5/30/2023 0 Comments Reconso gets alaunch slot![]() ![]() ![]() The guard time in 5G is not specified so it is up to the scheduler in the gNB to fix this by not scheduling other UEs for UL transmission. The larger the cell is, the larger guard time should be. To handle this, PRACH must have a guard time so that the UEs far away in the cell can still send a preamble without disturbing another UEs UL transmission. If not, the UL orthogonality gets lost within the cell. But in a system using OFDM with strict timing of symbols, it is important that symbols from users arrive at the receiver approximately the same time. The distance to the base station is unknown, when the UE starts a RA procedure. Below are some of the terms/characteristics associated with preamble transmission: 1. So, in general, it is up to gNB scheduler to ensure that there are no other transmissions in the UL resources in which preamble transmission may take place. Prior to preamble transmission, there is no closed-loop timing control in operation so there will be an uncertainty in the preamble reception timing which for larger cells could be in order of 100µs or even more. In 5G NR, transmission timing of UL transmission is typically controlled by the network by means of regularly provided time adjustment commands (called closed-loop timing control). Here in this blog, i will mostly focus on the RA Procedure during the standalone mode. Once the random access (RA) procedure is completed, UE moves to connected state and UE-NW communication can continue using normal dedicated transmission. Message 3 and Message 4 will be used to resolve an eventual collision between 2 or more UEs attempting to access the network with the same preamble in the same physical PRACH resource, which is done by a unique identity (Either the RAN identity or the Core Network identity). Random Access Response (RAR) sent by the network indicates which preamble it is related to, the Timing Advance ( TA) it should use, a scheduling grant for sending Message 3 and a temporary Cell Radio Network Temporary Identifier ( TC-RNTI). If not, UE will make a new attempt with the increased power. UE will start monitoring the DL channel to see if the base station (gNB) answers the request to connect to the network. Summary of the procedure is: UE selects a “ preamble” (a code sequence) and sends it at a random time on a UL channel called Physical Random Access Channel ( PRACH). ![]()
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