Application Notes. LTE Downlink Coverage Mapping using a Base Station Analyzer Measuring LTE Modulation Quality Over-The-Air with a Handheld

Transcription

1 Application Notes LTE Downlink Coverage Mapping using a Base Station Analyzer Measuring LTE Modulation Quality Over-The-Air with a Handheld

2 LTE Downlink Coverage Mapping Carriers are moving to Long Term Evolution (LTE) to deliver the exponential increases in data traffic that the market is demanding. Primary coverage maps are in most cases being developed with drive test systems that are so expensive and difficult to operate that most carriers have only a few of them. Yet, base station technicians and RF engineers frequently receive reports of coverage problems in areas under their responsibility. They don t want to and in many cases can t wait for a drive-test system to become available. They want to diagnose the problem as soon as possible so they can begin working on a fix. Anritsu has addressed this problem by providing coverage mapping as part of the LTE measurement options for handheld instruments such as the Anritsu BTS Master MT8221B. These instruments are currently used by many technicians and engineers servicing cellular base stations and are therefore readily available to help address coverage problems. By equipping their existing instruments with option 546 (LTE Over-the-Air Measurements), technicians will now be able to perform LTE coverage mapping whenever the need arises. The instrument can automatically measure and save the Sync Signal power from up to 6 Base Stations or sectors, also known as Evolved NodeBs (enodebs), every 5-10 seconds. If desired, the modulation quality of the strongest transmitter can also be measured and stored at the same time. Later, users can upload the data to a computer and view it on a map showing the coverage at each measured point with colour codes. Users can drill down on each point to see the detailed data LTE Technology Overview Service Layer USM Messaging Charging Content & Application Other Enablers B2B Control Core Network Internet SIP PSTN MSC IP Backbone MGW PLMN Access Network IP Access Network A-GW Packet Delivery Control A-GW IP Access Network xdsl WLAN WiMAX 2/3G/HSPA/ LTE Figure 1: LTE provides mobile access to all-ip core Most mobile carriers in the United States and many worldwide carriers have announced plans to convert their networks to LTE in order to increase the capacity and speed of their mobile networks. LTE is the mobile access network standard of the Next Generation Network (NGN) that provides a complete range of communications services on a flat all-ip core that interconnects multiple access technologies and provides a consistent user experience regardless of the access 2 LTE Downlink Coverage Mapping with a Handheld

3 method. LTE supports peak data rates of up to 100 Mbps on the downlink and 50 Mbps on the uplink when using a 20 MHz channel bandwidth, a single transmit and two receive antennas at the user equipment (UE) and two transmit and receive antennas at the base station. LTE uses Orthogonal Frequency Division Multiple Access (OFDMA) for the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) for the uplink. OFDMA is a variant of Orthogonal Frequency Division Multiplexing (OFDM) which splits the carrier frequency bandwidth into many small subcarriers spaced at 15 khz and then modulates each individual carrier using the QPSK, 16-QAM or 64-QAM digital modulation formats. OFDMA differs from OFDM in that multiple users share the bandwidth at each point in time, with each user assigned a set of subcarriers organized into groups called Resource Blocks. OFDMA subcarriers each transport unique data while in SC-FDMA data spreads across multiple subcarriers. Downlink physical signals include the Reference Signal (RS), Primary Synchronization Signal (P-SS) and Secondary Synchronization Signal (S-SS). The RS is used for downlink channel estimation. UEs use the P-SS for timing and frequency acquisition during cell search. UEs use the S-SS in cell search and for finer timing alignment. A Practical Coverage Mapping Tool Technicians responsible for installing and maintaining LTE enodebs are frequently called upon to address issues such as reports of dead spots or dropped calls. Diagnosing the problem requires that technicians take signal strength and sometimes modulation quality estimates over a wide range of locations. The drive test systems that are used for large scale coverage mapping during the implementation phase are usually too expensive, bulky and complicated for use as a practical troubleshooting tool. Carriers typically have a few of them and they are usually busy or far away when needed to address a troubleshooting area. Figure 2: Anritsu BTS Master MT8221B Anritsu has addressed these concerns by introducing LTE measurement options that enable its MT8221B BTS Master handheld base station analyzer and other instruments to perform LTE coverage mapping as well as broad range of other LTE measurements. The new LTE measurement options provide the first available suite of LTE tests in a handheld measurement solution. The measurement options accurately and easily measure all LTE bandwidths and frequencies. To make coverage mapping as simple as the push of a button, an autosave function lets the instrument make coverage measurements without user intervention using an affordable, handheld, easy-to-operate instrument that most base station technicians already have and use for other tasks. Besides the BTS Master 8221B, LTE coverage mapping can also be performed with the BTS Master MT8222A, Spectrum Master MS2712E, MS2713E, MS2721B, MS2723B and MS2724B and Cell Master MT8212E and MT8213E. The Anritsu handheld LTE measurement suite is comprised of several options. Options 541 (LTE RF Measurements), 542 (LTE Modulation Measurements), and 543 (15 & 20 MHz LTE Bandwidths) are used to validate the performance of the complete enodeb system and troubleshoot problems with LTE signals. Options 542 and 543 also expand the LTE coverage mapping capabilities of Anritsu instruments by providing a wider range of modulation quality, performance and troubleshooting measurements. These options display the different active modulation formats in the LTE signal, as well as measure the Error Vector Magnitude (EVM), carrier frequency and frequency error. Option 546 (LTE Over-the-Air Measurements) generates the measurements used for coverage maps. In combination with Option 542, Option 546 can be used to validate over-the-air performance. This is especially useful in the case of Remote Radio Head/Unit (RRH/RRU) installations. Option 546 identifies up to 6 different enodebs with the cell ID (consisting of sector ID and group ID), and also measures the Sync Signal power of each sector. It then calculates the dominance, which shows the difference in the signal level between the strongest sector and other sectors. Dominance is a useful measurement for determining the level of co-channel interference. 3

