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CLEAR_REQUEST：the message is sent from BSC to MSC, and it shows that BSC require MSC to release special channel resource.
As shown in the above diagram, major causes for TCH call drop are connection failure, error indication, Abis failure, O&M intervention. CLEAR_REQ is the clearance request message that BSC sends to MSC. ERR_IND is the link error message that BTS reports to BSC.
The immediate assignment process is triggered by the random access process . It is intended to assign a signaling channel for MS to implement signaling transmission in the call setup process. Normally SDCCH is assigned. When there is no SDCCH channel or an emergency call is originated, TCH can also be assigned (as early as possible). Therefore, when the CH_ACT_ACK is received during immediate assignment and the type of channel directly allocated is TCH, it will be counted into successful TCH seizures as a measurement point.
As shown in the above diagram, main causes of SDCCH call drop are connection failure, error indication, Abis failure, and so on.
Radio link fault: when radio link timeout timer is reduced to 0, the channel will be released, and a call drop will occur whose cause will be recorded as a radio link fault. In network running, such call drops are the most common.
Radio link fault means that the communication link is lost during communication. During communication, due to interference in the system or low receiving level, the voice or data often becomes too deteriorated. And finally as a result, MS or network cannot correctly decode the information sent from the opposite side; moreover this cannot be controlled in any other ways. In this case, the system will conclude that a radio link fault occurs and the MS will either start call re-establish or forcedly disconnect the link. Forced link disconnection will lead to a call drop. Therefore, a radio link fault will be concluded by the system only when the communication quality is unacceptable. In Huawei system, the parameter Radio Link Timeout is defined in system information data, which is used by MS to decide when to disconnect the call (downlink) if SACCH decoding fails. In cell property data, the parameter SACCH multi-frames is defined, which is used by BTS as a criterion to decide the disconnection of uplink and inform the BSC a radio link connection failure (uplink). The radio link fault algorithms at BTS and MS are consistent. That is, when a dedicated channel is assigned to MS, it will start counter S. Then each time when an SACCH message cannot be decoded, S will reduce by 1, and each time when an SACCH message is decoded correctly, S will increase by 2. When this value is reduced to 0, radio link failure will be reported.
If handover to the target cell fails, the MS should return to the original cell. So, in the source cell, the original channel should be reserved for some time to accept the MS if it fails in accessing the target cell. T3103 is the timer for remaining the original channel. T3103 timeout leads to a call drop. When BSC sends the Handover Command to BTS, counter T3103 will start counting. When receiving Handover Complete from the handover destination cell or Handover Failure from the original cell, BSC will reset counter T3103. After sending the Handover Command to BTS, if BSC still cannot receive the message after counter T3103 times out, it means the MS failed in accessing the target cell and cannot return to the original channel. Then the BSC will release the reserved channel in the original cell.
Interference mainly includes co-channel, adjacent-channel, inter-modulation and external interference. When MS receives intensive co-channel or adjacent-channel interference signals in the serving cell, BER will be deteriorated and will result in call drop. When there is serious inter-modulation interference in BTS, it will also result in call drop.
Equipment problems: due to self-excitation of TRX or tower-mount amplifier, the system noise coefficient becomes larger and the sensitivity is deteriorated. Inter-modulation of the antenna is also an equipment problem. Analyze according to drive test data: interference area, signal quality distribution and the overlapping that causes interference. Adjust the BTS antenna downtilt, transmission power, adjacent cell relationship, handover parameters of the relevant cells or adjust the frequency planning to avoid interference.
