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SDCCH Assignment Analysis & Solutions:: : 32-36pt:

:: : 24pt: OverviewInstructions on checking of SDCCH assignment failureContents> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.SDCCH: the Standalone Dedicated Control Channel is used to transmit information like channel assignment, in brief, the following processes shall be taken into consideration in the process of occupying SDCCH:Location update, periodical location update;IMSI attach/detachCall setupSMSDefinition of SDCCH> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.Signaling flow of immediate assignment

> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.1.Hard to MOC, MTC, CSSR is decreasing2.Number of SDCCH assignment failure is high in performance statistics3.TCH traffic is decreasing4.SMS is hard to be sentPhenomenon of SDCCH assignment failure> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.Number of SDCCH assignment success:Function: After BSC sends out the immediate assignment message(IMM_ASS), this counter counts the number of successful MS accesses to the corresponding SDCCH.Sampling: when BSC receive the correct EST_IND or the message of assignment complete.Number of SDCCH assignment failure:Function: After BSC sends out the immediate assignment message(IMM_ASS),this counter counts the number of failed MS accesses to the allocated SDCCH.Sampling: when BSC receives the wrong EST_IND, or when T3201 expires.Counters related to SDCCH assignment & corresponding signaling messages> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.Definition: Number of successful SDCCH assignment*100%/(Number of successful SDCCH assignment + Number of failed SDCCH assignment)SDCCH assignment success rate> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.Definition: Number of successful random accesses/Number of random access requests*100%Number of random access requestsDefinition: MS applies for a channel in the idle mode.Trigger point: it counts the message of channel required received by BSC from MS.(A1)Number of successful random accessesDefinition: BSC successfully assigns a dedicated channel for MS.Trigger point: it counts the message of IMMEDIATELY ASSIGNMENT sent from BSC to MS.(A2)Difference: Random access success rate> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.OverviewInstructions on checking of SDCCH assignment failureContents> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.MS frequently originates location update due to poor downlink quality;Improper setting of Tx-Integer;High SD assignment failure rate due to LAPD delay;The same frequency and the same BSICUplink interferenceCoverage overshootingExplanation on common causes of SDCCH assignment failure> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.The default of Tx-Integer is 14,which is also the max value.When the transmission link delay is long, while Tx-Integer is set with a small value, it will result in MS sending too many access requests. However, MS only responds to the first immediate assign it receives.Improper setting of Tx-Integer> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.Improper setting of Tx-Integer-Flow chart of repeated assignment failure

> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.Possible causes of LAPD delay:Heavy flow on LAPD leads to delay. For example, improper LAC division will lead to large amount of paging and hence LAPD flow control.Transmission equipment fault leads to loss of message on LAPD or long LAPD delay. These phenomena are often accompanied with SDCCH assignment failure.The transmission equipments own delay, such as the delay caused by satellite transmission at Abis interface.Impact of PS service: PS service is more sensitive to network delay. Any LAPD delay will leads to re-transmission of PS service message, which increases the flow on LAPD and causes longer LAPD delay, then a malicious circle will be resulted.LAPD delay> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.Two cells have same BCCH and same BSICThe Channel Request sent by MS is received by two cells and they assign SDCCH at the same time, but MS can only accept one SDCCH, therefore, one of the two cells will inevitably experience SDCCH assignment failure.Two cells have same BSIC and the TCH frequency of one cell is same as the BCCH frequency in the other cell.The handover access request occurring on the TCH timeslot is received as channel request by the other cell, which thereafter performs assignment. This certainly leads to SDCCH assignment failure. The same BCCH and the same BSIC> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.If the coverage of cell is too large, the DL Rxqual at the cell margin will be poor. In this case, BTS can receive channel request sent by MS, but MS can not receive immediate assign sent by BTS, because BTS is more sensitive than MS.If the coverage of cell is too large, the cell may share channel and BSIC with the cell which is far away.Solution to overshootingAdjust the engineering parameters of antenna to limit the cell coverage.TA_allowed can effectively decrease SDCCH assignment failures caused by overshooting. The side effect it bring is that the distant MS is not able to access network. Therefore, the threshold of TA_allowed shall be set a bit higher than the cells actual coverage. Besides, we should take into account the transmission distance of repeater when calculating the cell coverage range.Overshooting> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.BTS receiving sensitivity is -112dbm~-125dbm.If the random access signal strength received by BTS is lower than BTS sensitivity, it usually is confirmed to be interference. The interference can be decoded as random access, which is called as fake access, and will definitely lead to SDCCH assignment failure.Another feature of fake access is that TA is larger than that needed for the actual coverage range.Solution: TA_allowedUplink InterferenceFake Access> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.If MS needs to make location update, while the radio environment is poor, it will retransmit Channel Request with the cause of location update again and again, but it can never receive Immediate Assign message.The frequent location update will cause fluctuations in SDCCH assignment indicators.Frequent location update started by MS> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.Check Tx_Integer of the problem cell, along with LAPD delay observed from signaling.Check whether the LAPD link of BCCH TRX in the problem cell is multiplexed with other cells.Check whether any of the adjacent cells have same frequency and BSIC with the problem cell.Check whether the value of counter number of access attempts due to other causes is big. If so, and the counter number of access successes due to other causes is zero, it is possible that handover access on other TCH TRXs are decoded as channel request by the problem cell.Troubleshooting instructions> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved.Check SDCCH allocation KPI and transmission alarmsIf SDCCH & TCH assignment indicators are all bad, the problem shall be related to radio environment.Analyze signaling and check if Channel Request with large TA, If so, fake access exist and TA_allowed restriction can be used.Troubleshooting instructions> G91, B170G137, B207G174, B239G171, B189:: : 30-32pt:

(1-5):: : 28-12pt: ZTE Corporation. All rights reserved. ZTE Corporation. All rights reserved.Thanks!