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UE did not receive immediate assignment for channel request(RACH)

All the wireless devices intended to use the channel or common resources first send request to obtain the one. The network assigns the resources to the requested device (MS or UE).

In the ideal scenario channel request is transmitted by mobile subscriber (MS) also referred as UE in certain systems. It is received by Base station (BTS) or NodeB or eNodeB designated in different wireless systems.

In response to the received channel request, the network transmits immediate assignment or any other suitable message as per wireless system.

Let us understand this scenario with respect to GSM system. CHANNEL REQUEST is a request of an MS for a channel when in the idle state. Although only 1 byte long this message already contains the reason for the connection request (answer to PAGING, Emergency Call, etc.) and an identifier for the channel type that the MS prefers. The CHANNEL REQUEST has no hexadecimal message type, because the message does not conform to the regular format and is sent via an access burst.

First of all, depending on the channel configuration, it is possible to send a RACH, practically anywhere within a 51-multiframe.

One of the RACH control parameters sent in the BCCH/SYSTEM INFORMATION MESSAGES 1-4. MAX_RETRAN indicates how many consecutive times an MS may send the CHANNEL REQUEST message to the BTS. Valid values are 1, 2, 4, and 7.

If UE does not receive IMMEDIATE ASIGNMENT for the multiple channel requests (RACH), timer T3126 gets expired. This results into UE connection failure with the GSM network (I.e. base station or cell).

Following are the checkpoints to be analyzed for this issue. Check for both the transmit and receive path.

Let us examine transmission of channel request from the UE:
•  Check for whether channel request has been scheduled on the proper slots in the 51 frame multi frame as per combined or non combined channel type.
•  Check whether the power level for the access burst is fair enough.
•  Check for proper gain settings in the RFIC.
•  Check for upper layer values which need to be embedded into the access burst is as expected or not.
•  Check the channel conditions at the time of channel request initiation.

Let us examine reception of the same at the network side:
•  Check whether the Channel request has been received at the network side. Network log captured can help for the same. Equipments such as Agilent 8960 and Anritsu 8480 help capture the network log. If the channel request is not received then the problem might be with UE transmission or in the channel itself.
•  If the channel request has been received then whether response for the same has been transmitted back to UE/MS or not.
•  If the response such as IMMEDIATE ASSIGNMENT in GSM is already sent by network but is not received by the UE then there might be problem with the UE receive part of channel can be the cause of the issue.

Timer T3122 is used during random access or during dedicated channel assignment while in packet transfer mode (PS mode), after the receipt of an IMMEDIATE ASSIGN REJECT message at UE.

Often it has been observed that poor C/I can results into channel requests not reaching the network.

In LTE RACH is transmitted using PRACH physical channel. The similar issue if observed in LTE system can also be analyzed with the checklists mentioned above.

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