Call failure caused by low paging channel gain

Call failure caused by low paging channel gain
Typical case:
In the beginning of origination, MS Rx power is normal, pilot Ec/Io is normal, reverse Tx power is normal;
In the call procedure, MS moves far away from BTS, when MS is some distance from BTS (inside the coverage), access is failed;
At the point of access failure, MS can not stay in the current cell or always is in initialization state, or stay in other cell whose pilot Ec/Io is very low;
At the point of access failure, test pilot strength, the Ec/Io of the origination cell PN is normal;
Use the equipment (like Viper) to test the power of code domain, and find this cell paging channel power is not enough.

Analyzing:
Paging channel gain is set in background. If it is too low, even if pilot power is high, but MS can not stay in this cell because it can not demodulate paging message.
In the above case, when MS starts origination, all the signals are normal. But when MS is far away from BTS and exceeds the paging channel coverage, MS will not receive paging channel message correctly and call will be failed finally. After call failure, MS can still not stay in the current cell. Use the equipment (like Viper) to test the power of code domain, and can find this cell paging channel power is not enough

Optimization method:
Check background parameters setting and adjust paging channel gain.

CDMA Call failure caused by unsuitable access parameters

Call failure caused by unsuitable access parameters
Typical case:
MS Rx power is normal and pilot Ec/Io is normal;
Observe MS received signal messages in DT test, MS does not receive BTS ACK message after MS starts origination, then finally the access is failed;
In this period MS Rx power is normal, there is no large fading;
But MS Tx power is not high before access failure:

Analyzing:
According to call procedure principle, when BTS receives MS access request, BTS will send ACK message to MS. In above cases, MS does not receive BTS ACK message, this is the main reason for call failure.
MS does not receive ACK message, two possibilities:
1. BTS does not receive MS access request;
2. BTS has sent ACK message, but MS does not receive.

For the second, usually the reason is large fading in forward link and MS can not receive the message. According to DT test, if there are no large fading phenomena, this reason can be excluded.
For the first reason, according to DT test, it is very possible. There are also two possibilities:
There is large fading or strong interference in reverse link; In this case, MS Tx power will tend to maximum and it can be solved;
Access parameters setting are not suitable and access problem power is too low, so access request can not be received correctly.

The related parameters are like the following:
INIT_PWR
Definition: Initial power, the correction factor used by the mobile stations in the open-loop power estimation for initial transmission on an access channel expressed as a binary complement value in units of 1 dB
Range: -16~~15dB, default value is 0dB;
Note: High INIT_PWR value is better for access channel acquisition, but increases reverse interference; low INIT_PWR will make access channel acquisition become difficult.

NOM_PWR
Definition: Nominal transmit power offset, BTS use this parameter as correction factor for open-loop power control, expressed as a binary complement value in units of 1 dB
Range: -8~~7dB, default value is 0dB;

PWR_STEP
Definition: Power increment, the value by which mobile stations increase their transmit power between successive probes in an access probe sequence, in units of 1 dB.
Range: 0~~7dB

NUM_STEP
Definition: Number of access probes. This parameter is one less than the maximum number of access probes that mobile stations are to transmit in a single access probe sequence.
Range: 1~~15dB, default value is 6
Note: high NUM_STEP value can increase access probability, but also can increase access time; low NUM_STEP value will decrease access probability.

MAX_REQ_SEQ
Definition: The maximum number of access probe sequences for an access channel request. The value must be greater than 0.
Range: 1~~15, default value is 2
Note: high MAX_REQ_SEQ value can increase access probability, but also can delay access time; low MAX_REQ_SE value will decrease access probability, but decrease access time.

MAX_RSP_SEQ
Definition: The maximum number of access probe sequences for an access channel response. The value must be greater than 0.
Range: 1~~15, default value is 2
Note: high MAX_ RSP _SEQ value can increase access probability, but also can delay access time; low MAX_ RSP _SEQ value will decrease access probability, but decrease access time.

Note:
To confirm if BTS receives access request or sends ACK message, the best way is to start signal tracing for this MS.

Optimization method:
Check background parameter setting and adjust access parameters.

Some analyzing when MS termination call is not successful

Some analyzing when MS termination call is not successful
Typical case:
Originated MS signal is normal;
Originated MS finally can here voice record: “the number you dialed is not reachable”.

