JOURNAL OF TELECOMMUNICATIONS, VOLUME 8, ISSUE 2, MAY 2011 61

Incipient Failures in the Field for Base Transceiver Stations S. Wadud Ahmed and Shahriar Khan Independent University, Bangladesh

. Abstract- The mobile phone Base Transceiver Station (BTS) accounts for about 80 % of total equipment budget. Of great importance is appropriate deployment and maintenance of the BTS. Outage, call drop, and diminished operation of even a single voice channel can lead to significant loss of revenue. Data and records of BTS commissioning and failure were analyzed. This paper provides such data, with the goal of better understanding of commissioning and incipient failure of BTS. Both the VSWR and BER test results were given for selected cases. Faults due to cable-connectors were found to contribute to the largest number of outage hours. Improper commissioning and operation of the BTS has been identified as the cause of various failures. Index Terms – BTS, VSWR, BER,

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. Failure of this component can result in outage hours or operation at diminished capacity. Such operation at diminished capacity can be due to lack of proper installation, insufficient preventive maintenance, or any other reason. Diminished operation, also referred to here as incipient failure, causes significant outage hours and revenue loss. Although BTS failures have been the subject of some academic research [1,2], there is very little existing data on BTS failures in the field. In this study, a wide range of data was collected successfully from selected BTS in the fully operational system [3]. Various test results and fault data are analyzed for underlying patterns and characteristics. TESTING Testing is an essential part of a base station’s installation and commissioning process. Even though

the constituent parts of a BTS are initially tested by the manufacturer, they are disassembled for transport and reassembled on site. Many faults may arise during the complex installation process. For example, wiring errors will occur even in the best-managed working environment. Connectors will not always be correctly crimped, backplane switches can be incorrectly set, and RF connectors and cables can easily introduce faults. Even one bad voice channel can affect numerous connections during the course of a day and produce a large numbers of fault reports. 2. VSWR TEST Voltage Standing Wave Ratio (VSWR) is the ratio of the maximum voltage to the minimum voltage in the standing wave on a transmission line. Standing waves are the result of reflected RF energy. As the VSWR approaches 1:1, the reflections on the line approach zero and maximum power may be transmitted. Reflections occur at any place where the impedance of the transmission line changes. VSWR value for 900 & 1800 MHz bands are compared for characterization in 12 BTS sites.

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Table 1 : Data from VSWR Test in 12 BTS sites Station

BTS-1

BTS-2

BTS-3

BTS-4

BTS-5

BTS-6

BTS-7

BTS-8

BTS-9

BTS-10

BTS-11

BTS-12

Cell No Cell A Cell B Cell C Cell A Cell B Cell C Cell A Cell B Cell C Cell A Cell B Cell C Cell A Cell B Cell C Cell A Cell B Cell C Cell A Cell B Cell C Cell A Cell B Cell C Cell A Cell B Cell C Cell A Cell B Cell C Cell A Cell B Cell C Cell A Cell B Cell C

Freq. in MHZ (Left) 946.8 938.7 936.1 926.5 917.5 927.4 935 958.1 960 916.1 959.1 954 1823 1860.9 1862.9 939.7 940 948.4 939.8 940.5 943.6 935.5 948.5 931.5 957.4 893.6 935.2 1749.5 1786.4 1872.1 1715.9 1878.7 1876 952.9 908.4 959.4

VSWR 1.281 1.323 1.314 1.114 1.138 1.182 1.569 1.282 1.373 1.172 1.128 1.159 1.198 1.208 1.269 1.36 1.374 1.307 1.348 1.362 1.315 1.217 1.139 1.198 1.127 1.179 1.229 1.331 1.283 1.257 1.314 1.263 1.193 1.35 1.309 1.391

Freq. in MHZ (Right) 947.7 952 922 925.9 880.9 910.5 959.7 954.6 957.8 892.6 959.4 880 1830.3 1719.6 1818.1 946.4 944.3 934.6 936.3 949 943.7 941.4 937.2 931.8 960 885.6 954.6 1867.5 1733.1 1808.8 1754.1 1878.7 1872.8 897.1 905.9 910.1

Cable insertion loss due to feeder damage

VSWR 1.296 1.298 1.33 1.147 1.127 1.14 1.317 1.392 1.272 1.135 1.211 1.184 1.186 1.219 1.212 1.305 1.322 1.349 1.306 1.328 1.332 1.154 1.143 1.125 1.157 1.188 1.118 1.249 1.275 1.245 1.306 1.263 1.124 1.328 1.311 1.38

Dist. in meter (Left) 28.42 28.22 27.13

1.093 1.126 1.171

Dist. in meter. (Right) 26.93 29.23 26.72

1.38 12.44 17.55

1.057 1.061 1.054

1.16 12.44 0.95

1.041 1.049 1.044

28.83 28.96 29.84 16.69 16.55 17.97

1.09 1.109 1.134 1.094 1.683 1.077

29.1 29.51 23.81 16.69 18.89 18.38

1.099 1.163 1.01 1.114 1.102 1.093

53.26 52.67 51.16

1.201 1.142 1.101

53.37 53.56 51.28

1.085 1.116 1.075

VSWR

VSWR 1.127 1.108 1.146

loss which was not taken into consideration during feeder selection and cable loss calculation.

In BTS 3 and BTS 11 feeder cables were found to be damaged during installation. It was observed that this type of damage occurred to the cable during bringing the feeder cable drum from the warehouse and transporting to the BTS sites. This type of cable damage is associated with a certain amount of insertion

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JOURNAL OF TELECOMMUNICATIONS, VOLUME 8, ISSUE 2, MAY 2011 63

Figure 2: High VSWR for excessive cable bending at BTS 3 Figure 1: Feeder cable damaged at BTS 3

High VSWR for excessive feeder bending In BTS-1 it was found that Cell A, B and C were being affected by a sharp bending radius at a distance from 26 to 29 metre away from radio base station end.

