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Signals Ahead: By the Light of the Silvery Moon

  • ID: 2886029
  • Report
  • July 2014
  • Region: Global
  • 41 Pages
  • Signals Research Group, LLC
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Quantifying The Impact Of 4X2 Cl-MIMO In A Commercial LTE Network
When operators first launched LTE starting back in late 2009 they used a standard 2x2 MIMO implementation, meaning two transmit / receive antennas at the cell site and two receive antennas in the mobile devices. And until last year they also relied on Cat 3 (Category 3) devices which under certain circumstances became the bottleneck that limited downlink data rates in a 2x20 MHz network. The resultant data rates were still very impressive and dramatically higher than they were with DC-HSDPA - thanks, in large part to the wider channel bandwidths - but in hindsight they could have been even better.

Thanks to the technology advancements that the industry has made for infrastructure and devices / chipsets and the initiatives taken by some forward-thinking operators, LTE is undergoing another round of performance improvements. It is also providing many of these improvements under the auspices of the Release 8 standard. One noteworthy example, which also happens to be the subject of this Signals Ahead report, is the introduction of 4x2 antenna schemes.

KEY OBSERVATIONS INCLUDE THE FOLLOWING:

- The benefits of 2x20 MHz LTE, a Category 4 smartphone, and 4x2 CL- MIMO cannot be ignored
- peak data rate of 143.8 Mbps in the downlink (per TTI) and 46.93 Mbps in the uplink (per second)
- The median MAC (Media Access Control) Layer downlink throughput was 36.42 Mbps and the median MAC Layer uplink throughput was 28.15 Mbps - both values include 2x2 OL-MIMO results
- the downlink throughput exceeded 10 Mbps for nearly 95% of the time and the uplink throughput exceeded 10 Mbps for 87% of the time - both values include 2x2 OL-MIMO results
- The clincher for bullish views on a 4x2 cell site configuration appears after we filter the 4x2 and 2x2 results.
- Substantially higher uplink user data rates, in particular closer to the cell edge
- Far more efficient scheduling of uplink network resources, again when the mobile device was closer to the edge of cell
- A clear indication of improved battery life throughout a large portion of the cell
- Meaningful improvements in downlink data rates near the edge of the cell
- Consistent with our earlier study, 4x2 CL-MIMO delivers strong gains versus transmit diversity for a wide range of network conditions - perhaps even a greater range than we previously observed with 2x2 OL-MIMO.

SIGNALS AHEAD SUBSCRIPTION INFORMATION

This report is included as part of a subscription to Signals Ahead or it can be purchased separately.

Signals Ahead is a research-focused product that is published on a periodic basis. Its clientele include all facets of the wireless ecosystem, including some of the largest mobile operators, the top handset suppliers, the major infrastructure vendors, subsystem suppliers, semiconductor companies and financial institutions, including Wall Street, Private Equity and Venture Capitalists, spread across five continents.
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1.0 Executive Summary

2.0 Key Conclusions and Observations

3.0 Detailed Results - Uplink Performance

4.0 Detailed Results - Downlink Performance (4x2 CL-MIMO versus 2x2 OL-MIMO)

5.0 Detailed Results - Downlink Performance (4x2 CL-MIMO versus 4x2 Transmit Diversity)

6.0 Test Methodology

7.0 Final Thoughts

8.0 Appendix

Index of Figures

Figure 1. LTE FDD 20 MHz Channel Downlink Throughput Distribution - CDF and Pie Charts

Figure 2. LTE FDD 20 MHz Channel Uplink Throughput Distribution - CDF and Pie Charts

Figure 3. North Dallas Texas Drive Routes - geo plot

Figure 4. Category 4 Device Performance in a 4x2 CL-MIMO Network

Figure 5. Uplink MAC Layer Throughput Versus RSRP- 4x2 and 2x2 Results

Figure 6. Uplink MAC Layer Throughput Versus Downlink Path Loss - 4x2 and 2x2 Results

Figure 7. Uplink Resource Block Allocation Versus Downlink Path Loss - 4x2 and 2x2 Results

