December 16, 2016
As we explained in our last post, baseband unit (BBU) emulation is a highly effective method of managing and reducing time-driven costs during site build, commissioning, and troubleshooting of 4G networks. It allows wireless field engineers and technicians to test remote radio head (RRH) performance from the ground to validate and sign off on the installation before the BBU is installed, thereby reducing repeat site visits by crews.
Selecting the proper test solutions, such as the Anritsu BTS Master™ handheld base station tester, is only part of the equation that equals these benefits. Understanding how to conduct these measurements is also important. Here are eight steps to take when conducting tests using BBU emulation.
Step One: Set up the BBU
The first step is somewhat obvious. You need to make sure the RRH is powered up and that the Small Form Factor Pluggable (SFP) optical transceivers being installed support the required line rates of the test. Because you will be dealing with fiber optic cables it’s important to clean both ends of the fibers and connectors with proper cleaning gear.
Once the antenna has been installed, you will connect the duplex fiber pair between the RRH SFP and the tester SFP. You can then set the test solution into BBU emulation mode so it becomes the master.
As opposed to normal RF tests in which everything occurs almost instantaneously, BBU tests often involve delays of 20-40 seconds because commands and responses are sent and received. All the lights on the analyzer interface will turn green when a valid CPRI link between the test unit and the RRH has been established (Figure 1).
Step 2: Establish Communication
Over CPRI Link Once a link has been established between the RRH SFP and the tester SFP, communication with the RRH can begin. After sending commands, you will receive a response with the IP address of the RRH. Commands, which are determined by the RRH equipment manufacturers, can be sent to the instrument in multiple ways, including Ethernet link, High Level Data Link Control (HDLC) and proprietary methods.
All active RRHs in the daisy chain will request their own IP addresses, which will be allocated by the test instrument. A list of RRH IP addresses on the CPRI connection is displayed in the order in which they are communicated to the BBU.
Step 3: Identifying IP Addresses
When you click on the IP address displayed in the BTS Master interface (or similar instrument), you will be presented with a set of commands that ask for the manufacturer, model number, running firmware, frequency range, power levels and other specifications. This information helps validate that the correct equipment is installed in the proper location. It also helps prevent a common error – connecting the SFP in the wrong port. Figure 2 shows how it should be connected. When all IPs are identified, it becomes easier to determine which specific piece isn’t working properly in the event of a system error.
At this point, SFP tests can be conducted. You can query the SFP, which will report back data such as its wavelength and supported bit rate. Again, by having the vendor name and serial number displayed, it is easier to keep track of what is and is not working.
Step 4: LTE Waveform Test Models
Many of the functionality tests on an RRH require loading a waveform file into the RRH and then transmitting it. These waveforms come in different pattern lengths and different 3GPP test models:
• E-Tm 1.1 – RF-based test
o Test unwanted emissions, occupied bandwidth, ACLR, transmitter intermodulation and RS absolute accuracy
• E-Tm 1.2 – RF-based test
o Unwanted emissions and ACLR are measured
• E-TM 2 – Tests at Minimum Power
o Measures total power dynamic range, EVM of single 56QAM PRB allocation and frequency error
• E-TM 3.1 – Tests at Maximum Power
o Tests output tower dynamics, total power dynamic range, transmitted signal quality and EVM for 64QAM modulation
• E-TM 3.2
o Frequency error, transmitted signal quality and EVM for 16QAM modulation are measured
• E-TM 3.3
o Tests transmitted signal quality, frequency error and EVM for QPSK modulation
Note: 3.1, 3.2 and 3.3 send out three typical modulations that are used in LTE networks.
When the LTE wave file has been loaded into the unit an acknowledgement will be sent. You then can pick the proper center frequency for transmission; keep in mind that the BBU software running on the test unit dictates the proper frequency range and output power. After selecting the waveform test model, measurements can be made.
Step 5: Validate the Antenna
Using a spectrum analyzer, over the air (OTA) measurements should be conducted to confirm that the sector is transmitting a strong signal. To acquire specific data, the signal can also be demodulated. This latter step isn’t always necessary, but it is useful for troubleshooting existing antenna units.
Step 6: Measure the RRH Antenna Port VSWR
Return loss (VSWR) measurements should be made at this point in the process. When the radio is transmitting, you are basically using it to make the measurement. You can even set Pass/Fail limits for the results and determine if the values are within the necessary ranges.
Step 7: View PIM on the RRH Uplink
When the system is installed, tests for possible PIM can be conducted. Use E-TM 2 to establish a baseline, and then use E-TM 3.1 to transmit maximum power to see if the uplink is affected. Minimum hold trace and normal trace can also be used to validate if PIM is present (Figure 3). If the minimum trace stays flat, it is probably the result of a separate interference issue that is not PIM (Figure 4).
Figure 4: a flat noise floor
Step 8: Check Fully Commissioned Installation
Even after the BBUs have been fully installed and commissioned, you should still return and test the system to make sure everything is working properly. If a site needs to be troubleshooted, crews will typically use an optical TAP. Some carriers are installing TAPs in their sites, but in other cases, crews will have to use their own. It can be installed between the BBU and the RRH to monitor the existing live network. It is important to make sure that the test instrument is in normal CPRI mode rather than BBU mode when troubleshooting. In this case, you can make the uplink RF measurements to check for any interference or possible PIM issues.
You can also measure the downlink to verify that the BBU is transmitting its LTE signal according to specification. It is important to validate that there is no interference that will drastically affect your KPIs.
To learn more on this important topic that can save you time and money, watch a BBU Emulation webinar.