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munucool
01-30-2014, 05:02 PM
Case 2: Call drop caused by intra-network interference
[Problem description]
Customers in a place complaint that call drop happen frequently.
[Problem analysis and solution]
1) Through a careful test, engineers found that there were 12 channel numbers gathering at the call drop spot and Rxlev reached -73dBm. When the MS seized channel number 11, the interference from channel number 112 caused the call drop.
2) Through testing the CGI of channel number 12 using test MS, engineers found that this channel number was one of the BCCH number of D3.
3) Through surveying BTS D, engineers found that the antenna of D3 is installed at the top of a building. In addition, a house made of glass was found 8m away and 4m under the antenna. Engineers tested that the signal strength near the antenna was about -45dBm, and the signal strength at near the glass was -30dBm, which was beyond the expectation of engineers. In fact, the cause was that the signals reflected by the glass were reflected to the call drop spot.
4) It is suggested to change the antenna installation place and channel number. You should interchange the channel number 111 and channel number 114 of BTS A and increase the down tilt angle of A3 cell. In addition, to avoid the interference caused by channel number 111 after the interchange, you should adjust the direction of channel number 113 of C1 cell.
5) Test shows that everything is normal after the adjustment. The channel number 113 of BTS C has no effect against channel number 114. And the call drop disappears.

Case 3: Interference caused by repeater
[Problem description]
Users in an area complaint that the MS cannot seize a channel to hold conversation, or the noise is great after channel seizure and the channel and the MS signal is strong. Two BTSs are installed in this area. The antenna azimuth angle of cell1 rightly directs to the north. Before user complaint, the BTS in this area ran normally and the network indexes met the requirement. After the problem arisen, the traffic volume of the two BTSs dropped sharply from the perspective of traffic statistics indexes. In addition, the traffic volume of cell1 and cell3 also dropped sharply. Though the signals for the conversation were strong, the voice quality was quite poor. According to traffic statistics, the interference bands of the four cells were of level 3, level 4, and level 5, and 95% of the channels were interfered. In addition, other channels were interfered to some extent. However, no alarm messages were generated at the OMC.
[Problem analysis and solution]
1) According to user feedbacks, the possible reasons include transmission problem, antenna feeder problem, hardware problem, intra-network interference, and outside interference.
2) The uplink interference signals in the northwest direction might strong. Therefore, cell1, cell2, and cell3 of the two BTSs were interfered, in which cell1 and the cell3 were seriously interfered.
3) Through on-site dialing test, engineers found that it was hard to make calls in the areas covered by cell1 and cell3. Even if a call was put through, the voice quality was quite poor. In addition, the voice was discontinuous and the interference was strong. Through using MS to call a fixed phone, engineers found it was hard to hear the voice clearly. On the contrary, they could hear the voice from the fixed phone clearly. This has proved the above analysis. That is, the interference might be from the outside, or the standing wave problem was occurring at the antenna (from this perspective, it can be judged that the interference existed on the uplink only).
4) Through using antenna feeder analyzer to perform on-site test, engineers found no problem was existing at any BTS. A new repeater was found in this area, and it was located two kilometers away from the BTS in the northwest direction. Moreover, the interference appeared just when the repeater was enabled. On-site test found that the BTS became normal state once the repeater shut down, and the interference bands also became normal, so did the call. If the repeater was enabled, however, it was hard to make calls and the interference was strong. At last, the agreement to shut down the repeater was reached. After that, the conversation became normal.

scorpion
01-31-2014, 12:24 AM
excellent knowledge sharing, Can you provide the overall document now.