JOURNAL OF TELECOMMUNICATIONS, VOLUME 8, ISSUE 2, MAY 2011 25

Failures in the Field for Base Transceiver Stations Sheikh Wadud Ahmed and Shahriar Khan Independent University, Bangladesh ABSTRACT - The growing popularity of cell phones has greatly increased the importance of mobile-phone base stations. Outage, call drop, and diminished operation of even a single voice channel can lead to significant loss of revenue. This paper reports some failures in the field of BTS. Selected failures have been shown in detail. Faults due to cableconnectors were found to contribute to the largest number of outage hours. Microwave misalignment owing to bad weather was another cause of incipient failures. The analysis in this paper will help determine better design, operation, installation and maintenance for BTS. Index Terms: Failure, BTS, Base Transceiver Station

1 INTRODUCTION The Base Transceiver Station (BTS) is an important component of any mobile communication system, as about 80% of the total equipment budget goes to BTS equipment. BTS failure causes significant outage hours and revenue loss, and has been the subject of some academic research [1],[2],[3]. In spite of it’s great scientific and commercial value, there is very little existing data on BTS failures in the field. Data on field failures was obtained through a university-industry

collaboration [4]. A range of failures were observed and analyzed from selected BTS in a fully operational GSM system.

2 FAULT TREND ANALYSIS Certain failures and diminished operation at any BTS unit cause alarm at the alarm server, and are the focus of this study. Category wise fault occurrence (figure 1) and outage hours (figure 2) within a four-month period are listed below:

Categorywise Fault Occurance % Distribution, May-August 2006 BSC PROBLEM BTS HARDWARE 16% 3% 1%

1% 2%

4%

BTS SOFTWARE EXTERNAL POWER

1% 30%

HIGH CAPACITY INTERNAL POWER LOW CAPACITY MAINTENANCE WORK OPTICAL FIBER

30% 4%

8%

OTHERS PLANNED WORK TEMPERATURE PROBLEM UNIDENTIFIED

Figure 1. Category wise fault occurrence to BTS (May to Aug)

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JOURNAL OF TELECOMMUNICATIONS, VOLUME 8, ISSUE 2, MAY 2011 26

Categorywise Outage(Cell-Min)/BTS, 2006 1800.00 1600.00 1400.00 1200.00 1000.00 800.00 600.00 400.00 200.00 0.00

1550.83

843.15 593.96

500.04

376.79 3.45 BSC PROBLEM

60.79

166.32

150.25

BTS BTS HARDWARE SOFTWARE

90.39

23.38

EXTERNAL HIGH CAPACITY INTERNAL LOW CAPACITYMAINTENANCEOPTICAL FIBER OTHERS POWER POWER WORK

4.02

9.58

PLANNED TEMPERATUREUNIDENTIFIED WORK PROBLEM

Figure 2. Outage per BTS: It was found that most faults and diminished operations were due to external power failure. Low capacity faults were mainly due to cable-connector fault, and adverse weather changing antenna alignment. Faults due to cable-connector & antenna misalignment were considered for further level root cause analysis and investigation. Root cause analysis for cable-connector type of fault and antenna misalignment type of fault are analyzed. Physical site inspections were conducted for unusual and unidentified faults for which alarm data is not enough to identify the root cause. Some issues like grounding connectivity in the BTS, radio and transmission planning, design level issues etc need further improvement recommendations.

As cable–connectors and transmission type of faults were found to be predominant, further study had been done to categorize them. Physical site inspections were also done in several sites. 2.1 Cable-Connector Faults Faults due to Cable-connectors were found to contribute to the largest number of outage hours. 40% of total outage due to PDH fault was cable-connector related. A total of 458 faults occurred in the four month period, which contributed to significant outage hrs.

Table 1. Outage for cable-connector fault (May to Aug 2006)

Fault Type

Outage in Hrs

Cable Fault

1050.063

Connector Fault

1036.023

Other Faults (Equipment, Unidentified, Unusual)

Total

3201.34

5287.426

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JOURNAL OF TELECOMMUNICATIONS, VOLUME 8, ISSUE 2, MAY 2011 27

Figure 3. Outage for cable-connector fault (May to Aug 2006)

Figure 4. Category wise cable connector fault (May to Aug 2006)

3 MICROWAVE ANTENNA RELATED LOW CAPACITY FAULTS

reported that a total of 27 links lost their alignment, in the investigation it was found that several faults took place due to other reasons.

