Question

Why do we need only contention free RACH in LAA cells?

Answer 1

LAA is an technology which uses unlicensed spectrum, so channel availability and its duration cannot be guaranteed . Hence ue need to attach to the network as soon as possible so it uses contention free rach in LAA cell to achieve fast rach procedure.

Answer 2

This RACH procedure is performed between UE and eNB. LAA utilizes Secondary channel along with the primary channel for its transmission.
In RACH procedure, when UE request for the uplink/downlink access request from the eNB. Then eNB will send the scheduling information through the DCI packets. Then UE has to perform its operation with in 4ms time frame. where 4 ms is for the HARQ delay. So when eNB in Pcell sends the grant to UE in unlicensed spectrum, UE has to start contesting for the channel before transmitting the data in spectrum. So if they follow contention based RACH procedure, it is not much appreciable for the LAA systems.

Regards,
Balagopal.

Answer 3

As per my understanding, LAA (LTE augmented access) works on primary and secondary cell concepts.
UE always does RACH contention based in primary cell but for the secondary cell access information sent to UE using primary cell in RRC Connection Reconfiguration.

Let others respond on this query. I am completely doubtful.