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LTE RF Channel Troubleshooting Guide www.huawei.com
HUAWEI TECHNOLOGIES CO., LTD.
Huawei Confidential
Abstract
This document describes the common faults of the LTE RF
channels and ALD encountered during the IF/RF maintenance, symptoms, principles, diagnosis methods, data required for diagnosis, and analysis methods.
Abstract
This document describes the common faults of the LTE RF
channels and ALD encountered during the IF/RF maintenance, symptoms, principles, diagnosis methods, data required for diagnosis, and analysis methods.
Contents
Common RF Channel Faults
Uplink Channel Faults
Uplink Interference
Intra-eNodeB Inter-cell Reverse Connection of Antenna
Downlink Channel Faults
ALD Link Fault
Common RF Channel Faults Interference: Interference detection
Inter-cell reverse connection: Inter-cell handover statistics
Crossed pair: RTWP difference between main & diversity
The common faults related to the RF channels are uplink channel fault, uplink interference, intra-eNodeB intercell reverse connection of antenna, downlink channel fault, downlink antenna fault, and ALD link fault. This document describes the analysis procedure beginning from discovery of a symptom to conclusion.
Antenna fault: 1. VSWR alarm 2. Passive intermodulation interference 3. RTWP difference between main & diversity or low RTWP
Contents
Common RF Channel Faults
Uplink Channel Faults
Uplink Interference
Intra-eNodeB Inter-cell Reverse Connection of Antenna
Downlink Channel Faults
ALD Link Fault
Uplink Channel Faults When an uplink channel fault occurs, the symptoms are degraded cell KPI, RF Unit RX Channel RTWP/RSSI Too Low alarm, and RF Unit RX Channel RTWP/RSSI Unbalanced alarm. I. Hardware fault Choose Monitor > Signaling Trace > Signaling Trace Management on the M2000 or Web LMT. In the navigation tree, choose Cell Performance Monitoring > Interference Detect Monitoring. The RXA RSSI and RXB RSSI are displayed in a table, as shown in the next page. Check whether the values in this table are less than the RSSI values under empty load. The threshold for reporting RTWP/RSSI too low alarm is –114 dBm. The RSSI under empty load is calculated as follows:
–174+10*logBW+NF where BW is the bandwidth in Hz and NF is the noise factor of the RF module and its value is between 2 to 2.5. Assuming an LRRU 2.6G 2T2R cell of 5 MHz bandwidth, the RSSI under empty load is calculated as follows:
–174+10*log(5*10^6)+2.5= –104.5 dBm
Uplink Channel Faults
If no TMA is configured, the value of Rx Channel Attenuation contained in the MOD RXBRANCH command must be 0.
Uplink Channel Faults II. Passive intermodulation The root cause of intermodulation is nonlinearity of the channels. An index to measure the nonlinearity of an antenna system is intermodulation suppression. For a linear system, the output to a 2-signal input is also 2 signals. However, for a nonlinear system, a 2-signal input generates new frequency components known as intermodulation products. The phenomenon of generating new frequency components is called intermodulation. If the intermodulation products fall in the reception band and increase the uplink interference or RTWP, intermodulation interference arises.
Uplink Channel Faults The following figure shows the intermodulation products of signals of BW bandwidth after passing a nonlinear system.
2*(f1+BW/2)-f1=f1+BW Third-order intermodulation products generated by central frequency & high edge frequencies
2*(f1-BW/2)-f1=f1-BW Third-order intermodulation products generated by central frequency & low edge frequencies
IM3 Third-order intermodulation products generated by high & low edge frequencies
2*(f1-BW/2)-(f1+BW/2)=f13*BW/2
IM3 Central frequency f1 Low edge frequencies =f1BW/2
High edge frequencies = f1+BW/2
Third-order intermodulation products generated by high & low edge frequencies
2*(f1+BW/2)-(f1-BW/2)=f1+3*BW/2
Uplink Channel Faults Intermodulation products are governed by the following rules:
The magnitude of intermodulation products has a negative correlation with the intermodulation suppression of the antennas. Assuming third-order intermodulation suppression IM3= – 140dBc@2*43dBm, the third-order intermodulation product of two input signals of 43 dBm is calculated as follows: 43 dBm – 140 dBc = -97 dBm The magnitude of intermodulation products also depends on the power of the input signals. Theoretically, for every 1 dB increase of the input signals, IM3 is increased by 3 dB. In practice, for every 1dB increase of the input power, IM3 is increased by 2 to 3 dB. In other words, given the same intermodulation suppression, the magnitude of intermodulation products has a positive correlation with the input power and increases faster than the input power.