Significant outage hours were due to antenna misalignment and other type of faults. Though it was

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JOURNAL OF TELECOMMUNICATIONS, VOLUME 8, ISSUE 2, MAY 2011 28

25

186.57

Incidinets

20

149.4

15 10

10

9 6

5

50.52 2

0 April

May

June

200 180 160 140 120 100 80 60 40 20.38 20 0

Outage(Hrs.)

Monthly Incidents and Outage for Antenna Disalignment

July

Month Incidents

MTTR

Outage

Figure 5. Monthly incidents and Outage for Antenna Misalignment (April to July 2006)

4 ROOT CAUSE ANALYSIS FOR LOW CAPACITY FAULT

around 406.7 hrs. Loss of alignment for 10 links occurred within one year of going on air.

Out of total 27 links, 17 lost their alignment within the four-month time period. This contributed to outage of

Table 2. CATEGORY WISE ANTENNA MISALIGNMENT FAULT ( OUT OF 27 LINKS ) Deficiency Type Nos of Faults Antenna misalignment 17 IF cable & grounding cable lost 2 ODU connector change 2 Polarization mismatch from plan 1 Water entered into IF cable 1 Jam nut is tight though inside nut is loose 1 Insect problem into IDU 2 HOP distance was minimization 1 Total 27 Root cause of reported 17 misaligned antenna were taken care of for further level analysis and it was found that 13 no. of 1.2 m antenna are affected with this type of problem. Only a single antenna had the support pipe installed. The remaining antennas did not have any support pipe installed. Company A antennas was found to be more susceptible to misalignment than other companies B, and C.

5 SITE INSPECTIONS At the investigation phase of fault data analysis, site visits were performed to find the root cause of unusual

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JOURNAL OF TELECOMMUNICATIONS, VOLUME 8, ISSUE 2, MAY 2011 29

faults, which could not be identified from analyzing the fault data. Several deficiencies were observed, which are discussed for further improvement. Eleven sites were visited for the above purpose, and the findings are listed below:

Feeder Hole Open The feeder hole being open (figure 6) is a common problem in BTS sites. The open feeder hole also allows dust & insects to get inside the room. An open feeder hole increases room temperature and reduces BTS equipment life.

DHSHN3 CMKAI1 Col

Figure 6: Feeder Hole Open & Insect attack in IDU Temperature Alarm During investigation of BTS sites it was found that sometimes the temperature inside the BTS room exceeded the recommended value of temperature for

telecom equipment. Temperature related problems were found in 3 out of 11 site visits. Some sites had temperature alarm for 3 to 4 days.

Figure 7. Observed high temperatures, which may contribute to decreased equipment life.

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JOURNAL OF TELECOMMUNICATIONS, VOLUME 8, ISSUE 2, MAY 2011 30

5.1 IF Cable Damage It was found in a number of cases, that during expansion or installation work MW, the IF cable had

been damaged. A damaged I.F cable causes severe error second rates during transmission, and deteriorates network performance.

Figure 8. IF Cable Damage

5.2 Civil Works Design Problem Water logging above the BTS room and penetration inside the room were found to be a frequent problem in many BTS sites. Wrong air conditioner tilting outside caused the stagnant water to get inside the equipment

room and finally make the room damp. Water logging above the BTS room, in same cases, caused the Indoor transmission unit to go out of order. Long term accumulation of water may dampen the ceiling of equipment room, and reduce the life of the structure.

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A/C Drainage problem

Figure 9. Civil Works & Design Related Issues

5.3 Connector Sealing Problem On site inspection showed that in some cases, the connector was not sealed as required. Often, the protective layer was found to have deteriorated. In

some cases, there was no sealing or sometimes very poor sealing in the microwave connector at the outdoor unit. Water ingress into the feeder caused the link to go down.

Sealing lost or going to loose

No sealing

Figure 10. Connector sealing problem

5.4 Poor Grounding Condition In some cases, on site inspection revealed that grounding measurement was out of limit. In a few places grounding connections were found loose or disconnected. Outdoor and indoor grounding

connections were found to be connected 1 m below the BTS room in roof top sites. In case of a storm, a lightning strike would go inside the BTS room causing damage to equipment.

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Earthing cable lost Poor connection

High resistance

Figure 11. Poor Grounding Condition

for GSM Base Transceiver Station, Master’s Thesis, Independent University, May 2007.

6 CONCLUSION Failures over many months have been documented in this paper. The failures were both outage causing, and those causing diminished operation. In terms of no. of faults occurrence, faults due to external power failure are the most significant. Low capacity faults are mostly due to cable-connector fault and antenna misalignment caused by adverse weather. The data and their analysis are expected to be very useful for future design, operation, and maintenance of BTS.

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Sheikh A. Wadud, Acquisition and Characterization of Deployment and Fault Data

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