36.213 series (2) - Random Access

RACH is the interface between non-synchronized UEs and eNB.

Difference between LTE and WCDMA on RACH:

- RACH can't carry any user data, just pre-ambles. One exception is the scheduling request (SR), if the UE doesn't have any uplink resource block to send SR, it is allowed to send SR via RACH

- RACH is orthogonal with other resources such as PUSCH/PUCCH

When to RACH?

- A UE in RRC_CONNECTED state, but not uplink synchronized, need to send uplink data or control message (e.g. meas. report, ACK/NACK for downlink data)

- A UE in RRC_CONNECTED statebeing handover from one cell to another

- A transition from RRC_IDLE to RRC_CONNECTED (e.g. initial access or TAU)

- Recovering from radio link failure

Contention based Random Access

step 1) UE randomly choose RACH preamble signature. One UE says : "hello, I am Zhang-san", but the other can also say the same. A collision could happen and a collision resolution is required. The random access message is sent according to PRACH configuration (acquired by UE via system information or handover command). Initial transmission power is based on open loop estimation with full path-loss compensation.

step 2) eNodeB sends RAR (random access response) on PDSCH, and addressed with an ID, the Random Access Radio Network Temporary Identifier (RA-RNTI). RA-RNTI identifies the Time-frequency slot in which the preamble was detected. If multiple UEs had collided by selecting the same signature at the same time, they would each receive the RAR.

The RAR gives UE a C-RNTI (temporately), timing information and an initial uplink resource grant.

step 3) UE(s) sends the uplink message (For e.g. a ACK/NACK, measurement report, RRC connect request, etc) on the uplink resource granted by RAR. If a collision happened during step 1), they are going to collide again at this step. Each UE shall sent its own UE identity or C-RNTI (if its has one) in this uplink message, so that proper contention resolution can be done in the next step. Note in this step, the C-RNTI or UE identity is now unique for each UE.

Step 4) Possibly, the uplink message from none of the UE can be decoded, then all UE will have to re-RACH (triggered by guard timer expiry). Or, if one UE is succesfully decoded, it will receive a contention resolution message address to it (by C-RNTI or UE identity, which echos the message from UE in step 3). Other UE will also hear this message but the message is not addressing to them! Other UEs will now realize there was a collision and quit current RACH procedure to start a new one.

Contention free Random Access

UE use pre-allocated preamble signature. eNB can reserve a set of preambles signatures to be used for contention free Random Access (total number of available preamble signatures in LTE is 64).