A good reverse link connection between the MS and the base station is required when a call is required in a CDMA system. If this link is interrupted for any reason, the MS loses precise power control. For CDMA, a self-interference system, power control is the key to determining system capacity and performance. Therefore, if the MS loses control of the base station, it will adjust the power of its own transmission according to the power received by the base station. This may cause the MS to self. The maximum power transmission causes great interference to the entire system, so important processes such as power control and switching require good closed-loop channels.
MS call drop mechanism:
When the MS receives poor signal quality on the forward link, it causes a higher FER (Forword Error Rate), indicating that the forward link is not good. If the MS continuously receives 12 bad frames, the MS stops transmitting. At the same time, the T5m (usually set to 5 seconds) counter of the MS starts counting down. If the MS receives 2 consecutive good frames before the counter expires, the counter is reset and the MS retransmits; if the counter expires and there is no reset, the MS reinitializes, causing the call to fall. The other is that the MS does not receive the acknowledgment message: When the MS transmits the acknowledgment information on the traffic channel, if it does not receive the acknowledgment message from the base station after resending N1m times, the MS will also enter the initialization state.
Base station call drop trigger mechanism:
The CDMA system does not specify the call drop mechanism of the wireless subsystem, but the device manufacturer generally specifies the corresponding call drop mechanism according to the MS call drop situation. One is that the base station receives a certain number of bad frames, and the base station closes the forward link. The other is that after receiving the acknowledgment information, the system still considers that the call is dropped.
two. Analysis of the reasons for dropped calls1. Drop call caused by access/switch conflict
When the MS initiates a call at the edge of the cell coverage, the pilot cell strength of the serving cell is weak, and it is necessary to switch to the new pilot during the access process, while the IS-95A CDMA system does not support the process of accessing the system. Switching at the same time, it is very likely that a dropped call will occur. From the data analysis, the signal strength received by the MS is getting stronger and stronger in this case, but the dominant frequency Ec/Io is getting weaker. When the pilot frequency Ec/Io is below -15dB during the access process, the forward channel signal quality will be greatly degraded because the new pilot is a strong interferer. When the current link cannot be demodulated by the MS, the handset quickly receives 12 consecutive bad frames, the handset stops transmitting and starts the T5m counter. If the Ec/Io of the pilot frequency after the MS receives the channel allocation message 1~2 frame is less than -15dB, the MS will not be able to switch and the call will be dropped, and the MS reinitializes to a strong pilot signal. Currently, in the IS-95B and CDMA2000 systems, the switching problem of the access process has been solved.
2. Call drop caused by switching failure
The characteristics of such dropped calls are that the mobile station's transmit power is maximized, the mobile station's received power is increasing, and the pilot's Ec/Io is continuously decreasing, and it is rapidly increased after resynchronizing to the new pilot. TX-GAIN - The amplitude of ADJ remains flat. The Ec/Io of the pilot continues to decrease as the received power of the mobile station increases, indicating that a new strong pilot becomes the source of interference and should be switched. When the pilot strength is reduced below -15 dB, the quality of the forward link is severely degraded, and the mobile station will turn off its transmitter when the current link cannot be successfully demodulated. Since the mobile station no longer transmits a signal, the reverse closed-loop power control bits are ignored and the amplitude of the TX-GAIN-ADJ remains flat, typically a few dB. A very high mobile station receive power will cause the open loop power control process to underestimate the required mobile station transmit power.
The reasons for the dropped calls are:
1) Soft handover failure caused by handover grant algorithm: If the BS log shows that there is a suitable pilot in the PSMM message and there is available resources, but does not send a handover indication message (HDM) containing strong pilots, it is a handover grant algorithm problem. Possible reasons are: no more than 3 soft handovers are allowed; the handover algorithm is imperfect; it is not allowed to switch to pilots that are not in the neighbor list.
2) Soft handover failure caused by resource allocation: When performing soft handover, it is necessary to apply for resources to the target base station. The system must ensure that there are enough resources to support soft handover. If the system has a large number of active users or because the handover rate is too high, all resources are eventually exhausted, and the handover fails due to the lack of available resources. If excessive resources are reserved for handover calls, the probability of blocking new calls will increase. Therefore, the access control process may not reserve sufficient resources for handover, resulting in handover failure. Possible reasons are: the network load is too large; the switching rate is too high. It can be solved by adjusting the switching parameters T-ADD, T-COM, T-DROP, T-TDROP and the handover grant algorithm.
3) Soft handover failure caused by handover signaling: With the available resources, the handover grant algorithm also allows, whether the soft handover is successful or not depends on the timely transmission and reception of appropriate signaling messages. If the signaling used for handover is incomplete and timely, it may also cause the handover to fail. If the base station log records that the pilot strength measurement message (PSMM) including the strong pilot is not received or the PSMM is received for a long time, there is a problem with the handover signaling.
The main reasons are: strong available pilots are not detected: the mobile station reports the detected strong pilot to the base station. If the mobile station detects that the pilot is slow or does not detect all the pilots, the handover will not be performed in time. . If the mobile station does not transmit a PSMM that includes strong pilots or transmits very slowly, then the mobile station does not detect strong pilots. Possible reasons are: the search window is too small, T-ADD is too high, and the pilot search for the mobile station is too slow. The parameters that can be adjusted are: SERACH-WIN-A, SERACH-WIN-N, SERACH-WIN-R, T-ADD, PILOT-INC.
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