Spectrum & Power Measurements Using the E6474A Wireless Network Optimization Platform Application Note By Richard Komar

Transcription

1 Spectrum & Power Measurements Using the E6474A Wireless Network Optimization Platform Application Note By Richard Komar Contents Introduction...1 Band Clearing...2 Using the spectrum analyzer for band clearing...2 Site Evaluation...7 CW Power Measurements...7 Channel Power Measurement...8 Interference Control...11 Internal interference...11 Downlink interference...11 Uplink interference...11 Base station interference...11 External interference...11 Spectograph...12 Appendix: Optimization Features, Capabilities and Specifi cations...12 Events (Alarms)...12 Event confi guration...13 Basic event confi guration...13 User Events...13 Markers...15 Spectrum Noise Floor & Sensitivity Specifi cations...16 Lee s Criteria - Equation Introduction With the rapid development of wireless technologies, it has become increasingly important to maintain a high-performance and fully optimized network. Quality and reliability of service have grown exponentially in importance over the years and now greatly infl uence the customer s choice of a service provider. The E6474A Agilent drive-test system provides a low-cost, lightweight, customizable and easy-to-use platform solution for spectrum monitoring and interference detection. The system can perform the basic functions of a spectrum analyzer, but also has recording capabilities that allow data to be collected and then post-processed. With this spectrum measurement and analysis tool, the entire network can be examined, in both downlink and uplink, increasing the chance of discovering spurious transmissions. In addition, the W1314A family of receivers is designed with a signifi cantly low noise fi gure for easier noise fl oor characterization. Networks of all technologies - LTE, WiMAX, UMTS, EVDO, HSPA+ cdma2000, GSM and GPRS - can benefi t from spectrum monitoring for network optimization, troubleshooting, infrastructure installation and even manufacturing. This application note describes how the spectrum measurements contribute throughout the network lifecycle. Each stage in the cycle -- band clearing, site evaluation, optimization and quality of service -- can benefi t greatly by using the Agilent drive-test system. Figure 1. E6474A Agilent Drive-Test System

2 Band Clearing Prior to cellsite turn-up, a given area must be deemed interferer free. The intended coverage area should be driven, searching for spurious or illegal transmissions or interference. With excellent sensitivity, the spectrum analyzer enables the user to view and record interferers. Band clearing is used for the following applications: To eliminate interference prior to network turn-up To add new channels to the existing network To clear spectrum to provide space for new bands or additional channels To determine possible interferers Using the spectrum analyzer for band clearing 1. Open two spectrum analyzer views, one for uplink frequencies, one for downlink frequencies and horizontally tile them as shown in Figure 2. For more details on how to confi gure your hardware and how to use the E6474A software, refer to the User s Guide (E ) and the online help. Figure 2. Uplink and Downlink Spectrum Views 2

3 2. Open a properties panel for each spectrum analyzer view (Figures 3 and 4). Set the band and channelization for both views. Figure 3. Uplink Spectrum Analyzer View Properties Figure 4. Downlink Spectrum Analyzer View Properties 3

4 3. Open a properties panel for each spectrum analyzer measurement (Figures 5 and 6) and set the center frequency and span. Set the IF bandwidth to 200 KHz to lower the spectrum noise fl oor. Figure 5. Uplink Spectrum Analyzer Properties Figure 6. Downlink Spectrum Analyzer Properties 4

5 4. Select Event Wizard from the Confi guration pull-down menu, highlight the receiver device and highlight the Spectrum Clearing event. Press Next. 5. Since you had created two spectrum analyzer views, one for uplink and one for downlink, you will see a Resolve Additional Information screen which shows the two spectrum analyzer views. Highlight the Downlink spectrum analyzer and press Resolve button repeatedly until you see the Next option button activate. Press Next. 5

6 6. Click on Edit Event Conditions to view or change the desired threshold at which to trigger the event (in this example, the threshold was changed to -65 dbm). 7. In the Event Action view, select Add if you want to add a sound and/or message action in addition to the existent marker action. Highlight each action to customize it. Press Next. 8. Select View > System Panels > Active Events List and check Project. If you expand Project, you will see the Spectrum Clearing event and its actions. Right click on any item to change its properties. 6

