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LTE: What are all parameters considered by an eNodeB while giving UL grant for an UE ?

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posted May 27, 2014 by Vimal Kumar Mishra

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Actually this question came in my mind when I was going through UL-AMBR.
For the downlink data, PCEF imposes policy rules. But how eNodeB control UL-AMBR for an UE. Does eNodeB considers UL-AMBR while allocating UL resource for an UE ? As per my knowledge UL-AMBR information is not communicated to UE from eNodeB through any of the RRC message.

1 Answer

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There are many type of schedulers are available in LTE. Some of them are

Fair Fixed Assignment Scheduler:
It assigns the same a priori specified, number of resource blocks to each mobile station. The number of assigned resource blocks per MS, denoted m is hence the same for each mobile station, independently from its location, and is an operator-specified parameter.If the number n of active users is such that the total number of requested resource blocks is less than the available number of resource blocks per TTI, i.e. if n.m < M, then a number of resource blocks areleft idle. aturally this reflects a certain degree of resource inefficiency in the scheme, especially for situations with low traffic load and hence few active users.

Fair Work Conserving Scheduler:
The scheme's objective is to distribute the available resource blocks evenly over the active users within each individual TTI. FWC scheduler is optimal in the class of resource-fair Round Robin schedulers. In principle each of the n MSs is assigned M/n resource blocks in each TTI. Since M/n needs not be integral, in an implementable version of the FWC scheduler, a scheduling cycle is defined of multiple TTIs during which user-specific resource block assignments appropriately vary between low allocation high allocation in order to, onaverage, achieve the fair assignment of M/n resource blocks. More specifically, the cycle length is equal to the smallest integer c such that c =M/n is integral, which is at most equal to n.

Maximum Added Value Scheduler:
This scheme has as main objective to maximise the total data rate realised given the active users present in the system. The scheme assigns RBs to those users that can make best use of it. In particular, scheduling decisions are based on a metric termed added value, which, for a particular user, is the gain in data rate that a new resource block can deliver to that user. Of all active users the one with the highest added value is assigned the resource block.This procedure continues until
all resource blocks have been assigned thus resulting in cycle length c = 1. In the MAV scheduling it is possible that cell edge users are deprived from service
if the system is under high load - since other users can make better use of the available resource blocks cell edge users get none.

Apart from these schedulers SPS is used for voice call.

answer May 28, 2014 by anonymous
But this does not describes the original query?
Hi Sahil,

LTE Scheduler handles the resource allocation. I explain some of the scheduler how they are allocating the resources among different UEs.
For example, if we implement Fair Fixed Assignment Schedule then It will assigns the same a priori specified, number of resource blocks to each mobile station i.e. No of RB per UE = (total number of resource blocks)/(number of active UEs).

Hope I answer your query.
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In case of Rel 10 supported(Carrier Aggregation) eNodeb, how does the enodeb decide as to through PDCCH of which cell UL Grant is sent ?

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As far as I know eNodeB calculates the number of Physical Resource Blocks allocated to each UE, based on the specific metric, then takes the minimal modulation and coding scheme to deduce the transport block size that must be allocated to each user, Why this minimal MCS ? Isn't it too sub-optimal?

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