RACH Failure in LTE

Hi Exparts,

I am a new in Telecom Hall.Can any help me out for the below doubts?
1.In RACH Failure there is a reason of RAPID mismatch.How UE can measure this?
2.In RACH Failure there is a reason if PRACH sequence delay is greater than the Cyclic shift value.How this PRACH sequence delay is calculated?

Kindly for the first Question; check below is your first question or not. Too much complicate for RACH

Upon receiving the preamble, the eNodeB applies for a temporary cell RNTI (C-RNTI) and uplink and downlink scheduling resources. Then, the eNodeB sends a random access response over the downlink shared channel (DL-SCH) for each UE. The response contains the RA-preamble identifier, timing alignment information, initial uplink grant, and temporary C-RNTI. One DL-SCH can carry random access responses to multiple UEs.

After the UE sends the preamble, it monitors the physical dedicated control channel (PDCCH) and waits for a random access response within a random access response window:

If the UE receives a response containing an RA-preamble identifier which is the same as the identifier contained in the transmitted random access preamble, the response is successful. The UE then transmits uplink scheduling information.

If the UE does not receive a response within the random access window or fails to verify the response, the response fails. In this case, if the number of random access attempts is smaller than the upper limit, the UE retries random access. Otherwise, random access fails. The maximum number of random access attempts of the UE is specified by the RACHCFG.preambleTransMax* parameter and can be obtained from SIB2.

Seem also like detect UE excess cell radius or not, it is same like use preamble ID to identify. Like below case

Optimization of Non-Contention-based Random Access Beyond Cell Radius

This function can optimize the procedure of non-contention-based random access initiated by a UE beyond the cell radius to ensure the successful access.

The optimized procedure is as follows:

An eNodeB receives a dedicated preamble that is a retransmission preamble, and both of the following conditions are met:

  • The eNodeB detects that the index of the preamble is N but not N+1.
  • The eNodeB allocates preamble N+1 to a UE but not preamble N.

Then the eNodeB delivers the RAR message that carries the preamble index of N+1 to the UE.

Detection of non-contention-based random access beyond cell radius is controlled by the NonContRaOptSwitch option under the CellAlgoSwitch.RachAlgoSwitch parameter.

This function is recommended during off-peak hours when the non-contention-based random access success rate in a cell is lower than expected. This function takes effect only when all of the following conditions are met:

  • The cell is a normal low-speed cell.
  • The distance between the UE and the eNodeB is greater than the cell radius and is less than twice the cell radius.
  • The distance between the UE and the eNodeB is less than or equal to 100 km.

The distance between the UE and the eNodeB is greater than the cell radius and less than twice the cell radius. The eNodeB parse the preamble ID as a value less than 1 compare Assign preamble ID . If the following conditions are met, the eNodeB considers that the preamble ID of a non-contention UE exceeds the radius and sends an RAR message to the UE based on the normal procedure.

For example: at point 0,UE received dedicated preamble ID 63 which assigned by eNodeB, but at point 1 eNodeB calculate the preamble is 62 as it is based on UE access distance which is exceed cell radius. Preamble ID 62≠63,normally eNodeB will reject UE access,but after we enable NonContRaOptSwitch, eNodeB will consider it is ok as 62 is near 63 and eNodeB will send RAR with preamble ID 63 to UE at point 2,then UE access successful.

Thanks Alaa.It will be too helpfull.Once Again Thank you!!!