DL CA test.

Example: FTP Downlink TestAs an example of how the UXM can be used to test to TR 37.901, consider the case of a FTP DL test. All test cases for the LTE FTP DL are shown in Figure 4. We will look at the specific case of A.3.2.4, closed-loop spatial multiplexing testing. The test parameters for this test are listed in TR 37.901, while the test points are shown in Figure 5.

05 | Keysight | Solutions for Testing Data Throughput Performance in LTE-A User Equipment - Application Note

37.901 Title

A.3.2.1 LTE/FTP DL/PDSCH Single Antenna Port Performance (CSRS)

A.3.2.2 LTE/FTP DL/PDSCH Transmit Diversity Performance (CSRS)

A.3.2.3 LTE/FTP DL/PDSCH Open Loop Spatial Multiplexing Performance (CSRS)

A.3.2.4 LTE/FTP DL/PDSCH Closed Loop Spatial Multiplexing Performance (CSRS)

A.3.2.5 LTE/FTP DL/PDSCH Single Layer Spatial Multiplexing Performance (Port 5, UE-Specific RS)

A.3.2.6 LTE/FTP DL/PDSCH Closed Loop Spatial Multiplexing Performance (Port 7 or 8, UE-Specific RS)

A.3.2.7 LTE/FTP DL/PDSCH Dual-Layer Spatial Multiplexing Performance (Port 7 or 8, UE-Specific RS)

Figure 4. Tests cases for LTE FTP downlink testing are shown in this table.

06 | Keysight | Solutions for Testing Data Throughput Performance in LTE-A User Equipment - Application Note

To perform the closed loop spatial multiplexing testing using the UXM, several steps are required:

Step 1. Setup the UXM to respond to the UE’s reported Channel Quality Indicator (CQI), Rank Indicator (RI) and Pre-coding Matrix Indicator (PMI).

Step 2. Setup the UXM for the correct bandwidth, reference measurement channel, fading, noise, etc.

Step 3. Using the UDP client, begin UDP download for the test duration (60 seconds for static, 36.521 Annex G3.5 for faded tests—14 to 1500 seconds), see TR 37.901 Table A.3.1-1. Note that the faded test times vary depending on things like the fading profile, antenna configuration, correlation, and refer- ence measurement channel used. Also note that HSPA tests use 60 seconds for static and 164 seconds for all faded tests.

Step 4. Record the throughput, T, result for the first iteration (Figures 6 and 7).

Figure 5. This table contains the test points for closed-loop spatial multiplexing DL testing.

Figure 6. Shown here is a throughput/BLER measurement at 10 MHz (static, no noise), subtest 1, using the UXM. The maxi-mum throughput for a 10-MHz cell with the TR 37.901 test conditions is 50.73 Mbps.

Initial conditions

Channel bandwidth Note1

Test parameters for each bandwidth

Test Number Reference test point Propagation Conditions SNR (dB) Correlation

1 Note 2 Note 2 Note 2 N/A

2 LTE-1 Static No interference Low

3 LTE-2 EPA5 Note 3 Low

4 LTE-3 EVA5 20 Low

5 LTE-4 ETU70 10 Low

Note 1: See Annex B.3 for the recommended channel bandwidthNote 2: The test points is according to Table B.1.2-1in Annex B.1.2.Note 3: In the performance report, the tester shall indicate for the ‘No Interference’ condition,

the following note: In case of 'no interference', the throughput is expected to be maximal. This may be the maximum theoretical throughput or below. In the latter case it cannot be distinguished, whether UE limitations, or signal generator limitations with respect to EVM, or both contribute to this.

07 | Keysight | Solutions for Testing Data Throughput Performance in LTE-A User Equipment - Application Note

Step 5. Repeat for a further 2 to 5 iterations and calculate and record the average of the 5 iterations. Next, count and record the overall number of ACK and NACK/DTX on the PUSCH/PUCCH during the test interval. Finally, repeat the steps for each subtest as shown in Figure 5. The results of these steps are shown in Figures 8 to 15.

Figure 7. Shown here is the Channel State Information (CSI) for subtest 1 in Figure 6. As with “perfect” throughput, the result is a perfect CQI 15, Rank 2, with no noise or fading.

Figure 8. Shown here is a throughput/BLER measurement at 10 MHz (EPA5, 20-dB SNR), subtest 2, using the UXM. EPA5 in the presence of only a little noise (SNR 20 dB) results in very variable throughput and large variations in Transport Block Size (TBS), but only one or two transitions into Rank 1. In general, both codewords will be present. Within test case A.3.2.4, this step is the only one requiring averaging from one test to another, since tests can vary several Mbps from one another. All other steps result in regular repeatable results right down to 1/100th of a Mbps throughput.

Figure 9. Shown here is the CSI for subtest 2. Notice that with highly variable wide fades EPA5 results in a very wide range of CQI results and with only 20-dB SNR, most of this time will be limited to Rank 2.

Figure 10. Shown here are actual TR 37.901 results for a throughput/BLER measurement at 10 MHz (EVA5, 10-dB SNR), subtest 3, with the UXM.

Figure 11. Shown here is the CSI for subtest 3. This oscillation in Rank results in a strange CQI histogram. When Rank 2 is reported, there is a spread of CQI results. In the presence of similar noise levels, both CQI results for both code words will be roughly similar. When Rank 1 is reported, the single code word CQI values will typically be a little higher; on this histogram its CQI 10-13. Essentially, the lower SNR of 10 dB forces a much lower CQI in Rank 2 with frequent transitions into Rank 1. Switching Rank forces large swings in data rates as the UE recommends the use of either one or two codewords/streams.

08 | Keysight | Solutions for Testing Data Throughput Performance in LTE-A User Equipment - Application Note

Figure 12. Shown here are actual TR 37.901 results for a throughput/BLER measurement at 10 MHz (ETU70, 0-dB SNR), subtest 4, with the UXM. Although ETU70 would normally result in highly variable CQI, in the presence of 0-dB SNR, it results in very low throughput.

Figure 13. Shown here is the CSI for subtest 4. The 0-dB SNR also results in almost certain Rank 1, only rarely transitioning to Rank 2.

Figure 14. Shown here are actual TR 37.901 results for a throughput/BLER measurement at 10 MHz (ETU300, 0-dB SNR), subtest 5, with the UXM. Similar to ETU70, the ETU300 results in a slightly lower throughput, and Rank 1.

09 | Keysight | Solutions for Testing Data Throughput Performance in LTE-A User Equipment - Application Note

Figure 15. Shown here is the CSI for subtest 5. The ETU300 results in a slightly lower throughput and Rank 1.

Summary

Today’s wireless devices must perform in highly dynamic environments. Users expect a high-quality experience regardless of complexities created by things like higher data rates and the use of advanced technologies like MIMO and CA. These complexi-ties make testing UE data throughput performance to the TR 37.901 specification imperative. The UXM, with its processing power, digital bandwidth, ease-of-use, flex-ibility, and scalability offers the ideal solution for this challenging task. For LTE-A UE device designers, that means they now have an edge in ensuring their devices meet the data demands of wireless device users, both today and in the future.

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10 | Keysight | Solutions for Testing Data Throughput Performance in LTE-A User Equipment - Application Note

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