Uplink Channel Faults
The magnitude has a negative correlation with the order of
intermodulation products. The relationship is not quantitative.
The bandwidth of intermodulation products is widened. The bandwidth of nth-order intermodulation product is n times wider than the input signal. For an input of 2 equal-bandwidth signals, the bandwidth of third-order and fifth-order intermodulation products is 3 times and 5 times the input bandwidth, respectively.
Uplink Channel Faults
If the following two conditions are met, the eNodeB is interfered by intermodulation products: The intermodulation signals fall in the receiver band. The intermodulation signals are strong enough to affect demodulation.
Note If the source signals that generate intermodulation interference include one or two uplink signals, the intermodulation signals are negligible due to small uplink signal level. Intermodulation interference that propagates through the space causes little impact on other eNodeBs due to large propagation loss. Any two downlink signals in the same cell may generate intermodulation interference that is significant to the uplink but negligible to the downlink. Of great concern is the intermodulation interference generated by any two downlink signals and falling in the uplink band.
Uplink Channel Faults The passive intermodulation increases much faster than the increase of the transmit power. Therefore, the passive intermodulation can be detected by adding simulated load in off-peak hours. If the Interference Detect Monitoring on the M2000 shows that all the RSSIs are increased, the RSSI increase is caused by passive intermodulation. Note If the number of active users in a cell is more than 6, the scheduling performance cannot be guaranteed in this function is enabled. Therefore, this function is not recommended in such scenario.
Uplink Channel Faults The commercial intermodulation tester can only determine the magnitude of intermodulation of the antenna system and cannot determine the location where the intermodulation is generated. To determine the location of intermodulation, check the antenna system segment by segment. The following figure shows the schematic diagram of this method. The segmental inspection procedure is as follows: Disconnect point 1, 2, 3, or 4, connect it to a lowintermodulation load, and add a downlink simulated load. Observe whether the RTWP and RSSI are increased. If yes, the point is where intermodulation interference is introduced. An example is described as follows: Disconnect point 1, connect it to a lowintermodulation load, add a downlink simulated load, and observe RTWP and RSSI values. The RTWP and RSSI are unchanged, indicating that the RF module has no problem. Disconnect point 2, connect it to a low-intermodulation load, add a downlink simulated load, and observe RTWP/RSSI. The RTWP and RSSI are significantly increased, indicating that the lower jumper is where the intermodulation interference is introduced. An alternative is the replacement method. Replace the feeder line segment by segment and check the RTWP and RSSI.
Contents
Common RF Channel Faults
Uplink Channel Faults
Uplink Interference
Intra-eNodeB Inter-cell Reverse Connection of Antenna
Downlink Channel Faults
ALD Link Fault
Uplink Interference External interference affects nearly all KPIs, including RRC connection setup success rate, access success rate, call drop rate, handover success rate, and uplink and downlink throughput. The problem of external interference cannot be solved from within. We must find out the external interference source and contact the customer to remove the source. When a cell is interfered by an external source, both the main and diversity receive the interference simultaneously. Therefore, the RSSI of RXA and RXB fluctuates in similar pattern and is large even if no UE in the cell runs services. The interference type, interfered frequency, and bandwidth can be obtained by using the spectrum detection tool described in the next page.