Analyzing:
Commonly the reason may be the following:
The terminated MS is not in service area or the signal coverage is poor;
The terminated MS just crosses LAC and does not start location update registration;
The terminated MS is just in idle handoff state.
In order to confirm which reason causes the problem, need analyze the condition and locate the problem.
Acquire if the terminated MS is in coverage area and judge if the reason is MS has moved over coverage area or signal is too bad;
Acquire if the terminated MS is in the border of two LACs, judge if the reason is MS does not make registration in time (Signal tracing can be used to analyze), usually one city should be assigned only one LAC area, except for very big city;
If the above two cases can be excluded, it is very possible that MS is just in idle handoff and can not be terminated successfully.
In IS-95A 6.6.2.1.4.2 protocol:
In idle handoff, MS will go into non-slot mode. After idle handoff, MS will throw away all the messages from the old BTS, which has not been processed. This means: when BTS sends paging message to MS, MS is just in idle handoff and MS can not receive the message correctly; furthermore, even if MS has received the message, it can still not send paging response. These two cases will lead that MS can not be terminated successfully.

This is why we often meets the case: the first dialing to other MS, we will hear voice record: “the number you are dialing is not reachable” (because MS is just in idle handoff); but the second dialing will be successful (because MS has finished idle
handoff).

Note:
System not support access handoff also can cause terminated call failure. When MS is paged successfully and MS is just moving from the current cell to another cell, MS will access failure because the system does not support access handoff and the current cell pilot fades too fast.
In this case, MSC side often sends voice record to origination MS like: “The network is busy now, please redial later”. Certainly different operators may send different voice records.

Optimization method:
If the reason is coverage, it is normal;
If the reason is crossing LAC, check LAC planning. Usually the border of LAC should be set in the low traffic area;
If the reason is idle handoff, it is normal phenomenon. If the problem is very heavy,
consider adjusting the network topology structure and changing idle handoff area.

Drive Test Application in Call Drop Analyzing

1. BTS bad frame
when FER reaches to 90% in 100 frames and this happens 3 times consecutively, BTS will release the links.

Drive Test Application in Call Drop Analyzing
Call drop is the interruption of call and it is caused by BTS or MS releasing traffic channel without user permission.
Call drop mechanism
CDMA system is a closed-loop system, the closed-loop signal link is very necessary between BTS and MS in data or voice transmission. Some important process, like power control and handoff, all need one closed-loop link. If this link is broken for any reason, MS will lose control and need initialization again and go back to idle state. When closed-loop signal link is broken, call drop mechanism will be started. Closed-loop signal link includes forward link and reverse link, any one link is broken can cause call drop.

1 MS drop mechanism

1. MS bad frame
If the mobile station receives N2m consecutive bad frames on the Forward Traffic Channel, it shall disable its transmitter. Thereafter, if the mobile station receives N3m consecutive good frames, the mobile station should re-enable its transmitter.
(Extracted from 95 standard 6.4.4) In 95 standards, N2m is defined as constant 12; N3m is defined as constant 2.

2. MS fading timer
The mobile station shall establish a Forward Traffic Channel fade timer. The timer shall be enabled when the mobile station first enables its transmitter when in the Traffic Channel Initialization Sub state of the Mobile Station Control on the Traffic Channel State. The fade timer shall be reset for T5m seconds whenever N3m consecutive good frames are received on the Forward Traffic Channel. If the timer expires, the mobile station shall disable its transmitter and declare a loss of the Forward Traffic Channel. (Extracted from 95 standard 6.4.4) In 95 standards, N3m is defined as constant 2; T5m is defined as constant 5s.

3. MS acknowledgment failure
If the mobile station has not received an acknowledgement within T1m seconds after transmitting the message, the mobile station shall retransmit the message. (Extracted from 95 standard 6.6.4.1.3.1.1) The mobile has N1m attempts to transmit a message that required acknowledgment. If the mobile does not receive the acknowledgement T1m seconds after the N1m time, the mobile declares an acknowledgment failure. In 95 standards, N1m is defined as constant 3; T1m is defined as constant 0.4s.

2. BTS acknowledgement
There 9 times for BTS acknowledgement, and the waiting time between two acknowledgements is 0.4s.

Integrated analyzing
Call drop ratio can be used to evaluate network integrated performance. Call drop ratio improvement can decrease subscribers’ complaints and increase their satisfaction. For the large scale commercial network, call drop ratio observed from background OMC is more accurate and impersonal; for unloaded or light-load network, it can be got by DT test. In DT test, the call mode adopts “sequence call test”, call drop ratio is defined as: call drop number/total successful call number; if adopts “long call test”, total successful call number is total calling time (s)/90. For urban area, call drop ratio is less than 0.8% means network performance is good; for suburb or roads coverage, this index can be a litter low.

Call drop reason analyzing

1 Equipments problem
The method for equipments problem can refer to the section in call failure analyzing. TRX, CE, vocoders and transmission links are the key points which should be cared for.

2 Call drop caused by overstepping the coverage
Typical case:
MS Rx power is about -100dBm or less;
MS Tx power tends to maximum 23dBm;
The strongest pilot Ec/Io < -15dB or less;
MS Tx_Adj maintains in normal range: 0~-10dB; MS goes into system searching mode after call drop, and can not find system or system signal is very weak and is very easy to be lost.