1. If there are results in the interference band 3, 4, and 5, usually the interference problem should be taken into consideration. The interference band is reported to BSC by the BTS via the RF resource indication message when the carrier channel is idle. It should also be noted that the interference band condition of this channel will not be shown in the traffic measurement when the 3. In cell measurement function/inter-cell handover measurement function, or outgoing inter-cell channel is blocked or busy. 2. If there are too many times of high receiving level with low quality, it means that there is co-frequency or adjacent frequency interference or external interference. handover measurement function, outgoing handover attempts due to various causes are measured. If there are too many times of handover caused by low quality, it indicates possibly there is interference. The handover times due to low uplink and downlink quality shows how serious the uplink and downlink are interfered. 4. Take the average receiving quality level of TRX for reference. 5. Record the average level and quality upon call drop for reference.
1. Isolated island effect. For some uncertain reasons, the service cell may cover a too large area, superposing the adjacent cells. As a result, after MS goes beyond the coverage scope defined for the adjacent cell B and reaches cell C, it still occupies the signal of the original service cell A. However, cell A does not define cell C in the adjacent cell list and at this time MS will perform handover according to the adjacent cells table provided by the original service cell A. In this case, call drop will occur because the appropriate target cell cannot be found. (Isolated island phenomenon) 2. Coverage hole: there is uncovered area between cells. 3. Signal attenuation: serious fading occurs during signal propagation so that handover cannot be implemented in time and this causes a call drop. 4. The adjacent cell definition is incomplete so that MS keeps conversation in the current cell until it goes beyond this cell coverage edge and as a result, call drop occurs. 5. If the uplink signal coverage is larger than the downlink signal coverage, the downlink signal of the cell edge will become weak and can easily be “submerged” by the intensive signals of other cells. While if the downlink signal coverage is larger than the uplink signal coverage, MS has to remain under this intensive signal. However, if the uplink signal is too weak or the voice quality is too bad, call drop will occur.
Drive test is the most direct way to locate down a coverage problem. Traffic measurement can be another useful method. 1. In power control measurement function, the average uplink and downlink signal intensity is too low. 2. In receiving level measurement function, the proportion of low receiving level times is too large. 3. In cell measurement function/inter-cell handover measurement function, the level when originating a handover is too low and the average receiving level is too low. 4. In call drop measurement function, the level during a call drop is too low and the TA value before a call drop is abnormal. 5. In undefined adjacent cell measurement function, the undefined adjacent cell average receiving level is too high (over-shooting coverage). 6. The average level of undefined adjacent cells is too high (isolated island phenomenon). 7. In power control measurement function, the maximum distance between MS and BTS exceeds the normal value frequently. 8. In outgoing-cell handover measurement function, the handover success rate to a certain adjacent cell is low. 9. Register “uplink and downlink balance measurement function” in traffic measurement and analyze whether uplink and downlink are imbalanced.
1. According to traffic measurement analysis, make sure that the cell has a high call drop rate and a high outgoing inter-cell handover failure rate. And most handovers are caused by low signal level or bad quality, but all other indices are normal. If this is the case, it is necessary to check whether there is the coverage problem. Drive test is feasible to find out the areas with insufficient coverage. Analyze whether it is caused by terrain reasons, such as tunnels, large shopping centers, entrances of subway. Generally speaking, most of this kind of call drops occur in one direction and this problem can be solved by adding a micro-cell. 2. Find out the areas with insufficient coverage. The BTS coverage can be increased by adding new BTS or in some other ways such as increasing the maximum BTS transmission power, adopting the zero fill antenna, and changing azimuth and downtilt of the antenna. 3. Restrict the BTS coverage scope. In terms of hardware, adjust BTS downtilt or antenna height. As for software, lower the maximum transmission power or increase the minimum receiving level (RXLEV ACCESS MIN) and RACH minimum access level so as to restrict the coverage range. 4. Check whether the adjacent cells are defined resonably and whether they are in mutual symmetrical relation. Pay more attention to external adjacent cell data. 5. Judge whether the uplink and downlink are balanced. Check the tower-mount amplifier, power amplifier, antenna feeder and combiner, and check the alarm to make comprehensive analysis. Check the BTS antenna. If the diversity receiving antenna is wrongly connected, the diversity receiving gain of the antenna will be greatly reduced and the uplink signal will become worse than the downlink signal. According to the TA value, check whether BTS coverage scope is too large or whether the signal is reflected. In this case, some weak uplink MS signals will drop out very easily after accessing the network.