Uplink Interference Spectrum detection tool
Uplink Interference Tracing result indicating external interference
Uplink Interference Tracing result indicating no external interference
Contents
Common RF Channel Faults
Uplink Channel Faults
Uplink Interference
Intra-eNodeB Inter-cell Reverse Connection of Antenna
Downlink Channel Faults
ALD Link Fault
Intra-eNodeB Inter-cell Reverse Connection of Antenna Except in MIMO mutual-aid scenario where the RTWP unbalance alarm is not reported, the RXA and RXB of the RRU must belong to the same cell, in which case the direction and coverage area of RXA are the same as those of RXB to ensure RTWP balance. If RXA and RXB of the RRU are connected to different sectors, the problem of RTWP unbalance occurs and the RTWP unbalance alarm is reported.
If the RTWP unbalance alarm is caused by incorrect connection of RXA or RXB to another antenna, the symptoms are as follows: At least two cells in the same site report an RTWP unbalance alarm. The RTWP change of RXA is independent of that of RXB. The RTWP change of different sectors is similar.
Intra-eNodeB Inter-cell Reverse Connection of Antenna Analyze the engineering parameter table and traffic statistics about handovers to obtain cells that have no overlapping coverage but that are involved in handovers. Analyze these cells as follows: 1. In the case of reverse antenna connection, if the local cell and a neighboring cell that has no overlapping coverage are involved in handovers, there must be another intra-eNodeB cell that has overlapping coverage with this neighboring cell. 2. In the case of reverse antenna connection, there are at least two intraeNodeB cells involved in handover to a neighboring cell that has no overlapping coverage. If the preceding analysis shows that the RXA and RXB of the RRU are incorrectly connected to another antenna, go to the site to connect the feeder lines correctly.
Contents
Common RF Channel Faults
Uplink Channel Faults
Uplink Interference
Intra-eNodeB Inter-cell Reverse Connection of Antenna
Downlink Channel Faults
ALD Link Fault
Downlink Channel Faults
If the downlink channel gain is abnormal, ALM-26520 RF Unit TX Channel Gain Out of Range is reported. Principles: If the actual gain of the RF unit TX channel deviates from the standard gain by more than 2.5 dB, this alarm is reported. Impact: Large gain causes coverage overlap. Small gain causes coverage holes, or interruption of the services over the RF unit in worst cases.
Troubleshooting I. Run the RST BRDPWROFF command to reset the RRU, excluding the possibility that the problem is caused by soft failure. Check whether the alarm is cleared. II. If the alarm is not cleared, replace the RRU and return the replaced RRU to R&D department for further analysis.
Downlink Channel Faults If the VSWR value is abnormal, a VSWR alarm is reported. If the ALD connection and match are poor, most of the output microwave power is not emitted by the antenna but is reflected back. Large reflection power damages the module and causes reverse breakdown in worst cases. VSWR is an exponential function of the forward & reverse powers and is independent of the input signal power. The default VSWR alarm threshold is 2.0 and severe VSWR alarm threshold is 2.5. To avoid module damage, if the severe VSWR alarm is reported, the TX channel is automatically switched off and there is no output power. Therefore, if the severe VSWR alarm is reported, the cell performance deteriorates or the cell is unavailable, degrading the network coverage and performance.
Downlink Channel Faults PA
Coupler
DUP
Coupler
Forward Reverse
VSWR detection
If a VSWR alarm is reported, the possible causes are: 1. The VSWR alarm threshold is low. 2. The jumper is incorrectly installed; the feeder line connector is loosened or contains water; the feeder line is compressed, bent, damaged, or loosened. 3. The RF unit band is inconsistent with the band specification of the ALD components. 4. The VSWR detection circuit of the RF unit or the hardware of the RF unit is faulty.
Downlink Channel Faults Troubleshooting procedure 1. View the VSWR value contained in the alarm.
If the VSWR is greater than 100 (unit: 0.1), the reflection power is large. The cause is that the feeder line is not connected, bent, or damaged, 2. Run the LST RRU command to query the VSWR alarm threshold of the RF unit and check the appropriateness of the value according to the planning data. If inappropriate, run the MOD RRU command to modify the threshold.
