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Article
LTE Allocations explained - 2
by
Samir Amberkar
(published on 18-Aug-2012)
Abstract:
In this article, author attempts to describe LTE allocations further, covering details on uplink control signaling.
Let us start with following scenario. There are few packets at UE, waiting to be sent over uplink and few at eNode B to be sent over downlink, at (PDCP-)RLC buffer level. We will try to note the differences between DL and UL allocations.
In downlink, as radio resource usage is controlled by eNode B, allocations process is a single step process once downlink signal conditions are known. But in uplink, it is not that straightforward. UE does not control UL allocations and eNB, to give allocations to UE, does not know the amount of UL allocations needed. So how eNode B to do UL allocations or how eNode B to give an opportunity to UE(s) to request appropriate amount of allocations ?
Fig. 1.8.1
This is not specific to LTE, but generic to wireless resources (especially protocols targeted to be adaptive and highly efficient). There are number of ways to solve this: random access (UE uses shared collision prone resources to request or transfer UL data), regular UL allocations (eNB gives regular UL allocations to UE or to group of UEs) , irregular UL allocations piggybacked on DL allocations (DL allocation carry UL allocation information), or combinations of these methods etc.
In LTE, UL allocations are done by combination of MAC and PHY procedures. UE makes a scheduling request, eNode B gives UL allocation, UE while using UL allocation also includes "buffer status report" (BSR), eNode B gives further UL allocation based on BSR. This is illustrated by simplified example below: BSR simply contain logical channel group ID and buffer level. LCG ID is given during RRC Reconfig procedure [36.331:LogicalChannelConfig].
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