Understanding LTE Model Internals and Interfaces

1581 Understanding LTE Model Internals and InterfacesBuffer Status Reporting (BSR) for theUplink Data• UE sends BSRs as a MAC subheader in an uplink MPDU• Short (16 bits) or the long BSRs (32 bits) are sent as per the standard• After reading the BSR contents, eNodeB sends grants to serve that traffic, in whichnew BSRs can be sent, and so on.• When buffers are empty, the UE reports 0 traffic, at which point the eNodeB stopsissuing grants.• However in order to send the BSR, it needs an “initial grant”. There are 2 ways in whichthe UE gets it:• Case 1: UE has a dedicated uplink control channel (PUCCH): In this case, it sends ascheduling request (SR) bit at the first opportunity. The eNodeB issues it a grant of apredefined size, in which the UE can send the BSR.• Case 2: The UE has no dedicated uplink control channel. It this case, it uses therandom access channel and the random access procedure to get the initial grant.• When the UE is waiting for a new grant, it stays in either:• SR_TR or• BW_REQ_VIA_RACH• A UE can go in only 1 of these 2 red states depending upon whether it hasPUCCH allocation or not.CONFIDENTIAL ─ RESTRICTED ACCESS: This information may not be disclosed, copied, or transmitted in any format without the prior written consent of OPNET Technologies, Inc. © 2010 OPNET Technologies, Inc.61