Figure 8. Uplink MCS Allocation Versus Downlink Path Loss - 4x2 and 2x2 Results

Figure 9. Power Headroom Versus Downlink Path Loss - 4x2 and 2x2 Results

Figure 10. Transmit Power Versus Downlink Path Loss - 4x2 and 2x2 Results

Figure 11. Power Headroom Versus Downlink Path Loss with Uplink MAC Layer Throughput - 3 Dimensional Scatter Plot of 4x2 Results

Figure 12. Power Headroom Versus Downlink Path Loss with Uplink MAC Layer Throughput - 3 Dimensional Scatter Plot of 2x2 Results

Figure 13. Uplink Resource Block Allocation Versus Downlink Path Loss with Power Headroom - 3 Dimensional Scatter Plot of 4x2 Results

Figure 14. Uplink Resource Block Allocation Versus Downlink Path Loss with Power Headroom - 3 Dimensional Scatter Plot of 2x2 Results

Figure 15. Downlink MAC Layer RB Adjusted Throughput Versus Downlink Path Loss - 4x2 CL-MIMO and 2x2 OL-MIMO Results

Figure 16. Downlink MAC Layer RB Adjusted 4x2 CL-MIMO Throughput Performance Advantages Versus 2x2 OL-MIMO - Relative and Absolute

Figure 17. Downlink MAC Layer Throughput Versus Downlink Path Loss - 4x2 CL-MIMO and 2x2 OL-MIMO Results

Figure 18. Downlink MAC Layer RB Adjusted Throughput Versus Downlink Path Loss Scatter Plot - 4x2 CL-MIMO and 2x2 OL-MIMO Results

Figure 19. Downlink Physical Layer RB Adjusted Throughput Versus RSRP Including Individual Code Word Contributions (average values) - 4x2 CL-MIMO and 2x2 OL-MIMO Results

Figure 20. Downlink Physical Layer RB Adjusted Throughput Versus RSRP Including Individual Code Word Contributions (median values) - 4x2 CL-MIMO and 2x2 OL-MIMO Results

Figure 21. Downlink Physical Layer RB Adjusted Throughput Versus SINR Including Individual Code Word Contributions (median values) – 4x2 CL-MIMO and 2x2 OL-MIMO Results

Figure 22. Downlink MAC Layer RB Adjusted Throughput Versus RSRP Scatter Plot – 4x2 CL-MIMO and 2x2 OL-MIMO Results

Figure 23. SINR Versus Downlink Path Loss with RB Adjusted MAC Layer Downlink Throughput – 3 Dimensional Scatter Plot of CL-MIMO 4x2 Resultsan values) – 4x2 CL-MIMO and 2x2 OL-MIMO Results

Figure 24. SINR Versus Downlink Path Loss with RB Adjusted MAC Layer Downlink Throughput – 3 Dimensional Scatter Plot of OL-MIMO 2x2 Results

Figure 25. Downlink Physical Layer RB Adjusted Throughput Versus RSRP Including Individual Code Word Contributions (average values) – 4x2 CL-MIMO and 4x2 Transmit Diversity Results

Figure 26. Downlink Physical Layer RB Adjusted Throughput Versus RSRP Including Individual Code Word Contributions (median values) – 4x2 CL-MIMO and 4x2 Transmit Diversity Results

Figure 27. Downlink MAC Layer Throughput Versus RSRP – 4x2 CL-MIMO and 4x2 Transmit Diversity Results

Figure 29. Downlink MAC Layer RB Adjusted Throughput Versus RSRP Scatter Plot – 4x2 CL-MIMO and 4x2 Transmit Diversity Results

Figure 30. XCAL in Action

Figure 31. Downlink Path Loss by Groupings of Power Headroom Values – 4x2 and 2x2 Results

Figure 32. Power Headroom Versus RSRP – 4x2 and 2x2 Results

Figure 33. Downlink Path Loss by Groupings of Downlink Throughput Values – 4x2 CL-MIMO and 2x2 OL-MIMO Results

Figure 34. Uplink Resource Block Allocations Versus Downlink Path Loss with Transmit Power - 3 Dimensional Scatter Plot of 4x2 Results

