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White Paper Impact of Multipath Propagation on Downlink Power Control and Network Capacity September 2005 V 1.0 Spirent Communications, Inc. 541 Industrial Way West Eatontown, New Jersey 07724 USA Tel: +1 732-544-8700 Fax: +1 732-544-8347 Email: wireless.sales Web: North America +1-800-927-2660 Europe, Middle East, Africa +33-1-6137-2250 Asia Pacific +852-2166-8382 All Other Regions +1-818-676-2683 Copyright 2005 Spirent Communications, Inc. All Rights Reserved. All of the company names and/or brand names and/or product names referred to in this document, in particular, the name “Spirent” and its logo device, are either registered trademarks or trademarks of Spirent plc and its subsidiaries, pending registration in accordance with relevant national laws. All other registered trademarks or trademarks are the property of their respective owners. The information contained in this document is subject to change without notice and does not represent a commitment on the part of Spirent Communications. The information in this document is believed to be accurate and reliable, however, Spirent Communications assumes no responsibility or liability for any errors or inaccuracies that may appear in the document. Spirent Communications White Paper 1 Impact of Multipath Propagation on Downlink Power Control and Network Capacity This White Paper provides a discussion of observed downlink power control performance differences in WCDMA mobile devices tested under varying multipath conditions. These differences indicate that standards-based Conformance testing alone does not provide a complete picture of the impact of mobile device performance on network capacity. Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Power Control and Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Effects of Power Control on Channel Demodulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Appendix A - Fade Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Impact of Multipath Propagation Effects on Channel Demodulation and Network Capacity Introduction Spirent Communications White Paper 3 Introduction Successful commercial deployment of WCDMA is subject to the availability of mobile devices or user equipment (UE) that can provide an end-user experience that exceeds that currently available on existing GSM/GPRS/EDGE networks. Legacy GSM mobile device test methodologies are focused on traditional Conformance testing. These Conformance test cases are used by the operators and device manufacturers to ensure that the device meets a minimum acceptable level of RF performance. Although this approach is widely accepted throughout the GSM and UMTS communities, a more progressive, or integrated testing approach is required for Release 99 WCDMA devices (or user equipment (UE) that evaluates the performance of the UEs to ensure a successful launch. Test solutions for WCDMA must attempt to characterize the performance of the UE over a wide range of scenarios that are representative of typical end-user experiences. While Conformance testing plays a valuable role in establishing the minimum acceptable baseline, it does not provide a complete picture of just how a UE will perform when operating on a live network. In an environment in which better performing UEs can minimize the interference and/or require less power, both of which will have a direct impact on cell capacity, additional performance testing needs to be conducted. Performance testing offers the ability to benchmark the performance of multiple UEs beyond Conformance levels. In addition, it also offers the ability to determine how a UE will perform in typical end-to-end mobile scenarios, providing insight into how its performance will impact the quality of the network. Commercially deployed UEs, as might be expected, typically exceed the Pass/Fail criteria that are set in the standard Conformance test cases (which are specified in TS 34.121 1). Better performing UEs are likely to have a greater positive impact on network performance (in terms of cell capacity for example), therefore testing beyond the minimum Conformance requirements is necessary to determine which UEs perform better on a live network and by what margin. Being able to conduct this type of testing on lab-based test systems that offer repeatable testing scenarios will also allow for early diagnosis of potential performance and network capacity related issues prior to deployment of the UE on the live network. Capacity determination on WCDMA networks differs greatly from the GSM networks. The capacity available on a WCDMA cell is dependent on the level of acceptable performance needed by all the UEs using the cell simultaneously. Maximum capacity on a WCDMA cell translates directly to all UEs using the minimum power level necessary to achieve the minimum acceptable quality level. This indicates that the ability to precisely control the power levels needed by all of the UEs is a critical tool to optimize the network in terms of capacity. If a particular UE requires less power to achieve or maintain a specific Block Error Rate (BLER) target, that additional power can be redistributed to other UEs in the cell, which results in increased capacity. Precise Power Control will become even more important when networks introduce High Speed Packet Data Access (HSDPA) capability, which utilizes remaining or unused power available on the cell for high speed packet data connectivity. Impact of Multipath Propagation on Downlink Power Control and Network Capacity Power Control and Capacity Spirent Communications White Paper 3 It follows therefore that power control is one key area in which UE Performance could have a direct impact on a WCDMA network. Precisely managing the