RXLEV ACCESS MIN SD Drop
"The minimum received signal level at a MS for access to a cell"
ACH threshold is the minimum rxlev below which a RACH burst is not decoded RXLEV Access Min is the minimum rxlev below which a RACH burst is decoded and contains a Request for a Call (Normal or Emergency), but is rejected. Increasing the RLAM is usually enough to increase the SDCCH success... fyi, in alcatel, the RACH Threshold is not changeable.
andri - 28 May 2008
basically, both can reduce SDCCH Drop. Higher value will get better SDCCH drop. Based on my experience, RACH Threshold should be less than Rx LevAmi...
BSS - 28 May 2008
peyo dear RACH min access level is -105 and RX level Access min is -100
asically, both can reduce SDCCH Drop. Higher value will get better SDCCH drop. Based on my experience, RACH Threshold should be less than Rx LevAmi...
Hi, I can see some confusion.. there is a difference between "rxlev access min" and "rxlev min". rxlev_min = 96dBm means that the cell will accept incoming HO only if the incoming MS measures it with more than -96dBm. During the candidate cell evaluation, the HO is possible towards the neighbor n1 only if : AV_RXLEV_NCELL(n1) >= RLEV_MIN(n1)
RXLEV ACCESS MIN is the limit under which a BTS will not decode an incoming CHANNEL REQUEST "Reduce the coverage of the cell" is not really precise : if rxlev min is reduced, then it means the cell is less likely to receive an incoming HO. If rxlev access min is reduced, then the area in which a call setup is allowed is decreased, yes. But if a call is starting at -65dBm, and decreases below the RXLEV ACCESS MIN, it is not rejected. So the cell coverage is as big as possible IF the call is alread setup. Regards, Pix
SDCCH Drop
When we assigned SD for call origination and at that time due to some problem or any mismatch comes by which SD loss occurs, it is called as SD Drop.
It occurs between allocation of SD and before TCH allocation. Sometimes SD drop occurs because queuing is not activated in the system.
If SD drop is high plz look on parameters likeovershooting, shift the SD timeslot , may be hardware issue, interference, change the values of RXP, PMAX, may be issue of uplink or downlink issue in that cells for UL put a TMA in that cell and for DL provide tilt ,re orient that antenna.
If SD Drop is high: If SD drop is high plz look on parameters likeOvershooting Shift the SD time slot Hardware issue Interference Change the values of RXP PMAX It may be uplink or downlink issue in which cells for UL put a TMA in that cell and for DL provide tilt Re orient that antenna.
Reasons of SD Drop: Hardware Fault. Interference. MAIO mismatch.
Bad Coverage. High TR Fail. Outage. Overshooting. Abis Drop. High Path Loss. Wrong Parameter Planning. Due to ICM Band(CDMA) High LAPD Utilization Heavy blocking and DR feature being used extensively
Solutions for removal of SD Drop:
Interference:
Check the BCCH Plan (C/I or C/A). Co-BSIC & Co BCCH. Use latest ND 111 and MapInfo to find out proper frequency to reduce interference.
Arrange Drive Test:
The best way to find the real issues for Interference makes DT. Check interference by Interference scanning. Check clean BCCH by frequency scanning.
Overshooting: LAC Planning. If a cell is picking call from long distance,Check the sample log according to TA. Cell orientation need to be defined according to clutter. Mount position Effective Tilt
High TR Fail: Check and clear TR fail from OSS end.
Bad Coverage:
If the drop call is due to low signal strength uplink, check the receive path of this particular TRX. Check receiver sensitivity, VSWR, feeder connection and etc. Drops due to Low Signal Strength. If the drop call reason is due to low signal strength downlink, then, check the transmit path. Check cards, feeder and etc. Use MapInfo or Google Earth to find location of sites.
High LAPD Utilization:
Check LAPD util report from OSS, and define 32 kbps signaling instead of 16kbps.
Hardware Fault: Check Alarms. TRX condition. Check Path Imbalance. VSWR of the Cell. Connector Connection. Some times you will find issues on BCCH TRX.In this case BCCH shift from one to other TRX will reduce SD drop.
Check for parameter: Check the Timer T 3101 Check the Timer T 200(20ms) T11 Expired(10 s) MAIO check.
UMTS UL Link budget example, (c) UMTSWorld.com TX Mobile max power = 0.125W (dBm) 21 Body loss - Antenna gain (dB) 2 EIRP (dBm) 19 RX BTS noise density (dBm/Hz) =Thermal noise density + BTS noise figure -168 RX noise power (dBm) =-168+10*log(3840000) -102.2 Interference margin (dB) 3 RX interference power (dBm) =10*LOG(10^((-102.2+3)/10)-10^(-102.2/10)) -102.2 Noise & interference (dBm) =10*LOG(10^((-102.2)/10)+10^(-102.2/10)) -99.2 Process gain (dB), 12.2k voice =10*log(3840/12.2) 25.0 Required Eb/No for speech (dB) 5 Antenna gain (dBi) 17 Cable and connector losses (dB) 3
Fast fading margin (dB) =slow moving mobile 4 RX sensitivity (dBm) -129.1 Total available path loss (dB) 148.1 Dimensioning Log normal fading margin (dB) 7 Indoor / In-vehicle loss (dB) 0 Softhandover gain (dB) 3 Cell edge target propagation loss (dB) 144.1
Okamura-Hata cell range (km) L=137.4+35.2LOG(R) 1.55
Meaning
The TCH call drop rate refers to the ratio of call drops to successful TCH seizures after the BSC successfully assigns TCHs to MSs. The TCH call drop rate can be measured from the following aspects:
TCH call drop rate (including handover) TCH call drop rate (excluding handover)
The TCH call drop rate, one of the most significant KPIs for telecom operators, is related to retainability. It indicates the probability of call drops due to various reasons after MSs access TCHs. A too high TCH call drop rate adversely affects the user's experience.