Figure 35. Uplink Resource Block Allocations Versus Downlink Path Loss with Transmit Power - 3 Dimensional Scatter Plot of 2x2 Results

Figure 36. PUSCH Transmit Power per RB Versus Downlink Path Loss with Uplink MAC Layer Throughput - 3 Dimensional Scatter Plot of 4x2 Results

Figure 37. PUSCH Transmit Power per RB Versus Downlink Path Loss with Uplink MAC Layer Throughput - 3 Dimensional Scatter Plot of 2x2 Results

Figure 38. Downlink MAC Layer Throughput Versus RSRP - 4x2 CL-MIMO and 2x2 OL-MIMO Results
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The combination of 2x20 MHz LTE, Category 4 devices, and Closed Loop MIMO with four transmit/receive antennas at the cell site delivers a number of noteworthy benefits. The headline peak data rate of 143.8 Mbps in the downlink (per TTI) and 46.93 Mbps in the uplink (per second) are noteworthy, but the real benefits are observed elsewhere.

Due to the presence of four receive antennas at the cell sites, we documented substantial benefits in the uplink performance, including much higher user data rates, far more efficient scheduling of network resources, and clear indications that the battery life of the mobile device was greatly enhanced. Generally, the benefits increased the further the mobile device was from the center of the cell, but the improvements in battery life were observed throughout a large portion of the cell. We also identified meaningful benefits in the downlink performance, again closer to the edge of the cell. And since all LTE mobile devices inherently support the 4x2 cell site configuration, the benefits of 4x2 are immediate and applicable to the entire installed base of mobile devices.

Although we haven’t quantified the economic benefits associated with the 4x2 cell site configuration, it is difficult to imagine a scenario in which the ROI wouldn’t be extremely favorable. The ease of upgrade for legacy hardware, the impact on the cell site, including the logistics of mounting the new hardware (if required) and the impact on the lease agreement, could be mitigating factors that should be considered. Nonetheless, the triple whammy of higher user data rates, increased network efficiency, and a longer battery life shouldn’t be dismissed.

When operators first launched LTE starting back in late 2009 they used a standard 2x2 MIMO implementation, meaning two transmit / receive antennas at the cell site and two receive antennas in the mobile devices. And until last year they also relied on Cat 3 (Category 3) devices which under certain circumstances became the bottleneck that limited downlink data rates in a 2x20 MHz network. The resultant data rates were still very impressive and dramatically higher than they were with DC-HSDPA – thanks, in large part to the wider channel bandwidths – but in hindsight they could have been even better.

Thanks to the technology advancements that the industry has made for infrastructure and devices/ chipsets and the initiatives taken by some forward-thinking operators, LTE is undergoing another round of performance improvements. It is also providing many of these improvements under the auspices of the Release 8 standard. One noteworthy example, which also happens to be the subject of this Signals Ahead report, is the introduction of 4x2 antenna schemes. With 4x2 MIMO there are 4 transmit antennas at the cell site along with 2 receive antennas in the mobile devices. More importantly, there are also 4 receive antennas at the cell site which can create a diversity gain and lead to higher uplink performance. Throughout this report we use 4x2 to refer to the sites with 4 transmit/ receive antennas although we recognize this nomenclature could be a bit confusing to some readers.

In the United States, T-Mobile is leading the charge by upgrading its legacy 2x2 cell site configuration to support 4x2 capabilities. Additionally, it is using CL-MIMO (Closed Loop MIMO) at these upgraded cell sites instead of the more widely adopted OL-MIMO (Open Loop MIMO). Based on our request, T-Mobile provided logistical support by identifying the location of a 4x2 cluster in North Dallas, Texas and it reconfigured the ~120 km cluster on the second night of testing by “turning off” MIMO so that we could evaluate the performance of transmit diversity. The cluster and 2 much of the surrounding area also leveraged 2x20 MHz radio channels which enabled much higher data rates than what we have previously observed in our domestic testing campaigns. T-Mobile also provided us with a Datum license and access to a Datum UDP server so that we could generate massive amounts of downlink and uplink data traffic. Spirent Communications is the supplier of the Datum tool and we have used it numerous times in the past.
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