power levels needed by UEs will result in increased capacity on the network and also allow for more high speed packet data connections when HSDPA is introduced. To investigate this further, this paper analyzes the amount of power (in terms of Average DPCH_Ec/Ior) needed by different UE platforms to converge to a specified link quality which is set by the network, in the presence of various multipath propagation conditions. The goal of this analysis is to determine if performance differences exist between UEs in their ability to achieve a specified link quality level when their power control algorithm is active.Power Control and Capacity Power control is one of the most important concepts in WCDMA networks. Without power control (especially in the uplink) a single UE located close to the base station and transmitting at a high power could easily impact the performance of other UEs operating at the fringe (i.e. the near-far problem) or even block the entire cell, as shown in Figure 1. Figure 1. Power Control Example (Near-Far Problem) If there were no power control mechanism then it would be quite possible for UE 1 to over power UE 2, which is located at the fringe. This would reduce the cell coverage and capacity while at the same time block UE 2 from successfully communicating with the base station. Now although WCDMA networks support Open Loop Power Control, it is basically used to provide a coarse initial power setting for the UE at the beginning of the connection. In order to truly optimize the network to achieve maximum capacity, Closed (Inner) Loop Power Control must be used to equalize the received power per bit for all the mobiles that are active on the cell at all times. Inner Loop Power Control is used on both the uplink and the downlink. In the uplink, the base station performs frequent estimates of the received Signal-to-Interference Ratio (SIR) and calculates a target SIR level. It then compares the target value with the measured value and instructs the UE accordingly based on the level comparison made. In the uplink Inner Loop Power Control is intended to prevent power imbalances in the signal levels received at the base stations in the network. Reduced Coverage (due to interference)P1 LevelsP2 Levels UE 1UE 2Planned CoverageImpact of Multipath Propagation on Downlink Power Control and Network Capacity Effects of Power Control on Channel Demodulation Spirent Communications White Paper 4 In the downlink, the same Inner Loop Power Control technique is used; however the need is different in this direction. In the downlink, all the signals originate from the same point of origin (i.e. the base station), but the power level needs of each UE in the cell will be different based on their location, the multipath propagation conditions, and the services required. To ensure that the UEs are able to communicate successfully with the base station, each UE must determine its power needs based on the Radio Access Bearer (RAB) it intends to use. That information is then reported so that the correct amount of power is allocated on a per-UE basis to ensure optimal use of the available bandwidth. One final important area of power control is Outer Loop Power Control. Outer Loop Power Control adjusts the target SIR at the base station based on the needs and the desired quality levels of the individual radio links. The required or desired SIR is dependent on the multipath propagation conditions for the UE under test. Since the environment is constantly changing, choosing a worst-case configuration for setting the SIR would waste a large amount of capacity, especially with low-speed connections. The best approach to maximum capacity is therefore to allow the target SIR value to float around the minimum value that just fulfills the specified target quality. One way to investigate the role played by power control is to test the UEs ability to manage the average power level (DPCH_Ec/Ior) needed to achieve a specified QoS level in the presence of various multipath propagation conditions. Effects of Power Control on Channel Demodulation Performance baseline capability comparisons across different UE platforms can be determined using test case 7.8.1 (Power Control in the Downlink) from TS 34.121. This test establishes the UEs ability to converge to a required link quality set by the network, while using the lowest possible power in the downlink. The standard conformance tests focus on ensuring that the power level required to achieve the specified BLER target is below a pre-configured threshold 90% of the time. However, the average DPCH_Ec/Ior level (which is not required as part of the Standard Conformance Test) can be calculated and used as comparative data when analyzing the performance of UEs on the network with power control enabled. This test methodology, although focused on different UE platforms (from different vendors), could very easily be applied to different core UE platforms being developed by a UE manufacturer, or even to firmware upgrades under development for the same platform. The standard Section 7.8.1 Conformance test cases employ multipath propagation conditions which are defined in Annex D of TS 34.121 (refer to Appendix A of this paper for additional details on the multipath propagation conditions). Figure 2 contains both the BLER and the Average DPCH_Ec/Ior levels needed by each of the three UEs tested, when configured for a 1% BLER Target (+/- 0.3), using the Standard Conformance Fade Model (Case 4) for the environmental conditions. Impact of Multipath Propagation on Downlink Power Control and Network Capacity Effects of Power Control on Channel Demodulation Spirent Communications White Paper 5 Figure 2. Performance using the Standard Conformance Test Case Conditions In analyzing