1. Radio Link Timeout and SACCH Multi-Frames When these two parameters are too small, the radio link fault can easily occur and thus call drop will happen. If difficult access or serious call drop is caused by too bad downlink radio environment, properly increase the value of the Radio Link Timeout parameter. If difficult access or serious call drop is caused by too bad uplink radio environment, properly increase SACCH multi-frames. 2. RACH minimum access level It indicates the signal level threshold that the system uses to judge effective MS random access. It is used to control uplink access (call, paging response, handover). Adjustment of this parameter can get a compromise between coverage and call drop rate. 3. RXLEV_ACCESS_MIN RXLEV_ACCESS_MIN indicates the minimum receiving signal level for MS to access the cell. If this parameter is too low, as a result, many MSs with low receiving level will reside in the cell, so the cell load will be increased and call drop occurs more frequently. Therefore, it should be set according to the link balance between uplink and downlink. 4. Call re-establishment permitted In case of the call drop due to a radio link fault because of sudden interference or coverage hole, MS will start the call re-establishment process to recover conversation if permitted. Call re-establishment can make the service better, but it takes long time .It can be applied to suburbs or city areas with bad coverage.
5. NCC permitted NCC permitted, NCC permitted is sent in system messages 2 and 6. It lists the combinations of NCCs that MS must measure. MS will not report the adjacent cell’s measurement report whose NCC is not permitted. If this parameter is not set properly, call drop will be caused for necessary handover cannot be triggered. 6. Set handover-related parameters properly to lower the call drop rate. For example, let PBGT handover easily take place, so as to avoid interference and lower the call drop rate. Set the emergency handover threshold properly so that emergency handover can be triggered before a call drop so as to reduce call drops. 7. Optimize the power control parameters to make it more sensitive. In addition, the level after power control must be intensive enough to ensure normal conversation. 8. Unsuitable frequency planning will bring interference in the system. A good frequency planning can minimize the call drops obviously.
System information data: Radio Link Timeout (effective on MS), 32 or 56 recommended. Cell data: SACCH multi-frames, 31 recommended.. Radio link connection timer, maximum resend times of physical information is related to call drop during handover. Find out call drop causes through call drop performance traffic measurement. Judge from the cause of call drop: the cause “error indication” is usually caused by T200 timeout.
Hardware problem may cause instability when it processes traffic leading to call drop; hardware fault may also generate internal interference that leads to a call drop. Due to possible fault in TRX or performance decrease of some parts, TRX amplification circuit self-activation may occur, thus internal interference will be generated. If the transmission link is unstable, it will cause signaling loss and low-layer link instability, and even call drop. Many Abis call drops can be avoided if stable transmission quality is ensured. If the azimuths of two antennas of one sector are different, possibly the MS can receive SDCCH, MS may drop because of the TCH signal strength very poor.
Frequency planning principles: 1. Same frequency cannot exist in the same BTS. 2. The frequency interval of BCCH and TCH in the same cell should better be above 400K. 3. When there is no frequency hopping, the frequency interval between TCHs in the same cell should better be above 400K. 4. In non- 1*3 frequency reuse mode, the immediate adjacent BTSs cannot use the same frequency (even if the directions of the antenna main lobes are not the same, the interference of side lobes and back lobes may cause strong interference). 5. In consideration of the complexity of antenna height and propagation environment, the two opposite cells cannot be arranged same frequency. 6. Usually, 1*3 frequency reuse should ensure that the number of hopping frequencies should be more than twice of the hopping carriers. 7. Make sure to avoid the situation that the same BCCH or BSIC exists in adjacent areas.
From the above we can see, the interference goes along with the traffic. This is the characteristic of internal interference. But we cannot exclude the possibility of that the interference come from another wireless communication system.
