OME200301 GSM BTS Trouble Shooting docx

Downlink signal weakenedPoor conversion quality, BTS coverage shrink Feeder system faults Standing wave alarm Standing wave alarm occurs at CDU LNA alarm LNA alarm occurs at CDU TTA alar

Upon completion of this course, you will be able to:

Know how to find the fault in BTS

Know the common fault types

Grasp BTS fault disposal method

Know how to prevent the fault

Chapter 1 General Introduction

Chapter 2 Typical Cases

Chapter 3 BTS Fault Prevention

Chapter 1 General Introduction

Section 1 The Way To Locate Fault Section 2 Basic Disposal Method

The ways to locate faults

Locating fault equipment

 If BSC has fault, usually it will affect some Sites or all of Sites

 If BTS has fault, usually it just affect itself and the handover success rate of adjacent cells

 During implementation or expansion, we can “Interchange ” BTS and judge whether the fault is because of BSC or BTS

 During maintenance, BSC faults don’t just affect one BTS

Common fault types

Chapter 1 General Introduction

Section 1 The Way To Locate Fault Section 2 Basic Disposal Method

Basic disposal method

 Analyzing the Alarm Information

Alarm information come from the BSS alarm system, indicated

usually through sound, light, LED, and screen output, etc

It includes the detailed description of the abnormality, possible

causes and restoration suggestions, involves the hardware, link, trunk and CPU loading ratio, etc It is a gist for the fault analysis and location

 Analyzing the Indicator Status

Indicators can indicate the work status of circuit, link, optical path, node and active/standby mode besides that of the corresponding boards, usually used along with the alarm information generally

 Analyzing Traffic Statistics Result

It is the most useful method for locating RNO fault.

Basic disposal method

 Analyzing Apparatus & Meter Test Result

It reflects the fault nature with the visual and quantified data

Some common Apparatus & Meter

− Signaling Analyzer, Test phone, Sitemaster, etc

 Tracing the Interface

It is applied in locating the failure causes of subscriber call connection and inter-office signaling cooperation, etc

The trace result can help to find the cause of call failure directly and locate the problem or to get the index for the subsequent analysis

 Calling Test

It is a simple and quick method to judge whether the call processing function and relative modules of the BSS are normal or not

Basic disposal method

 Comparison/Interchange

Comparison: compare the faulted components or phenomenon with the normal ones, and find the differences so as to find the problem

− It is usually used in the situation with simple fault range

Interchange: if the fault can not be located even after the standby

components are replaced, you can interchange the normal components (such as board, optical fiber, etc.) with the potential faulted components, and to compare the work status so as to specify the fault range or part

− It is usually used in the situation with complicated fault range

 Switching/Resetting

It can not locate the fault cause accurately, and due to the randomness

of software running, the fault may be not able to recur, thus it is difficult

to know the real fault and solve the problem

This method is just an emergency method, applicable only in the

Chapter 1 General Introduction

Chapter 2 Typical Cases

Chapter 3 BTS Fault Prevention

Chapter 2 Typical Cases

Section 1 Antenna & Feeder Fault Section 2 Transmission Fault

Section 3 Hardware connection fault Section 4 Hardware fault

BTS feeder system structure

TX RX RXD

TTA

Feede r

arreste r

optional

Antenn a

arreste r

Downlink signal weakened

Poor conversion quality, BTS coverage shrink

Feeder system faults

Standing wave alarm Standing wave alarm occurs at CDU LNA alarm LNA alarm occurs at CDU

TTA alarm TTA alarm occurs at CDU TTA feeding fails No DC feeding voltage at CDU antenna

Common faults on downlink

− Step1: View the history alarms and real-time alarm at OMC

or local maintenance console

− Step2: If there is emergent standing wave alarm at CDU, it is the most possible cause for which TMU turns off the

transmitter power amplifier resulting in no downlink signal

▪ check the standing wave ratio at jumper side of CDU antenna port

▪ If the standing wave ratio is beyond limits, locate the

Common faults on downlink

 Analysis

No downlink signal

− Step3: Since there is no downlink signal, there must be a broken point in the RF signal path If this point is located at the part from CDU antenna port to tower top, then CDU should be able to detect the emergent standing wave alarm Otherwise, it can be concluded that the broken point is

located between TRX output to CDU antenna point

▪ Check whether the cable connection between CDU COM and TX-DUP is correct

TX-− Step4: If the operations above fail to locate the failure, change the CDU

Common faults on downlink

 Analysis

Downlink signal weakened: The symptom of this failure is that the coverage of BTS or carrier shrinks Follow the steps below to handle this problem:

