RACH Failure Troubleshooting

RACH Access Failure Troubleshooting (I) •Why are RACH parameter VERY important? Because it impacts strongly user experience (also called E2E=end-to-end user experience) No performance indicators. Only estimation by RTWP, load of the RACH channel etc..

Enough performance counters

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RACH Access Failure Troubleshooting (II) The answer on AICH must be a specific positive response for the specific RACH sent Max_TX_power_on_PRACH NB01

Power_Ramp_Step :

….

Preamble_Initial_Power :

Uplink/UE/PRACH

Preamble 1

Pp-m :

….

Message part

Preamble n AICH_Transmission_Timing

Preamble_Retrans_Max :

MMax Parameters for RACH/PRACH: •NBO1( 0 NBO1min  NBO1  NBO1max ) is the time between 2 ramping power of the preamble within the same preamble cycle. •Preamble_Retrans_Max is the maximum number of preamble that can be sent in a cycle. •Mmax is the maximum number of preamble cycles. •Preamble_Initial_Power = Primary CPICH TX power – CPICH_RSCP + UL interference + Constant Value •Constant value is an initial value to start the first preamble power usually is -24. •UL interference is the latest value broadcasted by the NodeB in SIB7. Ue needs to decode this value before being able to transmit RACH. •Power_Ramp_Step is the how much the preamble power should be increased after each No ack received on AICH. •Power offset P p-m = Pmessage-control – Ppreamble, measured in dB, between the power of the last transmitted preamble and the control part of the random-access message. •AICH_Transmission_Timing is the time when the RACH message must be transmitted after positive AICH was received( there are other parameters too) RACH is a common type transport channel in the uplink. RACHs are always mapped one-to-one onto physical channels (PRACHs), i.e. there is no physical layer multiplexing of RACHs, and there can only be one RACH TrCH and no other TrCH in a RACH CCTrCH. Service multiplexing is handled by the MAC layer. In one cell several RACHs/PRACHs may be configured. If more than one PRACH is configured in a cell, the UE performs PRACH selection RACH message mandatory parameters: -UE identity( IMSI,IMSI+LAI, TMSI, IMEI-when no USIM is inserted) -RRC establishment cause (31 causes) -radio bearer ID( AS or NAS, UM or TM or AM) -release5 indicator -measurements results on RACH(like EcNo of the serving cell).

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RACH Access Failure Troubleshooting-(III) From all this information what do you need to know?: • Current RACH parameters are not optimal: allows the UE to increase the power 20 dBm more than the RTWP(CONSTANTVALUE=-20, PREAMBLERETRANSMAX=20, POWERRAMPSTEP=2). Due to this RTWP increase, due to this RACH increases and so on(it creates an avalanche effect). Better have longer call setup time for one UE (RACH failures due to missing NB relations of overshooting cells) instead of having entire cell shrinked due to one UE not being able to transmit RACH message.

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Missing neighbours, lack of best server area and poor UL coverage influence a lot the RACH success rate.

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Cell radius is now at 29.000 km. Make sure there are no UE from a larger distance(path distance) else will fail on RACH.

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Spreaders inside the Node-B are limited. Multipath ( long distance) is not good for resource consumptions and so RACH messages might be missed.

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Huawei Confidential