Recommended Fo rmulas
TCH call drop rate (including handover) = Number of call drops on TCH/(Number of successful TCH seizures (signaling channel) + Number of successful TCH seizures (TCH) + Number of successful TCH Seizures in TCH handovers (TCH)) x 100% TCH call drop rate (excluding handover) = Number of call drops on TCH/Number of successful TCH seizures (TCH) x 100%
Factors That Affect the TCH Call Drop Rate
According to user complaints and network optimization experience, the major factors that affect the TCH call drop rate are as follows:
Hardware failure Transmission problem Version upgrade Parameter setting Intra-network and inter-network interference Coverage problem Antenna system problem Imbalance between uplink and downlink Repeater problem
1.1 Hardwar e Fai lure
When a TRX or a combiner is faulty, seizing the TCH becomes difficult, and thus the TCH call drop rate increases.
1.2 T ransmission Problem
The TCH call drop rate increases in the following conditions:
The transmission quality on the A or Abis interface is poor for various reasons. Transmission links are unstable.
1.3 Version Upgr ade
After the BTS version or BSC version is upgraded, the BTS version may be incompatible with the BSC version, and the parameters and algorithms in the new version may be changed. In this case, the TCH call drop rate increases.
1.4 Parameter Se tting
The settings of some parameters on the BSC and MSC sides may affect the TCH call drop rate. If the following situations occur, the TCH call drop rate may increase:
1. 2. 3. 4. set.
The parameters SACCH Multi-Frames and Radio Link Timeout are set to too small values. The parameter RXLEV_ACCESS_MIN is set to a too small value. The parameter RACH Min.Access Level is set to a too small value.
The parameters Min DL Power on HO Candidate Cell and Min Access Level Offset are inappropriately
5. The length of timer T3103 (this timer is set to wait for a Handover Complete message) is set to a too small value. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. The length of timer T3109 (this timer is set to wait for a Release Indication message) is set to a too small value. The length of timer T3111 (this timer specifies the connection release delay) is set to a too small value. The length of timer T305/T308 is set to an invalid or too great value. The parameter TCH Traffic Busy Threshold is set to a too small value. The parameter Call Reestablishment Forbidden is set to Yes. The parameters related to edge handover are inappropriately set. The parameters related to BQ handover are inappropriately set. The parameters related to interference handover are inappropriately set. The parameters related to concentric cell handover are inappropriately set. The parameters related to power control are inappropriately set. T200 and N200 are set to too small values. Some neighboring cell relations are not configured. The parameter MAIO is inappropriately set. The parameter Disconnect Handover Protect Timer is set to a too small value. The parameter TR1N is set to a too small value. 21. The parameters Software Parameter 13 and MAX TA are set to too small
values. 22. If a repeater is used, the parameter Directly Magnifier Site Flag is set to No.
1.5 Intra-Net work and Int er -Net wor k Interfer ence
If inter-network interference and repeater interference exist, or if severe intra-network interference occurs because of tight frequency reuse, call drops may occur on TCHs because of poor QoS. This adversely affects the TCH call drop rate. The following types of interference may occur: 1. 2. 3. 4. 5. Inter-network interference from scramblers or privately installed antennas Interference from the CDMA network of China Unicom Repeater interference Inter-modulation interference from BTSs Intra-network co-channel and adjacent-channel interference
1.6 Coverage Pr oblem
The following coverage problems may affect the TCH call drop rate. 1. Discontinuous coverage (blind areas) The voice quality at the edge of an isolated BTS is poor and calls cannot be handed over to other cells. In this case, call drops may occur. In complex terrains such as mountainous regions, the signals are blocked and thus the transmission is discontinuous, leading to call drops. 2. Poor indoor coverage Densely distributed buildings and thick walls cause great attenuation and low indoor signal level, which causes call drops. 3. Cross coverage (isolated BTS) The serving cell causes cross coverage due to various reasons (such as excess power). An MS cannot be handed over to another cell due to no suitable neighboring cells. In this case, the signal level becomes low and the voice quality of the MS deteriorates. Thus, call drops occur. 4. Insufficient coverage If the signal from an antenna is blocked or the BCCH TRX is faulty, call drops may occur because of discontinuous coverage.
1.7 Antenna S ys t em Problem
The following antenna system problems may affect the TCH call drop rate 1. If the transmit antennas of two cells are improperly connected, the uplink signal level in each cell is much lower than the downlink signal level in the cell. Therefore, call drops are likely to occur at places far away from the BTS. 2. If a directional cell has main and diversity antennas, the BCCH and SDCCH of the cell may be transmitted from different antennas. If the two antennas have different pitch angles or azimuths, the coverage areas of the two antennas are different. In this case, the following result may occur: An MS can receive the BCCH signals from one antenna; when a call is made, the MS cannot seize the SDCCH transmitted by the other antenna and thus a call drop occurs. 3. If the feeder is damaged, water leaks in the feeder, or the feeder and the connector are not securely
connected, both the transmit power and receiver sensitivity of the antenna are reduced. Thus, call drops may occur.
1.8 Imbalance B e tween U plink and Do wnlin k
The difference between the uplink signal level and the downlink signal level may be great in the following conditions: The transmit power of the BTS is high. The tower mounted amplifier (TMA) or BTS amplifier does not work properly. The antenna and the connector are not securely connected. As a result, call drops may occur at the edge of the BTS coverage area.