7 9. Start either Live Mode or Logging Mode to capture band clearing data (Figure 7). Figure 7. Band Clearing using a Spectrum Analyzer View and an Event 10. Select View > System Panels > Event Output View to see a description of the event when it is triggered (See Figure 8). Notice all events are listed by frequency where the signal exceeded -65 dbm. Figure 8. Event Output View for Band Clearing 11. Repeat Steps 5 through 10 above for the uplink band. As an alternative to setting up an event, if not logging data, the Max Hold function can be used to monitor the network without having to constantly observe the screen. It will show the maximum values over all measurements since the option was selected. The Max Hold selection is in the Averaging pull-down menu in the spectrum analyzer measurement properties panel (Figure 9). Figure 9. The Max Hold Selection Site Evaluation Before a new cell site is installed, site evaluation tests are run and data is analyzed. These tests are done in the following manner: Setting up a test transmitter at the candidate site Driving the intended coverage area and recording the transmitted signal strength Plotting the collected data on a map corresponding to the drive route Tuning the predictive model using the drive test data Many candidate sites may be tested before choosing a cell location. However, instead of testing each location individually, the CW power analyzer can measure multiple CW signals simultaneously. Site evaluation can then be determined by a single drive of the proposed coverage area, reducing the time spent in the fi eld. CW Power Measurements Continuous Wave (CW) power is defi ned as the peak power of a transmitted signal in a user-defi ned resolution bandwidth. CW testing is the most important task executed for site 7

8 evaluation. When collecting data using the CW power analyzer, the At Least averaging technique is used to give confi dent results. In order to have 90 percent confi dence in the predictive model, the data used to tune that model needs to meet Lee s Criteria. Lee s Criteria states that during data collection, at least 50 measurements must be taken every 40 wavelengths of distance in meters. (Distance = 40λ = 40(c/f) = 14.1 m at 850 MHz, 6.3 m at 1900 MHz, and 5.7 m at 2.1 GHz.). See Lee s Criteria - Equation 1 on page 16 and Table 1 in the Appendix for additional values. The At Least averaging feature is found on the Averaging pull-down menu of the CW power analyzer measurement properties panel (Figure 10) and is an essential key to collecting accurate data. Figure 10. Averaging set to At Least In addition, when measuring multiple CW signals simultaneously, as described above, the test frequencies should be contained within 1 MHz to maintain Lee s Criteria. For example, 870, 870.2, 870.4, 870.6, and MHz would be suffi cient test transmissions. Channel Power Measurement Channel power is the total integrated power in a user-defi ned channel width. A modulated transmitter can be used in conjunction with channel power measurements to further characterize a site. Using the CW power analyzer for site evaluation testing 1. Set up a signal source that will transmit a CW signal at the desired power level. 2. Open a receiver bar chart common view (Figure 11) to use as a CW Power Analyzer view Figure 11. CW Power Analyzer View 3. Open a properties panel for the CW power analyzer view (Figure 12). Set the band, channelization, and chart title. 8

9 Figure 12. CW Power Analyzer View Properties 4. Open a properties panel for the CW power analyzer measurement (Figure 13) and set the preselector, channelization band, channelization, CW Power List Yes, User List, Averaging ( At Least), Averages (50) and Measurement by Distance (for your frequency band per Equation 1 and Table 1 in the Appendix). Figure 13. CW Power Analyzer Properties 5. Select Event Wizard from the Confi guration pull-down menu, highlight the receiver device and highlight the CW Low Power event. Press Next. 9

10 6. Click on Edit Event Conditions to view or change the desired threshold at which to trigger the event (in this example, the threshold was changed to -90 dbm). 7. In the Event Action view, select Add if you want to add a sound and/or message action in addition to the existent marker action. Highlight each action to customize it. Press Next and Finish. 8. Select View > System Panels > Active Events List and check Project. If you expand Project, you will see the CW Low Power event and its actions. Right click on the event if you want to change its properties. 9. Turn on your transmitter and turn on Live or Logging Mode, start driving and collect CW power data for your site evaluation (Figure 14). Figure 14. Site Evaluation using a CW Power Analyzer View & an Event 10. Select View > System Panels > Event Output View to see a description of the event when it is triggered (See Figure 15). Figure 15. CW Low Power Event 11. You can also set up your own custom events by referring to the Appendix for event confi guration. 10

