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1、 White Paper White Paper Impact of Multipath Propagation on Downlink Power Control and Network Capacity 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
2、-544-8700 Fax: +1 732-544-8347 Email: 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 pro
3、duct 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 pr
4、operty 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 assum
5、es 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
6、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 . . . . . . . . . . . . .
7、 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Power Control and Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Effects of Power Control on Channel Demodulation . . . . . . . .
8、 . . . . . . . . . . . . . . . . . . . . . . . . 4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9、 . . . . . . . . . . . . . . . . . 8 Appendix A - Fade Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Impact of Multipath Propagation Effects on Channel Demodulation and Network Capacity Introduction Spirent Communications White Paper
10、 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
11、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
12、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
13、 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
14、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 abili
15、ty 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 case
16、s (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 networ
17、k 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 determinatio
18、n 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 nec
19、essary 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
20、 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 avai
21、lable 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 direc
22、t 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 Avera
23、ge 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 spec
24、ified 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 cou
25、ld 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
26、 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 fo
27、r 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 bot
28、h 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 ma
29、de. 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
30、 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 ba
31、se 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
32、 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 adj
33、usts 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 configu
34、ration 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 rol
35、e 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 acros
36、s 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 focu
37、s 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
38、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 developme
39、nt 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 Av
40、erage 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 Contro
41、l 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 th
42、e 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 p
43、ower 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 cond
44、itions 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
45、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 d
46、B(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 AUE BUE CBLER 1.02% BLER 1.02% BLER 0.99% BLER 1.15% BLER 1.11% BLER 0.99% Te
47、st 1Test 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 t
48、o 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 criteri
49、a. 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%
50、. 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
51、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
52、. 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
53、Constant at -60 dBm/3.84 MHz)Average DPCH_Ec/Ior (dB)UE AUE BUE 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 N
54、etwork 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
55、(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
56、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,
57、 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
58、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 origin
59、ally 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 (U
60、L/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-16-15-14-13-12-11-10-9-8-7-6Ior/Ioc = -0.4 dBInit. DPCH = -8.9 dB(Fade Model = TU3)Ior/Ioc = -0.4 dBInit. DPCH = -8.9 dB(Fade Model =
61、 ITU-A3)Ior/Ioc = -0.4 dBInit. DPCH = -8.9 dB(Fade Model = TU50)Ior/Ioc = -0.4 dBInit. DPCH = -8.9 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)Ave
62、rage DPCH_Ec/Ior (dB)UE AUE BUE CTest 7 Test 8 Test 9 Test 10 BLER 1.08% BLER 1.05%BLER 1.03%UE A & B Failed to Converge BLER 1.28%BLER 1.05%BLER 1.11%BLER 0.98% BLER 1.17% BLER 0.99%UE B Failed to Converge Impact of Multipath Propagation on Downlink Power Control and Network Capacity References Spi
63、rent Communications White Paper 8 It is interesting to speculate on the impact that testing under this real-world representative multipath model will have on the performance of high speed data calls. Optimization of power control in Release 99 is critical to successful deployment of HSPDA, which use
64、s that available power. This paper focused on analyzing the amount of power that would be needed to obtain a specified BLER. However, there are a whole variety of performance tests cases (for example changing the BLER Target, the Ior/Ioc ratios, the data rates, etc) outside of what this paper has sh
65、own that can be used to truly evaluate how a UE will perform prior to being deployed on a live network. Simply taking the Average DPCH_Ec/Ior levels from the 7.8.1 testing shown into account, it is apparent that UE C performed better than the others (i.e. required the least amount of power to obtain
66、 a specified BLER target) over the entire range of environmental conditions tested. When compared to UE A, UE C on average required 1.5 dB less power to achieve the same BLER target regardless of the environmental conditions tested. Therefore, taking only the downlink resources into account, if ther
67、e are two cells with the same configuration and usage model, one with only UE A models allowed on it and the second only UE C models allowed on it, the cell with only UE C models could likely support 40% more capacity at the same Quality of Service, given the difference in average power. Of course t
68、his analysis does not take other factors, such as the bandwidth availability on the Uplink, into account. However it does provide a clear indication that, in a WCDMA network, differences in UE performance can significantly impact available power, in an environment where even 1 dB of additional avail
69、able power can make a large impact on network capacity. References 1. 3GPP Technical Specification 34.121, V6.1.0 (2005-06) 3rd Generation Partnership Project, Technical Specification Group Radio Access Network, Terminal Conformance Specification, Radio Transmission and Reception (FDD) 2. 3GPP Techn
70、ical Report 25.943, V 6.00 (2004-12) 3rd Generation Partnership Project, Technical Specification Group Radio Access Network, Deployment Aspects 3. Recommendation ITU-R M.1225 (1997) Guidelines for Evaluation of Radio Transmission Technologies for IMT-2000.Impact of Multipath Propagation Effects on C
71、hannel Demodulation and Network Capacity Appendix A Fade Models Spirent Communications White Paper 9 Appendix A Fade Model Details The standard Section 7.8.1 conformance test cases utilize multipath propagation conditions which are defined in Annex D of TS 34.121. Table A-1 lists the details (Relati
72、ve Delay, Relative Power, and Vehicle Speed) for the Multipath Propagation Model (Case 4) which is used in the standard conformance testing for the 7.8.1 test cases. Table A-1. Multipath Propagation Conditions for Standard Conformance Testing for Test 7.8.1 Case 4 Speed = 3 km/hr (Band 1) Tap Rel. D
73、elay (ns) Rel. Power (dB) 1 0 0 2 976 -10 Two additional multipath propagation models can be used for additional investigations into determining the performance of the UE platform under more representative real-world conditions. Table A-2 contains the details of two additional multipath propagation
74、models from 3GPP and the ITU that can be used to test under these conditions. Table A-2. Additional Representative Real World Multipath Propagation Conditions Typical Urban Channel Model (TUx) (x = speed) (From 3GPP TR 25.943 - Simplified using time resolution of 195 ns - Refer to Annex B). ITU Chan
75、nel Model (ITU-Ax) (x = speed) Outdoor to Indoor Pedestrian Model (Channel A) (From Rec. ITU-R M.1225) Tap Rel. Delay (ns) Rel. Power (dB) Rel. Delay (ns) Rel. Power (dB) 1 0 -5.7 0 0 2 195 -7.6 110 -9.7 3 585 -4.4 190 -19.2 4 975 -13.4 410 -22.8 5 1170 -16.3 6 1365 -12.4 7 1560 -14.5 8 1755 -18.5 9
76、 1950 -17.9 10 2145 -24.3 The 3GPPs Typical Urban channel model, defined in TR 25.943, is intended to establish a channel model for use in deployment evaluation of WCDMA UEs. Note that, for the purposes of this testing, the 20-path TUx fade model defined in Section 5.1 of TR 25.943 was simplified to
77、 the 10-path fade model shown in Table A-2, using the technique specified in Annex B of TR 25.943, with a time resolution of 195 ns. The ITUs Channel Models, defined in ITU-R M.1225, were developed to provide a representative environment in which to test the performance of radio transmission Impact of Multipath Propagation Effects on Channel Demodulation and Network Capacity Appendix A Fade Models Spirent Communications White Paper 10 technologies for IMT-2000. The model selected for this testing is the Outdoor to Indoor and Pedestrian Test Environment Channel A.