Figure 3: VSWR value for BTS-1 Cell-B against distance Sharp bending radius was found to be a common problem associated with the installation works in several BTS. The distance of fault was measured using the distance to fault identification test. The same tests were done in BTS 3, 6, 7 and 12

away during preparation of feeder connector (figure 4). It is a widespread problem in this company’s network and the fault was identified by distance to fault test.

High VSWR due to feeder connector In BTS 3 Cell A the VSWR was found to be 1.569 which is totally unacceptable. Then the distance to fault test were performed to identify the fault. Finally it was found that the fault was associated with jumper connector 1.69 m away from radio base station. The insulation layer inside the connector was not taken out © 2011 JOT http://sites.google.com/site/journaloftelecommunications/

JOURNAL OF TELECOMMUNICATIONS, VOLUME 8, ISSUE 2, MAY 2011 64

3. BIT ERROR RATE TEST BER test meter is used to measure the performance of the newly installed microwave link and also to check the transmission path from the BTS to the BSC. The test of the transmission path was done for 12 BTS sites for around 3 - 9 hours. A transmission having a BER of 10-6 means that out of 1,000,000 bits transmitted, one bit was in error.

Figure 4: High VSWR for feeder connector faults

Table 2: BER Test Result Station BTS-1 BTS-2 BTS-3 BTS-4 BTS-5 BTS-6 BTS-7 BTS-8 BTS-9 BTS-10 BTS-11 BTS-12

% Error Second 0 0 0 0 0 0 0 6.7 0 2.1 0 0

% Error Free Second 100 100 100 100 100 100 100 93.3 100 97.9 100 100

Avalibale Second 76091 32400 32400 44515 32400 32400 44240 32400 32400 32400 44515 32400

BER 10^-11 10^-10 10^-10 10^-10 10^-10 10^-10 10^-10 10^-7 10^-10 10^-8 10^-10 10^-10

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Link Status OK OK OK OK OK OK OK OK OK OK OK OK

JOURNAL OF TELECOMMUNICATIONS, VOLUME 8, ISSUE 2, MAY 2011 65

A/C Cover not found

BOBLG1 Reg Outside screw missing in L-connector Old U-link used during relocation

Different u-link & L-connector used

Figure 5: Different brand of U-link & L connector used

Connector Problem Different brand of u-link & L-connector were found to be used in the BTS sites causing design mismatch and specification problem. Different brand of u-link & Lconnector causes design oriented and specification mismatch.

Undersized Battery Cable Undersized cable in between cell of battery was found in the BTS sites having burnt sign in the cable layer. The cable used to interconnect the battery cell were undersized causing the cable to burn.

During relocation, often old U-link, L-connector & PCM cables are used which creates shouldering & grounding related problems.

Example: No Cell Number

Sign of burn

Figure 6. Undersized cable in Battery Polarization not as per plan A mismatch in the transmission plan was found during site inspection. The mismatch was in the polarization of the microwave antenna. This had caused severe

error second and unavailable second during BER test. There had been significant call drop and diminished performance of the cell.

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As per plan polarity Horizontal but found Vertical during rectification.

Figure 7: Transmission plan not followed properly

Lightening Arrestor Problem During on-site inspection, it was found in a few sites, the lightening arrestor for the tower was missing.

Lightning could have damaged equipment with significant outage.

the

expensive

No lightning arrester

Figure 8. Lightening Arrestor is missing

Connector sealing and bad weather condition In BTS 9 the results showed that there was 6.7 percent error second during transmission. It was observed that

the connector sealing was improper, causing ingress of water into the connector, with severe error seconds in

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[4]

Transceiver Station, Master’s Thesis, Independent University Bangladesh, May 2007. Kuo-Chung Chua, Frank Yeong-Sung Linb, “Survivability and performance optimization of mobile wireless communication networks in the event of base station failure,” Computers & Electrical Engineering, Recent Advances in Wireless Networks and Systems, Volume 32, Issues 1-3, Pages 50-64, JanuaryMay 2006

Figure 9: Microwave outdoor unit connector problem

] the transmission path. It was because the microwave frequency range is more susceptible to rainwater drops or dusts and snow molecules. It was observed that the link performance deteriorated, showing error seconds in the transmission path, during a moderate storm in the month of April. Microwave alignment problem It was noticed that error seconds were observed at BTS 10. It was observed that the fault was associated with microwave alignment problem and there was no support pipe to give protection to the antenna against the wind thrust imposed on it. The problem was rectified after providing the support pipe and fixing the antenna in proper alignment.

4. CONCLUSION Field data on VSWR and BER tests have been given. Incipient failures, or the type that cause diminished operation, are the focus of this paper. Incipient faults, such as these studied in this paper, deserve more attention, because of the difficulties in detection, and rectification. This field data is expected to be useful in determining future strategies on maintenance, installation, and design.

REFERENCES [1] N. Purohit, S. Tokekar, “Performance Analysis of Downlink GPRS Traffic after a BTS Failure,” Wireless and Optical Communications Networks, 2007. WOCN '07. IFIP International Conference on 2-4 July 2007, pp 1 - 5, Singapore [2] Purohit, N.; Tokekar, S, “Performnance Analysis of EDGE after a BTS Failure,” Wireless Communication and Sensor Networks, 2007. WCSN '07. Third International Conference on, 2007 , pp 100 - 104 [3] Sheikh A. Wadud Feroz Khan, Acquisition and Characterization of Deployment and Fault Data for GSM Base

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