Downlink Channel Faults 3. Run the DSP VSWR command to query the current VSWR. Run the STR VSWRTEST command to perform VSWR test. Note that this operation interrupts the service.
(1) Repeat step 3 for five times at an interval of 2 minutes. Check whether the STR VSWRTEST command output shows significant fluctuation of the VSWR. If yes, we can suspect that the ALD is loosely connected. If not, compare the VSWR in the DSP VSWR command output with the VSWR in the STR VSWRTEST command output. If the comparison shows that the VSWRs are equivalent, go to the site to check the equipment.
Downlink Channel Faults (2) If there are other special situations, send the operation result, LMPT log, and RRU log to the R&D department for further analysis.
The procedure of onsite inspection is as follows: (1) Based on the network planning data, check whether the RF unit band is consistent with the band specification of the ALD components such as antenna, feeder, jumper, combiner, divider, filter, and TMA. (2) Prepare Sitemaster, a VSWR tester, for onsite inspection. Sitemaster can show the distance between the test point and the point of VSWR generation.
Downlink Channel Faults (3) During a field inspection, if Sitemaster is not available, use the segmental isolation method as follows: add a load to one segment of the ALD at a time, or replace the feeder line, and run the STR VSWRTEST command to check the result. Incorrect connection, damaged cable, or water or alien matter in the joint can cause standing wave. (4) After checking the ALD connection or replacing the ALD, run the STR VSWRTEST command to check the result. If the command output shows that the VSWR is normal, reset the RRU to clear the alarm. Troubleshooting ends. If the command output shows that the VSWR still exceeds the threshold, replace the RRU to check whether the RRU is faulty. Return the replaced RRU to R&D department for further analysis.
Contents
Common RF Channel Faults
Uplink Channel Faults
Uplink Interference
Intra-eNodeB Inter-cell Reverse Connection of Antenna
Downlink Channel Faults
ALD Link Fault
ALD Link Fault A remote electrical tilt (RET) can be remotely controlled by a remote control unit (RCU) that is remotely controlled. The RCU is installed outside the antenna and consists of numerically controlled step motor, control circuit, and gearing device. The function of the control circuit is to communicate with the controller and control the motor. The motor includes a gear that is coupled to a transmission shaft. The gear that is driven by the motor can drive the transmission shaft, changing the downtilt of the antenna.
ALD Link Fault The following figure shows the mechanism of remote electrical tilt.
ALD Link Fault A common error during TMA installation is that RXA and RXB of the RRU are inversely connected to the TMA, leading to TMA failure. The power supply to the RCU can be from the RET port on the RRU, RCU port on the SBT, or the AISG port on the STMA. Only one power supply switch can be on at any time.
ALD Link Fault The current intensity at the RET port or Antenna port to the TMA or RCU are monitored and the TMA gain and alarm information are detected. The result of the monitoring and detection is used to determine the well-being of the TMA and RCU. During the routine maintenance, alarms are good indications of the ALD link status. Note: During routine maintenance, each time an operation is completed, check whether the active alarm is cleared before proceeding to the next step. The following describes handling of ALD alarms, the first three of which are common:
(1) ALM-26530 RF Unit ALD Current Out of Range Check the current intensity and details contained in the alarm.
[1] If the alarm cause is overcurrent/undercurrent, the troubleshooting method is as follows:
ALD Link Fault a) Ensure that the power supply is consistent with the configuration. b) Check whether the current thresholds are correctly configured. The current threshold is the sum of the rated currents of all ALDs. An example is described as follows: Assuming that the rated currents of the SBT is 50 mA, the rated current of the RCU in standby mode less than 83 mA, and the rated current of the 12-dB TMA is 90 mA, then Overcurrent threshold = (50*SBT quantity+83*RCU quantity+90*TMA quantity)*150%~200% = XXX mA Undercurrent threshold = (50*SBT quantity+83*RCU quantity+90*TMA quantity)*20%~30% = XXX mA c) If the current thresholds are correct and the alarm persists, go to the site to check the following: I. Check whether the ALD is correctly connected. Turn on the power supply switch from the M2000 and run the DSP ANTENNAPORT or DSP RETPORT command to view the current intensity.
