Why separate "RACH" Channel needed? why cant it send in PUCCH ?

Question

why ue cant transmit on PUCCH and PUSCH simulteneously

Answer 1

Three uplink channels (PUCCH, PUSCH, PRAH) are defined in LTE.
PRACH (Physical RACH channel) is a common channel where UEs can send initial small size information to get the uplink shared resources. While PUCCH comes into picture when UE has to send dedicated minimal information (for example: Scheduling Request) to eNodeB.

If UE does not have PUCCH resource and it has something to send to eNodeB, it initiates RACH procedure using the PRACH. For example: UE is in RRC-IDLE state and it wants to send "RRC Connection Request" to eNodeB, since eNodeB does not have PUCCH resource to inform eNodeB, it has to execute RACH procedure to get the initial grant over the PUSCH channel.

Comments

Hi Thank you ... So RACH initiation  doesnt required any scheduling .. if so why RAR needs scheduling ?

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Since there is no dedicated resources are assigned for an UE to send initial Random Access Preamble that's the reason collision may occur. There is a probability when more than one UE can receive Random Access Response for the same Preamble sent to the eNodeB in "Random Access Preamble". Therefore, Contention resolution is required.

But in case of handover procedure when target eNodeB/cell can assign a dedicated preamble that will used while doing RACH as target eNodeB/cell. In this case there is no chance of contention. In order words, Preamble was scheduled.

Since channel name itself is "Random", means any UE can send Preamble request at any time but location of Random access channel is common for all UEs. SIB2 carries random access channel information.

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The PRACH is a fixed resource in LTE whose locations are defined in the Broadcast information, specifically SIB 2.  In FDD, the 2 primary parameters are the PRACH configuration index, and the PRACH Frequency offset.  Having established time synchronization on the downlink, the UE can make it's best guess as to how to send a RACH Preamble aligned with PRACH uplink resources.  

The RACH has to be designed so a UE that has never joined the cell/network before can seamless enter.

The part of the attach procedure that gets to the root of your question is the so-called timing advance.  As I mentioned earlier, the UE can only make a good guess as to when to transmit to align with uplink signaling.  Time alignment is required in order to enable efficient processing of the LTE uplink by the eNB.  Further, if a UE transmits on any other channel but the PRACH, and it is out of time alignment, it is quite likely it's transmissions will not be able to be read at all.

So without time alignment an uplink PUCCH transmission doesn't really make sense.  Further complicating the PUCCH concept you bring up is that the eNB knows where to look in the PUCCH resource for transmissions of specific UEs based on aspects of previous resource assignments.  So not only is timing an issue, but the PUCCH design would not support a random access of UEs.

So if a UE is already connected, the tower has full context on it, so it makes sense that a connected UE can request uplink resources without a more formal attach process.

A UE that is not connected doesn't have any identifiers, in particular it doesn't have a C-RNTI.  The C-RNTI is the "address" through which a UE will be granted uplink/downlink resources.  

If you notice the random access response is tagged in control channel by the so-called RA-RNTI (random access RNTI) because it cannot be not known by the tower which UE PRACH it has successfully processed.  The RAR contains a Temporary C-RNTI which does not become permanent until after contention resolution has been successful.  

I hope this helps answer your question.