Analyzing:
In the edge of coverage area, for the forward and reverse links are all very bad, the call drop is one normal phenomenon
When pilot Ec/Io decreases to some extent, the forward link quality will be bad greatly and can not be demodulated well, F-FER will increase very fast. If MS oversteps coverage area too long (more than 5s), MS fading timer expires in 5s, MS will initialize again, that is call drop. MS will go into system searching mode after initialization, because MS has overstepped coverage area, it can not find service system or system signal is very weak and is very easy to be lost.

Note:
The above time is 5 seconds, in fact MS oversteps the coverage area in shorter time (less than 5s) also can cause call drop. In this case usually the reason is BTS call drop mechanism is faster than MS fading timer. When MS oversteps the coverage area, reverse link is also weak and R-FER is also high, then BTS call drop mechanism will be triggered. BTS will release forward link in short time (<5s), when reverse link is too bad. Even if MS returns to coverage area at this time, the call drop will happen also. Because the forward link does not exist, so the call drop must happen, though the pilot has been resumed.

Optimization method:
The ultimate way for this kind of problem is add new BTS or repeater in blind coverage area or poor coverage area.
If adding new BTS is not possible, other methods also can be used to improve coverage, like increase antenna height, select large gain antenna, and adjust antenna azimuth and down tilt. But these methods can not solve problems ultimately, and it should be very careful when change these parameters.

Call drop caused by access and handoff collision

Typical case:
MS origination call may be failed in poor coverage area in DT test, or call drop will happen very soon after successful origination. MS Rx level and pilot Ec/Io are all low, and MS Tx power is very high. As MS moves, MS Rx power will become larger and larger, but pilot Ec/Io is too bad to satisfy the demodulation requirement;
MS initializes again after call drop and stays in one new strong pilot; If MS originates successfully in good coverage area and passes the same DT rote, call drop will not happen;

Analyzing:
If system can not support access handoff (access handoff also need MS to support), then during MS access procedure, MS can only start handoff after finish access. The above case is belonging to call drop caused by access and handoff collision. If one MS starts origination in the edge of coverage area, because this is near to another cell, it is possible to meet handoff. But our system does not support access handoff, access and handoff will have collision and access has priority in system. Because access need some time, so MS Rx power will become higher and higher, but pilot Ec/Io will become lower and lower. At this time, the objective handoff pilot is becoming strong interference, when current pilot Ec/Io is lower enough; the forward link will become very bad and can not be demodulated successfully. The access will be failed. After access failure, MS will stay in new pilot (It is the objective handoff pilot). If MS originates successfully in good coverage area and passes the same DT rote, call drop will not happen, because at this time MS can carry the normal handoff procedure.

Note:
Access handoff can be supported after 5.4 version;
For terminals, 1X handset can support, but 95 handset can not.

Optimization method:
If system and terminals can support access handoff, this kind of problem will not happen;
Adjust network structure and soft handoff area. Extend soft handoff area in the area where the above problems are heavy, then MS will firstly handoff to another cell before access and MS will have enough time to finish access.

Call drop caused by forward and reverse imbalance

Typical case:
MS received power and strongest pilot Ec/Io maintain in good state, such as Rx Power>-100dBm, Ec/Io>-15dB;
As MS moves, MS reverse Tx power is increasing continuously to 23dBm, until call drop happen;
As MS moves, MS Tx_Adj is increasing continuously and it is one positive value, until call drop happen;
After call drops, MS will initialize and stay in previous pilot. There is signal in MS, but origination call is difficult, even if origination is successful, call drop is very possible.

Analyzing:
In the above case, MS Rx power and pilot Ec/Io are all good, and this means the forward link is good. But in MS origination procedure MS is increasing transmitting power until maximum, and this means the reverse link is bad. That is forward and reverse imbalance. Tx_Adj>0 also means forward is better than reverse.
Because forward is better than reverse, so in the edge of coverage BTS can not receive the signal from MS correctly and BTS call drop mechanism is triggered, and then call drop will happen.

Note:
Forward and reverse link imbalance includes two kinds: forward is better than reverse; reverse is better than forward.
The above case is the former. It popular in practice and effect is large. Because subscriber can not tolerate that call failure and call drop happen when there is signal in MS, so this case should be cared for very much.

Optimization method:
Find the rootstock of imbalance:
For the case that forward is better than reverse:
Judge if the cell power configuration is too high;
Judge if the pilot gain is too high;

Judge if there is reverse interference;
For the case that reverses is better than forward:
Judge if the cell power configuration is too low;
Judge if the pilot gain is too low;
Judge if there is forward interference;