1. Interference bands as high as level 3-5 appear. 2. Interference is quite a possible cause of congestion. 3. The call drop rate is far higher than the normal. 4. High BER. Sometimes even if the uplink receiving level is up to -70dBm, the receiving BER may also be bigger than 12.8%. 5. Check the traffic statistic of handover causes to make judgment If there are many handovers triggered by uplink signal quality deterioration, it can be caused by uplink interference or hardware fault. If there are many handovers triggered by downlink signal quality deterioration, it can be caused by downlink interference or hardware fault cause it. If there are many handovers triggered by both uplink and downlink signal quality deterioration, hardware fault should be ruled out firstly, and then check interference.
The repeater is widely used in early phase of network construction to extend the BTS coverage. Due to its own characteristics, it will bring interference when being used improperly. The repeater has the two following interference modes: 1. As installation of the repeater doesn’t conform to relevant standard, there is not enough separation between the donor antenna and transmit antenna. As a result, self-activation is formed and the normal work of the BTS which this repeater relies to is affected. 2. Since the repeater is a broad frequency band and non-linear amplifier, its intermodulation indices are far larger than protocol requirements. If the power is too large, its intermodulation signal will also be strong and it can interfere with adjacent BTS very easily.
Clear uplink interference Currently this is the major interference, which mainly occurs in peak traffic time and originates from co-channel interference, or external interference. Co-channel interference is related to the traffic of the co-channel cell. If the traffic is high, the interference is also high. External interference is mainly intermodulation interference. The uplink interference can be cleared by modifying the co-channel frequency of the co-channel cell base on analyzing related results in drive test, increasing the distance between two co-channel cells. Interference can also be reduced by frequency hopping, DTX and effective power control. Clear downlink interference Downlink interference are mainly co-channel interference and adjacent-channel interference of some cells due to inappropriate frequency planning. The interference source can be found out with the spectrum analyzer.
1. Because no matter which TRX of this cell is blocked, the congestion rate is always relatively high. There can be interference or the terrain in the coverage range of the cell is possibly complex. 2. It is concluded that, by viewing and analyzing the traffic statistic data, the interference band of cell 3 basically stays at 4 or 5 in daytime, and it stays at band 1 or band 2 between 23:00 PM and 7:00 AM. In addition, the call drop rate and the interference band are regular. 3. First take co-channel and adjacent-channel interference into consideration. Change the frequency. The frequency interval of cell 3 is changed to 1M . But the problem persists. 4. Then consider the equipment problems. Interchange the antenna and feeder of cell 3 with that of cell 1, but cell 3 interference remains the same. Therefore, it can basically be concluded that there is no problem with the BTS devices below the antenna and feeder. After the above possibilities are excluded, the fault can be located as external interference.
1. Although there is a 10MHz distance between this frequency band and that used in this cell, it is a continuous signal and it can be more possibly to conflict and inter-modulate with other signals. Some parts of intermodulation components may fall in the receiving band and form interference. 2. In daytime the traffic is larger than that at night，so the intermodulation components (interference) are also more than those at night.
It is found that, after multiple on-site dialing tests, there really exist call drops and noise. However, it can be seen from the test MS that it always stays in a service cell of a remote BTS A before call drop, and its TA value is about 17, and the receiving signal strength is about -80dBm.
When there is an isolated coverage island from a cell in an area, if MS stays in this cell at the island area and make a call, no matter how the signal changes, handover cannot be implemented normally and a call drop occurs. To avoid such situation, two means can be used. The better one is to adjust the antenna of the cell to eliminate the isolated island phenomenon. However, due to the complexity of radio propagation, usually multiple experiments are required to eliminate the isolated island effect while the coverage area is not obviously affected. In addition, it is difficult to completely eliminate the isolated island phenomenon of high buildings. The another means is to define new adjacent cells for the cell with isolated island.