4 LTE Downlink Coverage Mapping Figure 3: Option 546 Over-the-Air Measurements with modulation option A Versatile Base Station Measurement Platform Figure 4: Option 542 LTE Modulation Measurements The MT8221B BTS Master base station analyzer was selected as the primary platform for LTE coverage mapping because it was developed specifically to support emerging 4G standards such as LTE including 20 MHz demodulation capability. It also provides a complete suite of measurement capabilities for measuring all key aspects of base station performance, including line sweep, spectrum measurements, interference hunting, and backhaul verification. When equipped with the new LTE measurement options, the MT8221B BTS Master provides RF and modulation quality measurements for LTE bandwidths from 1.4 MHz all the way through to 20 MHz. No other handheld instrument provides similar LTE coverage and measurement capability. Another advantage of BTS Master handheld base station analyzers is that they are already widely used by base stations technicians and RF engineers for accurately and quickly testing and verifying the installation and the 4 LTE Downlink Coverage Mapping with a Handheld

5 commissioning of base stations and cell sites for optimal wireless network performance. BTS Masters are also used for on-going maintenance and troubleshooting to keep the wireless network infrastructure running smoothly. The BTS Master MT8221B is small, lightweight and battery operated, making it easy to use anywhere at a cell site. The instrument warms up in less than 5 minutes, making it possible to get started quickly and increasing useful battery life. How to Perform LTE Coverage Mapping The BTS Master and other Anritsu handheld instruments equipped with LTE measurement options make it easy to perform LTE coverage mapping. The technician or RF engineer simply connects the receive and GPS antennas to the instrument, tunes to the LTE signal, switches to the OTA measurement, and pushes the Autosave button. The instrument automatically stores the cell ID, group ID and sector ID of every received signal as well as the time and location of the measurement and the sync signal power for each signal. For more detail, the user can enable saving measurements on the dominant signal including the rms and peak Error Vector Magnitude (EVM), and the carrier frequency and frequency error in both Hz and parts per million. The instrument makes and saves measurements approximately every 5 seconds with modulation measurements disabled, and every 10 seconds with modulation enabled. Sync Signal power is the best single metric for coverage of an area, and capturing just this measurement optimizes the speed of the instrument. The modulation quality can also be captured, which helps determine whether or not the signal is receivable. If the car is traveling at 25 miles per hour and modulation is disabled, the instrument will make a measurement approximately once every 150 feet. The data can be stored in the internal instrument memory or on a USB stick. The user then can transfer the data to a computer in one of several ways -- connecting the instrument to computer using an Ethernet or USB cable, or by removing the USB stick from the instrument and plugging it into the computer. The user then opens the file using the Anritsu Master Software Tools, which is included with the instrument as well as available as a free download from the Anritsu web site. Master Software Tools is then used to export the measurement data to a Google Earth KML file. The user then opens the KML file in Google Earth, which is also a free download. When the file is opened the user sees a satellite view of the area whose coverage was mapped. Each point where data was captured appears on the map. The coverage map is colourcoded based on the sync signal power. This view helps to identify patterns, such as a deadspot that might be caused because a large building or other obstruction is blocking the signal. The user can mouse over any data point to see more detailed results as shown in Figure 5. Technicians or engineers can also use the instrument to take more detailed measurements while on foot in the critical area. The ability to use a single handheld instrument for LTE coverage mapping as well as base station installation, commissioning, maintenance and troubleshooting for 2G/3G and LTE networks helps improve productivity for more reliable and efficient base station operations. Figure 5: LTE coverage map opened in Google Earth 5