− Step1: Check whether the output power of TRX is normal

− Step2: Check whether the standing wave ratio at jumper side

of CDU antenna port is normal

− Step3: Check insertion loss of CDU transmitting path

− Step4: Check whether the connectors involved in the RF signal path are tightened

Common faults on uplink

− Step1: Try another antenna feeder (CDU excluded) which has proven

to be normal to substitute the one without uplink signal

▪ If the uplink signal at the new feeder recovered while the one at the original feeder fails, then the original antenna feeder has problems

▪ If the phenomenon remains, then CDU has problems Check whether the cable connection between RXD OUT and HL_IN or between HL_OUT and HL_IN is correct

Common faults on uplink

− When changing the antenna feeder, make sure that:

▪ The two corresponding antenna feeders should be in the same cell/sector

▪ The antenna connection should be restored to the original status after locating the failure Otherwise, the coverage of the cell may

be affected This is the basic principle to obey when using this method to locate the problem

Common faults on uplink

 Analysis

BTS sensitivity weakened

− If TTA is configured, first check whether there is any TTA alarm

▪ If so, the TTA is working abnormally

▪ Otherwise, check the CDU antenna port feeding

» If no feeding is detected, then the CDU is faulty and needs to be changed

» If DC voltage is normal, then it is considered that the TTA is normal

− After confirming that TTA is normal, check the standing wave ratio of antenna feeder

▪ If it is too large, then the connection of antenna feeder RF path is poor or something else

▪ If the standing wave is normal, check the performance of CDU

Common faults on uplink

 Analysis

BTS sensitivity weakened

− The common faults can be located by adopting the methods above But it is inevitable that there are some problems which can not be located by this method since

it is not a comprehensive test For example, if the gain decrease and noise factor increase of TTA is not

reflected in the working current, the problem cannot be detected

− On such occasions, make clear records of the operations which have been done so far for further analysis

Common faults on feeder system

 Description

CDU alarm (SWR, TTA)

 Analysis

Standing wave alarm

− Check the SWR of antenna feeder (CDU excluded) If it is lower than 1.5, while CDU SWR alarm has been generated, this alarm should be regarded as a mis-alarm, and the CDU needs to be changed

− If the SWR is higher than 1.5, it is necessary to adjust the connection of antenna feeder until it is lower than 1.5

− The installation specification requires the SWR be lower than1.3

Common faults on feeder system

 Analysis

TTA alarm

− CDU measures the TTA feeding current through the antenna port If the current is not in the normal TTA working current range

(45~170mA), CDU generates TTA alarm

− If the feeding current is normal while there is TTA alarm, then it can be considered as a TTA mis-alarm Use another CDU to substitute the faulty CDU Keep the faulty CDU for further analysis

− If the feeding current is beyond limits, TTA is faulty and needs to be changed

− For the migration site, it is also necessary to confirm the type of

Chapter 2 Typical Cases

Section 1 Antenna & Feeder Fault Section 2 Transmission Fault

Section 3 Hardware connection fault Section 4 Hardware fault

Transmission Fault

 Description

Alarm console

− “BIE board PCM loss of sync.”, “LAPD_OML alarm”

Traffic Statistic console

− The handover successful rate, call drop rate of the cell is abnormal

Consumer complaint

− Cannot take a call, bad quality, call drop

Transmission Fault

 Possible causes

Transmission device, board or E1 is faulty

− More transmission device, more fault

Transmission code is different (use CRC4 or not)

E1 connection quality is bad

− It will cause bad quality, even transmission broken

High BER (bit error rate)

− Microwave, HDSL transmission, especially in rainy weather

The grounding system is faulty

Transmission Fault 1: E1 broken

− Step2: perform self-loop test over BSC and check whether the E1 indicator of BIE board is OFF If not, it can be considered that the problem lies in the transmission device

− Step3: check the transmission NM and check whether a transmission related alarm is given Based on the alarm (if any), you may judge whether the problem lies in the transmission device

− Step 4: if neither of them is faulty, it can be considered that the problem lies in the cooperation between the transmission device and

Transmission Fault 2: OML alarm frequently

Transmission Fault 2: OML alarm frequently

 Handling process

− Check whether the system is in the single-point-grounded state If not, modify the system to the single-point-grounded state, then check whether the trouble is removed

− If the trouble is still not removed after the above measures, it may be considered that the problem lies in the transmission device, E1 or E1 interface board Check the connection and perform loop test

segment by segment to locate the fault

− Check the transmission NM and check whether a transmission related alarm is given If yes, please handle it as the related alarm details