the enhanced results (i.e. the Average DPCH_Ec/Ior levels) when testing with the standard conformance conditions, it is apparent that UE C performs the best. On average UE C needed approximately 1.5 dB less power than UE A to obtain the same BLER target. One key aspect to remember when analyzing these results is that the downlink power control algorithm in the UE under test is active during test case 7.8.1; therefore any differences observed in power levels between UE platforms are analogous to the UE performance on a live network under similar faded conditions. Although UE C appears to have performed the best under the standard conformance test cases, additional investigations using more representative real-world multipath propagation conditions need to be conducted. Two additional fade models, the Typical Urban channel model defined in TR 25.493 2 and one of the ITU Channel Models defined in ITU-R M.1225 3 can be used for this analysis (refer to Appendix A for additional details on these multipath channel models). Figure 3 contains the results for the three UEs tested, when configured for a 1% BLER Target (+/- 0.3), using the TU and ITU-A Fade Models with high Signal to Interference (Ior/Ioc) ratios.-22-21-20-19-18-17-16-15-14-13-12Ior/Ioc = +9.6 dBInit. DPCH = -15.9 dB(Fade Model = Case 4)Ior/Ioc = -0.4 dBInit. DPCH = -8.9 dB(Fade Model = Case 4)Test Case Configurations Average DPCH_Ec/Ior Levels (dB) w/ BLER Target = 1% (+/- 0.3) for 7.8.1 (Data Rate = 12.2 Uplink & Downlink & Ioc Level Constant at -60 dBm/3.84 MHz)Average DPCH_Ec/Ior(dB)UE A UE B UE CBLER 1.02% BLER 1.02% BLER 0.99% BLER 1.15% BLER 1.11% BLER 0.99% Test 1 Test 2 Impact of Multipath Propagation on Downlink Power Control and Network Capacity Effects of Power Control on Channel Demodulation Spirent Communications White Paper 6 Figure 3. TU & ITU Channel Model Test Case Results with High Ior/Ioc Ratios Analyzing the results, once again UE C appears to have performed the best even under these more representative real-world multipath propagation conditions. Again, on average UE C required 1.5 dB less power when compared to UE A to satisfy the same target BLER. In the case of Test 4 UE A was not even able to satisfy the initial convergence criteria. In addition, the BLER values calculated when UE C was tested are much more consistent when compared to the other UEs. For example in Test 4, UE B only needed less then 1 dB more power when compared to UE C, however it converged to a BLER of 1.25% given a target of 1%, while UE C converged to .98%. In Test 6, although UE B was able to initially converge it was not able to satisfy the target BLER and thus failed the test requirements, resulting in a BLER of 1.45% as shown in Figure 3. Additional testing can be conducted using the same multipath conditions, but at lower Ior/Ioc ratios. Figure 4 contains the results for the three UEs tested, when configured for a 1% BLER Target (+/- 0.3), using the TU and ITU-A Fade Models with lower Signal to Interference (Ior/Ioc) ratios.-24-23-22-21-20-19-18-17-16-15-14-13-12Ior /Ioc = +9.6 dBInit. DPCH = -15.9 dB(Fade Model = TU3)Ior/Ioc = +9.6 dBInit. DPCH = -15.9 dB(Fade Model = ITU-A3)Ior/Ioc = +9.6 dBInit. DPCH = -15.9 dB(Fade Model = TU50)Ior/Ioc = +9.6 dBInit. DPCH = -15.9 dB(Fade Model = ITU-A50)Test Case Configurations Average DPCH_Ec/Ior Levels (dB) w/ BLER Target = 1% (+/- 0.3) for 7.8.1(Data Rate = 12.2 Uplink & Downlink & Ioc Level Constant at -60 dBm/3.84 MHz)Average DPCH_Ec/Ior(dB)UE A UE B UE CBLER 1.00% BLER 1.01%BLER 0.97%UE A Failed to Converge Test 3 Test 4 Test 5 Test 6 BLER 1.25%BLER 0.98%BLER 1.03%BLER 1.07%BLER 0.98% BLER 1.14% BLER 1.45%(Fail) BLER 0.99%Impact of Multipath Propagation on Downlink Power Control and Network Capacity Conclusion Spirent Communications White Paper 7 Figure 5. ITU-A Channel Model Results Figure 4. TU & ITU Channel Model Test Case Results with Low Ior/Ioc Ratios The results shown in Figure 4 continue to support the previous observations in this paper. Regardless of the configuration (i.e. Fade Model used, Ior/Ioc level setting, or Initial DPCH value) UE C still requires the lowest power levels (when compared to the other UEs) necessary to achieve the target BLER level. In Test 8, UE C is the only UE that was successfully able to converge to the specified target. In Test 10, UE C not only converges to the specified target, it does so using 3 dB less power then UE A, while UE B was once again unable to successfully initially coverage. So although all 3 UE platforms tested pass the standard conformance tests, there is a clear benefit, at least in terms of lower power levels, to using UE C on a live network. Conclusion Network operators and UE manufacturers are concerned with predicting the performance of the UE and its impact on the end-user experience and network capacity. While Conformance testing attempts to offer a systematic approach to establishing a performance baseline, UEs truly need to be evaluated past the standard conformance specifications so that performance benefits can be determined, especially when operating in more real-world multipath conditions in the lab prior to deployment. Although the ITU-A Pedestrian multipath propagation model was originally specified for testing IMT-2000-based technologies, it is important to note that it has also been incorporated in TS 34.121 as one of the multipath propagation models to be used when evaluating the performance of HSPDA. Looking back on the results, which are based on a low Radio Access Bearer (UL/DL = 12.2) typically used for Voice, this multipath propagation model clearly impacts the performance of the UE. UE A & UE BUnable to ConvergeUE BUnable to Converge-16-15-14-13-12-11-10-9-8-7-6
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