6 Measuring LTE Modulation Quality Over-the-Air Introduction Most major mobile carriers in the United States and many worldwide carriers are in the process of converting their networks to Long Term Evolution (LTE), the next generation of radio technologies designed to increase the capacity and speed of mobile networks. LTE utilizes multi-antenna techniques such as spatial multiplexing, beamforming, and transmit diversity to support higher data rates and improve coverage. These same multi-antenna techniques, however, create challenges for base station engineers and technicians in performing basic measurements such as modulation quality. These challenges can be circumvented by connecting the instrument directly to the transmitter, but this method has problems of its own such requiring considerable additional time and possibly having to take the transmitter off the air. At the same time, wireless operators are being pressured to reduce costs, yet improve network quality. By making Over-the-Air (OTA) modulation quality measurements, along with Pass/Fail, Scanner, and throughput tests, LTE enodeb performance can be quickly verified and trouble spots easily detected. This application note presents a simpler and faster method of measuring modulation quality making the measurement over-the-air with a handheld instrument. This method is based on the fact that control channels do not use spatial multiplexing or beamforming because they have to operate over the entire cell, including at the cell edges. Over-the-air measurements of LTE modulation quality are not intended to replace direct-connect measurements because direct-connect measurements are more accurate and comprehensive. However, over-the-air measurements are much faster and more convenient so they provide a valuable tool to help meet the challenge of delivering seamless LTE service. LTE is a step toward the 4th generation (4G) of radio technologies designed to increase the capacity and speed of mobile telephone networks. The main advantages of LTE are high throughput, low latency, plug and play, an improved end-user experience and a simple architecture resulting in low operating costs. A variety of multi-antenna techniques play a key role in delivering these performance improvements. Transmit diversity uses signals that originate from two or more transmitters with identical data streams, but different coding; this helps overcome the effects of fading, which is one of the major limitations of wireless systems especially at the cell edge where the signal strength is low. Spatial multiplexing uses multiple input multiple output (MIMO) wireless communications to transmit independent and separately encoded data signals from each of several transmit antennas. Receivers use matrix mathematics to separate the two data streams and demodulate the data. Transmitting data in multiple streams in parallel improves bandwidth but requires a relatively high signal-to-noise ratio. Beamforming uses patterns of constructive and destructive interference on the wavefront to increase and decrease the signal in specific areas, thus improving the SNR at the receiver while decreasing interference. Figure 1: Transmit Diversity and Spatial Multiplexing are two key multi-antenna techniques 6 Measuring LTE Modulation Quality Over-the-Air with a Handheld

7 Challenge of LTE modulation quality measurements Multi-antenna methods also increase the difficulty of basic operational and troubleshooting techniques that were much easier with the previous generation of network technology. Spatial multiplexing and beamforming provide the biggest challenges for over-the-air measurements; the dynamic nature of which multi-antenna technique is used at any given moment adds even greater complexity. With spatial multiplexing the different antennas appear to be co-channel interference to a single receiver, thus necessitating a very expensive and heavy measurement device with multiple receivers. Beamforming can also present problems because it increases or decreases the amount of power received in particular areas on a continually changing basis making reliable measurements of the signal impossible for a passive measurement device. While transmit diversity does not present a measurement problem (because multiple antennas signals can be recovered with a single receive channel), each Physical Downlink Shared Channel (PDSCH) used to transmit LTE data can change multi-antenna mode dynamically based on signal conditions per user. When looking at a captured signal with a measuring instrument, it s impossible to know if each resource block uses spatial multiplexing, beamforming, or transmit diversity. Of course, you can avoid these complications by directly connecting the instrument to the transmitter. This approach provides the most thorough and accurate measurements of modulation quality so it is essential in many cases. However, there are a number of limitations associated with direct-connect measurements. It takes time to open up a shelter or building. If the transmitter has a test port then connecting to the instrument is not a problem. If there is no test port, then you have to disconnect the transmitter from the antenna which is usually difficult and time-consuming. If the site uses a Remote Radio Head (RRH) or Remote Radio Unit (RRU), then you need to gain physical access to the RF signal. This may not be difficult if the RRH/RRU is mounted inside a building or on a roof with reasonable access, but if the RRH/RRU is mounted on a tower or inaccessible roof, then you need to climb the tower or otherwise get access to the transmitter usually a difficult and expensive process. Over-the-air modulation measurements with a handheld analyzer Making measurements over the air is much easier and faster which is always important for technicians and engineers with many responsibilities and limited time. Speed is particularly important when troubleshooting a reported problem. Anritsu has introduced LTE measurement options that enable its MT8221B BTS Master handheld base station analyzer and other instruments to perform over-the-air modulation quality measurements as well as a broad range of other LTE measurements. The new measurement options on the MT822221B accurately and easily measure all LTE bandwidths and frequencies. Besides the BTS Master MT822 1B, LTE modulation quality measurements can also be performed with the Cell Master MT8212E and MT8213E, Spectrum Master MS2712E, MS2713E, MS2721B, MS2723B and MS2724B, and BTS Master MT8222 A. The MT8221B BTS Master base station analyzer was selected as the primary platform for over-the-air modulation quality measurements because it was developed specifically to support emerging 4G standards such as LTE, including 20 MHz demodulation capability. The BTS Master MT8221B is small, lightweight and battery operated, making it easy to use anywhere at a cell site. It also includes a complete suite of measurement capabilities for measuring all key aspects of base station performance, including line sweep, spectrum measurements, interference hunting, and backhaul verification. Another advantage of BTS Master handheld base station analyzers is that they are already widely used by base stations technicians and RF engineers for accurately and quickly testing and verifying the installation and the commissioning of base stations and cell sites for optimal wireless network performance. These instruments can be easily upgraded to LTE capability. The Anritsu handheld LTE measurement suite is comprised of several options. Options 541 (LTE RF Measurements), 542 (LTE Modulation Measurements), and 543 (15 & 20 MHz LTE Bandwidths) are used to validate the performance of the complete enodeb system and troubleshoot problems with LTE signals. These options provide a wide range of modulation quality and performance measurements. Option 546 (LTE Over-the-Air Measurements) generates measurements used for checking coverage and co-channel interference. This option adds the ability to measure EVM 7