Typical case: OML broken for E1 grounding error

 Description

A site’s OML was frequently interrupted and the indicator

(corresponding to the E1) at BSC flashed

− The equipment room was located at the top of a 300m-high hill The microwave transmission equipment room was 20m away

− On the site, the maintenance engineer found the following

▪ The E1 was grounded, as was checked from the DIP switch

▪ The E1 connector was insulated from the cabinet enclosure The working grounding cable of the rack was connected with that of the equipment room

▪ The DDF, an all-metal frame, was connected to the grounding cable of the equipment room The E1 connector contacted the metal of the DDF

▪ No lightning arrester was configured for the E1

▪ The E1 indicator flashed fast

Typical case: OML broken for E1 grounding error

 Handling process

Step1: self-loop the E1 at the top of BTS cabinet and found the

indicator of the E1 cable was OFF It means BTS is OK

Step2: self-loop the E1 on the DDF and found the indicator of the E1 cable was OFF It means the E1 from BTS to DDF is OK

Step3: self-loop the BSC on the DDF and found the E1 indicator is

OFF It means the E1 from BSC to DDF is OK

Step4: power the TMU off and then on, the trouble still existed

Step5: remove the E1 from the DDF, the trouble still existed

Step6: disconnect the E1 at the top of BTS cabinet, power off the rack and removed the TMU Test the resistance between the cabinet-top E1 connector case and the grounding cable of the rack and found they were insulated from each other (normal)

Typical case: OML broken for E1 grounding error

 Handling process

Step7: change the TMU DIP switch that corresponded to the

grounding of the E1 cable to OFF (ungrounded), the trouble still existed

Step8: remove the E1 connector from the DDF and change the TMU DIP switch that corresponded to the grounding of the E1 cable to OFF (ungrounded) The trouble disappeared

Step9: for confirming the reason, replace the TMU (with the E1 cable ungrounded) Let the E1 connector case contact the DDF and found the TMU E1 indicator flash fast

Restored the TMU to the original one and removed the E1

connector from DDF, the trouble disappeared

Chapter 2 Typical Cases

Section 1 Antenna & Feeder Fault Section 2 Transmission Fault

Section 3 Hardware connection fault Section 4 Hardware fault

Typical case: VSWR alarm for cable broken

 Description

On BTS maintenance console, one TRX board in a sector was red, alarm console showed TRX VSWR alarm

− The site was just established

− On the site, the maintenance engineer found the following

▪ The RF cable between TRX and CDU was well connected

▪ The RF jumper in the CDU was well connected and also the connection was right

▪ The RF cable between CDU and lightening arrester was well connected

Typical case: VSWR alarm for cable broken

 Handling process

Step1: check all the connection from TRX to main feeder and lightening arrester detailed and also re-tighten all the connections , the problem was still there, meant the connection had no problem

Step1: connect the CDU in current sector to the antenna in another

sector that was ok before, the TRX was still red, that meant the

antenna and feeder system had no problem , recovered the connection

Step2: connected the TRX to a CDU in another sector that had nothing alarm before, the TRX was still red, that meant the problem was in the TRX side, recovered the connection

Step3: change the faulty TRX to a new one, the TRX was still red ,

meant the TRX had no problem, recovered the connection

Step4: change the cables between the TRX and CDU , problem

disappeared The problem was that the cable was broken during

Chapter 2 Typical Cases

Section 1 Antenna & Feeder Fault Section 2 Transmission Fault

Section 3 Hardware connection fault Section 4 Hardware fault

Typical case: site failed for TRX fault

 Description

A site had no traffic and customer complained that they could not make a call

− It was a omni-directional site and had only one TRX

− On the site, the maintenance engineer found the following

▪ All the boards was well in place and the indicators showed that all the boards had no alarm

▪ Antenna and feeder was well connected

▪ The whole site was well grounded

▪ The power supply had no problem

Typical case: site failed for TRX fault

Step3: queried all the boards software, all were correct

Step4: changed the TMU board to a new one, problem was still there, recovered it

Step5: reset TRX and changed all the connections to the TRX to new one , problem was still there, recovered it

Step6: changed the TRX to a new one , problem disappeared, meant the problem was in TRX

Chapter 1 General Introduction

Chapter 2 Typical Cases

Chapter 3 BTS Fault Prevention

BTS fault prevention

nip the fault

in the bud

 Hardware: installation specifications is most important!

BTS fault prevention