11 Interference Control Internal interference Internally generated interference, or interference that occurs as a result of one s own network, is a major cause of performance problems. Internal interference can occur in both the uplink and downlink frequency bands either at the base station or in the intended coverage area. Downlink interference Internal interference in the downlink can be from a variety of causes such as: Adjacent channel interference (GSM) Co-channel interference (GSM) Pilot pollution (CDMA) Scrambling code pollution (UMTS) Transmitted base station noise Faulty power amplifi er stages Internally generated interference from this list will cause poor network performance. To insure quality service, the E6474A Agilent drive-test system is used to monitor pilot pollution, scrambling code pollution, adjacent channel interference and co-channel interference. Uplink interference The E6474A Agilent drive-test system can make measurements in the uplink frequency band. Internal uplink interference can be adjacent or can be co-channel interference from frequencies in reuse. Uplink interference can be very diffi cult to detect in the case of GSM, because of the time-burst transmissions. Also, carrier-to-interference (C/I) measurements will be different at different periods due to power control (when the mobile powers up or down depending on its proximity to the base station). The spectrum analyzer measurement can easily be connected to the receive antenna of the base station to make uplink interference measurements. This will display the uplink spectrum as seen by the cell s antenna. This data can be viewed, recorded, and analyzed either at the site or at a later time. The measurement procedure is similar to that of band clearing. So, for this application, simply connect the base station receive antenna to the receiver input and follow the steps in Using the spectrum analyzer for band clearing on page 2, opening only one spectrum analyzer measurement view for the uplink frequencies. Base station interference The E6474A Agilent drive-test system can be used to monitor interference at the base station. It is often necessary to troubleshoot this area when switch events have been triggered. The receiver can be used to investigate any internal station interferers. Possible interferers include RF leakage within or between cabinets, antenna base plates, cables, or connectors. External interferers can be detected by connecting the base station receive antenna directly to the receiver. The RF environment in the immediate vicinity and selected radius of the base station can then be examined on the spectrum analyzer. The spectrum analyzer can help detect both manufacturing fl aws and quality control issues. External interference External interference can occur in either the uplink or downlink frequency bands. Examples include: Paging transmitters Competitor networks (adjacent bands) Illegal transmissions Spurs and harmonics from other transmitters Radar 11

12 Industrial appliances Special mobile radios (SMR) Cordless telephones Interference caused by external sources, such as 900-MHz cordless phones, can be continuous or time-bursted. For time-bursted interferers, long-term monitoring may be required. In the example of the cordless phone, an event for a signal occurring above a particular threshold (for a given duration of time) would need to be set. This is because the cordless phone generates interference when it turns on and remains continuous for the duration of the cordless phone call. Spectograph The spectrograph view provides a shaded mapping of a spectrum type value. This is useful if you want to see if you have any period interferers in your received RF signal. Description of properties Chart Title Auto Scale? Min Max Base Color Rainbow Chart Type Color Setup Threshold 1 Threshold 2 Remove Action Enter the text you wish displayed above the chart. Enable this option to have the chart automatically scale the X and Y axis. Enter the minimum value to be used for the Y axis. Enter the maximum value to be used for the Y axis. Select the base color to be used for coloring the lines displayed on the chart. Enable this option to change the color range to be spread across the color spectrum. Select either Spectrogram or Fast Line type charts. The spectrogram type chart displays a view over time looking down onto a spectrum type data item. Using this type of display it is possible, for example, to easily see repeating interferers - these will appear as dashed marks going up the display. The Fast Line type display changes the chart to display data in the same way as the line chart view. Select this dialog to open the color and binning dialog box. This dialog box is used for adding binning regions for the displayed data. Enter the upper and lower ranges and then select the autofi ll button to automatically insert equally spaced values and colors. This positions a horizontal line on the Fast Line type chart. This positions a horizontal line on the Fast Line type chart. Select this property to open a dialog box that lists the events associated with this view. To remove an event from the view: 1. Select this property to open the events overview dialog box. 2. Select the event you wish to remove. In project mode, select the event listed under the project option. In fi le mode, select the event listed under fi le option. 3. Select the Remove button. 12

