Understanding LTE Model Internals and Interfaces
Understanding LTE Model Internals and Interfaces
Understanding LTE Model Internals and Interfaces
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1581 <strong>Underst<strong>and</strong>ing</strong> <strong>LTE</strong> <strong>Model</strong> <strong>Internals</strong> <strong>and</strong> <strong>Interfaces</strong>Buffer 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 st<strong>and</strong>ard• After reading the BSR contents, eNodeB sends grants to serve that traffic, in whichnew BSRs can be sent, <strong>and</strong> 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 ther<strong>and</strong>om access channel <strong>and</strong> the r<strong>and</strong>om 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