AVOID RACH Collision •
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Check CO-BCCH and CO-BSIC in same BSC and close in cells of other BSC’s Change BSIC (In case of CO-BCCH, CO-BSIC) Decrease value of Max RACH Transmit (MAX_RET) (Decrease it’s value . values are 1,2,4 or 7) change to 2. High TA [give ACC_MIN ACC_MIN (tune nearby -100) RX_LEVEL_Min_Access(RXP)] TX_Integer.
Tx-integer Tx-integer Value range: 3~12, 14, 16, 20, 25, 32, 50 Unit: Number of RACH timeslots (equivalent to one timeslot, 0.577ms) Content: Extended Transmission Timeslots(T), used to calculate the number of timeslots in the interval between two channel requests sent by MS. Recommendation: 25 (If the cell immediate assignment success rate is low, set the S with a larger value. Select 32 for satellite transmission, so as to reduce satellite transmission delay).
It is set to reduce the collisions on RACH. It mainly affects the execution efficiency of the immediate assignment process. The value of this parameter is related to CCCH configuration mode, both work together to determine parameter S. The number of timeslots (excluding the timeslot from where the message is sent) from MS starts the immediate assignment process to the first channel request message sent is a random value in the set {0, 1, …, MAX(T, 8)--1}. And the number of timeslots between any two adjacent channel request messages is a random value in the set {S, S+1, …, S+T-1}. Generally, parameter S should be as small as possible (in order to shorten the access time of MS) when parameter T is fixed, but AGCH and SDCCH must not be overloaded. During the operation, a random value can be selected for parameter T for the cell with unknown traffic to ensure the minimum value of S. If AGCH or SDCCH of the cell is overloaded, then parameter T can be changed to make parameter S larger, until AGCH or SDCCH of the cell is not overloaded. The range of value T can be specified according to the above principles (multiple values can be selected for parameter T corresponding to various values of parameter S). When the RACH collisions is serious, value T should be large. When the number of RACH collisions is small, value T should be as small as possible.
If the the mobi mobile les s requ reques estt thro throug ugh h RACH RACH is not not answ answer ered ed with with in a specif specified ied time time the the MS assume assumes s that a colli collisi sion on has occurre occurred d and repeat repeats s the reques request. t. Mobile Mobile must must allow allow a random random delay delay before before reinitiating the request to avoid repeated collision.
Agar TA high hoga to UL poor hoga RACH is UL MS power is not able to reach BTS So RACH failure U can change parameter MAXRET Its no of RACH transmissin Iski vaue decrease kar de Maxmium RACH Transmission For TA ACCMIN tune kar de around -100 kar de ji n site par high hai Poor BSIC Plan: A high number of RA failures often are caused by bad BSIC plan. If a BSC is seen to have poor RA Success with all the other root causes not fitting the “profile” of the concerned BSC, then check for excess use of co-BCCH/BSIC plan within the BSC and for cells from immediate surrounding BSCs. Minimize excess use of co-BCCH/BSIC across the same BSC and also for close in cells from the surrounding BSCs. • Poor BCCH plan: When an MS tries to access the network either in response to a page from the network or in the form of a mobile originating call, it does so using the timeslot carrying the BCCH frequency. Excess levels of interference (co and adjacent) on the BCCH carrier can cause poor RA Succ Rate ACCMIN bhi kam kar de try kar sakta hai Pahle BSIC change kar siteka
Ghost RACH or Phantom RACH Phantom RACH is a process when specific mobile is sending Random Access burst to aserving cell (cell which specific mobile is camping on). The Random Access burst containsTSC (Training Sequence) the TSC is derived from BCC number which is part of BSICtransmitted on Synchronization Channel (SCH) on BCCH TRX i.e. TSC=BCC for BCCH TRX.Now when mobile transmits Random Access burst, this transmission is OMNI directionali.e. transmitted not only toward serving cell but to all surrounding cells as well.When network utilizes a very tight BCCH reuse (less than 25 BCCH in use) in urban area,then the BCCH reused with in the limited geographical area many times and BCCHallocation requires proper allocation of BCC in order to prevent BCCH/BCC (not BSIC)reuse.More over the Phantom RACH is related to UL performance of the cell. The ULperformance of the cell is 100% depends on antenna configuration i.e. antenna type andtilt. Correctly designed antenna configuration will prevent appearance of Phantom RACHas well.
BCCH Freq is frequency channel that is used to carry or transmit BCCH.One Cell has one BCCH.* BSIC (Base Station Identity Code) is Code that is used to distinguish Co-Channel BTSs. It is comprised of NCC(Network Color Code) + BCC(Bas Station Color Code).If there are two BTSs are close each others with different BCCH anddifferent BSIC, It will not be a problem. Each BTS will erceive RACH from itsown MSs.But If those two BTSs have different BSICs but have same BCCHs, It maycause Phantom RACH. MS will transmit a RACH to those two BTSs with justone certain BSIC. The BTS who has this BSIC will responce
this RACH, but theother BTS who doesn’t have this BISC will not response RACH. This is whatwe call Phantom RACH or Ghost RACH.
Post subject: Re: What is Phantom RACH?
Occurs primarily due to sporadic noise and interference, channel requests from distant MS can be affected by such noise when MS output power is low and if MS output power is high could cause interference to other co-adjacent channel cells.....the channel coder decodes the message on the RACH from the radio if message is decoded channel request is forwarded to Layer 1 of the BTS (Layer 1 does no filtering of RACH) --> BTS Abis receives the channel request and determines valid establishment cause value and if determined forwards the channel request to Call Processing --> Call Processing does no filtering of channel request and attempts to allocate an SDCCH if MS does not respond to immediate assignment that is no MS ever existed for RACH or MS is far away or turned off " CHANNELREQUESTMSFAIL" is pegged..........