13 If an interfering signal is intermittent, the Spectrograph view can be used to visualize the spectrum signal over a scrolling time window. The width of the color bar shows the extent of the interfering frequency and the color shows the amplitude. Appendix: Optimization Features, Capabilities and Specifications Events (Alarms) Event configuration One of the major benefi ts of the drive-test system is its ability to record data. A number of complex events can be set to mark trouble spots or areas that require attention. These events can be made up of single or multiple conditions. Events can be confi gured to respond with a sound, notifi cation or a particular action when the measurement result meets a user-specifi ed criteria. Events notify the user that the drive-test system has encountered specifi c conditions, such as a power level greater than or less than a specifi ed value. Any given measurement can carry more than one event. The user can also specify a minimum duration of time before an event is triggered or re-triggered. Basic event configuration Refer to the online help for a full animated demonstration of how to create an event and how to add custom events to your views and logged data. User Events When driving a network, actions may occur that have not been pre-set as events. User events can be inserted to mark these points during data collection. In other words, if data is being recorded and an area of interference is encountered, that segment of the data can be marked and played back later for in-depth analysis. The User Event tab is within the Options dialog box. To open it, click Tool > Options on the menu bar, then click the User Event tab. It allows you to confi gure the User Event feature. User Events are events that are dynamically added during logging using a pre-defi ned function key. You can also defi ne the marker and text used for the user event. 13

14 There are two types of user events: User Event with text entry on pop-up box - When you select the assigned function key (F2 default) a dialog box appears and you can enter any text to go with the marked event. When you close the text box, even though you may have moved further on from the marker point, the text information is automatically saved with the position of the marker. A marker is added to the views in your project that support event display. The user event marker is added at the point you selected the F2 key even though the data display had moved on after you had entered your text. An event is added to the Event Output View automatically as well. 14 User Event with pre-defined text message - When you select the assigned function key (F12 default) a marker is added to your open views and the event is displayed in the event output view. However no dialog box opens and you do not have

15 to enter any text. Instead the event uses the text you defi ne in the options dialog box (see above) and this text is used for all of these types of events. This is useful if you have a known event or situation you want to mark during data logging, but do not want to keep entering the same text. During playback the text pop-up box message is displayed on the screen and remains open until you close it. To quickly edit a user event double-click on the event listed in the Active Events Lists system panel. During playback, you can cycle through the user events that occurred while logging the data. This allows you to skip directly to each marked or evented segment rather than reviewing the complete recorded section. Markers Markers can be used on spectrum traces to display the numeric value of the trace at a particular X coordinate. The following commands are features of the spectrum analyzer view: The marker controls at the top of the spectrum analyzer view are used for adding, deleting and control the markers that you can place on a signal trace. The following summarizes what each marker control does: of marker controls Add a marker Select this icon to add a marker to the chart Delete a marker Select this icon to delete the highlighted marker. Delta Select this icon to display the difference between the last two added markers. The delta is displayed under the last selected marker. 15

16 To Max To Min Center on Marker Left Peak Right Peak Auto Peak Select this icon to move the highlighted marker to the maximum peak value on the display. Select this icon to move the highlighted marker to the minimum value on the display. Select this icon to move the displayed line to the center based on the center marker. Select this icon to move the marker to the far left peak of the display Select this icon to move the marker to the far right peak of the display. Select the drop-down arrow next to this marker to enable the marker conditions. There are Auto peak off, Full trace, +/- 1 point, +/- 2 points, +/- 3 points. Spectrum Noise Floor & Sensitivity Specifications Agilent W1314A Series Receivers: Accuracy ±1.5 db typical (0 to 55 C) ±1.0 typical (20 to 30 C) Internally generated spurious noise: -120 dbm Spectrum Analyzer (30 KHz RBW) Updates / Second Span (4 MHz, IFBW 5 MHz) 25 Span (10 MHz, IFBW 5 MHz) 14 Span (25 MHz, IFBW 5 MHz) 6 Span (50 MHz, IFBW 5 MHz) 3 For complete receiver technical specifi cations, refer to the Agilent W1314A Multi-band Wireless Measurement Receiver Data Sheet, EN. Lee s Criteria - Equation 1 At least 50 measurements within 40 wavelengths: Distance = 40 wavelengths = 40(c/f) c = speed of light (300,000,000 m/s), f = transmit frequency Table 1. Values for distance measurement interval to satisfy Lee s Criteria Frequency Distance 850 MHz 14.1 m 900 MHz 13.3 m 960 MHz 12.5 m 1800 MHz 6.7 m 1900 MHz 6.3 m 2.0 GHz 6.0 m 2.1 GHz 5.7 m This information is subject to change without notice Agilent Technologies, Inc. 2010, Printed in UK, February 8, EN