ALD Link Fault II. If the ALD is correctly connected and the current intensity is still abnormal, replace the ALD cables or ALD. [2] If the alarm cause is overcurrent protection, the troubleshooting method is as follows: a) Ensure that the power supply switch is consistent with the configuration. b) Go to the site and use a multimeter to check whether the RET port, Antenna port, feeder line, ALD AISG cable, or ALD is shortcircuited, or the AISG cable connector contains alien things.
ALD Link Fault If a multimeter is unavailable, use the replacement method and replace the ALD AISG cable, ALD, feeder line, and RRU
sequentially. [3] If the alarm cause is open-circuit protection, the alarm is caused by cable connection or hardware fault.
The troubleshooting method is as follows: a) Ensure that the ALD power supply switch is consistent with the configuration. b) Go to the site to check whether the ALD AISG cable is correctly connected. If the cable is correctly connected and the alarm persists, use the replacement method to replace the ALD AISG cable, ALD, and RRU sequentially.
ALD Link Fault (2) ALM-26541 ALD Maintenance Link Failure [1] In the single-RCU scenario, check whether the software and configuration files loaded to the ALD are the original files of the vendor. Depending on the power supply method, run the DSP ANTENNAPORT or DSP RETPORT command to query the actual power switch and current intensity.
a) If the command output shows that the ALD current is normal, run the MOD ANTENNAPORT or MOD RETPORT command to turn off the RF unit ALD power supply switch, and then turn it on.
ALD Link Fault Check whether the alarm is cleared. If not, go to the site to check
whether the RCU is properly connected to the RRU. Replace the ALD. b) If the command output shows that the ALD current is null, go to the site to check whether the RCU is properly connected to the RRU. Use the replacement method to replace the cables, ALD, and RRU to check where the problem is.
ALD Link Fault [2] In case of RCU cascade, run the LST RET, LST TMA, DSP RET, LST TMASUBUNIT, or LST RETSUBUNIT command to check whether the RF unit ALD configuration is consistent with the vendor code and device sequence number that can be obtained by running the SCN ALD command. If inconsistent, run the MOD RET, MOD TMA, MOD RETSUBUNIT, or MOD TMASUBUNIT command to make them consistent. If the alarm persists after the preceding handling, further handling is the same as in the single-RCU scenario.
ALD Link Fault (3) ALM-26531 RF Unit ALD Switch Configuration Mismatch [1] Check whether the ALD switch configuration is correct. a) If this alarm is reported when the RET port is turned on by the MML command, the cause of the alarm is that the RET port of the RRU is not connected to an AISG cable. b) This alarm is reported if the antenna port is turned on by the MML command but actually the RET port is connected to the RCU.
(4) ALM-26754 RET Antenna Data Loss, ALM-26757 RET Antenna Running Data and Configuration Mismatch [1] Run the MOD ANTENNAPORT or MOD RETPORT command to power off & on the RCU. [2] Run the DLD ALDSW command to load the configuration file, and then reset the ALD. [3] Go to the site to replace the ALD or RCU.
ALD Link Fault (5) ALM-26751 RET Antenna Motor Fault [1] Check whether the software and configuration file installed in the ALD are the original files of the vendor. [2] Turn off and on the ALD power supply switch, or reset the RET. [3] If the alarm is not cleared, go to the site to check whether the RCU is properly connected to the ALD. [4] Go to the site to replace the ALD or RCU.
(6) ALM-26753 RET Antenna Not Calibrated [1] Check whether the software and configuration file installed in the ALD are the original files of the vendor. [2] Run the CLB RET command to calibrate the RET. [3] Go to the site to replace the ALD or RCU.