8 Measuring LTE Modulation Quality Over-the-Air on over-the-air signals with transmit diversity when used in combination with Option 542. Option 546 identifies up to 6 different enodebs with the cell ID, sector ID and group ID, and also measures the sync signal power of each sector. It then calculates the dominance, which shows the difference in the signal level between the strongest sector and other sectors. How to make modulation quality measurements Using an instrument with both Option 542 and Option 546, switch to the Over-The-Air measurement menu, select the Scanner measurement, and turn on Modulation Measurements in the sub-menu. This measures the EVM of the Physical Broadcast Channel (PBCH) which uses Transmit Diversity. Tune the instrument to the signal of interest and attach an appropriate antenna through a short cable. Next, find a sweet spot, a place where the signal strength of the enodeb to be measured is high and interference, especially from other enodebs, is low. Option 546 is an excellent tool for finding a sweet spot because it shows the signal strength (using the LTE Sync Signal or SS), as well as dominance of one enodeb over others. The sweet spot will be a short distance from the transmitter, and near the center of the antenna pattern. If you are too close, the antenna beam will be above you; if you are too far the signal strength will be too low, there will be too much multipath, and too much interference from adjacent transmitters. Being in the center of the sector s transmit beam reduces the co-channel interference from adjacent sectors. The recommended approach is to start measuring several hundred feet from the antenna in the beam center, then walk or drive around to find the best available location and then mark it using GPS coordinates. Figure 2: Performing EVM measurements with a Yagi antenna Omnidirectional or omni antennas are much more convenient for making over-the-air measurements because of their small size. A good trade-off is to make initial measurements with the omni antenna. Then if a problem is detected, connect the larger Yagi directional antenna. Move the directional antenna around to find the best measurements; while usually this is when pointed directly at the transmit antenna, this is not always the case. If the directional antenna can be oriented so that EVM levels are reduced within the specification, this is a good indication that the transmitter is fine and any signal problems are coming from external interference. Modulation quality specifications for LTE transmitters are 8% EVM or less at 64QAM. This limit should be used when making a direct connect measurement, with the addition of a small factor for instrument contribution. In the case of 8 Measuring LTE Modulation Quality Over-the-Air with a Handheld

9 an over-the-air measurement, an even larger factor should be added to take account of the signal path. As a general rule, readings under 10% are OK. A good approach is to find a sweet spot and take an over-the-air measurement when the base station is commissioned. This will provide a benchmark value to maintain going forward. Figure 3: One quick test - OTA Pass/Fail - checks health of cell site. Conclusion The ability to make over-the-air modulation quality measurements with a handheld instrument can substantially improve troubleshooting efficiency. This is especially true for cases with inaccessible Remote Radio Heads/Units, but can also be used for quick verification of transmitter quality in traditional installations. When OTA modulation quality tests are combined with the BTS Master s LTE Pass Fail and OTA Scanner tests, as well as a simple throughput test using a PC with wireless modem, technicians can have high confidence in the entire base station. This includes the transmitter & receiver, antennas & transmission lines, backhaul, and co-channel interference. These quick tests provide a simple method to help ensure optimal network performance, with minimal time spent. When trouble spots are found, the Anritsu BTS Master also has all of the needed tools for troubleshooting the problem, so the correct fix can be made. Finding faults early has the added benefit that repairs can be scheduled at a convenient time, rather than under emergency conditions. 9

10 Notes 10 LTE Testing

11 11

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